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+<!DOCTYPE html>
+<html>
+<head>
+<meta http-equiv="content-type" content="text/html; charset=UTF-8">
+<title>Google C++ Style Guide</title>
+<link rel="stylesheet" type="text/css" href="include/styleguide.css">
+<script language="javascript" src="include/styleguide.js"></script>
+<link rel="shortcut icon" type="image/x-icon" href="https://www.google.com/favicon.ico" />
+</head>
+<body onload="initStyleGuide();">
+<div id="content">
+<h1>Google C++ Style Guide</h1>
+<div class="horizontal_toc" id="tocDiv"></div>
+
+<div class="main_body">
+
+<h2 class="ignoreLink" id="Background">Background</h2>
+
+<p>C++ is one of the main development languages used by
+many of Google's open-source projects. As every C++
+programmer knows, the language has many powerful features, but
+this power brings with it complexity, which in turn can make
+code more bug-prone and harder to read and maintain.</p>
+
+<p>The goal of this guide is to manage this complexity by
+describing in detail the dos and don'ts of writing C++ code.
+These rules exist to
+keep the code base manageable while still allowing
+coders to use C++ language features productively.</p>
+
+<p><em>Style</em>, also known as readability, is what we call
+the conventions that govern our C++ code. The term Style is a
+bit of a misnomer, since these conventions cover far more than
+just source file formatting.</p>
+
+<p>
+Most open-source projects developed by
+Google conform to the requirements in this guide.
+</p>
+
+
+
+
+
+<p>Note that this guide is not a C++ tutorial: we assume that
+the reader is familiar with the language. </p>
+
+<h3 id="Goals">Goals of the Style Guide</h3>
+<div class="stylebody">
+<p>Why do we have this document?</p>
+
+<p>There are a few core goals that we believe this guide should
+serve. These are the fundamental <b>why</b>s that
+underlie all of the individual rules. By bringing these ideas to
+the fore, we hope to ground discussions and make it clearer to our
+broader community why the rules are in place and why particular
+decisions have been made. If you understand what goals each rule is
+serving, it should be clearer to everyone when a rule may be waived
+(some can be), and what sort of argument or alternative would be
+necessary to change a rule in the guide.</p>
+
+<p>The goals of the style guide as we currently see them are as follows:</p>
+<dl>
+<dt>Style rules should pull their weight</dt>
+<dd>The benefit of a style rule
+must be large enough to justify asking all of our engineers to
+remember it. The benefit is measured relative to the codebase we would
+get without the rule, so a rule against a very harmful practice may
+still have a small benefit if people are unlikely to do it
+anyway. This principle mostly explains the rules we don’t have, rather
+than the rules we do: for example, <code>goto</code> contravenes many
+of the following principles, but is already vanishingly rare, so the Style
+Guide doesn’t discuss it.</dd>
+
+<dt>Optimize for the reader, not the writer</dt>
+<dd>Our codebase (and most individual components submitted to it) is
+expected to continue for quite some time. As a result, more time will
+be spent reading most of our code than writing it. We explicitly
+choose to optimize for the experience of our average software engineer
+reading, maintaining, and debugging code in our codebase rather than
+ease when writing said code. "Leave a trace for the reader" is a
+particularly common sub-point of this principle: When something
+surprising or unusual is happening in a snippet of code (for example,
+transfer of pointer ownership), leaving textual hints for the reader
+at the point of use is valuable (<code>std::unique_ptr</code>
+demonstrates the ownership transfer unambiguously at the call
+site). </dd>
+
+<dt>Be consistent with existing code</dt>
+<dd>Using one style consistently through our codebase lets us focus on
+other (more important) issues. Consistency also allows for
+automation: tools that format your code or adjust
+your <code>#include</code>s only work properly when your code is
+consistent with the expectations of the tooling. In many cases, rules
+that are attributed to "Be Consistent" boil down to "Just pick one and
+stop worrying about it"; the potential value of allowing flexibility
+on these points is outweighed by the cost of having people argue over
+them. </dd>
+
+<dt>Be consistent with the broader C++ community when appropriate</dt>
+<dd>Consistency with the way other organizations use C++ has value for
+the same reasons as consistency within our code base. If a feature in
+the C++ standard solves a problem, or if some idiom is widely known
+and accepted, that's an argument for using it. However, sometimes
+standard features and idioms are flawed, or were just designed without
+our codebase's needs in mind. In those cases (as described below) it's
+appropriate to constrain or ban standard features. In some cases we
+prefer a homegrown or third-party library over a library defined in
+the C++ Standard, either out of perceived superiority or insufficient
+value to transition the codebase to the standard interface.</dd>
+
+<dt>Avoid surprising or dangerous constructs</dt>
+<dd>C++ has features that are more surprising or dangerous than one
+might think at a glance. Some style guide restrictions are in place to
+prevent falling into these pitfalls. There is a high bar for style
+guide waivers on such restrictions, because waiving such rules often
+directly risks compromising program correctness.
+</dd>
+
+<dt>Avoid constructs that our average C++ programmer would find tricky
+or hard to maintain</dt>
+<dd>C++ has features that may not be generally appropriate because of
+the complexity they introduce to the code. In widely used
+code, it may be more acceptable to use
+trickier language constructs, because any benefits of more complex
+implementation are multiplied widely by usage, and the cost in understanding
+the complexity does not need to be paid again when working with new
+portions of the codebase. When in doubt, waivers to rules of this type
+can be sought by asking
+your project leads. This is specifically
+important for our codebase because code ownership and team membership
+changes over time: even if everyone that works with some piece of code
+currently understands it, such understanding is not guaranteed to hold a
+few years from now.</dd>
+
+<dt>Be mindful of our scale</dt>
+<dd>With a codebase of 100+ million lines and thousands of engineers,
+some mistakes and simplifications for one engineer can become costly
+for many. For instance it's particularly important to
+avoid polluting the global namespace: name collisions across a
+codebase of hundreds of millions of lines are difficult to work with
+and hard to avoid if everyone puts things into the global
+namespace.</dd>
+
+<dt>Concede to optimization when necessary</dt>
+<dd>Performance optimizations can sometimes be necessary and
+appropriate, even when they conflict with the other principles of this
+document.</dd>
+</dl>
+
+<p>The intent of this document is to provide maximal guidance with
+reasonable restriction. As always, common sense and good taste should
+prevail. By this we specifically refer to the established conventions
+of the entire Google C++ community, not just your personal preferences
+or those of your team. Be skeptical about and reluctant to use
+clever or unusual constructs: the absence of a prohibition is not the
+same as a license to proceed. Use your judgment, and if you are
+unsure, please don't hesitate to ask your project leads to get additional
+input.</p>
+
+</div>
+
+
+
+<h2 id="Header_Files">Header Files</h2>
+
+<p>In general, every <code>.cc</code> file should have an
+associated <code>.h</code> file. There are some common
+exceptions, such as unittests and
+small <code>.cc</code> files containing just a
+<code>main()</code> function.</p>
+
+<p>Correct use of header files can make a huge difference to
+the readability, size and performance of your code.</p>
+
+<p>The following rules will guide you through the various
+pitfalls of using header files.</p>
+
+<a id="The_-inl.h_Files"></a>
+<h3 id="Self_contained_Headers">Self-contained Headers</h3>
+
+<div class="summary">
+<p>Header files should be self-contained (compile on their own) and
+end in <code>.h</code>. Non-header files that are meant for inclusion
+should end in <code>.inc</code> and be used sparingly.</p>
+</div>
+
+<div class="stylebody">
+<p>All header files should be self-contained. Users and refactoring
+tools should not have to adhere to special conditions to include the
+header. Specifically, a header should
+have <a href="#The__define_Guard">header guards</a> and include all
+other headers it needs.</p>
+
+<p>Prefer placing the definitions for template and inline functions in
+the same file as their declarations. The definitions of these
+constructs must be included into every <code>.cc</code> file that uses
+them, or the program may fail to link in some build configurations. If
+declarations and definitions are in different files, including the
+former should transitively include the latter. Do not move these
+definitions to separately included header files (<code>-inl.h</code>);
+this practice was common in the past, but is no longer allowed.</p>
+
+<p>As an exception, a template that is explicitly instantiated for
+all relevant sets of template arguments, or that is a private
+implementation detail of a class, is allowed to be defined in the one
+and only <code>.cc</code> file that instantiates the template.</p>
+
+<p>There are rare cases where a file designed to be included is not
+self-contained. These are typically intended to be included at unusual
+locations, such as the middle of another file. They might not
+use <a href="#The__define_Guard">header guards</a>, and might not include
+their prerequisites. Name such files with the <code>.inc</code>
+extension. Use sparingly, and prefer self-contained headers when
+possible.</p>
+
+</div>
+
+<h3 id="The__define_Guard">The #define Guard</h3>
+
+<div class="summary">
+<p>All header files should have <code>#define</code> guards to
+prevent multiple inclusion. The format of the symbol name
+should be
+<code><i><PROJECT></i>_<i><PATH></i>_<i><FILE></i>_H_</code>.</p>
+</div>
+
+<div class="stylebody">
+
+
+
+<p>To guarantee uniqueness, they should
+be based on the full path in a project's source tree. For
+example, the file <code>foo/src/bar/baz.h</code> in
+project <code>foo</code> should have the following
+guard:</p>
+
+<pre>#ifndef FOO_BAR_BAZ_H_
+#define FOO_BAR_BAZ_H_
+
+...
+
+#endif // FOO_BAR_BAZ_H_
+</pre>
+
+
+
+
+</div>
+
+<h3 id="Forward_Declarations">Forward Declarations</h3>
+
+<div class="summary">
+ <p>Avoid using forward declarations where possible.
+ Just <code>#include</code> the headers you need.</p>
+</div>
+
+<div class="stylebody">
+
+<div class="definition">
+<p>A "forward declaration" is a declaration of a class,
+function, or template without an associated definition.</p>
+</div>
+
+<div class="pros">
+<ul>
+ <li>Forward declarations can save compile time, as
+ <code>#include</code>s force the compiler to open
+ more files and process more input.</li>
+
+ <li>Forward declarations can save on unnecessary
+ recompilation. <code>#include</code>s can force
+ your code to be recompiled more often, due to unrelated
+ changes in the header.</li>
+</ul>
+</div>
+
+<div class="cons">
+<ul>
+ <li>Forward declarations can hide a dependency, allowing
+ user code to skip necessary recompilation when headers
+ change.</li>
+
+ <li>A forward declaration may be broken by subsequent
+ changes to the library. Forward declarations of functions
+ and templates can prevent the header owners from making
+ otherwise-compatible changes to their APIs, such as
+ widening a parameter type, adding a template parameter
+ with a default value, or migrating to a new namespace.</li>
+
+ <li>Forward declaring symbols from namespace
+ <code>std::</code> yields undefined behavior.</li>
+
+ <li>It can be difficult to determine whether a forward
+ declaration or a full <code>#include</code> is needed.
+ Replacing an <code>#include</code> with a forward
+ declaration can silently change the meaning of
+ code:
+ <pre> // b.h:
+ struct B {};
+ struct D : B {};
+
+ // good_user.cc:
+ #include "b.h"
+ void f(B*);
+ void f(void*);
+ void test(D* x) { f(x); } // calls f(B*)
+ </pre>
+ If the <code>#include</code> was replaced with forward
+ decls for <code>B</code> and <code>D</code>,
+ <code>test()</code> would call <code>f(void*)</code>.
+ </li>
+
+ <li>Forward declaring multiple symbols from a header
+ can be more verbose than simply
+ <code>#include</code>ing the header.</li>
+
+ <li>Structuring code to enable forward declarations
+ (e.g. using pointer members instead of object members)
+ can make the code slower and more complex.</li>
+
+
+</ul>
+</div>
+
+<div class="decision">
+<ul>
+ <li>Try to avoid forward declarations of entities
+ defined in another project.</li>
+
+ <li>When using a function declared in a header file,
+ always <code>#include</code> that header.</li>
+
+ <li>When using a class template, prefer to
+ <code>#include</code> its header file.</li>
+</ul>
+
+<p>Please see <a href="#Names_and_Order_of_Includes">Names and Order
+of Includes</a> for rules about when to #include a header.</p>
+</div>
+
+</div>
+
+<h3 id="Inline_Functions">Inline Functions</h3>
+
+<div class="summary">
+<p>Define functions inline only when they are small, say, 10
+lines or fewer.</p>
+</div>
+
+<div class="stylebody">
+
+<div class="definition">
+<p>You can declare functions in a way that allows the compiler to expand
+them inline rather than calling them through the usual
+function call mechanism.</p>
+</div>
+
+<div class="pros">
+<p>Inlining a function can generate more efficient object
+code, as long as the inlined function is small. Feel free
+to inline accessors and mutators, and other short,
+performance-critical functions.</p>
+</div>
+
+<div class="cons">
+<p>Overuse of inlining can actually make programs slower.
+Depending on a function's size, inlining it can cause the
+code size to increase or decrease. Inlining a very small
+accessor function will usually decrease code size while
+inlining a very large function can dramatically increase
+code size. On modern processors smaller code usually runs
+faster due to better use of the instruction cache.</p>
+</div>
+
+<div class="decision">
+<p>A decent rule of thumb is to not inline a function if
+it is more than 10 lines long. Beware of destructors,
+which are often longer than they appear because of
+implicit member- and base-destructor calls!</p>
+
+<p>Another useful rule of thumb: it's typically not cost
+effective to inline functions with loops or switch
+statements (unless, in the common case, the loop or
+switch statement is never executed).</p>
+
+<p>It is important to know that functions are not always
+inlined even if they are declared as such; for example,
+virtual and recursive functions are not normally inlined.
+Usually recursive functions should not be inline. The
+main reason for making a virtual function inline is to
+place its definition in the class, either for convenience
+or to document its behavior, e.g., for accessors and
+mutators.</p>
+</div>
+
+</div>
+
+<h3 id="Names_and_Order_of_Includes">Names and Order of Includes</h3>
+
+<div class="summary">
+<p>Use standard order for readability and to avoid hidden
+dependencies: Related header, C library, C++ library, other libraries'
+<code>.h</code>, your project's <code>.h</code>.</p>
+</div>
+
+<div class="stylebody">
+<p>
+All of a project's header files should be
+listed as descendants of the project's source
+directory without use of UNIX directory shortcuts
+<code>.</code> (the current directory) or <code>..</code>
+(the parent directory). For example,
+
+<code>google-awesome-project/src/base/logging.h</code>
+should be included as:</p>
+
+<pre>#include "base/logging.h"
+</pre>
+
+<p>In <code><var>dir/foo</var>.cc</code> or
+<code><var>dir/foo_test</var>.cc</code>, whose main
+purpose is to implement or test the stuff in
+<code><var>dir2/foo2</var>.h</code>, order your includes
+as follows:</p>
+
+<ol>
+ <li><code><var>dir2/foo2</var>.h</code>.</li>
+
+ <li>C system files.</li>
+
+ <li>C++ system files.</li>
+
+ <li>Other libraries' <code>.h</code>
+ files.</li>
+
+ <li>
+ Your project's <code>.h</code>
+ files.</li>
+</ol>
+
+<p>With the preferred ordering, if
+<code><var>dir2/foo2</var>.h</code> omits any necessary
+includes, the build of <code><var>dir/foo</var>.cc</code>
+or <code><var>dir/foo</var>_test.cc</code> will break.
+Thus, this rule ensures that build breaks show up first
+for the people working on these files, not for innocent
+people in other packages.</p>
+
+<p><code><var>dir/foo</var>.cc</code> and
+<code><var>dir2/foo2</var>.h</code> are usually in the same
+directory (e.g. <code>base/basictypes_test.cc</code> and
+<code>base/basictypes.h</code>), but may sometimes be in different
+directories too.</p>
+
+
+
+<p>Within each section the includes should be ordered
+alphabetically. Note that older code might not conform to
+this rule and should be fixed when convenient.</p>
+
+<p>You should include all the headers that define the symbols you rely
+upon, except in the unusual case of <a href="#Forward_Declarations">forward
+declaration</a>. If you rely on symbols from <code>bar.h</code>,
+don't count on the fact that you included <code>foo.h</code> which
+(currently) includes <code>bar.h</code>: include <code>bar.h</code>
+yourself, unless <code>foo.h</code> explicitly demonstrates its intent
+to provide you the symbols of <code>bar.h</code>. However, any
+includes present in the related header do not need to be included
+again in the related <code>cc</code> (i.e., <code>foo.cc</code> can
+rely on <code>foo.h</code>'s includes).</p>
+
+<p>For example, the includes in
+
+<code>google-awesome-project/src/foo/internal/fooserver.cc</code>
+might look like this:</p>
+
+
+<pre>#include "foo/server/fooserver.h"
+
+#include <sys/types.h>
+#include <unistd.h>
+
+#include <hash_map>
+#include <vector>
+
+#include "base/basictypes.h"
+#include "base/commandlineflags.h"
+#include "foo/server/bar.h"
+</pre>
+
+<p class="exception">Sometimes, system-specific code needs
+conditional includes. Such code can put conditional
+includes after other includes. Of course, keep your
+system-specific code small and localized. Example:</p>
+
+<pre>#include "foo/public/fooserver.h"
+
+#include "base/port.h" // For LANG_CXX11.
+
+#ifdef LANG_CXX11
+#include <initializer_list>
+#endif // LANG_CXX11
+</pre>
+
+</div>
+
+<h2 id="Scoping">Scoping</h2>
+
+<h3 id="Namespaces">Namespaces</h3>
+
+<div class="summary">
+<p>With few exceptions, place code in a namespace. Namespaces
+should have unique names based on the project name, and possibly
+its path. Do not use <i>using-directives</i> (e.g.
+<code>using namespace foo</code>). Do not use
+inline namespaces. For unnamed namespaces, see
+<a href="#Unnamed_Namespaces_and_Static_Variables">Unnamed Namespaces and
+Static Variables</a>.
+</p></div>
+
+<div class="stylebody">
+
+<div class="definition">
+<p>Namespaces subdivide the global scope
+into distinct, named scopes, and so are useful for preventing
+name collisions in the global scope.</p>
+</div>
+
+<div class="pros">
+
+<p>Namespaces provide a method for preventing name conflicts
+in large programs while allowing most code to use reasonably
+short names.</p>
+
+<p>For example, if two different projects have a class
+<code>Foo</code> in the global scope, these symbols may
+collide at compile time or at runtime. If each project
+places their code in a namespace, <code>project1::Foo</code>
+and <code>project2::Foo</code> are now distinct symbols that
+do not collide, and code within each project's namespace
+can continue to refer to <code>Foo</code> without the prefix.</p>
+
+<p>Inline namespaces automatically place their names in
+the enclosing scope. Consider the following snippet, for
+example:</p>
+
+<pre>namespace X {
+inline namespace Y {
+ void foo();
+} // namespace Y
+} // namespace X
+</pre>
+
+<p>The expressions <code>X::Y::foo()</code> and
+<code>X::foo()</code> are interchangeable. Inline
+namespaces are primarily intended for ABI compatibility
+across versions.</p>
+</div>
+
+<div class="cons">
+
+<p>Namespaces can be confusing, because they complicate
+the mechanics of figuring out what definition a name refers
+to.</p>
+
+<p>Inline namespaces, in particular, can be confusing
+because names aren't actually restricted to the namespace
+where they are declared. They are only useful as part of
+some larger versioning policy.</p>
+
+<p>In some contexts, it's necessary to repeatedly refer to
+symbols by their fully-qualified names. For deeply-nested
+namespaces, this can add a lot of clutter.</p>
+</div>
+
+<div class="decision">
+
+<p>Namespaces should be used as follows:</p>
+
+<ul>
+ <li>Follow the rules on <a href="#Namespace_Names">Namespace Names</a>.
+ </li><li>Terminate namespaces with comments as shown in the given examples.
+ </li><li>
+
+ <p>Namespaces wrap the entire source file after
+ includes,
+ <a href="https://gflags.github.io/gflags/">
+ gflags</a> definitions/declarations
+ and forward declarations of classes from other namespaces.</p>
+
+<pre>// In the .h file
+namespace mynamespace {
+
+// All declarations are within the namespace scope.
+// Notice the lack of indentation.
+class MyClass {
+ public:
+ ...
+ void Foo();
+};
+
+} // namespace mynamespace
+</pre>
+
+<pre>// In the .cc file
+namespace mynamespace {
+
+// Definition of functions is within scope of the namespace.
+void MyClass::Foo() {
+ ...
+}
+
+} // namespace mynamespace
+</pre>
+
+ <p>More complex <code>.cc</code> files might have additional details,
+ like flags or using-declarations.</p>
+
+<pre>#include "a.h"
+
+DEFINE_FLAG(bool, someflag, false, "dummy flag");
+
+namespace a {
+
+using ::foo::bar;
+
+...code for a... // Code goes against the left margin.
+
+} // namespace a
+</pre>
+ </li>
+
+
+
+ <li>Do not declare anything in namespace
+ <code>std</code>, including forward declarations of
+ standard library classes. Declaring entities in
+ namespace <code>std</code> is undefined behavior, i.e.,
+ not portable. To declare entities from the standard
+ library, include the appropriate header file.</li>
+
+ <li><p>You may not use a <i>using-directive</i>
+ to make all names from a namespace available.</p>
+
+<pre class="badcode">// Forbidden -- This pollutes the namespace.
+using namespace foo;
+</pre>
+ </li>
+
+ <li><p>Do not use <i>Namespace aliases</i> at namespace scope
+ in header files except in explicitly marked
+ internal-only namespaces, because anything imported into a namespace
+ in a header file becomes part of the public
+ API exported by that file.</p>
+
+<pre>// Shorten access to some commonly used names in .cc files.
+namespace baz = ::foo::bar::baz;
+</pre>
+
+<pre>// Shorten access to some commonly used names (in a .h file).
+namespace librarian {
+namespace impl { // Internal, not part of the API.
+namespace sidetable = ::pipeline_diagnostics::sidetable;
+} // namespace impl
+
+inline void my_inline_function() {
+ // namespace alias local to a function (or method).
+ namespace baz = ::foo::bar::baz;
+ ...
+}
+} // namespace librarian
+</pre>
+
+ </li><li>Do not use inline namespaces.</li>
+</ul>
+</div>
+</div>
+
+<h3 id="Unnamed_Namespaces_and_Static_Variables">Unnamed Namespaces and Static
+Variables</h3>
+
+<div class="summary">
+<p>When definitions in a <code>.cc</code> file do not need to be
+referenced outside that file, place them in an unnamed
+namespace or declare them <code>static</code>. Do not use either
+of these constructs in <code>.h</code> files.
+</p></div>
+
+<div class="stylebody">
+
+<div class="definition">
+<p>All declarations can be given internal linkage by placing them in
+unnamed namespaces, and functions and variables can be given internal linkage by
+declaring them <code>static</code>. This means that anything you're declaring
+can't be accessed from another file. If a different file declares something
+with the same name, then the two entities are completely independent.</p>
+</div>
+
+<div class="decision">
+
+<p>Use of internal linkage in <code>.cc</code> files is encouraged
+for all code that does not need to be referenced elsewhere.
+Do not use internal linkage in <code>.h</code> files.</p>
+
+<p>Format unnamed namespaces like named namespaces. In the
+ terminating comment, leave the namespace name empty:</p>
+
+<pre>namespace {
+...
+} // namespace
+</pre>
+</div>
+</div>
+
+<h3 id="Nonmember,_Static_Member,_and_Global_Functions">Nonmember, Static Member, and Global Functions</h3>
+
+<div class="summary">
+<p>Prefer placing nonmember functions in a namespace; use completely global
+functions rarely. Prefer grouping functions with a namespace instead of
+using a class as if it were a namespace. Static methods of a class should
+generally be closely related to instances of the class or the class's static
+data.</p>
+</div>
+
+ <div class="stylebody">
+
+ <div class="pros">
+ <p>Nonmember and static member functions can be useful in
+ some situations. Putting nonmember functions in a
+ namespace avoids polluting the global namespace.</p>
+ </div>
+
+<div class="cons">
+<p>Nonmember and static member functions may make more sense
+as members of a new class, especially if they access
+external resources or have significant dependencies.</p>
+</div>
+
+<div class="decision">
+<p>Sometimes it is useful to define a
+function not bound to a class instance. Such a function
+can be either a static member or a nonmember function.
+Nonmember functions should not depend on external
+variables, and should nearly always exist in a namespace.
+Rather than creating classes only to group static member
+functions which do not share static data, use
+<a href="#Namespaces">namespaces</a> instead. For a header
+<code>myproject/foo_bar.h</code>, for example, write</p>
+<pre>namespace myproject {
+namespace foo_bar {
+void Function1();
+void Function2();
+} // namespace foo_bar
+} // namespace myproject
+</pre>
+<p>instead of</p>
+<pre class="badcode">namespace myproject {
+class FooBar {
+ public:
+ static void Function1();
+ static void Function2();
+};
+} // namespace myproject
+</pre>
+
+<p>If you define a nonmember function and it is only
+needed in its <code>.cc</code> file, use
+<a href="#Unnamed_Namespaces_and_Static_Variables">internal linkage</a> to limit
+its scope.</p>
+</div>
+
+</div>
+
+<h3 id="Local_Variables">Local Variables</h3>
+
+<div class="summary">
+<p>Place a function's variables in the narrowest scope
+possible, and initialize variables in the declaration.</p>
+</div>
+
+<div class="stylebody">
+
+<p>C++ allows you to declare variables anywhere in a
+function. We encourage you to declare them in as local a
+scope as possible, and as close to the first use as
+possible. This makes it easier for the reader to find the
+declaration and see what type the variable is and what it
+was initialized to. In particular, initialization should
+be used instead of declaration and assignment, e.g.:</p>
+
+<pre class="badcode">int i;
+i = f(); // Bad -- initialization separate from declaration.
+</pre>
+
+<pre>int j = g(); // Good -- declaration has initialization.
+</pre>
+
+<pre class="badcode">std::vector<int> v;
+v.push_back(1); // Prefer initializing using brace initialization.
+v.push_back(2);
+</pre>
+
+<pre>std::vector<int> v = {1, 2}; // Good -- v starts initialized.
+</pre>
+
+<p>Variables needed for <code>if</code>, <code>while</code>
+and <code>for</code> statements should normally be declared
+within those statements, so that such variables are confined
+to those scopes. E.g.:</p>
+
+<pre>while (const char* p = strchr(str, '/')) str = p + 1;
+</pre>
+
+<p>There is one caveat: if the variable is an object, its
+constructor is invoked every time it enters scope and is
+created, and its destructor is invoked every time it goes
+out of scope.</p>
+
+<pre class="badcode">// Inefficient implementation:
+for (int i = 0; i < 1000000; ++i) {
+ Foo f; // My ctor and dtor get called 1000000 times each.
+ f.DoSomething(i);
+}
+</pre>
+
+<p>It may be more efficient to declare such a variable
+used in a loop outside that loop:</p>
+
+<pre>Foo f; // My ctor and dtor get called once each.
+for (int i = 0; i < 1000000; ++i) {
+ f.DoSomething(i);
+}
+</pre>
+
+</div>
+
+<h3 id="Static_and_Global_Variables">Static and Global Variables</h3>
+
+<div class="summary">
+ <p>Variables of class type with <a href="http://en.cppreference.com/w/cpp/language/storage_duration#Storage_duration">
+ static storage duration</a> are forbidden: they cause hard-to-find bugs due
+ to indeterminate order of construction and destruction. However, such
+ variables are allowed if they are <code>constexpr</code>: they have no
+ dynamic initialization or destruction.</p>
+</div>
+
+<div class="stylebody">
+
+<p>Objects with static storage duration, including global
+variables, static variables, static class member
+variables, and function static variables, must be Plain
+Old Data (POD): only ints, chars, floats, or pointers, or
+arrays/structs of POD.</p>
+
+<p>The order in which class constructors and initializers
+for static variables are called is only partially
+specified in C++ and can even change from build to build,
+which can cause bugs that are difficult to find.
+Therefore in addition to banning globals of class type,
+we do not allow non-local static variables to be initialized
+with the result of a function, unless that function (such
+as getenv(), or getpid()) does not itself depend on any
+other globals. However, a static POD variable within
+function scope may be initialized with the result of a
+function, since its initialization order is well-defined
+and does not occur until control passes through its
+declaration.</p>
+
+<p>Likewise, global and static variables are destroyed
+when the program terminates, regardless of whether the
+termination is by returning from <code>main()</code> or
+by calling <code>exit()</code>. The order in which
+destructors are called is defined to be the reverse of
+the order in which the constructors were called. Since
+constructor order is indeterminate, so is destructor
+order. For example, at program-end time a static variable
+might have been destroyed, but code still running
+— perhaps in another thread
+— tries to access it and fails. Or the
+destructor for a static <code>string</code> variable
+might be run prior to the destructor for another variable
+that contains a reference to that string.</p>
+
+<p>One way to alleviate the destructor problem is to
+terminate the program by calling
+<code>quick_exit()</code> instead of <code>exit()</code>.
+The difference is that <code>quick_exit()</code> does not
+invoke destructors and does not invoke any handlers that
+were registered by calling <code>atexit()</code>. If you
+have a handler that needs to run when a program
+terminates via <code>quick_exit()</code> (flushing logs,
+for example), you can register it using
+<code>at_quick_exit()</code>. (If you have a handler that
+needs to run at both <code>exit()</code> and
+<code>quick_exit()</code>, you need to register it in
+both places.)</p>
+
+<p>As a result we only allow static variables to contain
+POD data. This rule completely disallows
+<code>std::vector</code> (use C arrays instead), or
+<code>string</code> (use <code>const char []</code>).</p>
+
+
+
+<p>If you need a static or global
+variable of a class type, consider initializing a pointer
+(which will never be freed), from either your main()
+function or from pthread_once(). Note that this must be a
+raw pointer, not a "smart" pointer, since the smart
+pointer's destructor will have the order-of-destructor
+issue that we are trying to avoid.</p>
+
+
+
+
+
+</div>
+
+<h2 id="Classes">Classes</h2>
+
+<p>Classes are the fundamental unit of code in C++. Naturally,
+we use them extensively. This section lists the main dos and
+don'ts you should follow when writing a class.</p>
+
+<h3 id="Doing_Work_in_Constructors">Doing Work in Constructors</h3>
+
+<div class="summary">
+<p>Avoid virtual method calls in constructors, and avoid
+initialization that can fail if you can't signal an error.</p>
+</div>
+
+<div class="stylebody">
+
+<div class="definition">
+<p>It is possible to perform arbitrary initialization in the body
+of the constructor.</p>
+</div>
+
+<div class="pros">
+<ul>
+ <li>No need to worry about whether the class has been initialized or
+ not.</li>
+
+ <li>Objects that are fully initialized by constructor call can
+ be <code>const</code> and may also be easier to use with standard containers
+ or algorithms.</li>
+</ul>
+
+</div>
+
+<div class="cons">
+<ul>
+ <li>If the work calls virtual functions, these calls
+ will not get dispatched to the subclass
+ implementations. Future modification to your class can
+ quietly introduce this problem even if your class is
+ not currently subclassed, causing much confusion.</li>
+
+ <li>There is no easy way for constructors to signal errors, short of
+ crashing the program (not always appropriate) or using exceptions
+ (which are <a href="#Exceptions">forbidden</a>).</li>
+
+ <li>If the work fails, we now have an object whose initialization
+ code failed, so it may be an unusual state requiring a <code>bool
+ IsValid()</code> state checking mechanism (or similar) which is easy
+ to forget to call.</li>
+
+ <li>You cannot take the address of a constructor, so whatever work
+ is done in the constructor cannot easily be handed off to, for
+ example, another thread.</li>
+</ul>
+</div>
+
+
+<div class="decision">
+<p>Constructors should never call virtual functions. If appropriate
+for your code
+,
+terminating the program may be an appropriate error handling
+response. Otherwise, consider a factory function
+or <code>Init()</code> method. Avoid <code>Init()</code> methods on objects with
+no other states that affect which public methods may be called
+(semi-constructed objects of this form are particularly hard to work
+with correctly).</p>
+</div>
+
+</div>
+
+<a id="Explicit_Constructors"></a>
+<h3 id="Implicit_Conversions">Implicit Conversions</h3>
+
+<div class="summary">
+<p>Do not define implicit conversions. Use the <code>explicit</code>
+keyword for conversion operators and single-argument
+constructors.</p>
+</div>
+
+<div class="stylebody">
+
+<div class="definition">
+<p>Implicit conversions allow an
+object of one type (called the <dfn>source type</dfn>) to
+be used where a different type (called the <dfn>destination
+type</dfn>) is expected, such as when passing an
+<code>int</code> argument to a function that takes a
+<code>double</code> parameter.</p>
+
+<p>In addition to the implicit conversions defined by the language,
+users can define their own, by adding appropriate members to the
+class definition of the source or destination type. An implicit
+conversion in the source type is defined by a type conversion operator
+named after the destination type (e.g. <code>operator
+bool()</code>). An implicit conversion in the destination
+type is defined by a constructor that can take the source type as
+its only argument (or only argument with no default value).</p>
+
+<p>The <code>explicit</code> keyword can be applied to a constructor
+or (since C++11) a conversion operator, to ensure that it can only be
+used when the destination type is explicit at the point of use,
+e.g. with a cast. This applies not only to implicit conversions, but to
+C++11's list initialization syntax:</p>
+<pre>class Foo {
+ explicit Foo(int x, double y);
+ ...
+};
+
+void Func(Foo f);
+</pre>
+<pre class="badcode">Func({42, 3.14}); // Error
+</pre>
+This kind of code isn't technically an implicit conversion, but the
+language treats it as one as far as <code>explicit</code> is concerned.
+</div>
+
+<div class="pros">
+<ul>
+<li>Implicit conversions can make a type more usable and
+ expressive by eliminating the need to explicitly name a type
+ when it's obvious.</li>
+<li>Implicit conversions can be a simpler alternative to
+ overloading.</li>
+<li>List initialization syntax is a concise and expressive
+ way of initializing objects.</li>
+</ul>
+</div>
+
+<div class="cons">
+<ul>
+<li>Implicit conversions can hide type-mismatch bugs, where the
+ destination type does not match the user's expectation, or
+ the user is unaware that any conversion will take place.</li>
+
+<li>Implicit conversions can make code harder to read, particularly
+ in the presence of overloading, by making it less obvious what
+ code is actually getting called.</li>
+
+<li>Constructors that take a single argument may accidentally
+ be usable as implicit type conversions, even if they are not
+ intended to do so.</li>
+
+<li>When a single-argument constructor is not marked
+ <code>explicit</code>, there's no reliable way to tell whether
+ it's intended to define an implicit conversion, or the author
+ simply forgot to mark it.</li>
+
+<li>It's not always clear which type should provide the conversion,
+ and if they both do, the code becomes ambiguous.</li>
+
+<li>List initialization can suffer from the same problems if
+ the destination type is implicit, particularly if the
+ list has only a single element.</li>
+</ul>
+</div>
+
+<div class="decision">
+<p>Type conversion operators, and constructors that are
+callable with a single argument, must be marked
+<code>explicit</code> in the class definition. As an
+exception, copy and move constructors should not be
+<code>explicit</code>, since they do not perform type
+conversion. Implicit conversions can sometimes be necessary and
+appropriate for types that are designed to transparently wrap other
+types. In that case, contact
+your project leads to request
+a waiver of this rule.</p>
+
+<p>Constructors that cannot be called with a single argument
+should usually omit <code>explicit</code>. Constructors that
+take a single <code>std::initializer_list</code> parameter should
+also omit <code>explicit</code>, in order to support copy-initialization
+(e.g. <code>MyType m = {1, 2};</code>).</p>
+</div>
+
+</div>
+
+<h3 id="Copyable_Movable_Types">Copyable and Movable Types</h3>
+<a id="Copy_Constructors"></a>
+<div class="summary">
+<p>Support copying and/or moving if these operations are clear and meaningful
+for your type. Otherwise, disable the implicitly generated special functions
+that perform copies and moves.
+</p></div>
+
+<div class="stylebody">
+
+<div class="definition">
+<p>A copyable type allows its objects to be initialized or assigned
+from any other object of the same type, without changing the value of the source.
+For user-defined types, the copy behavior is defined by the copy
+constructor and the copy-assignment operator.
+<code>string</code> is an example of a copyable type.</p>
+
+<p>A movable type is one that can be initialized and assigned
+from temporaries (all copyable types are therefore movable).
+<code>std::unique_ptr<int></code> is an example of a movable but not
+copyable type. For user-defined types, the move behavior is defined by the move
+constructor and the move-assignment operator.</p>
+
+<p>The copy/move constructors can be implicitly invoked by the compiler
+in some situations, e.g. when passing objects by value.</p>
+</div>
+
+<div class="pros">
+<p>Objects of copyable and movable types can be passed and returned by value,
+which makes APIs simpler, safer, and more general. Unlike when passing objects
+by pointer or reference, there's no risk of confusion over ownership,
+lifetime, mutability, and similar issues, and no need to specify them in the
+contract. It also prevents non-local interactions between the client and the
+implementation, which makes them easier to understand, maintain, and optimize by
+the compiler. Further, such objects can be used with generic APIs that
+require pass-by-value, such as most containers, and they allow for additional
+flexibility in e.g., type composition.</p>
+
+<p>Copy/move constructors and assignment operators are usually
+easier to define correctly than alternatives
+like <code>Clone()</code>, <code>CopyFrom()</code> or <code>Swap()</code>,
+because they can be generated by the compiler, either implicitly or
+with <code>= default</code>. They are concise, and ensure
+that all data members are copied. Copy and move
+constructors are also generally more efficient, because they don't
+require heap allocation or separate initialization and assignment
+steps, and they're eligible for optimizations such as
+
+<a href="http://en.cppreference.com/w/cpp/language/copy_elision">
+copy elision</a>.</p>
+
+<p>Move operations allow the implicit and efficient transfer of
+resources out of rvalue objects. This allows a plainer coding style
+in some cases.</p>
+</div>
+
+<div class="cons">
+<p>Some types do not need to be copyable, and providing copy
+operations for such types can be confusing, nonsensical, or outright
+incorrect. Types representing singleton objects (<code>Registerer</code>),
+objects tied to a specific scope (<code>Cleanup</code>), or closely coupled to
+object identity (<code>Mutex</code>) cannot be copied meaningfully.
+Copy operations for base class types that are to be used
+polymorphically are hazardous, because use of them can lead to
+<a href="https://en.wikipedia.org/wiki/Object_slicing">object slicing</a>.
+Defaulted or carelessly-implemented copy operations can be incorrect, and the
+resulting bugs can be confusing and difficult to diagnose.</p>
+
+<p>Copy constructors are invoked implicitly, which makes the
+invocation easy to miss. This may cause confusion for programmers used to
+languages where pass-by-reference is conventional or mandatory. It may also
+encourage excessive copying, which can cause performance problems.</p>
+</div>
+
+<div class="decision">
+
+<p>Provide the copy and move operations if their meaning is clear to a casual
+user and the copying/moving does not incur unexpected costs. If you define a
+copy or move constructor, define the corresponding assignment operator, and
+vice-versa. If your type is copyable, do not define move operations unless they
+are significantly more efficient than the corresponding copy operations. If your
+type is not copyable, but the correctness of a move is obvious to users of the
+type, you may make the type move-only by defining both of the move operations.
+</p>
+
+<p>If your type provides copy operations, it is recommended that you design
+your class so that the default implementation of those operations is correct.
+Remember to review the correctness of any defaulted operations as you would any
+other code, and to document that your class is copyable and/or cheaply movable
+if that's an API guarantee.</p>
+
+<pre class="badcode">class Foo {
+ public:
+ Foo(Foo&& other) : field_(other.field) {}
+ // Bad, defines only move constructor, but not operator=.
+
+ private:
+ Field field_;
+};
+</pre>
+
+<p>Due to the risk of slicing, avoid providing an assignment
+operator or public copy/move constructor for a class that's
+intended to be derived from (and avoid deriving from a class
+with such members). If your base class needs to be
+copyable, provide a public virtual <code>Clone()</code>
+method, and a protected copy constructor that derived classes
+can use to implement it.</p>
+
+<p>If you do not want to support copy/move operations on your type,
+explicitly disable them using <code>= delete</code> in
+the <code>public:</code> section:</p>
+
+<pre class="code">// MyClass is neither copyable nor movable.
+MyClass(const MyClass&) = delete;
+MyClass& operator=(const MyClass&) = delete;
+</pre>
+
+<p></p>
+
+</div>
+</div>
+
+<h3 id="Structs_vs._Classes">Structs vs. Classes</h3>
+
+<div class="summary">
+<p>Use a <code>struct</code> only for passive objects that
+ carry data; everything else is a <code>class</code>.</p>
+</div>
+
+<div class="stylebody">
+
+<p>The <code>struct</code> and <code>class</code>
+keywords behave almost identically in C++. We add our own
+semantic meanings to each keyword, so you should use the
+appropriate keyword for the data-type you're
+defining.</p>
+
+<p><code>structs</code> should be used for passive
+objects that carry data, and may have associated
+constants, but lack any functionality other than
+access/setting the data members. The accessing/setting of
+fields is done by directly accessing the fields rather
+than through method invocations. Methods should not
+provide behavior but should only be used to set up the
+data members, e.g., constructor, destructor,
+<code>Initialize()</code>, <code>Reset()</code>,
+<code>Validate()</code>.</p>
+
+<p>If more functionality is required, a
+<code>class</code> is more appropriate. If in doubt, make
+it a <code>class</code>.</p>
+
+<p>For consistency with STL, you can use
+<code>struct</code> instead of <code>class</code> for
+functors and traits.</p>
+
+<p>Note that member variables in structs and classes have
+<a href="#Variable_Names">different naming rules</a>.</p>
+
+</div>
+
+<h3 id="Inheritance">Inheritance</h3>
+
+<div class="summary">
+<p>Composition is often more appropriate than inheritance.
+When using inheritance, make it <code>public</code>.</p>
+</div>
+
+<div class="stylebody">
+
+<div class="definition">
+<p> When a sub-class
+inherits from a base class, it includes the definitions
+of all the data and operations that the parent base class
+defines. In practice, inheritance is used in two major
+ways in C++: implementation inheritance, in which actual
+code is inherited by the child, and
+<a href="#Interfaces">interface inheritance</a>, in which
+only method names are inherited.</p>
+</div>
+
+<div class="pros">
+<p>Implementation inheritance reduces code size by re-using
+the base class code as it specializes an existing type.
+Because inheritance is a compile-time declaration, you
+and the compiler can understand the operation and detect
+errors. Interface inheritance can be used to
+programmatically enforce that a class expose a particular
+API. Again, the compiler can detect errors, in this case,
+when a class does not define a necessary method of the
+API.</p>
+</div>
+
+<div class="cons">
+<p>For implementation inheritance, because the code
+implementing a sub-class is spread between the base and
+the sub-class, it can be more difficult to understand an
+implementation. The sub-class cannot override functions
+that are not virtual, so the sub-class cannot change
+implementation. The base class may also define some data
+members, so that specifies physical layout of the base
+class.</p>
+</div>
+
+<div class="decision">
+
+<p>All inheritance should be <code>public</code>. If you
+want to do private inheritance, you should be including
+an instance of the base class as a member instead.</p>
+
+<p>Do not overuse implementation inheritance. Composition
+is often more appropriate. Try to restrict use of
+inheritance to the "is-a" case: <code>Bar</code>
+subclasses <code>Foo</code> if it can reasonably be said
+that <code>Bar</code> "is a kind of"
+<code>Foo</code>.</p>
+
+<p>Make your destructor <code>virtual</code> if
+necessary. If your class has virtual methods, its
+destructor should be virtual.</p>
+
+<p>Limit the use of <code>protected</code> to those
+member functions that might need to be accessed from
+subclasses. Note that <a href="#Access_Control">data
+members should be private</a>.</p>
+
+<p>Explicitly annotate overrides of virtual functions
+or virtual destructors with an <code>override</code>
+or (less frequently) <code>final</code> specifier.
+Older (pre-C++11) code will use the
+<code>virtual</code> keyword as an inferior
+alternative annotation. For clarity, use exactly one of
+<code>override</code>, <code>final</code>, or
+<code>virtual</code> when declaring an override.
+Rationale: A function or destructor marked
+<code>override</code> or <code>final</code> that is
+not an override of a base class virtual function will
+not compile, and this helps catch common errors. The
+specifiers serve as documentation; if no specifier is
+present, the reader has to check all ancestors of the
+class in question to determine if the function or
+destructor is virtual or not.</p>
+</div>
+
+</div>
+
+<h3 id="Multiple_Inheritance">Multiple Inheritance</h3>
+
+<div class="summary">
+<p>Only very rarely is multiple implementation inheritance
+actually useful. We allow multiple inheritance only when at
+most one of the base classes has an implementation; all
+other base classes must be <a href="#Interfaces">pure
+interface</a> classes tagged with the
+<code>Interface</code> suffix.</p>
+</div>
+
+<div class="stylebody">
+
+<div class="definition">
+<p>Multiple inheritance allows a sub-class to have more than
+one base class. We distinguish between base classes that are
+<em>pure interfaces</em> and those that have an
+<em>implementation</em>.</p>
+</div>
+
+<div class="pros">
+<p>Multiple implementation inheritance may let you re-use
+even more code than single inheritance (see <a href="#Inheritance">Inheritance</a>).</p>
+</div>
+
+<div class="cons">
+<p>Only very rarely is multiple <em>implementation</em>
+inheritance actually useful. When multiple implementation
+inheritance seems like the solution, you can usually find
+a different, more explicit, and cleaner solution.</p>
+</div>
+
+<div class="decision">
+<p> Multiple inheritance is allowed only when all
+superclasses, with the possible exception of the first one,
+are <a href="#Interfaces">pure interfaces</a>. In order to
+ensure that they remain pure interfaces, they must end with
+the <code>Interface</code> suffix.</p>
+</div>
+
+<div class="note">
+<p>There is an <a href="#Windows_Code">exception</a> to
+this rule on Windows.</p>
+</div>
+
+</div>
+
+<h3 id="Interfaces">Interfaces</h3>
+
+<div class="summary">
+<p>Classes that satisfy certain conditions are allowed, but
+not required, to end with an <code>Interface</code> suffix.</p>
+</div>
+
+<div class="stylebody">
+
+<div class="definition">
+<p>A class is a pure interface if it meets the following
+requirements:</p>
+
+<ul>
+ <li>It has only public pure virtual ("<code>=
+ 0</code>") methods and static methods (but see below
+ for destructor).</li>
+
+ <li>It may not have non-static data members.</li>
+
+ <li>It need not have any constructors defined. If a
+ constructor is provided, it must take no arguments and
+ it must be protected.</li>
+
+ <li>If it is a subclass, it may only be derived from
+ classes that satisfy these conditions and are tagged
+ with the <code>Interface</code> suffix.</li>
+</ul>
+
+<p>An interface class can never be directly instantiated
+because of the pure virtual method(s) it declares. To
+make sure all implementations of the interface can be
+destroyed correctly, the interface must also declare a
+virtual destructor (in an exception to the first rule,
+this should not be pure). See Stroustrup, <cite>The C++
+Programming Language</cite>, 3rd edition, section 12.4
+for details.</p>
+</div>
+
+<div class="pros">
+<p>Tagging a class with the <code>Interface</code> suffix
+lets others know that they must not add implemented
+methods or non static data members. This is particularly
+important in the case of <a href="#Multiple_Inheritance">multiple inheritance</a>.
+Additionally, the interface concept is already
+well-understood by Java programmers.</p>
+</div>
+
+<div class="cons">
+<p>The <code>Interface</code> suffix lengthens the class
+name, which can make it harder to read and understand.
+Also, the interface property may be considered an
+implementation detail that shouldn't be exposed to
+clients.</p>
+</div>
+
+<div class="decision">
+<p>A class may end
+with <code>Interface</code> only if it meets the above
+requirements. We do not require the converse, however:
+classes that meet the above requirements are not required
+to end with <code>Interface</code>.</p>
+</div>
+
+</div>
+
+<h3 id="Operator_Overloading">Operator Overloading</h3>
+
+<div class="summary">
+<p>Overload operators judiciously. Do not create user-defined literals.</p>
+</div>
+
+<div class="stylebody">
+
+<div class="definition">
+<p>C++ permits user code to
+<a href="http://en.cppreference.com/w/cpp/language/operators">declare
+overloaded versions of the built-in operators</a> using the
+<code>operator</code> keyword, so long as one of the parameters
+is a user-defined type. The <code>operator</code> keyword also
+permits user code to define new kinds of literals using
+<code>operator""</code>, and to define type-conversion functions
+such as <code>operator bool()</code>.</p>
+</div>
+
+<div class="pros">
+<p>Operator overloading can make code more concise and
+intuitive by enabling user-defined types to behave the same
+as built-in types. Overloaded operators are the idiomatic names
+for certain operations (e.g. <code>==</code>, <code><</code>,
+<code>=</code>, and <code><<</code>), and adhering to
+those conventions can make user-defined types more readable
+and enable them to interoperate with libraries that expect
+those names.</p>
+
+<p>User-defined literals are a very concise notation for
+creating objects of user-defined types.</p>
+</div>
+
+<div class="cons">
+<ul>
+ <li>Providing a correct, consistent, and unsurprising
+ set of operator overloads requires some care, and failure
+ to do so can lead to confusion and bugs.</li>
+
+ <li>Overuse of operators can lead to obfuscated code,
+ particularly if the overloaded operator's semantics
+ don't follow convention.</li>
+
+ <li>The hazards of function overloading apply just as
+ much to operator overloading, if not more so.</li>
+
+ <li>Operator overloads can fool our intuition into
+ thinking that expensive operations are cheap, built-in
+ operations.</li>
+
+ <li>Finding the call sites for overloaded operators may
+ require a search tool that's aware of C++ syntax, rather
+ than e.g. grep.</li>
+
+ <li>If you get the argument type of an overloaded operator
+ wrong, you may get a different overload rather than a
+ compiler error. For example, <code>foo < bar</code>
+ may do one thing, while <code>&foo < &bar</code>
+ does something totally different.</li>
+
+ <li>Certain operator overloads are inherently hazardous.
+ Overloading unary <code>&</code> can cause the same
+ code to have different meanings depending on whether
+ the overload declaration is visible. Overloads of
+ <code>&&</code>, <code>||</code>, and <code>,</code>
+ (comma) cannot match the evaluation-order semantics of the
+ built-in operators.</li>
+
+ <li>Operators are often defined outside the class,
+ so there's a risk of different files introducing
+ different definitions of the same operator. If both
+ definitions are linked into the same binary, this results
+ in undefined behavior, which can manifest as subtle
+ run-time bugs.</li>
+
+ <li>User-defined literals allow the creation of new
+ syntactic forms that are unfamiliar even to experienced C++
+ programmers.</li>
+</ul>
+</div>
+
+<div class="decision">
+<p>Define overloaded operators only if their meaning is
+obvious, unsurprising, and consistent with the corresponding
+built-in operators. For example, use <code>|</code> as a
+bitwise- or logical-or, not as a shell-style pipe.</p>
+
+<p>Define operators only on your own types. More precisely,
+define them in the same headers, .cc files, and namespaces
+as the types they operate on. That way, the operators are available
+wherever the type is, minimizing the risk of multiple
+definitions. If possible, avoid defining operators as templates,
+because they must satisfy this rule for any possible template
+arguments. If you define an operator, also define
+any related operators that make sense, and make sure they
+are defined consistently. For example, if you overload
+<code><</code>, overload all the comparison operators,
+and make sure <code><</code> and <code>></code> never
+return true for the same arguments.</p>
+
+<p>Prefer to define non-modifying binary operators as
+non-member functions. If a binary operator is defined as a
+class member, implicit conversions will apply to the
+right-hand argument, but not the left-hand one. It will
+confuse your users if <code>a < b</code> compiles but
+<code>b < a</code> doesn't.</p>
+
+<p>Don't go out of your way to avoid defining operator
+overloads. For example, prefer to define <code>==</code>,
+<code>=</code>, and <code><<</code>, rather than
+<code>Equals()</code>, <code>CopyFrom()</code>, and
+<code>PrintTo()</code>. Conversely, don't define
+operator overloads just because other libraries expect
+them. For example, if your type doesn't have a natural
+ordering, but you want to store it in a <code>std::set</code>,
+use a custom comparator rather than overloading
+<code><</code>.</p>
+
+<p>Do not overload <code>&&</code>, <code>||</code>,
+<code>,</code> (comma), or unary <code>&</code>. Do not overload
+<code>operator""</code>, i.e. do not introduce user-defined
+literals.</p>
+
+<p>Type conversion operators are covered in the section on
+<a href="#Implicit_Conversions">implicit conversions</a>.
+The <code>=</code> operator is covered in the section on
+<a href="#Copy_Constructors">copy constructors</a>. Overloading
+<code><<</code> for use with streams is covered in the
+section on <a href="#Streams">streams</a>. See also the rules on
+<a href="#Function_Overloading">function overloading</a>, which
+apply to operator overloading as well.</p>
+</div>
+
+</div>
+
+<h3 id="Access_Control">Access Control</h3>
+
+<div class="summary">
+<p> Make data members <code>private</code>, unless they are
+<code>static const</code> (and follow the <a href="#Constant_Names">
+naming convention for constants</a>). For technical
+reasons, we allow data members of a test fixture class to
+be <code>protected</code> when using
+
+
+<a href="https://github.com/google/googletest">Google
+Test</a>).</p>
+</div>
+
+<h3 id="Declaration_Order">Declaration Order</h3>
+
+<div class="summary">
+<p>Group similar declarations together, placing public parts
+earlier.</p>
+</div>
+
+<div class="stylebody">
+
+<p>A class definition should usually start with a
+<code>public:</code> section, followed by
+<code>protected:</code>, then <code>private:</code>. Omit
+sections that would be empty.</p>
+
+<p>Within each section, generally prefer grouping similar
+kinds of declarations together, and generally prefer the
+following order: types (including <code>typedef</code>,
+<code>using</code>, and nested structs and classes),
+constants, factory functions, constructors, assignment
+operators, destructor, all other methods, data members.</p>
+
+<p>Do not put large method definitions inline in the
+class definition. Usually, only trivial or
+performance-critical, and very short, methods may be
+defined inline. See <a href="#Inline_Functions">Inline
+Functions</a> for more details.</p>
+
+</div>
+
+<h2 id="Functions">Functions</h2>
+
+<h3 id="Function_Parameter_Ordering">Parameter Ordering</h3>
+
+<div class="summary">
+<p>When defining a function, parameter order is: inputs, then
+outputs.</p>
+</div>
+
+<div class="stylebody">
+<p>Parameters to C/C++ functions are either input to the
+function, output from the function, or both. Input
+parameters are usually values or <code>const</code>
+references, while output and input/output parameters will
+be pointers to non-<code>const</code>. When ordering
+function parameters, put all input-only parameters before
+any output parameters. In particular, do not add new
+parameters to the end of the function just because they
+are new; place new input-only parameters before the
+output parameters.</p>
+
+<p>This is not a hard-and-fast rule. Parameters that are
+both input and output (often classes/structs) muddy the
+waters, and, as always, consistency with related
+functions may require you to bend the rule.</p>
+
+</div>
+
+<h3 id="Write_Short_Functions">Write Short Functions</h3>
+
+<div class="summary">
+<p>Prefer small and focused functions.</p>
+</div>
+
+<div class="stylebody">
+<p>We recognize that long functions are sometimes
+appropriate, so no hard limit is placed on functions
+length. If a function exceeds about 40 lines, think about
+whether it can be broken up without harming the structure
+of the program.</p>
+
+<p>Even if your long function works perfectly now,
+someone modifying it in a few months may add new
+behavior. This could result in bugs that are hard to
+find. Keeping your functions short and simple makes it
+easier for other people to read and modify your code.</p>
+
+<p>You could find long and complicated functions when
+working with
+some code. Do not be
+intimidated by modifying existing code: if working with
+such a function proves to be difficult, you find that
+errors are hard to debug, or you want to use a piece of
+it in several different contexts, consider breaking up
+the function into smaller and more manageable pieces.</p>
+
+</div>
+
+<h3 id="Reference_Arguments">Reference Arguments</h3>
+
+<div class="summary">
+<p>All parameters passed by reference must be labeled
+<code>const</code>.</p>
+</div>
+
+<div class="stylebody">
+
+<div class="definition">
+<p>In C, if a
+function needs to modify a variable, the parameter must
+use a pointer, eg <code>int foo(int *pval)</code>. In
+C++, the function can alternatively declare a reference
+parameter: <code>int foo(int &val)</code>.</p>
+</div>
+
+<div class="pros">
+<p>Defining a parameter as reference avoids ugly code like
+<code>(*pval)++</code>. Necessary for some applications
+like copy constructors. Makes it clear, unlike with
+pointers, that a null pointer is not a possible
+value.</p>
+</div>
+
+<div class="cons">
+<p>References can be confusing, as they have value syntax
+but pointer semantics.</p>
+</div>
+
+<div class="decision">
+<p>Within function parameter lists all references must be
+<code>const</code>:</p>
+
+<pre>void Foo(const string &in, string *out);
+</pre>
+
+<p>In fact it is a very strong convention in Google code
+that input arguments are values or <code>const</code>
+references while output arguments are pointers. Input
+parameters may be <code>const</code> pointers, but we
+never allow non-<code>const</code> reference parameters
+except when required by convention, e.g.,
+<code>swap()</code>.</p>
+
+<p>However, there are some instances where using
+<code>const T*</code> is preferable to <code>const
+T&</code> for input parameters. For example:</p>
+
+<ul>
+ <li>You want to pass in a null pointer.</li>
+
+ <li>The function saves a pointer or reference to the
+ input.</li>
+</ul>
+
+<p> Remember that most of the time input
+parameters are going to be specified as <code>const
+T&</code>. Using <code>const T*</code> instead
+communicates to the reader that the input is somehow
+treated differently. So if you choose <code>const
+T*</code> rather than <code>const T&</code>, do so
+for a concrete reason; otherwise it will likely confuse
+readers by making them look for an explanation that
+doesn't exist.</p>
+</div>
+
+</div>
+
+<h3 id="Function_Overloading">Function Overloading</h3>
+
+<div class="summary">
+<p>Use overloaded functions (including constructors) only if a
+reader looking at a call site can get a good idea of what
+is happening without having to first figure out exactly
+which overload is being called.</p>
+</div>
+
+<div class="stylebody">
+
+<div class="definition">
+<p>You may write a function that takes a <code>const
+string&</code> and overload it with another that
+takes <code>const char*</code>.</p>
+
+<pre>class MyClass {
+ public:
+ void Analyze(const string &text);
+ void Analyze(const char *text, size_t textlen);
+};
+</pre>
+</div>
+
+<div class="pros">
+<p>Overloading can make code more intuitive by allowing an
+identically-named function to take different arguments.
+It may be necessary for templatized code, and it can be
+convenient for Visitors.</p>
+</div>
+
+<div class="cons">
+<p>If a function is overloaded by the argument types alone,
+a reader may have to understand C++'s complex matching
+rules in order to tell what's going on. Also many people
+are confused by the semantics of inheritance if a derived
+class overrides only some of the variants of a
+function.</p>
+</div>
+
+<div class="decision">
+<p>If you want to overload a function, consider qualifying
+the name with some information about the arguments, e.g.,
+<code>AppendString()</code>, <code>AppendInt()</code>
+rather than just <code>Append()</code>. If you are
+overloading a function to support variable number of
+arguments of the same type, consider making it take a
+<code>std::vector</code> so that the user can use an
+<a href="#Braced_Initializer_List">initializer list
+</a> to specify the arguments.</p>
+</div>
+
+</div>
+
+<h3 id="Default_Arguments">Default Arguments</h3>
+
+<div class="summary">
+<p>Default arguments are allowed on non-virtual functions
+when the default is guaranteed to always have the same
+value. Follow the same restrictions as for <a href="#Function_Overloading">function overloading</a>, and
+prefer overloaded functions if the readability gained with
+default arguments doesn't outweigh the downsides below.</p>
+</div>
+
+<div class="stylebody">
+
+<div class="pros">
+<p>Often you have a function that uses default values, but
+occasionally you want to override the defaults. Default
+parameters allow an easy way to do this without having to
+define many functions for the rare exceptions. Compared
+to overloading the function, default arguments have a
+cleaner syntax, with less boilerplate and a clearer
+distinction between 'required' and 'optional'
+arguments.</p>
+</div>
+
+<div class="cons">
+<p>Defaulted arguments are another way to achieve the
+semantics of overloaded functions, so all the <a href="#Function_Overloading">reasons not to overload
+functions</a> apply.</p>
+
+<p>The defaults for arguments in a virtual function call are
+determined by the static type of the target object, and
+there's no guarantee that all overrides of a given function
+declare the same defaults.</p>
+
+<p>Default parameters are re-evaluated at each call site,
+which can bloat the generated code. Readers may also expect
+the default's value to be fixed at the declaration instead
+of varying at each call.</p>
+
+<p>Function pointers are confusing in the presence of
+default arguments, since the function signature often
+doesn't match the call signature. Adding
+function overloads avoids these problems.</p>
+</div>
+
+<div class="decision">
+<p>Default arguments are banned on virtual functions, where
+they don't work properly, and in cases where the specified
+default might not evaluate to the same value depending on
+when it was evaluated. (For example, don't write <code>void
+f(int n = counter++);</code>.)</p>
+
+<p>In some other cases, default arguments can improve the
+readability of their function declarations enough to
+overcome the downsides above, so they are allowed. When in
+doubt, use overloads.</p>
+</div>
+
+</div>
+
+<h3 id="trailing_return">Trailing Return Type Syntax</h3>
+<div class="summary">
+<p>Use trailing return types only where using the ordinary syntax (leading
+ return types) is impractical or much less readable.</p>
+</div>
+
+<div class="definition">
+<p>C++ allows two different forms of function declarations. In the older
+ form, the return type appears before the function name. For example:</p>
+<pre>int foo(int x);
+</pre>
+<p>The new form, introduced in C++11, uses the <code>auto</code>
+ keyword before the function name and a trailing return type after
+ the argument list. For example, the declaration above could
+ equivalently be written:</p>
+<pre>auto foo(int x) -> int;
+</pre>
+<p>The trailing return type is in the function's scope. This doesn't
+ make a difference for a simple case like <code>int</code> but it matters
+ for more complicated cases, like types declared in class scope or
+ types written in terms of the function parameters.</p>
+</div>
+
+<div class="stylebody">
+<div class="pros">
+<p>Trailing return types are the only way to explicitly specify the
+ return type of a <a href="#Lambda_expressions">lambda expression</a>.
+ In some cases the compiler is able to deduce a lambda's return type,
+ but not in all cases. Even when the compiler can deduce it automatically,
+ sometimes specifying it explicitly would be clearer for readers.
+</p>
+<p>Sometimes it's easier and more readable to specify a return type
+ after the function's parameter list has already appeared. This is
+ particularly true when the return type depends on template parameters.
+ For example:</p>
+ <pre>template <class T, class U> auto add(T t, U u) -> decltype(t + u);</pre>
+ versus
+ <pre>template <class T, class U> decltype(declval<T&>() + declval<U&>()) add(T t, U u);</pre>
+</div>
+
+<div class="cons">
+<p>Trailing return type syntax is relatively new and it has no
+ analogue in C++-like languages like C and Java, so some readers may
+ find it unfamiliar.</p>
+<p>Existing code bases have an enormous number of function
+ declarations that aren't going to get changed to use the new syntax,
+ so the realistic choices are using the old syntax only or using a mixture
+ of the two. Using a single version is better for uniformity of style.</p>
+</div>
+
+<div class="decision">
+<p>In most cases, continue to use the older style of function
+ declaration where the return type goes before the function name.
+ Use the new trailing-return-type form only in cases where it's
+ required (such as lambdas) or where, by putting the type after the
+ function's parameter list, it allows you to write the type in a much
+ more readable way. The latter case should be rare; it's mostly an
+ issue in fairly complicated template code, which is
+ <a href="#Template_metaprogramming">discouraged in most cases</a>.</p>
+
+</div>
+</div>
+
+<h2 id="Google-Specific_Magic">Google-Specific Magic</h2>
+
+
+
+<p>There are various tricks and utilities that
+we use to make C++ code more robust, and various ways we use
+C++ that may differ from what you see elsewhere.</p>
+
+
+
+<h3 id="Ownership_and_Smart_Pointers">Ownership and Smart Pointers</h3>
+
+<div class="summary">
+<p>Prefer to have single, fixed owners for dynamically
+allocated objects. Prefer to transfer ownership with smart
+pointers.</p>
+</div>
+
+<div class="stylebody">
+
+<div class="definition">
+<p>"Ownership" is a bookkeeping technique for managing
+dynamically allocated memory (and other resources). The
+owner of a dynamically allocated object is an object or
+function that is responsible for ensuring that it is
+deleted when no longer needed. Ownership can sometimes be
+shared, in which case the last owner is typically
+responsible for deleting it. Even when ownership is not
+shared, it can be transferred from one piece of code to
+another.</p>
+
+<p>"Smart" pointers are classes that act like pointers,
+e.g. by overloading the <code>*</code> and
+<code>-></code> operators. Some smart pointer types
+can be used to automate ownership bookkeeping, to ensure
+these responsibilities are met.
+<a href="http://en.cppreference.com/w/cpp/memory/unique_ptr">
+<code>std::unique_ptr</code></a> is a smart pointer type
+introduced in C++11, which expresses exclusive ownership
+of a dynamically allocated object; the object is deleted
+when the <code>std::unique_ptr</code> goes out of scope.
+It cannot be copied, but can be <em>moved</em> to
+represent ownership transfer.
+<a href="http://en.cppreference.com/w/cpp/memory/shared_ptr">
+<code>std::shared_ptr</code></a> is a smart pointer type
+that expresses shared ownership of
+a dynamically allocated object. <code>std::shared_ptr</code>s
+can be copied; ownership of the object is shared among
+all copies, and the object is deleted when the last
+<code>std::shared_ptr</code> is destroyed. </p>
+</div>
+
+<div class="pros">
+<ul>
+ <li>It's virtually impossible to manage dynamically
+ allocated memory without some sort of ownership
+ logic.</li>
+
+ <li>Transferring ownership of an object can be cheaper
+ than copying it (if copying it is even possible).</li>
+
+ <li>Transferring ownership can be simpler than
+ 'borrowing' a pointer or reference, because it reduces
+ the need to coordinate the lifetime of the object
+ between the two users.</li>
+
+ <li>Smart pointers can improve readability by making
+ ownership logic explicit, self-documenting, and
+ unambiguous.</li>
+
+ <li>Smart pointers can eliminate manual ownership
+ bookkeeping, simplifying the code and ruling out large
+ classes of errors.</li>
+
+ <li>For const objects, shared ownership can be a simple
+ and efficient alternative to deep copying.</li>
+</ul>
+</div>
+
+<div class="cons">
+<ul>
+ <li>Ownership must be represented and transferred via
+ pointers (whether smart or plain). Pointer semantics
+ are more complicated than value semantics, especially
+ in APIs: you have to worry not just about ownership,
+ but also aliasing, lifetime, and mutability, among
+ other issues.</li>
+
+ <li>The performance costs of value semantics are often
+ overestimated, so the performance benefits of ownership
+ transfer might not justify the readability and
+ complexity costs.</li>
+
+ <li>APIs that transfer ownership force their clients
+ into a single memory management model.</li>
+
+ <li>Code using smart pointers is less explicit about
+ where the resource releases take place.</li>
+
+ <li><code>std::unique_ptr</code> expresses ownership
+ transfer using C++11's move semantics, which are
+ relatively new and may confuse some programmers.</li>
+
+ <li>Shared ownership can be a tempting alternative to
+ careful ownership design, obfuscating the design of a
+ system.</li>
+
+ <li>Shared ownership requires explicit bookkeeping at
+ run-time, which can be costly.</li>
+
+ <li>In some cases (e.g. cyclic references), objects
+ with shared ownership may never be deleted.</li>
+
+ <li>Smart pointers are not perfect substitutes for
+ plain pointers.</li>
+</ul>
+</div>
+
+<div class="decision">
+<p>If dynamic allocation is necessary, prefer to keep
+ownership with the code that allocated it. If other code
+needs access to the object, consider passing it a copy,
+or passing a pointer or reference without transferring
+ownership. Prefer to use <code>std::unique_ptr</code> to
+make ownership transfer explicit. For example:</p>
+
+<pre>std::unique_ptr<Foo> FooFactory();
+void FooConsumer(std::unique_ptr<Foo> ptr);
+</pre>
+
+
+
+<p>Do not design your code to use shared ownership
+without a very good reason. One such reason is to avoid
+expensive copy operations, but you should only do this if
+the performance benefits are significant, and the
+underlying object is immutable (i.e.
+<code>std::shared_ptr<const Foo></code>). If you
+do use shared ownership, prefer to use
+<code>std::shared_ptr</code>.</p>
+
+<p>Never use <code>std::auto_ptr</code>. Instead, use
+<code>std::unique_ptr</code>.</p>
+</div>
+
+</div>
+
+<h3 id="cpplint">cpplint</h3>
+
+<div class="summary">
+<p>Use <code>cpplint.py</code>
+to detect style errors.</p>
+</div>
+
+<div class="stylebody">
+
+<p><code>cpplint.py</code>
+is a tool that reads a source file and identifies many
+style errors. It is not perfect, and has both false
+positives and false negatives, but it is still a valuable
+tool. False positives can be ignored by putting <code>//
+NOLINT</code> at the end of the line or
+<code>// NOLINTNEXTLINE</code> in the previous line.</p>
+
+
+
+<p>Some projects have instructions on
+how to run <code>cpplint.py</code> from their project
+tools. If the project you are contributing to does not,
+you can download
+<a href="https://raw.githubusercontent.com/google/styleguide/gh-pages/cpplint/cpplint.py">
+<code>cpplint.py</code></a> separately.</p>
+
+</div>
+
+
+
+<h2 id="Other_C++_Features">Other C++ Features</h2>
+
+<h3 id="Rvalue_references">Rvalue References</h3>
+
+<div class="summary">
+<p>Use rvalue references only to define move constructors and move assignment
+operators, or for perfect forwarding.
+</p>
+</div>
+
+<div class="stylebody">
+
+<div class="definition">
+<p> Rvalue references
+are a type of reference that can only bind to temporary
+objects. The syntax is similar to traditional reference
+syntax. For example, <code>void f(string&&
+s);</code> declares a function whose argument is an
+rvalue reference to a string.</p>
+</div>
+
+<div class="pros">
+<ul>
+ <li>Defining a move constructor (a constructor taking
+ an rvalue reference to the class type) makes it
+ possible to move a value instead of copying it. If
+ <code>v1</code> is a <code>std::vector<string></code>,
+ for example, then <code>auto v2(std::move(v1))</code>
+ will probably just result in some simple pointer
+ manipulation instead of copying a large amount of data.
+ In some cases this can result in a major performance
+ improvement.</li>
+
+ <li>Rvalue references make it possible to write a
+ generic function wrapper that forwards its arguments to
+ another function, and works whether or not its
+ arguments are temporary objects. (This is sometimes called
+ "perfect forwarding".)</li>
+
+ <li>Rvalue references make it possible to implement
+ types that are movable but not copyable, which can be
+ useful for types that have no sensible definition of
+ copying but where you might still want to pass them as
+ function arguments, put them in containers, etc.</li>
+
+ <li><code>std::move</code> is necessary to make
+ effective use of some standard-library types, such as
+ <code>std::unique_ptr</code>.</li>
+</ul>
+</div>
+
+<div class="cons">
+<ul>
+ <li>Rvalue references are a relatively new feature
+ (introduced as part of C++11), and not yet widely
+ understood. Rules like reference collapsing, and
+ automatic synthesis of move constructors, are
+ complicated.</li>
+</ul>
+</div>
+
+<div class="decision">
+ <p>Use rvalue references only to define move constructors and move assignment
+ operators (as described in <a href="#Copyable_Movable_Types">Copyable and
+ Movable Types</a>) and, in conjunction with <code><a href="http://en.cppreference.com/w/cpp/utility/forward">std::forward</a></code>,
+to support perfect forwarding. You may use <code>std::move</code> to express
+moving a value from one object to another rather than copying it. </p>
+</div>
+
+</div>
+
+<h3 id="Friends">Friends</h3>
+
+<div class="summary">
+<p>We allow use of <code>friend</code> classes and functions,
+within reason.</p>
+</div>
+
+<div class="stylebody">
+
+<p>Friends should usually be defined in the same file so
+that the reader does not have to look in another file to
+find uses of the private members of a class. A common use
+of <code>friend</code> is to have a
+<code>FooBuilder</code> class be a friend of
+<code>Foo</code> so that it can construct the inner state
+of <code>Foo</code> correctly, without exposing this
+state to the world. In some cases it may be useful to
+make a unittest class a friend of the class it tests.</p>
+
+<p>Friends extend, but do not break, the encapsulation
+boundary of a class. In some cases this is better than
+making a member public when you want to give only one
+other class access to it. However, most classes should
+interact with other classes solely through their public
+members.</p>
+
+</div>
+
+<h3 id="Exceptions">Exceptions</h3>
+
+<div class="summary">
+<p>We do not use C++ exceptions.</p>
+</div>
+
+<div class="stylebody">
+
+<div class="pros">
+<ul>
+ <li>Exceptions allow higher levels of an application to
+ decide how to handle "can't happen" failures in deeply
+ nested functions, without the obscuring and error-prone
+ bookkeeping of error codes.</li>
+
+
+
+ <li>Exceptions are used by most other
+ modern languages. Using them in C++ would make it more
+ consistent with Python, Java, and the C++ that others
+ are familiar with.</li>
+
+ <li>Some third-party C++ libraries use exceptions, and
+ turning them off internally makes it harder to
+ integrate with those libraries.</li>
+
+ <li>Exceptions are the only way for a constructor to
+ fail. We can simulate this with a factory function or
+ an <code>Init()</code> method, but these require heap
+ allocation or a new "invalid" state, respectively.</li>
+
+ <li>Exceptions are really handy in testing
+ frameworks.</li>
+</ul>
+</div>
+
+<div class="cons">
+<ul>
+ <li>When you add a <code>throw</code> statement to an
+ existing function, you must examine all of its
+ transitive callers. Either they must make at least the
+ basic exception safety guarantee, or they must never
+ catch the exception and be happy with the program
+ terminating as a result. For instance, if
+ <code>f()</code> calls <code>g()</code> calls
+ <code>h()</code>, and <code>h</code> throws an
+ exception that <code>f</code> catches, <code>g</code>
+ has to be careful or it may not clean up properly.</li>
+
+ <li>More generally, exceptions make the control flow of
+ programs difficult to evaluate by looking at code:
+ functions may return in places you don't expect. This
+ causes maintainability and debugging difficulties. You
+ can minimize this cost via some rules on how and where
+ exceptions can be used, but at the cost of more that a
+ developer needs to know and understand.</li>
+
+ <li>Exception safety requires both RAII and different
+ coding practices. Lots of supporting machinery is
+ needed to make writing correct exception-safe code
+ easy. Further, to avoid requiring readers to understand
+ the entire call graph, exception-safe code must isolate
+ logic that writes to persistent state into a "commit"
+ phase. This will have both benefits and costs (perhaps
+ where you're forced to obfuscate code to isolate the
+ commit). Allowing exceptions would force us to always
+ pay those costs even when they're not worth it.</li>
+
+ <li>Turning on exceptions adds data to each binary
+ produced, increasing compile time (probably slightly)
+ and possibly increasing address space pressure.
+ </li>
+
+ <li>The availability of exceptions may encourage
+ developers to throw them when they are not appropriate
+ or recover from them when it's not safe to do so. For
+ example, invalid user input should not cause exceptions
+ to be thrown. We would need to make the style guide
+ even longer to document these restrictions!</li>
+</ul>
+</div>
+
+<div class="decision">
+<p>On their face, the benefits of using exceptions
+outweigh the costs, especially in new projects. However,
+for existing code, the introduction of exceptions has
+implications on all dependent code. If exceptions can be
+propagated beyond a new project, it also becomes
+problematic to integrate the new project into existing
+exception-free code. Because most existing C++ code at
+Google is not prepared to deal with exceptions, it is
+comparatively difficult to adopt new code that generates
+exceptions.</p>
+
+<p>Given that Google's existing code is not
+exception-tolerant, the costs of using exceptions are
+somewhat greater than the costs in a new project. The
+conversion process would be slow and error-prone. We
+don't believe that the available alternatives to
+exceptions, such as error codes and assertions, introduce
+a significant burden. </p>
+
+<p>Our advice against using exceptions is not predicated
+on philosophical or moral grounds, but practical ones.
+ Because we'd like to use our open-source
+projects at Google and it's difficult to do so if those
+projects use exceptions, we need to advise against
+exceptions in Google open-source projects as well.
+Things would probably be different if we had to do it all
+over again from scratch.</p>
+
+<p>This prohibition also applies to the exception-related
+features added in C++11, such as <code>noexcept</code>,
+<code>std::exception_ptr</code>, and
+<code>std::nested_exception</code>.</p>
+
+<p>There is an <a href="#Windows_Code">exception</a> to
+this rule (no pun intended) for Windows code.</p>
+</div>
+
+</div>
+
+<h3 id="Run-Time_Type_Information__RTTI_">Run-Time Type
+Information (RTTI)</h3>
+
+<div class="summary">
+<p>Avoid using Run Time Type Information (RTTI).</p>
+</div>
+
+<div class="stylebody">
+
+<div class="definition">
+<p> RTTI allows a
+programmer to query the C++ class of an object at run
+time. This is done by use of <code>typeid</code> or
+<code>dynamic_cast</code>.</p>
+</div>
+
+<div class="cons">
+<p>Querying the type of an object at run-time frequently
+means a design problem. Needing to know the type of an
+object at runtime is often an indication that the design
+of your class hierarchy is flawed.</p>
+
+<p>Undisciplined use of RTTI makes code hard to maintain.
+It can lead to type-based decision trees or switch
+statements scattered throughout the code, all of which
+must be examined when making further changes.</p>
+</div>
+
+<div class="pros">
+<p>The standard alternatives to RTTI (described below)
+require modification or redesign of the class hierarchy
+in question. Sometimes such modifications are infeasible
+or undesirable, particularly in widely-used or mature
+code.</p>
+
+<p>RTTI can be useful in some unit tests. For example, it
+is useful in tests of factory classes where the test has
+to verify that a newly created object has the expected
+dynamic type. It is also useful in managing the
+relationship between objects and their mocks.</p>
+
+<p>RTTI is useful when considering multiple abstract
+objects. Consider</p>
+
+<pre>bool Base::Equal(Base* other) = 0;
+bool Derived::Equal(Base* other) {
+ Derived* that = dynamic_cast<Derived*>(other);
+ if (that == NULL)
+ return false;
+ ...
+}
+</pre>
+</div>
+
+<div class="decision">
+<p>RTTI has legitimate uses but is prone to abuse, so you
+must be careful when using it. You may use it freely in
+unittests, but avoid it when possible in other code. In
+particular, think twice before using RTTI in new code. If
+you find yourself needing to write code that behaves
+differently based on the class of an object, consider one
+of the following alternatives to querying the type:</p>
+
+<ul>
+ <li>Virtual methods are the preferred way of executing
+ different code paths depending on a specific subclass
+ type. This puts the work within the object itself.</li>
+
+ <li>If the work belongs outside the object and instead
+ in some processing code, consider a double-dispatch
+ solution, such as the Visitor design pattern. This
+ allows a facility outside the object itself to
+ determine the type of class using the built-in type
+ system.</li>
+</ul>
+
+<p>When the logic of a program guarantees that a given
+instance of a base class is in fact an instance of a
+particular derived class, then a
+<code>dynamic_cast</code> may be used freely on the
+object. Usually one
+can use a <code>static_cast</code> as an alternative in
+such situations.</p>
+
+<p>Decision trees based on type are a strong indication
+that your code is on the wrong track.</p>
+
+<pre class="badcode">if (typeid(*data) == typeid(D1)) {
+ ...
+} else if (typeid(*data) == typeid(D2)) {
+ ...
+} else if (typeid(*data) == typeid(D3)) {
+...
+</pre>
+
+<p>Code such as this usually breaks when additional
+subclasses are added to the class hierarchy. Moreover,
+when properties of a subclass change, it is difficult to
+find and modify all the affected code segments.</p>
+
+<p>Do not hand-implement an RTTI-like workaround. The
+arguments against RTTI apply just as much to workarounds
+like class hierarchies with type tags. Moreover,
+workarounds disguise your true intent.</p>
+</div>
+
+</div>
+
+<h3 id="Casting">Casting</h3>
+
+<div class="summary">
+<p>Use C++-style casts
+like <code>static_cast<float>(double_value)</code>, or brace
+initialization for conversion of arithmetic types like
+<code>int64 y = int64{1} << 42</code>. Do not use
+cast formats like
+<code>int y = (int)x</code> or <code>int y = int(x)</code> (but the latter
+is okay when invoking a constructor of a class type).</p>
+</div>
+
+<div class="stylebody">
+
+<div class="definition">
+<p> C++ introduced a
+different cast system from C that distinguishes the types
+of cast operations.</p>
+</div>
+
+<div class="pros">
+<p>The problem with C casts is the ambiguity of the operation;
+sometimes you are doing a <em>conversion</em>
+(e.g., <code>(int)3.5</code>) and sometimes you are doing
+a <em>cast</em> (e.g., <code>(int)"hello"</code>). Brace
+initialization and C++ casts can often help avoid this
+ambiguity. Additionally, C++ casts are more visible when searching for
+them.</p>
+</div>
+
+<div class="cons">
+<p>The C++-style cast syntax is verbose and cumbersome.</p>
+</div>
+
+<div class="decision">
+<p>Do not use C-style casts. Instead, use these C++-style casts when
+explicit type conversion is necessary. </p>
+
+<ul>
+ <li>Use brace initialization to convert arithmetic types
+ (e.g. <code>int64{x}</code>). This is the safest approach because code
+ will not compile if conversion can result in information loss. The
+ syntax is also concise.</li>
+
+
+
+ <li>Use <code>static_cast</code> as the equivalent of a C-style cast
+ that does value conversion, when you need to
+ explicitly up-cast a pointer from a class to its superclass, or when
+ you need to explicitly cast a pointer from a superclass to a
+ subclass. In this last case, you must be sure your object is
+ actually an instance of the subclass.</li>
+
+
+
+ <li>Use <code>const_cast</code> to remove the
+ <code>const</code> qualifier (see <a href="#Use_of_const">const</a>).</li>
+
+ <li>Use <code>reinterpret_cast</code> to do unsafe
+ conversions of pointer types to and from integer and
+ other pointer types. Use this only if you know what you
+ are doing and you understand the aliasing issues.
+ </li>
+
+
+</ul>
+
+<p>See the <a href="#Run-Time_Type_Information__RTTI_">
+RTTI section</a> for guidance on the use of
+<code>dynamic_cast</code>.</p>
+</div>
+
+</div>
+
+<h3 id="Streams">Streams</h3>
+
+<div class="summary">
+<p>Use streams where appropriate, and stick to "simple"
+usages.</p>
+</div>
+
+<div class="stylebody">
+
+<div class="definition">
+<p>Streams are the standard I/O abstraction in C++, as
+exemplified by the standard header <code><iostream></code>.
+They are widely used in Google code, but only for debug logging
+and test diagnostics.</p>
+</div>
+
+<div class="pros">
+<p>The <code><<</code> and <code>>></code>
+stream operators provide an API for formatted I/O that
+is easily learned, portable, reusable, and extensible.
+<code>printf</code>, by contrast, doesn't even support
+<code>string</code>, to say nothing of user-defined types,
+and is very difficult to use portably.
+<code>printf</code> also obliges you to choose among the
+numerous slightly different versions of that function,
+and navigate the dozens of conversion specifiers.</p>
+
+<p>Streams provide first-class support for console I/O
+via <code>std::cin</code>, <code>std::cout</code>,
+<code>std::cerr</code>, and <code>std::clog</code>.
+The C APIs do as well, but are hampered by the need to
+manually buffer the input. </p>
+</div>
+
+<div class="cons">
+<ul>
+<li>Stream formatting can be configured by mutating the
+state of the stream. Such mutations are persistent, so
+the behavior of your code can be affected by the entire
+previous history of the stream, unless you go out of your
+way to restore it to a known state every time other code
+might have touched it. User code can not only modify the
+built-in state, it can add new state variables and behaviors
+through a registration system.</li>
+
+<li>It is difficult to precisely control stream output, due
+to the above issues, the way code and data are mixed in
+streaming code, and the use of operator overloading (which
+may select a different overload than you expect).</li>
+
+<li>The practice of building up output through chains
+of <code><<</code> operators interferes with
+internationalization, because it bakes word order into the
+code, and streams' support for localization is <a href="http://www.boost.org/doc/libs/1_48_0/libs/locale/doc/html/rationale.html#rationale_why">
+flawed</a>.</li>
+
+
+
+
+
+<li>The streams API is subtle and complex, so programmers must
+develop experience with it in order to use it effectively.
+However, streams were historically banned in Google code (except
+for logging and diagnostics), so Google engineers tend not to
+have that experience. Consequently, streams-based code is likely
+to be less readable and maintainable by Googlers than code based
+on more familiar abstractions.</li>
+
+<li>Resolving the many overloads of <code><<</code> is
+extremely costly for the compiler. When used pervasively in a
+large code base, it can consume as much as 20% of the parsing
+and semantic analysis time.</li>
+</ul>
+</div>
+
+<div class="decision">
+<p>Use streams only when they are the best tool for the job.
+This is typically the case when the I/O is ad-hoc, local,
+human-readable, and targeted at other developers rather than
+end-users. Be consistent with the code around you, and with the
+codebase as a whole; if there's an established tool for
+your problem, use that tool instead. </p>
+
+<p>Avoid using streams for I/O that faces external users or
+handles untrusted data. Instead, find and use the appropriate
+templating libraries to handle issues like internationalization,
+localization, and security hardening.</p>
+
+<p>If you do use streams, avoid the stateful parts of the
+streams API (other than error state), such as <code>imbue()</code>,
+<code>xalloc()</code>, and <code>register_callback()</code>.
+Use explicit formatting functions rather than
+stream manipulators or formatting flags to control formatting
+details such as number base, precision, or padding.</p>
+
+<p>Overload <code><<</code> as a streaming operator
+for your type only if your type represents a value, and
+<code><<</code> writes out a human-readable string
+representation of that value. Avoid exposing implementation
+details in the output of <code><<</code>; if you need to print
+object internals for debugging, use named functions instead
+(a method named <code>DebugString()</code> is the most common
+convention).</p>
+</div>
+
+</div>
+
+<h3 id="Preincrement_and_Predecrement">Preincrement and Predecrement</h3>
+
+<div class="summary">
+<p>Use prefix form (<code>++i</code>) of the increment and
+decrement operators with iterators and other template
+objects.</p>
+</div>
+
+<div class="stylebody">
+
+<div class="definition">
+<p> When a variable
+is incremented (<code>++i</code> or <code>i++</code>) or
+decremented (<code>--i</code> or <code>i--</code>) and
+the value of the expression is not used, one must decide
+whether to preincrement (decrement) or postincrement
+(decrement).</p>
+</div>
+
+<div class="pros">
+<p>When the return value is ignored, the "pre" form
+(<code>++i</code>) is never less efficient than the
+"post" form (<code>i++</code>), and is often more
+efficient. This is because post-increment (or decrement)
+requires a copy of <code>i</code> to be made, which is
+the value of the expression. If <code>i</code> is an
+iterator or other non-scalar type, copying <code>i</code>
+could be expensive. Since the two types of increment
+behave the same when the value is ignored, why not just
+always pre-increment?</p>
+</div>
+
+<div class="cons">
+<p>The tradition developed, in C, of using post-increment
+when the expression value is not used, especially in
+<code>for</code> loops. Some find post-increment easier
+to read, since the "subject" (<code>i</code>) precedes
+the "verb" (<code>++</code>), just like in English.</p>
+</div>
+
+<div class="decision">
+<p> For simple scalar
+(non-object) values there is no reason to prefer one form
+and we allow either. For iterators and other template
+types, use pre-increment.</p>
+</div>
+
+</div>
+
+<h3 id="Use_of_const">Use of const</h3>
+
+<div class="summary">
+<p>Use <code>const</code> whenever it makes sense. With C++11,
+<code>constexpr</code> is a better choice for some uses of
+const.</p>
+</div>
+
+<div class="stylebody">
+
+<div class="definition">
+<p> Declared variables and parameters can be preceded
+by the keyword <code>const</code> to indicate the variables
+are not changed (e.g., <code>const int foo</code>). Class
+functions can have the <code>const</code> qualifier to
+indicate the function does not change the state of the
+class member variables (e.g., <code>class Foo { int
+Bar(char c) const; };</code>).</p>
+</div>
+
+<div class="pros">
+<p>Easier for people to understand how variables are being
+used. Allows the compiler to do better type checking,
+and, conceivably, generate better code. Helps people
+convince themselves of program correctness because they
+know the functions they call are limited in how they can
+modify your variables. Helps people know what functions
+are safe to use without locks in multi-threaded
+programs.</p>
+</div>
+
+<div class="cons">
+<p><code>const</code> is viral: if you pass a
+<code>const</code> variable to a function, that function
+must have <code>const</code> in its prototype (or the
+variable will need a <code>const_cast</code>). This can
+be a particular problem when calling library
+functions.</p>
+</div>
+
+<div class="decision">
+<p><code>const</code> variables, data members, methods
+and arguments add a level of compile-time type checking;
+it is better to detect errors as soon as possible.
+Therefore we strongly recommend that you use
+<code>const</code> whenever it makes sense to do so:</p>
+
+<ul>
+ <li>If a function guarantees that it will not modify an argument
+ passed by reference or by pointer, the corresponding function parameter
+ should be a reference-to-const (<code>const T&</code>) or
+ pointer-to-const (<code>const T*</code>), respectively.</li>
+
+ <li>Declare methods to be <code>const</code> whenever
+ possible. Accessors should almost always be
+ <code>const</code>. Other methods should be const if
+ they do not modify any data members, do not call any
+ non-<code>const</code> methods, and do not return a
+ non-<code>const</code> pointer or
+ non-<code>const</code> reference to a data member.</li>
+
+ <li>Consider making data members <code>const</code>
+ whenever they do not need to be modified after
+ construction.</li>
+</ul>
+
+<p>The <code>mutable</code> keyword is allowed but is
+unsafe when used with threads, so thread safety should be
+carefully considered first.</p>
+</div>
+
+<div class="stylepoint_subsection">
+<h4>Where to put the const</h4>
+
+<p>Some people favor the form <code>int const *foo</code>
+to <code>const int* foo</code>. They argue that this is
+more readable because it's more consistent: it keeps the
+rule that <code>const</code> always follows the object
+it's describing. However, this consistency argument
+doesn't apply in codebases with few deeply-nested pointer
+expressions since most <code>const</code> expressions
+have only one <code>const</code>, and it applies to the
+underlying value. In such cases, there's no consistency
+to maintain. Putting the <code>const</code> first is
+arguably more readable, since it follows English in
+putting the "adjective" (<code>const</code>) before the
+"noun" (<code>int</code>).</p>
+
+<p>That said, while we encourage putting
+<code>const</code> first, we do not require it. But be
+consistent with the code around you!</p>
+</div>
+
+</div>
+
+<h3 id="Use_of_constexpr">Use of constexpr</h3>
+
+<div class="summary">
+<p>In C++11, use <code>constexpr</code> to define true
+constants or to ensure constant initialization.</p>
+</div>
+
+<div class="stylebody">
+
+<div class="definition">
+<p> Some variables can be declared <code>constexpr</code>
+to indicate the variables are true constants, i.e. fixed at
+compilation/link time. Some functions and constructors
+can be declared <code>constexpr</code> which enables them
+to be used in defining a <code>constexpr</code>
+variable.</p>
+</div>
+
+<div class="pros">
+<p>Use of <code>constexpr</code> enables definition of
+constants with floating-point expressions rather than
+just literals; definition of constants of user-defined
+types; and definition of constants with function
+calls.</p>
+</div>
+
+<div class="cons">
+<p>Prematurely marking something as constexpr may cause
+migration problems if later on it has to be downgraded.
+Current restrictions on what is allowed in constexpr
+functions and constructors may invite obscure workarounds
+in these definitions.</p>
+</div>
+
+<div class="decision">
+<p><code>constexpr</code> definitions enable a more
+robust specification of the constant parts of an
+interface. Use <code>constexpr</code> to specify true
+constants and the functions that support their
+definitions. Avoid complexifying function definitions to
+enable their use with <code>constexpr</code>. Do not use
+<code>constexpr</code> to force inlining.</p>
+</div>
+
+</div>
+
+<h3 id="Integer_Types">Integer Types</h3>
+
+<div class="summary">
+<p>Of the built-in C++ integer types, the only one used
+ is
+<code>int</code>. If a program needs a variable of a
+different size, use
+a precise-width integer type from
+<code><stdint.h></code>, such as
+<code>int16_t</code>. If your variable represents a
+value that could ever be greater than or equal to 2^31
+(2GiB), use a 64-bit type such as
+<code>int64_t</code>.
+Keep in mind that even if your value won't ever be too large
+for an <code>int</code>, it may be used in intermediate
+calculations which may require a larger type. When in doubt,
+choose a larger type.</p>
+</div>
+
+<div class="stylebody">
+
+<div class="definition">
+<p> C++ does not specify the sizes of its integer types.
+Typically people assume that <code>short</code> is 16 bits,
+<code>int</code> is 32 bits, <code>long</code> is 32 bits
+and <code>long long</code> is 64 bits.</p>
+</div>
+
+<div class="pros">
+<p>Uniformity of declaration.</p>
+</div>
+
+<div class="cons">
+<p>The sizes of integral types in C++ can vary based on
+compiler and architecture.</p>
+</div>
+
+<div class="decision">
+
+<p>
+<code><stdint.h></code> defines types
+like <code>int16_t</code>, <code>uint32_t</code>,
+<code>int64_t</code>, etc. You should always use
+those in preference to <code>short</code>, <code>unsigned
+long long</code> and the like, when you need a guarantee
+on the size of an integer. Of the C integer types, only
+<code>int</code> should be used. When appropriate, you
+are welcome to use standard types like
+<code>size_t</code> and <code>ptrdiff_t</code>.</p>
+
+<p>We use <code>int</code> very often, for integers we
+know are not going to be too big, e.g., loop counters.
+Use plain old <code>int</code> for such things. You
+should assume that an <code>int</code> is
+
+at least 32 bits, but don't
+assume that it has more than 32 bits. If you need a 64-bit
+integer type, use
+<code>int64_t</code>
+or
+<code>uint64_t</code>.</p>
+
+<p>For integers we know can be "big",
+ use
+<code>int64_t</code>.
+</p>
+
+<p>You should not use the unsigned integer types such as
+<code>uint32_t</code>, unless there is a valid
+reason such as representing a bit pattern rather than a
+number, or you need defined overflow modulo 2^N. In
+particular, do not use unsigned types to say a number
+will never be negative. Instead, use
+assertions for this.</p>
+
+
+
+<p>If your code is a container that returns a size, be
+sure to use a type that will accommodate any possible
+usage of your container. When in doubt, use a larger type
+rather than a smaller type.</p>
+
+<p>Use care when converting integer types. Integer
+conversions and promotions can cause non-intuitive
+behavior. </p>
+</div>
+
+<div class="stylepoint_subsection">
+
+<h4>On Unsigned Integers</h4>
+
+<p>Some people, including some textbook authors,
+recommend using unsigned types to represent numbers that
+are never negative. This is intended as a form of
+self-documentation. However, in C, the advantages of such
+documentation are outweighed by the real bugs it can
+introduce. Consider:</p>
+
+<pre>for (unsigned int i = foo.Length()-1; i >= 0; --i) ...
+</pre>
+
+<p>This code will never terminate! Sometimes gcc will
+notice this bug and warn you, but often it will not.
+Equally bad bugs can occur when comparing signed and
+unsigned variables. Basically, C's type-promotion scheme
+causes unsigned types to behave differently than one
+might expect.</p>
+
+<p>So, document that a variable is non-negative using
+assertions. Don't use an unsigned
+type.</p>
+</div>
+
+</div>
+
+<h3 id="64-bit_Portability">64-bit Portability</h3>
+
+<div class="summary">
+<p>Code should be 64-bit and 32-bit friendly. Bear in mind
+problems of printing, comparisons, and structure alignment.</p>
+</div>
+
+<div class="stylebody">
+
+<ul>
+ <li>
+ <p><code>printf()</code> specifiers for some types
+ are not cleanly portable between 32-bit and 64-bit
+ systems. C99 defines some portable format specifiers.
+ Unfortunately, MSVC 7.1 does not understand some of
+ these specifiers and the standard is missing a few,
+ so we
+ have to define our own ugly versions in some cases
+ (in the style of the standard include file
+ <code>inttypes.h</code>):</p>
+
+ <div>
+ <pre>// printf macros for size_t, in the style of inttypes.h
+#ifdef _LP64
+#define __PRIS_PREFIX "z"
+#else
+#define __PRIS_PREFIX
+#endif
+
+// Use these macros after a % in a printf format string
+// to get correct 32/64 bit behavior, like this:
+// size_t size = records.size();
+// printf("%" PRIuS "\n", size);
+
+#define PRIdS __PRIS_PREFIX "d"
+#define PRIxS __PRIS_PREFIX "x"
+#define PRIuS __PRIS_PREFIX "u"
+#define PRIXS __PRIS_PREFIX "X"
+#define PRIoS __PRIS_PREFIX "o"
+ </pre>
+ </div>
+
+ <table border="1" summary="portable printf specifiers">
+ <tbody><tr align="center">
+ <th>Type</th>
+ <th>DO NOT use</th>
+ <th>DO use</th>
+ <th>Notes</th>
+ </tr>
+
+ <tr align="center">
+ <td><code>void *</code> (or any pointer)</td>
+ <td><code>%lx</code></td>
+ <td><code>%p</code></td>
+ <td></td>
+ </tr>
+
+
+
+ <tr align="center">
+ <td><code>int64_t</code></td>
+ <td><code>%qd</code>, <code>%lld</code></td>
+ <td><code>%" PRId64 "</code></td>
+ <td></td>
+ </tr>
+
+
+
+ <tr align="center">
+ <td><code>uint64_t</code></td>
+ <td><code>%qu</code>, <code>%llu</code>,
+ <code>%llx</code></td>
+ <td><code>%" PRIu64 "</code>,
+ <code>%" PRIx64 "</code></td>
+ <td></td>
+ </tr>
+
+
+
+ <tr align="center">
+ <td><code>size_t</code></td>
+ <td><code>%u</code></td>
+ <td><code>%" PRIuS "</code>, <code>%" PRIxS "</code></td>
+ <td>
+ C99 specifies <code>%zu</code></td>
+ </tr>
+
+ <tr align="center">
+ <td><code>ptrdiff_t</code></td>
+ <td><code>%d</code></td>
+ <td><code>%" PRIdS "</code></td>
+ <td>
+ C99 specifies <code>%td</code></td>
+ </tr>
+
+
+ </tbody></table>
+
+ <p>Note that the <code>PRI*</code> macros expand to
+ independent strings which are concatenated by the
+ compiler. Hence if you are using a non-constant
+ format string, you need to insert the value of the
+ macro into the format, rather than the name. Note also
+ that spaces are required around the macro identifier to
+ separate it from the string literal. It is
+ still possible, as usual, to include length
+ specifiers, etc., after the <code>%</code> when using
+ the <code>PRI*</code> macros. So, e.g.
+ <code>printf("x = %30" PRIuS "\n", x)</code> would
+ expand on 32-bit Linux to <code>printf("x = %30" "u"
+ "\n", x)</code>, which the compiler will treat as
+ <code>printf("x = %30u\n", x)</code>.</p>
+
+
+ </li>
+
+ <li>Remember that <code>sizeof(void *)</code> !=
+ <code>sizeof(int)</code>. Use <code>intptr_t</code> if
+ you want a pointer-sized integer.</li>
+
+ <li>You may need to be careful with structure
+ alignments, particularly for structures being stored on
+ disk. Any class/structure with a
+ <code>int64_t</code>/<code>uint64_t</code>
+ member will by default end up being 8-byte aligned on a
+ 64-bit system. If you have such structures being shared
+ on disk between 32-bit and 64-bit code, you will need
+ to ensure that they are packed the same on both
+ architectures.
+ Most compilers offer a way to
+ alter structure alignment. For gcc, you can use
+ <code>__attribute__((packed))</code>. MSVC offers
+ <code>#pragma pack()</code> and
+ <code>__declspec(align())</code>.</li>
+
+ <li>
+ <p>Use the <code>LL</code> or <code>ULL</code>
+ suffixes as needed to create 64-bit constants. For
+ example:</p>
+
+
+<pre>int64_t my_value = 0x123456789LL;
+uint64_t my_mask = 3ULL << 48;
+</pre>
+ </li>
+</ul>
+
+</div>
+
+<h3 id="Preprocessor_Macros">Preprocessor Macros</h3>
+
+<div class="summary">
+<p>Avoid defining macros, especially in headers; prefer
+inline functions, enums, and <code>const</code> variables.
+Name macros with a project-specific prefix. Do not use
+macros to define pieces of a C++ API.</p>
+</div>
+
+<div class="stylebody">
+
+<p>Macros mean that the code you see is not the same as
+the code the compiler sees. This can introduce unexpected
+behavior, especially since macros have global scope.</p>
+
+<p>The problems introduced by macros are especially severe
+when they are used to define pieces of a C++ API,
+and still more so for public APIs. Every error message from
+the compiler when developers incorrectly use that interface
+now must explain how the macros formed the interface.
+Refactoring and analysis tools have a dramatically harder
+time updating the interface. As a consequence, we
+specifically disallow using macros in this way.
+For example, avoid patterns like:</p>
+
+<pre class="badcode">class WOMBAT_TYPE(Foo) {
+ // ...
+
+ public:
+ EXPAND_PUBLIC_WOMBAT_API(Foo)
+
+ EXPAND_WOMBAT_COMPARISONS(Foo, ==, <)
+};
+</pre>
+
+<p>Luckily, macros are not nearly as necessary in C++ as
+they are in C. Instead of using a macro to inline
+performance-critical code, use an inline function.
+Instead of using a macro to store a constant, use a
+<code>const</code> variable. Instead of using a macro to
+"abbreviate" a long variable name, use a reference.
+Instead of using a macro to conditionally compile code
+... well, don't do that at all (except, of course, for
+the <code>#define</code> guards to prevent double
+inclusion of header files). It makes testing much more
+difficult.</p>
+
+<p>Macros can do things these other techniques cannot,
+and you do see them in the codebase, especially in the
+lower-level libraries. And some of their special features
+(like stringifying, concatenation, and so forth) are not
+available through the language proper. But before using a
+macro, consider carefully whether there's a non-macro way
+to achieve the same result. If you need to use a macro to
+define an interface, contact
+your project leads to request
+a waiver of this rule.</p>
+
+<p>The following usage pattern will avoid many problems
+with macros; if you use macros, follow it whenever
+possible:</p>
+
+<ul>
+ <li>Don't define macros in a <code>.h</code> file.</li>
+
+ <li><code>#define</code> macros right before you use
+ them, and <code>#undef</code> them right after.</li>
+
+ <li>Do not just <code>#undef</code> an existing macro
+ before replacing it with your own; instead, pick a name
+ that's likely to be unique.</li>
+
+ <li>Try not to use macros that expand to unbalanced C++
+ constructs, or at least document that behavior
+ well.</li>
+
+ <li>Prefer not using <code>##</code> to generate
+ function/class/variable names.</li>
+</ul>
+
+<p>Exporting macros from headers (i.e. defining them in a header
+without <code>#undef</code>ing them before the end of the header)
+is extremely strongly discouraged. If you do export a macro from a
+header, it must have a globally unique name. To achieve this, it
+must be named with a prefix consisting of your project's namespace
+name (but upper case). </p>
+
+</div>
+
+<h3 id="0_and_nullptr/NULL">0 and nullptr/NULL</h3>
+
+<div class="summary">
+<p>Use <code>0</code> for integers, <code>0.0</code> for
+reals, <code>nullptr</code> (or <code>NULL</code>) for
+pointers, and <code>'\0'</code> for chars.</p>
+</div>
+
+<div class="stylebody">
+
+<p>Use <code>0</code> for integers and <code>0.0</code>
+for reals. This is not controversial.</p>
+
+<p> For
+pointers (address values), there is a choice between
+<code>0</code>, <code>NULL</code>, and
+<code>nullptr</code>. For projects that allow C++11
+features, use <code>nullptr</code>. For C++03 projects,
+we prefer <code>NULL</code> because it looks like a
+pointer. In fact, some C++ compilers provide special
+definitions of <code>NULL</code> which enable them to
+give useful warnings, particularly in situations where
+<code>sizeof(NULL)</code> is not equal to
+<code>sizeof(0)</code>.</p>
+
+<p>Use <code>'\0'</code> for chars. This is the correct
+type and also makes code more readable.</p>
+
+</div>
+
+<h3 id="sizeof">sizeof</h3>
+
+<div class="summary">
+<p>Prefer <code>sizeof(<var>varname</var>)</code> to
+<code>sizeof(<var>type</var>)</code>.</p>
+</div>
+
+<div class="stylebody">
+
+<p>Use <code>sizeof(<var>varname</var>)</code> when you
+take the size of a particular variable.
+<code>sizeof(<var>varname</var>)</code> will update
+appropriately if someone changes the variable type either
+now or later. You may use
+<code>sizeof(<var>type</var>)</code> for code unrelated
+to any particular variable, such as code that manages an
+external or internal data format where a variable of an
+appropriate C++ type is not convenient.</p>
+
+<pre>Struct data;
+memset(&data, 0, sizeof(data));
+</pre>
+
+<pre class="badcode">memset(&data, 0, sizeof(Struct));
+</pre>
+
+<pre>if (raw_size < sizeof(int)) {
+ LOG(ERROR) << "compressed record not big enough for count: " << raw_size;
+ return false;
+}
+</pre>
+
+</div>
+
+<h3 id="auto">auto</h3>
+
+<div class="summary">
+<p>Use <code>auto</code> to avoid type names that are noisy, obvious,
+or unimportant - cases where the type doesn't aid in clarity for the
+reader. Continue to use manifest type declarations when it helps
+readability.</p>
+</div>
+
+<div class="stylebody">
+
+<div class="pros">
+<p>
+</p><ul>
+<li>C++ type names can be long and cumbersome, especially when they
+involve templates or namespaces.</li>
+<li>When a C++ type name is repeated within a single declaration or a
+small code region, the repetition may not be aiding readability.</li>
+<li>It is sometimes safer to let the type be specified by the type of
+the initialization expression, since that avoids the possibility of
+unintended copies or type conversions.</li>
+</ul>
+</div>
+<div class="cons">
+
+<p>Sometimes code is clearer when types are manifest,
+especially when a variable's initialization depends on
+things that were declared far away. In expressions
+like:</p>
+
+<pre class="badcode">auto foo = x.add_foo();
+auto i = y.Find(key);
+</pre>
+
+<p>it may not be obvious what the resulting types are if the type
+of <code>y</code> isn't very well known, or if <code>y</code> was
+declared many lines earlier.</p>
+
+<p>Programmers have to understand the difference between
+<code>auto</code> and <code>const auto&</code> or
+they'll get copies when they didn't mean to.</p>
+
+<p>If an <code>auto</code> variable is used as part of an
+interface, e.g. as a constant in a header, then a
+programmer might change its type while only intending to
+change its value, leading to a more radical API change
+than intended.</p>
+</div>
+
+<div class="decision">
+
+<p><code>auto</code> is permitted when it increases readability,
+particularly as described below. Never initialize an <code>auto</code>-typed
+variable with a braced initializer list.</p>
+
+<p>Specific cases where <code>auto</code> is allowed or encouraged:
+</p><ul>
+<li>(Encouraged) For iterators and other long/cluttery type names, particularly
+when the type is clear from context (calls
+to <code>find</code>, <code>begin</code>, or <code>end</code> for
+instance).</li>
+<li>(Allowed) When the type is clear from local context (in the same expression
+or within a few lines). Initialization of a pointer or smart pointer
+with calls
+to <code>new</code>
+commonly falls into this category, as does use of <code>auto</code> in
+a range-based loop over a container whose type is spelled out
+nearby.</li>
+<li>(Allowed) When the type doesn't matter because it isn't being used for
+anything other than equality comparison.</li>
+<li>(Encouraged) When iterating over a map with a range-based loop
+(because it is often assumed that the correct type
+is <code>std::pair<KeyType, ValueType></code> whereas it is actually
+<code>std::pair<const KeyType, ValueType></code>). This is
+particularly well paired with local <code>key</code>
+and <code>value</code> aliases for <code>.first</code>
+and <code>.second</code> (often const-ref).
+<pre class="code">for (const auto& item : some_map) {
+ const KeyType& key = item.first;
+ const ValType& value = item.second;
+ // The rest of the loop can now just refer to key and value,
+ // a reader can see the types in question, and we've avoided
+ // the too-common case of extra copies in this iteration.
+}
+</pre>
+</li>
+</ul>
+
+</div>
+
+</div>
+
+<h3 id="Braced_Initializer_List">Braced Initializer List</h3>
+
+<div class="summary">
+<p>You may use braced initializer lists.</p>
+</div>
+
+<div class="stylebody">
+
+<p>In C++03, aggregate types (arrays and structs with no
+constructor) could be initialized with braced initializer lists.</p>
+
+<pre>struct Point { int x; int y; };
+Point p = {1, 2};
+</pre>
+
+<p>In C++11, this syntax was generalized, and any object type can now
+be created with a braced initializer list, known as a
+<i>braced-init-list</i> in the C++ grammar. Here are a few examples
+of its use.</p>
+
+<pre>// Vector takes a braced-init-list of elements.
+std::vector<string> v{"foo", "bar"};
+
+// Basically the same, ignoring some small technicalities.
+// You may choose to use either form.
+std::vector<string> v = {"foo", "bar"};
+
+// Usable with 'new' expressions.
+auto p = new vector<string>{"foo", "bar"};
+
+// A map can take a list of pairs. Nested braced-init-lists work.
+std::map<int, string> m = {{1, "one"}, {2, "2"}};
+
+// A braced-init-list can be implicitly converted to a return type.
+std::vector<int> test_function() { return {1, 2, 3}; }
+
+// Iterate over a braced-init-list.
+for (int i : {-1, -2, -3}) {}
+
+// Call a function using a braced-init-list.
+void TestFunction2(std::vector<int> v) {}
+TestFunction2({1, 2, 3});
+</pre>
+
+<p>A user-defined type can also define a constructor and/or assignment operator
+that take <code>std::initializer_list<T></code>, which is automatically
+created from <i>braced-init-list</i>:</p>
+
+<pre>class MyType {
+ public:
+ // std::initializer_list references the underlying init list.
+ // It should be passed by value.
+ MyType(std::initializer_list<int> init_list) {
+ for (int i : init_list) append(i);
+ }
+ MyType& operator=(std::initializer_list<int> init_list) {
+ clear();
+ for (int i : init_list) append(i);
+ }
+};
+MyType m{2, 3, 5, 7};
+</pre>
+
+<p>Finally, brace initialization can also call ordinary
+constructors of data types, even if they do not have
+<code>std::initializer_list<T></code> constructors.</p>
+
+<pre>double d{1.23};
+// Calls ordinary constructor as long as MyOtherType has no
+// std::initializer_list constructor.
+class MyOtherType {
+ public:
+ explicit MyOtherType(string);
+ MyOtherType(int, string);
+};
+MyOtherType m = {1, "b"};
+// If the constructor is explicit, you can't use the "= {}" form.
+MyOtherType m{"b"};
+</pre>
+
+<p>Never assign a <i>braced-init-list</i> to an auto
+local variable. In the single element case, what this
+means can be confusing.</p>
+
+<pre class="badcode">auto d = {1.23}; // d is a std::initializer_list<double>
+</pre>
+
+<pre>auto d = double{1.23}; // Good -- d is a double, not a std::initializer_list.
+</pre>
+
+<p>See <a href="#Braced_Initializer_List_Format">Braced_Initializer_List_Format</a> for formatting.</p>
+
+</div>
+
+<h3 id="Lambda_expressions">Lambda expressions</h3>
+
+<div class="summary">
+<p>Use lambda expressions where appropriate. Prefer explicit captures
+when the lambda will escape the current scope.</p>
+</div>
+
+<div class="stylebody">
+
+<div class="definition">
+
+<p> Lambda expressions are a concise way of creating anonymous
+function objects. They're often useful when passing
+functions as arguments. For example:</p>
+
+<pre>std::sort(v.begin(), v.end(), [](int x, int y) {
+ return Weight(x) < Weight(y);
+});
+</pre>
+
+<p> They further allow capturing variables from the enclosing scope either
+explicitly by name, or implicitly using a default capture. Explicit captures
+require each variable to be listed, as
+either a value or reference capture:</p>
+
+<pre>int weight = 3;
+int sum = 0;
+// Captures `weight` by value and `sum` by reference.
+std::for_each(v.begin(), v.end(), [weight, &sum](int x) {
+ sum += weight * x;
+});
+</pre>
+
+
+Default captures implicitly capture any variable referenced in the
+lambda body, including <code>this</code> if any members are used:
+
+<pre>const std::vector<int> lookup_table = ...;
+std::vector<int> indices = ...;
+// Captures `lookup_table` by reference, sorts `indices` by the value
+// of the associated element in `lookup_table`.
+std::sort(indices.begin(), indices.end(), [&](int a, int b) {
+ return lookup_table[a] < lookup_table[b];
+});
+</pre>
+
+<p>Lambdas were introduced in C++11 along with a set of utilities
+for working with function objects, such as the polymorphic
+wrapper <code>std::function</code>.
+</p>
+</div>
+
+<div class="pros">
+<ul>
+ <li>Lambdas are much more concise than other ways of
+ defining function objects to be passed to STL
+ algorithms, which can be a readability
+ improvement.</li>
+
+ <li>Appropriate use of default captures can remove
+ redundancy and highlight important exceptions from
+ the default.</li>
+
+ <li>Lambdas, <code>std::function</code>, and
+ <code>std::bind</code> can be used in combination as a
+ general purpose callback mechanism; they make it easy
+ to write functions that take bound functions as
+ arguments.</li>
+</ul>
+</div>
+
+<div class="cons">
+<ul>
+ <li>Variable capture in lambdas can be a source of dangling-pointer
+ bugs, particularly if a lambda escapes the current scope.</li>
+
+ <li>Default captures by value can be misleading because they do not prevent
+ dangling-pointer bugs. Capturing a pointer by value doesn't cause a deep
+ copy, so it often has the same lifetime issues as capture by reference.
+ This is especially confusing when capturing 'this' by value, since the use
+ of 'this' is often implicit.</li>
+
+ <li>It's possible for use of lambdas to get out of
+ hand; very long nested anonymous functions can make
+ code harder to understand.</li>
+
+</ul>
+</div>
+
+<div class="decision">
+<ul>
+<li>Use lambda expressions where appropriate, with formatting as
+described <a href="#Formatting_Lambda_Expressions">below</a>.</li>
+<li>Prefer explicit captures if the lambda may escape the current scope.
+For example, instead of:
+<pre class="badcode">{
+ Foo foo;
+ ...
+ executor->Schedule([&] { Frobnicate(foo); })
+ ...
+}
+// BAD! The fact that the lambda makes use of a reference to `foo` and
+// possibly `this` (if `Frobnicate` is a member function) may not be
+// apparent on a cursory inspection. If the lambda is invoked after
+// the function returns, that would be bad, because both `foo`
+// and the enclosing object could have been destroyed.
+</pre>
+prefer to write:
+<pre>{
+ Foo foo;
+ ...
+ executor->Schedule([&foo] { Frobnicate(foo); })
+ ...
+}
+// BETTER - The compile will fail if `Frobnicate` is a member
+// function, and it's clearer that `foo` is dangerously captured by
+// reference.
+</pre>
+</li>
+<li>Use default capture by reference ([&]) only when the
+lifetime of the lambda is obviously shorter than any potential
+captures.
+</li>
+<li>Use default capture by value ([=]) only as a means of binding a
+few variables for a short lambda, where the set of captured
+variables is obvious at a glance. Prefer not to write long or
+complex lambdas with default capture by value.
+</li>
+<li>Keep unnamed lambdas short. If a lambda body is more than
+maybe five lines long, prefer to give the lambda a name, or to
+use a named function instead of a lambda.</li>
+<li>Specify the return type of the lambda explicitly if that will
+make it more obvious to readers, as with
+<a href="#auto"><code>auto</code></a>.</li>
+
+</ul>
+</div>
+
+</div>
+
+<h3 id="Template_metaprogramming">Template metaprogramming</h3>
+<div class="summary">
+<p>Avoid complicated template programming.</p>
+</div>
+
+<div class="stylebody">
+
+<div class="definition">
+<p>Template metaprogramming refers to a family of techniques that
+exploit the fact that the C++ template instantiation mechanism is
+Turing complete and can be used to perform arbitrary compile-time
+computation in the type domain.</p>
+</div>
+
+<div class="pros">
+<p>Template metaprogramming allows extremely flexible interfaces that
+are type safe and high performance. Facilities like
+
+<a href="https://code.google.com/p/googletest/">Google Test</a>,
+<code>std::tuple</code>, <code>std::function</code>, and
+Boost.Spirit would be impossible without it.</p>
+</div>
+
+<div class="cons">
+<p>The techniques used in template metaprogramming are often obscure
+to anyone but language experts. Code that uses templates in
+complicated ways is often unreadable, and is hard to debug or
+maintain.</p>
+
+<p>Template metaprogramming often leads to extremely poor compiler
+time error messages: even if an interface is simple, the complicated
+implementation details become visible when the user does something
+wrong.</p>
+
+<p>Template metaprogramming interferes with large scale refactoring by
+making the job of refactoring tools harder. First, the template code
+is expanded in multiple contexts, and it's hard to verify that the
+transformation makes sense in all of them. Second, some refactoring
+tools work with an AST that only represents the structure of the code
+after template expansion. It can be difficult to automatically work
+back to the original source construct that needs to be
+rewritten.</p>
+</div>
+
+<div class="decision">
+<p>Template metaprogramming sometimes allows cleaner and easier-to-use
+interfaces than would be possible without it, but it's also often a
+temptation to be overly clever. It's best used in a small number of
+low level components where the extra maintenance burden is spread out
+over a large number of uses.</p>
+
+<p>Think twice before using template metaprogramming or other
+complicated template techniques; think about whether the average
+member of your team will be able to understand your code well enough
+to maintain it after you switch to another project, or whether a
+non-C++ programmer or someone casually browsing the code base will be
+able to understand the error messages or trace the flow of a function
+they want to call. If you're using recursive template instantiations
+or type lists or metafunctions or expression templates, or relying on
+SFINAE or on the <code>sizeof</code> trick for detecting function
+overload resolution, then there's a good chance you've gone too
+far.</p>
+
+<p>If you use template metaprogramming, you should expect to put
+considerable effort into minimizing and isolating the complexity. You
+should hide metaprogramming as an implementation detail whenever
+possible, so that user-facing headers are readable, and you should
+make sure that tricky code is especially well commented. You should
+carefully document how the code is used, and you should say something
+about what the "generated" code looks like. Pay extra attention to the
+error messages that the compiler emits when users make mistakes. The
+error messages are part of your user interface, and your code should
+be tweaked as necessary so that the error messages are understandable
+and actionable from a user point of view.</p>
+
+</div>
+</div>
+
+
+<h3 id="Boost">Boost</h3>
+
+<div class="summary">
+<p>Use only approved libraries from the Boost library
+collection.</p>
+</div>
+
+<div class="stylebody">
+
+<div class="definition">
+<p> The
+<a href="https://www.boost.org/">
+Boost library collection</a> is a popular collection of
+peer-reviewed, free, open-source C++ libraries.</p>
+</div>
+
+<div class="pros">
+<p>Boost code is generally very high-quality, is widely
+portable, and fills many important gaps in the C++
+standard library, such as type traits and better binders.</p>
+</div>
+
+<div class="cons">
+<p>Some Boost libraries encourage coding practices which can
+hamper readability, such as metaprogramming and other
+advanced template techniques, and an excessively
+"functional" style of programming. </p>
+</div>
+
+<div class="decision">
+
+
+
+<div>
+<p>In order to maintain a high level of readability for
+all contributors who might read and maintain code, we
+only allow an approved subset of Boost features.
+Currently, the following libraries are permitted:</p>
+
+<ul>
+ <li>
+ <a href="https://www.boost.org/libs/utility/call_traits.htm">
+ Call Traits</a> from <code>boost/call_traits.hpp</code></li>
+
+ <li><a href="https://www.boost.org/libs/utility/compressed_pair.htm">
+ Compressed Pair</a> from <code>boost/compressed_pair.hpp</code></li>
+
+ <li><a href="https://www.boost.org/libs/graph/">
+ The Boost Graph Library (BGL)</a> from <code>boost/graph</code>,
+ except serialization (<code>adj_list_serialize.hpp</code>) and
+ parallel/distributed algorithms and data structures
+ (<code>boost/graph/parallel/*</code> and
+ <code>boost/graph/distributed/*</code>).</li>
+
+ <li><a href="https://www.boost.org/libs/property_map/">
+ Property Map</a> from <code>boost/property_map</code>, except
+ parallel/distributed property maps (<code>boost/property_map/parallel/*</code>).</li>
+
+ <li><a href="https://www.boost.org/libs/iterator/">
+ Iterator</a> from <code>boost/iterator</code></li>
+
+ <li>The part of <a href="https://www.boost.org/libs/polygon/">
+ Polygon</a> that deals with Voronoi diagram
+ construction and doesn't depend on the rest of
+ Polygon:
+ <code>boost/polygon/voronoi_builder.hpp</code>,
+ <code>boost/polygon/voronoi_diagram.hpp</code>, and
+ <code>boost/polygon/voronoi_geometry_type.hpp</code></li>
+
+ <li><a href="https://www.boost.org/libs/bimap/">
+ Bimap</a> from <code>boost/bimap</code></li>
+
+ <li><a href="https://www.boost.org/libs/math/doc/html/dist.html">
+ Statistical Distributions and Functions</a> from
+ <code>boost/math/distributions</code></li>
+
+ <li><a href="https://www.boost.org/libs/math/doc/html/special.html">
+ Special Functions</a> from <code>boost/math/special_functions</code></li>
+
+ <li><a href="https://www.boost.org/libs/multi_index/">
+ Multi-index</a> from <code>boost/multi_index</code></li>
+
+ <li><a href="https://www.boost.org/libs/heap/">
+ Heap</a> from <code>boost/heap</code></li>
+
+ <li>The flat containers from
+ <a href="https://www.boost.org/libs/container/">Container</a>:
+ <code>boost/container/flat_map</code>, and
+ <code>boost/container/flat_set</code></li>
+
+ <li><a href="https://www.boost.org/libs/intrusive/">Intrusive</a>
+ from <code>boost/intrusive</code>.</li>
+
+ <li><a href="https://www.boost.org/libs/sort/">The
+ <code>boost/sort</code> library</a>.</li>
+
+ <li><a href="https://www.boost.org/libs/preprocessor/">Preprocessor</a>
+ from <code>boost/preprocessor</code>.</li>
+</ul>
+
+<p>We are actively considering adding other Boost
+features to the list, so this list may be expanded in
+the future.</p>
+</div>
+
+<p>The following libraries are permitted, but their use
+is discouraged because they've been superseded by
+standard libraries in C++11:</p>
+
+<ul>
+ <li><a href="https://www.boost.org/libs/array/">
+ Array</a> from <code>boost/array.hpp</code>: use
+ <a href="http://en.cppreference.com/w/cpp/container/array">
+ <code>std::array</code></a> instead.</li>
+
+ <li><a href="https://www.boost.org/libs/ptr_container/">
+ Pointer Container</a> from <code>boost/ptr_container</code>: use containers of
+ <a href="http://en.cppreference.com/w/cpp/memory/unique_ptr">
+ <code>std::unique_ptr</code></a> instead.</li>
+</ul>
+</div>
+
+</div>
+
+
+
+<h3 id="std_hash">std::hash</h3>
+
+<div class="summary">
+<p>Do not define specializations of <code>std::hash</code>.</p>
+</div>
+
+<div class="stylebody">
+
+<div class="definition">
+<p><code>std::hash<T></code> is the function object that the
+C++11 hash containers use to hash keys of type <code>T</code>,
+unless the user explicitly specifies a different hash function. For
+example, <code>std::unordered_map<int, string></code> is a hash
+map that uses <code>std::hash<int></code> to hash its keys,
+whereas <code>std::unordered_map<int, string, MyIntHash></code>
+uses <code>MyIntHash</code>.</p>
+
+<p><code>std::hash</code> is defined for all integral, floating-point,
+pointer, and <code>enum</code> types, as well as some standard library
+types such as <code>string</code> and <code>unique_ptr</code>. Users
+can enable it to work for their own types by defining specializations
+of it for those types.</p>
+</div>
+
+<div class="pros">
+<p><code>std::hash</code> is easy to use, and simplifies the code
+since you don't have to name it explicitly. Specializing
+<code>std::hash</code> is the standard way of specifying how to
+hash a type, so it's what outside resources will teach, and what
+new engineers will expect.</p>
+</div>
+
+<div class="cons">
+<p><code>std::hash</code> is hard to specialize. It requires a lot
+of boilerplate code, and more importantly, it combines responsibility
+for identifying the hash inputs with responsibility for executing the
+hashing algorithm itself. The type author has to be responsible for
+the former, but the latter requires expertise that a type author
+usually doesn't have, and shouldn't need. The stakes here are high
+because low-quality hash functions can be security vulnerabilities,
+due to the emergence of
+<a href="https://emboss.github.io/blog/2012/12/14/breaking-murmur-hash-flooding-dos-reloaded/">
+hash flooding attacks</a>.</p>
+
+<p>Even for experts, <code>std::hash</code> specializations are
+inordinately difficult to implement correctly for compound types,
+because the implementation cannot recursively call <code>std::hash</code>
+on data members. High-quality hash algorithms maintain large
+amounts of internal state, and reducing that state to the
+<code>size_t</code> bytes that <code>std::hash</code>
+returns is usually the slowest part of the computation, so it
+should not be done more than once.</p>
+
+<p>Due to exactly that issue, <code>std::hash</code> does not work
+with <code>std::pair</code> or <code>std::tuple</code>, and the
+language does not allow us to extend it to support them.</p>
+</div>
+
+<div class="decision">
+<p>You can use <code>std::hash</code> with the types that it supports
+"out of the box", but do not specialize it to support additional types.
+If you need a hash table with a key type that <code>std::hash</code>
+does not support, consider using legacy hash containers (e.g.
+<code>hash_map</code>) for now; they use a different default hasher,
+which is unaffected by this prohibition.</p>
+
+<p>If you want to use the standard hash containers anyway, you will
+need to specify a custom hasher for the key type, e.g.</p>
+<pre>std::unordered_map<MyKeyType, Value, MyKeyTypeHasher> my_map;
+</pre><p>
+Consult with the type's owners to see if there is an existing hasher
+that you can use; otherwise work with them to provide one,
+ or roll your own.</p>
+
+<p>We are planning to provide a hash function that can work with any type,
+using a new customization mechanism that doesn't have the drawbacks of
+<code>std::hash</code>.</p>
+</div>
+
+</div>
+
+<h3 id="C++11">C++11</h3>
+
+<div class="summary">
+<p>Use libraries and language extensions from C++11 when appropriate.
+Consider portability to other environments
+before using C++11 features in your
+project. </p>
+
+</div>
+
+<div class="stylebody">
+
+<div class="definition">
+<p> C++11 contains <a href="https://en.wikipedia.org/wiki/C%2B%2B11">
+significant changes</a> both to the language and
+libraries. </p>
+</div>
+
+<div class="pros">
+<p>C++11 was the official standard until august 2014, and
+is supported by most C++ compilers. It standardizes
+some common C++ extensions that we use already, allows
+shorthands for some operations, and has some performance
+and safety improvements.</p>
+</div>
+
+<div class="cons">
+<p>The C++11 standard is substantially more complex than
+its predecessor (1,300 pages versus 800 pages), and is
+unfamiliar to many developers. The long-term effects of
+some features on code readability and maintenance are
+unknown. We cannot predict when its various features will
+be implemented uniformly by tools that may be of
+interest, particularly in the case of projects that are
+forced to use older versions of tools.</p>
+
+<p>As with <a href="#Boost">Boost</a>, some C++11
+extensions encourage coding practices that hamper
+readability—for example by removing
+checked redundancy (such as type names) that may be
+helpful to readers, or by encouraging template
+metaprogramming. Other extensions duplicate functionality
+available through existing mechanisms, which may lead to confusion
+and conversion costs.</p>
+
+
+</div>
+
+<div class="decision">
+
+<p>C++11 features may be used unless specified otherwise.
+In addition to what's described in the rest of the style
+guide, the following C++11 features may not be used:</p>
+
+<ul>
+
+
+
+
+
+
+
+
+ <li>Compile-time rational numbers
+ (<code><ratio></code>), because of concerns that
+ it's tied to a more template-heavy interface
+ style.</li>
+
+ <li>The <code><cfenv></code> and
+ <code><fenv.h></code> headers, because many
+ compilers do not support those features reliably.</li>
+
+ <li>Ref-qualifiers on member functions, such as <code>void X::Foo()
+ &</code> or <code>void X::Foo() &&</code>, because of concerns
+ that they're an overly obscure feature.</li>
+
+
+
+
+</ul>
+</div>
+
+</div>
+
+<h3 id="Nonstandard_Extensions">Nonstandard Extensions</h3>
+
+<div class="summary">
+<p>Nonstandard extensions to C++ may not be used unless otherwise specified.</p>
+</div>
+<div class="stylebody">
+<div class="definition">
+<p>Compilers support various extensions that are not part of standard C++. Such
+ extensions include GCC's <code>__attribute__</code>, intrinsic functions such
+ as <code>__builtin_prefetch</code>, designated initializers (e.g.
+ <code>Foo f = {.field = 3}</code>), inline assembly, <code>__COUNTER__</code>,
+ <code>__PRETTY_FUNCTION__</code>, compound statement expressions (e.g.
+ <code>foo = ({ int x; Bar(&x); x })</code>, variable-length arrays and
+ <code>alloca()</code>, and the <code>a?:b</code> syntax.</p>
+</div>
+
+<div class="pros">
+ <ul>
+ <li>Nonstandard extensions may provide useful features that do not exist
+ in standard C++. For example, some people think that designated
+ initializers are more readable than standard C++ features like
+ constructors.</li>
+ <li>Important performance guidance to the compiler can only be specified
+ using extensions.</li>
+ </ul>
+</div>
+
+<div class="cons">
+ <ul>
+ <li>Nonstandard extensions do not work in all compilers. Use of nonstandard
+ extensions reduces portability of code.</li>
+ <li>Even if they are supported in all targeted compilers, the extensions
+ are often not well-specified, and there may be subtle behavior differences
+ between compilers.</li>
+ <li>Nonstandard extensions add to the language features that a reader must
+ know to understand the code.</li>
+ </ul>
+</div>
+
+<div class="decision">
+<p>Do not use nonstandard extensions. You may use portability wrappers that
+ are implemented using nonstandard extensions, so long as those wrappers
+
+ are provided by a designated project-wide
+ portability header.</p>
+</div>
+</div>
+
+<h3 id="Aliases">Aliases</h3>
+
+<div class="summary">
+<p>Public aliases are for the benefit of an API's user, and should be clearly documented.</p>
+</div>
+<div class="stylebody">
+<div class="definition">
+<p>There are several ways to create names that are aliases of other entities:</p>
+<pre>typedef Foo Bar;
+using Bar = Foo;
+using other_namespace::Foo;
+</pre>
+
+ <p>Like other declarations, aliases declared in a header file are part of that
+ header's public API unless they're in a function definition, in the private portion of a class,
+ or in an explicitly-marked internal namespace. Aliases in such areas or in .cc files are
+ implementation details (because client code can't refer to them), and are not restricted by this
+ rule.</p>
+</div>
+
+<div class="pros">
+ <ul>
+ <li>Aliases can improve readability by simplifying a long or complicated name.</li>
+ <li>Aliases can reduce duplication by naming in one place a type used repeatedly in an API,
+ which <em>might</em> make it easier to change the type later.
+ </li>
+ </ul>
+</div>
+
+<div class="cons">
+ <ul>
+ <li>When placed in a header where client code can refer to them, aliases increase the
+ number of entities in that header's API, increasing its complexity.</li>
+ <li>Clients can easily rely on unintended details of public aliases, making
+ changes difficult.</li>
+ <li>It can be tempting to create a public alias that is only intended for use
+ in the implementation, without considering its impact on the API, or on maintainability.</li>
+ <li>Aliases can create risk of name collisions</li>
+ <li>Aliases can reduce readability by giving a familiar construct an unfamiliar name</li>
+ <li>Type aliases can create an unclear API contract:
+ it is unclear whether the alias is guaranteed to be identical to the type it aliases,
+ to have the same API, or only to be usable in specified narrow ways</li>
+ </ul>
+</div>
+
+<div class="decision">
+<p>Don't put an alias in your public API just to save typing in the implementation;
+ do so only if you intend it to be used by your clients.</p>
+<p>When defining a public alias, document the intent of
+the new name, including whether it is guaranteed to always be the same as the type
+it's currently aliased to, or whether a more limited compatibility is
+intended. This lets the user know whether they can treat the types as
+substitutable or whether more specific rules must be followed, and can help the
+implementation retain some degree of freedom to change the alias.</p>
+<p>Don't put namespace aliases in your public API. (See also <a href="#Namespaces">Namespaces</a>).
+</p>
+
+<p>For example, these aliases document how they are intended to be used in client code:</p>
+<pre>namespace a {
+// Used to store field measurements. DataPoint may change from Bar* to some internal type.
+// Client code should treat it as an opaque pointer.
+using DataPoint = foo::bar::Bar*;
+
+// A set of measurements. Just an alias for user convenience.
+using TimeSeries = std::unordered_set<DataPoint, std::hash<DataPoint>, DataPointComparator>;
+} // namespace a
+</pre>
+
+<p>These aliases don't document intended use, and half of them aren't meant for client use:</p>
+
+<pre class="badcode">namespace a {
+// Bad: none of these say how they should be used.
+using DataPoint = foo::bar::Bar*;
+using std::unordered_set; // Bad: just for local convenience
+using std::hash; // Bad: just for local convenience
+typedef unordered_set<DataPoint, hash<DataPoint>, DataPointComparator> TimeSeries;
+} // namespace a
+</pre>
+
+<p>However, local convenience aliases are fine in function definitions, private sections of
+ classes, explicitly marked internal namespaces, and in .cc files:</p>
+
+<pre>// In a .cc file
+using std::unordered_set;
+</pre>
+
+</div>
+</div>
+
+<h2 id="Naming">Naming</h2>
+
+<p>The most important consistency rules are those that govern
+naming. The style of a name immediately informs us what sort of
+thing the named entity is: a type, a variable, a function, a
+constant, a macro, etc., without requiring us to search for the
+declaration of that entity. The pattern-matching engine in our
+brains relies a great deal on these naming rules.
+</p>
+
+<p>Naming rules are pretty arbitrary, but
+ we feel that
+consistency is more important than individual preferences in this
+area, so regardless of whether you find them sensible or not,
+the rules are the rules.</p>
+
+<h3 id="General_Naming_Rules">General Naming Rules</h3>
+
+<div class="summary">
+<p>Names should be descriptive; avoid abbreviation.</p>
+</div>
+
+<div class="stylebody">
+<p>Give as descriptive a name as possible, within reason.
+Do not worry about saving horizontal space as it is far
+more important to make your code immediately
+understandable by a new reader. Do not use abbreviations
+that are ambiguous or unfamiliar to readers outside your
+project, and do not abbreviate by deleting letters within
+a word.</p>
+
+<pre>int price_count_reader; // No abbreviation.
+int num_errors; // "num" is a widespread convention.
+int num_dns_connections; // Most people know what "DNS" stands for.
+</pre>
+
+<pre class="badcode">int n; // Meaningless.
+int nerr; // Ambiguous abbreviation.
+int n_comp_conns; // Ambiguous abbreviation.
+int wgc_connections; // Only your group knows what this stands for.
+int pc_reader; // Lots of things can be abbreviated "pc".
+int cstmr_id; // Deletes internal letters.
+</pre>
+
+<p>Note that certain universally-known abbreviations are OK, such as
+<code>i</code> for an iteration variable and <code>T</code> for a
+template parameter.</p>
+
+<p>Template parameters should follow the naming style for their
+category: type template parameters should follow the rules for
+<a href="#Type_Names">type names</a>, and non-type template
+parameters should follow the rules for <a href="#Variable_Names">
+variable names</a>.
+
+</p></div>
+
+<h3 id="File_Names">File Names</h3>
+
+<div class="summary">
+<p>Filenames should be all lowercase and can include
+underscores (<code>_</code>) or dashes (<code>-</code>).
+Follow the convention that your
+
+project uses. If there is no consistent
+local pattern to follow, prefer "_".</p>
+</div>
+
+<div class="stylebody">
+
+<p>Examples of acceptable file names:</p>
+
+<ul>
+ <li><code>my_useful_class.cc</code></li>
+ <li><code>my-useful-class.cc</code></li>
+ <li><code>myusefulclass.cc</code></li>
+ <li><code>myusefulclass_test.cc // _unittest and _regtest are deprecated.</code></li>
+</ul>
+
+<p>C++ files should end in <code>.cc</code> and header files should end in
+<code>.h</code>. Files that rely on being textually included at specific points
+should end in <code>.inc</code> (see also the section on
+<a href="#Self_contained_Headers">self-contained headers</a>).</p>
+
+<p>Do not use filenames that already exist in
+<code>/usr/include</code>, such as <code>db.h</code>.</p>
+
+<p>In general, make your filenames very specific. For
+example, use <code>http_server_logs.h</code> rather than
+<code>logs.h</code>. A very common case is to have a pair
+of files called, e.g., <code>foo_bar.h</code> and
+<code>foo_bar.cc</code>, defining a class called
+<code>FooBar</code>.</p>
+
+<p>Inline functions must be in a <code>.h</code> file. If
+your inline functions are very short, they should go
+directly into your <code>.h</code> file. </p>
+
+</div>
+
+<h3 id="Type_Names">Type Names</h3>
+
+<div class="summary">
+<p>Type names start with a capital letter and have a capital
+letter for each new word, with no underscores:
+<code>MyExcitingClass</code>, <code>MyExcitingEnum</code>.</p>
+</div>
+
+<div class="stylebody">
+
+<p>The names of all types — classes, structs, type aliases,
+enums, and type template parameters — have the same naming convention.
+Type names should start with a capital letter and have a capital letter
+for each new word. No underscores. For example:</p>
+
+<pre>// classes and structs
+class UrlTable { ...
+class UrlTableTester { ...
+struct UrlTableProperties { ...
+
+// typedefs
+typedef hash_map<UrlTableProperties *, string> PropertiesMap;
+
+// using aliases
+using PropertiesMap = hash_map<UrlTableProperties *, string>;
+
+// enums
+enum UrlTableErrors { ...
+</pre>
+
+</div>
+
+<h3 id="Variable_Names">Variable Names</h3>
+
+<div class="summary">
+<p>The names of variables (including function parameters) and data members are
+all lowercase, with underscores between words. Data members of classes (but not
+structs) additionally have trailing underscores. For instance:
+<code>a_local_variable</code>, <code>a_struct_data_member</code>,
+<code>a_class_data_member_</code>.</p>
+</div>
+
+<div class="stylebody">
+
+<h4 class="stylepoint_subsection">Common Variable names</h4>
+
+<p>For example:</p>
+
+<pre>string table_name; // OK - uses underscore.
+string tablename; // OK - all lowercase.
+</pre>
+
+<pre class="badcode">string tableName; // Bad - mixed case.
+</pre>
+
+<h4 class="stylepoint_subsection">Class Data Members</h4>
+
+<p>Data members of classes, both static and non-static, are
+named like ordinary nonmember variables, but with a
+trailing underscore.</p>
+
+<pre>class TableInfo {
+ ...
+ private:
+ string table_name_; // OK - underscore at end.
+ string tablename_; // OK.
+ static Pool<TableInfo>* pool_; // OK.
+};
+</pre>
+
+<h4 class="stylepoint_subsection">Struct Data Members</h4>
+
+<p>Data members of structs, both static and non-static,
+are named like ordinary nonmember variables. They do not have
+the trailing underscores that data members in classes have.</p>
+
+<pre>struct UrlTableProperties {
+ string name;
+ int num_entries;
+ static Pool<UrlTableProperties>* pool;
+};
+</pre>
+
+
+<p>See <a href="#Structs_vs._Classes">Structs vs.
+Classes</a> for a discussion of when to use a struct
+versus a class.</p>
+
+</div>
+
+<h3 id="Constant_Names">Constant Names</h3>
+
+<div class="summary">
+ <p>Variables declared constexpr or const, and whose value is fixed for
+ the duration of the program, are named with a leading "k" followed
+ by mixed case. For example:</p>
+</div>
+
+<pre>const int kDaysInAWeek = 7;
+</pre>
+
+<div class="stylebody">
+
+ <p>All such variables with static storage duration (i.e. statics and globals,
+ see <a href="http://en.cppreference.com/w/cpp/language/storage_duration#Storage_duration">
+ Storage Duration</a> for details) should be named this way. This
+ convention is optional for variables of other storage classes, e.g. automatic
+ variables, otherwise the usual variable naming rules apply.</p><p>
+
+</p></div>
+
+<h3 id="Function_Names">Function Names</h3>
+
+<div class="summary">
+<p>Regular functions have mixed case; accessors and mutators may be named
+like variables.</p>
+</div>
+
+<div class="stylebody">
+
+<p>Ordinarily, functions should start with a capital letter and have a
+capital letter for each new word
+(a.k.a. "<a href="https://en.wikipedia.org/wiki/Camel_case">Camel
+Case</a>" or "Pascal case"). Such names should not have
+underscores. Prefer to capitalize acronyms as single words
+(i.e. <code>StartRpc()</code>, not <code>StartRPC()</code>).</p>
+
+<pre>AddTableEntry()
+DeleteUrl()
+OpenFileOrDie()
+</pre>
+
+<p>(The same naming rule applies to class- and namespace-scope
+constants that are exposed as part of an API and that are intended to look
+like functions, because the fact that they're
+objects rather than functions is an unimportant implementation detail.)</p>
+
+<p>Accessors and mutators (get and set functions) may be named like
+variables. These often correspond to actual member variables, but this is
+not required. For example, <code>int count()</code> and <code>void
+set_count(int count)</code>.</p>
+
+</div>
+
+<h3 id="Namespace_Names">Namespace Names</h3>
+
+<div class="summary">
+Namespace names are all lower-case. Top-level namespace names are
+based on the project name
+. Avoid collisions
+between nested namespaces and well-known top-level namespaces.
+</div>
+
+<div class="stylebody">
+<p>The name of a top-level namespace should usually be the
+name of the project or team whose code is contained in that
+namespace. The code in that namespace should usually be in
+a directory whose basename matches the namespace name (or
+subdirectories thereof).</p>
+
+
+
+
+
+<p>Keep in mind that the <a href="#General_Naming_Rules">rule
+against abbreviated names</a> applies to namespaces just as much
+as variable names. Code inside the namespace seldom needs to
+mention the namespace name, so there's usually no particular need
+for abbreviation anyway.</p>
+
+<p>Avoid nested namespaces that match well-known top-level
+namespaces. Collisions between namespace names can lead to surprising
+build breaks because of name lookup rules. In particular, do not
+create any nested <code>std</code> namespaces. Prefer unique project
+identifiers
+(<code>websearch::index</code>, <code>websearch::index_util</code>)
+over collision-prone names like <code>websearch::util</code>.</p>
+
+<p>For <code>internal</code> namespaces, be wary of other code being
+added to the same <code>internal</code> namespace causing a collision
+(internal helpers within a team tend to be related and may lead to
+collisions). In such a situation, using the filename to make a unique
+internal name is helpful
+(<code>websearch::index::frobber_internal</code> for use
+in <code>frobber.h</code>)</p>
+
+</div>
+
+<h3 id="Enumerator_Names">Enumerator Names</h3>
+
+<div class="summary">
+<p>Enumerators (for both scoped and unscoped enums) should be named <i>either</i> like
+<a href="#Constant_Names">constants</a> or like
+<a href="#Macro_Names">macros</a>: either <code>kEnumName</code> or
+<code>ENUM_NAME</code>.</p>
+</div>
+
+<div class="stylebody">
+
+<p>Preferably, the individual enumerators should be named
+like <a href="#Constant_Names">constants</a>. However, it
+is also acceptable to name them like
+<a href="#Macro_Names">macros</a>. The enumeration name,
+<code>UrlTableErrors</code> (and
+<code>AlternateUrlTableErrors</code>), is a type, and
+therefore mixed case.</p>
+
+<pre>enum UrlTableErrors {
+ kOK = 0,
+ kErrorOutOfMemory,
+ kErrorMalformedInput,
+};
+enum AlternateUrlTableErrors {
+ OK = 0,
+ OUT_OF_MEMORY = 1,
+ MALFORMED_INPUT = 2,
+};
+</pre>
+
+<p>Until January 2009, the style was to name enum values
+like <a href="#Macro_Names">macros</a>. This caused
+problems with name collisions between enum values and
+macros. Hence, the change to prefer constant-style naming
+was put in place. New code should prefer constant-style
+naming if possible. However, there is no reason to change
+old code to use constant-style names, unless the old
+names are actually causing a compile-time problem.</p>
+
+
+
+</div>
+
+<h3 id="Macro_Names">Macro Names</h3>
+
+<div class="summary">
+<p>You're not really going to <a href="#Preprocessor_Macros">
+define a macro</a>, are you? If you do, they're like this:
+<code>MY_MACRO_THAT_SCARES_SMALL_CHILDREN</code>.</p>
+</div>
+
+<div class="stylebody">
+
+<p>Please see the <a href="#Preprocessor_Macros">description
+of macros</a>; in general macros should <em>not</em> be used.
+However, if they are absolutely needed, then they should be
+named with all capitals and underscores.</p>
+
+<pre>#define ROUND(x) ...
+#define PI_ROUNDED 3.0
+</pre>
+
+</div>
+
+<h3 id="Exceptions_to_Naming_Rules">Exceptions to Naming Rules</h3>
+
+<div class="summary">
+<p>If you are naming something that is analogous to an
+existing C or C++ entity then you can follow the existing
+naming convention scheme.</p>
+</div>
+
+<div class="stylebody">
+
+<dl>
+ <dt><code>bigopen()</code></dt>
+ <dd>function name, follows form of <code>open()</code></dd>
+
+ <dt><code>uint</code></dt>
+ <dd><code>typedef</code></dd>
+
+ <dt><code>bigpos</code></dt>
+ <dd><code>struct</code> or <code>class</code>, follows
+ form of <code>pos</code></dd>
+
+ <dt><code>sparse_hash_map</code></dt>
+ <dd>STL-like entity; follows STL naming conventions</dd>
+
+ <dt><code>LONGLONG_MAX</code></dt>
+ <dd>a constant, as in <code>INT_MAX</code></dd>
+</dl>
+
+</div>
+
+<h2 id="Comments">Comments</h2>
+
+<p>Though a pain to write, comments are absolutely vital to
+keeping our code readable. The following rules describe what
+you should comment and where. But remember: while comments are
+very important, the best code is self-documenting. Giving
+sensible names to types and variables is much better than using
+obscure names that you must then explain through comments.</p>
+
+<p>When writing your comments, write for your audience: the
+next
+contributor who will need to
+understand your code. Be generous — the next
+one may be you!</p>
+
+<h3 id="Comment_Style">Comment Style</h3>
+
+<div class="summary">
+<p>Use either the <code>//</code> or <code>/* */</code>
+syntax, as long as you are consistent.</p>
+</div>
+
+<div class="stylebody">
+
+<p>You can use either the <code>//</code> or the <code>/*
+*/</code> syntax; however, <code>//</code> is
+<em>much</em> more common. Be consistent with how you
+comment and what style you use where.</p>
+
+</div>
+
+<h3 id="File_Comments">File Comments</h3>
+
+<div class="summary">
+<p>Start each file with license boilerplate.</p>
+
+<p>File comments describe the contents of a file. If a file declares,
+implements, or tests exactly one abstraction that is documented by a comment
+at the point of declaration, file comments are not required. All other files
+must have file comments.</p>
+
+</div>
+
+<div class="stylebody">
+
+<h4 class="stylepoint_subsection">Legal Notice and Author
+Line</h4>
+
+
+
+<p>Every file should contain license
+boilerplate. Choose the appropriate boilerplate for the
+license used by the project (for example, Apache 2.0,
+BSD, LGPL, GPL).</p>
+
+<p>If you make significant changes to a file with an
+author line, consider deleting the author line.</p>
+
+<h4 class="stylepoint_subsection">File Contents</h4>
+
+<p>If a <code>.h</code> declares multiple abstractions, the file-level comment
+should broadly describe the contents of the file, and how the abstractions are
+related. A 1 or 2 sentence file-level comment may be sufficient. The detailed
+documentation about individual abstractions belongs with those abstractions,
+not at the file level.</p>
+
+<p>Do not duplicate comments in both the <code>.h</code> and the
+<code>.cc</code>. Duplicated comments diverge.</p>
+
+</div>
+
+<h3 id="Class_Comments">Class Comments</h3>
+
+<div class="summary">
+<p>Every non-obvious class declaration should have an accompanying
+comment that describes what it is for and how it should be used.</p>
+</div>
+
+<div class="stylebody">
+
+<pre>// Iterates over the contents of a GargantuanTable.
+// Example:
+// GargantuanTableIterator* iter = table->NewIterator();
+// for (iter->Seek("foo"); !iter->done(); iter->Next()) {
+// process(iter->key(), iter->value());
+// }
+// delete iter;
+class GargantuanTableIterator {
+ ...
+};
+</pre>
+
+<p>The class comment should provide the reader with enough information to know
+how and when to use the class, as well as any additional considerations
+necessary to correctly use the class. Document the synchronization assumptions
+the class makes, if any. If an instance of the class can be accessed by
+multiple threads, take extra care to document the rules and invariants
+surrounding multithreaded use.</p>
+
+<p>The class comment is often a good place for a small example code snippet
+demonstrating a simple and focused usage of the class.</p>
+
+<p>When sufficiently separated (e.g. <code>.h</code> and <code>.cc</code>
+files), comments describing the use of the class should go together with its
+interface definition; comments about the class operation and implementation
+should accompany the implementation of the class's methods.</p>
+
+</div>
+
+<h3 id="Function_Comments">Function Comments</h3>
+
+<div class="summary">
+<p>Declaration comments describe use of the function (when it is
+non-obvious); comments at the definition of a function describe
+operation.</p>
+</div>
+
+<div class="stylebody">
+
+<h4 class="stylepoint_subsection">Function Declarations</h4>
+
+<p>Almost every function declaration should have comments immediately
+preceding it that describe what the function does and how to use
+it. These comments may be omitted only if the function is simple and
+obvious (e.g. simple accessors for obvious properties of the
+class). These comments should be descriptive ("Opens the file")
+rather than imperative ("Open the file"); the comment describes the
+function, it does not tell the function what to do. In general, these
+comments do not describe how the function performs its task. Instead,
+that should be left to comments in the function definition.</p>
+
+<p>Types of things to mention in comments at the function
+declaration:</p>
+
+<ul>
+ <li>What the inputs and outputs are.</li>
+
+ <li>For class member functions: whether the object
+ remembers reference arguments beyond the duration of
+ the method call, and whether it will free them or
+ not.</li>
+
+ <li>If the function allocates memory that the caller
+ must free.</li>
+
+ <li>Whether any of the arguments can be a null
+ pointer.</li>
+
+ <li>If there are any performance implications of how a
+ function is used.</li>
+
+ <li>If the function is re-entrant. What are its
+ synchronization assumptions?</li>
+ </ul>
+
+<p>Here is an example:</p>
+
+<pre>// Returns an iterator for this table. It is the client's
+// responsibility to delete the iterator when it is done with it,
+// and it must not use the iterator once the GargantuanTable object
+// on which the iterator was created has been deleted.
+//
+// The iterator is initially positioned at the beginning of the table.
+//
+// This method is equivalent to:
+// Iterator* iter = table->NewIterator();
+// iter->Seek("");
+// return iter;
+// If you are going to immediately seek to another place in the
+// returned iterator, it will be faster to use NewIterator()
+// and avoid the extra seek.
+Iterator* GetIterator() const;
+</pre>
+
+<p>However, do not be unnecessarily verbose or state the
+completely obvious. Notice below that it is not necessary
+ to say "returns false otherwise" because this is
+implied.</p>
+
+<pre>// Returns true if the table cannot hold any more entries.
+bool IsTableFull();
+</pre>
+
+<p>When documenting function overrides, focus on the
+specifics of the override itself, rather than repeating
+the comment from the overridden function. In many of these
+cases, the override needs no additional documentation and
+thus no comment is required.</p>
+
+<p>When commenting constructors and destructors, remember
+that the person reading your code knows what constructors
+and destructors are for, so comments that just say
+something like "destroys this object" are not useful.
+Document what constructors do with their arguments (for
+example, if they take ownership of pointers), and what
+cleanup the destructor does. If this is trivial, just
+skip the comment. It is quite common for destructors not
+to have a header comment.</p>
+
+<h4 class="stylepoint_subsection">Function Definitions</h4>
+
+<p>If there is anything tricky about how a function does
+its job, the function definition should have an
+explanatory comment. For example, in the definition
+comment you might describe any coding tricks you use,
+give an overview of the steps you go through, or explain
+why you chose to implement the function in the way you
+did rather than using a viable alternative. For instance,
+you might mention why it must acquire a lock for the
+first half of the function but why it is not needed for
+the second half.</p>
+
+<p>Note you should <em>not</em> just repeat the comments
+given with the function declaration, in the
+<code>.h</code> file or wherever. It's okay to
+recapitulate briefly what the function does, but the
+focus of the comments should be on how it does it.</p>
+
+</div>
+
+<h3 id="Variable_Comments">Variable Comments</h3>
+
+<div class="summary">
+<p>In general the actual name of the variable should be
+descriptive enough to give a good idea of what the variable
+is used for. In certain cases, more comments are required.</p>
+</div>
+
+<div class="stylebody">
+
+<h4 class="stylepoint_subsection">Class Data Members</h4>
+
+<p>The purpose of each class data member (also called an instance
+variable or member variable) must be clear. If there are any
+invariants (special values, relationships between members, lifetime
+requirements) not clearly expressed by the type and name, they must be
+commented. However, if the type and name suffice (<code>int
+num_events_;</code>), no comment is needed.</p>
+
+<p>In particular, add comments to describe the existence and meaning
+of sentinel values, such as nullptr or -1, when they are not
+obvious. For example:</p>
+
+<pre>private:
+ // Used to bounds-check table accesses. -1 means
+ // that we don't yet know how many entries the table has.
+ int num_total_entries_;
+</pre>
+
+<h4 class="stylepoint_subsection">Global Variables</h4>
+
+<p>All global variables should have a comment describing what they
+are, what they are used for, and (if unclear) why it needs to be
+global. For example:</p>
+
+<pre>// The total number of tests cases that we run through in this regression test.
+const int kNumTestCases = 6;
+</pre>
+
+</div>
+
+<h3 id="Implementation_Comments">Implementation Comments</h3>
+
+<div class="summary">
+<p>In your implementation you should have comments in tricky,
+non-obvious, interesting, or important parts of your code.</p>
+</div>
+
+<div class="stylebody">
+
+<h4 class="stylepoint_subsection">Explanatory Comments</h4>
+
+<p>Tricky or complicated code blocks should have comments
+before them. Example:</p>
+
+<pre>// Divide result by two, taking into account that x
+// contains the carry from the add.
+for (int i = 0; i < result->size(); i++) {
+ x = (x << 8) + (*result)[i];
+ (*result)[i] = x >> 1;
+ x &= 1;
+}
+</pre>
+
+<h4 class="stylepoint_subsection">Line Comments</h4>
+
+<p>Also, lines that are non-obvious should get a comment
+at the end of the line. These end-of-line comments should
+be separated from the code by 2 spaces. Example:</p>
+
+<pre>// If we have enough memory, mmap the data portion too.
+mmap_budget = max<int64>(0, mmap_budget - index_->length());
+if (mmap_budget >= data_size_ && !MmapData(mmap_chunk_bytes, mlock))
+ return; // Error already logged.
+</pre>
+
+<p>Note that there are both comments that describe what
+the code is doing, and comments that mention that an
+error has already been logged when the function
+returns.</p>
+
+<p>If you have several comments on subsequent lines, it
+can often be more readable to line them up:</p>
+
+<pre>DoSomething(); // Comment here so the comments line up.
+DoSomethingElseThatIsLonger(); // Two spaces between the code and the comment.
+{ // One space before comment when opening a new scope is allowed,
+ // thus the comment lines up with the following comments and code.
+ DoSomethingElse(); // Two spaces before line comments normally.
+}
+std::vector<string> list{
+ // Comments in braced lists describe the next element...
+ "First item",
+ // .. and should be aligned appropriately.
+ "Second item"};
+DoSomething(); /* For trailing block comments, one space is fine. */
+</pre>
+
+<h4 class="stylepoint_subsection">Function Argument Comments</h4>
+
+<p>When the meaning of a function argument is nonobvious, consider
+one of the following remedies:</p>
+
+<ul>
+ <li>If the argument is a literal constant, and the same constant is
+ used in multiple function calls in a way that tacitly assumes they're
+ the same, you should use a named constant to make that constraint
+ explicit, and to guarantee that it holds.</li>
+
+ <li>Consider changing the function signature to replace a <code>bool</code>
+ argument with an <code>enum</code> argument. This will make the argument
+ values self-describing.</li>
+
+ <li>For functions that have several configuration options, consider
+ defining a single class or struct to hold all the options
+ ,
+ and pass an instance of that.
+ This approach has several advantages. Options are referenced by name
+ at the call site, which clarifies their meaning. It also reduces
+ function argument count, which makes function calls easier to read and
+ write. As an added benefit, you don't have to change call sites when
+ you add another option.
+ </li>
+
+ <li>Replace large or complex nested expressions with named variables.</li>
+
+ <li>As a last resort, use comments to clarify argument meanings at the
+ call site.</li>
+</ul>
+
+Consider the following example:
+
+<pre class="badcode">// What are these arguments?
+const DecimalNumber product = CalculateProduct(values, 7, false, nullptr);
+</pre>
+
+<p>versus:</p>
+
+<pre>ProductOptions options;
+options.set_precision_decimals(7);
+options.set_use_cache(ProductOptions::kDontUseCache);
+const DecimalNumber product =
+ CalculateProduct(values, options, /*completion_callback=*/nullptr);
+</pre>
+
+<h4 class="stylepoint_subsection">Don'ts</h4>
+
+<p>Do not state the obvious. In particular, don't literally describe what
+code does, unless the behavior is nonobvious to a reader who understands
+C++ well. Instead, provide higher level comments that describe <i>why</i>
+the code does what it does, or make the code self describing.</p>
+
+Compare this:
+
+<pre class="badcode">// Find the element in the vector. <-- Bad: obvious!
+auto iter = std::find(v.begin(), v.end(), element);
+if (iter != v.end()) {
+ Process(element);
+}
+</pre>
+
+To this:
+
+<pre>// Process "element" unless it was already processed.
+auto iter = std::find(v.begin(), v.end(), element);
+if (iter != v.end()) {
+ Process(element);
+}
+</pre>
+
+Self-describing code doesn't need a comment. The comment from
+the example above would be obvious:
+
+<pre>if (!IsAlreadyProcessed(element)) {
+ Process(element);
+}
+</pre>
+
+</div>
+
+<h3 id="Punctuation,_Spelling_and_Grammar">Punctuation, Spelling and Grammar</h3>
+
+<div class="summary">
+<p>Pay attention to punctuation, spelling, and grammar; it is
+easier to read well-written comments than badly written
+ones.</p>
+</div>
+
+<div class="stylebody">
+
+<p>Comments should be as readable as narrative text, with
+proper capitalization and punctuation. In many cases,
+complete sentences are more readable than sentence
+fragments. Shorter comments, such as comments at the end
+of a line of code, can sometimes be less formal, but you
+should be consistent with your style.</p>
+
+<p>Although it can be frustrating to have a code reviewer
+point out that you are using a comma when you should be
+using a semicolon, it is very important that source code
+maintain a high level of clarity and readability. Proper
+punctuation, spelling, and grammar help with that
+goal.</p>
+
+</div>
+
+<h3 id="TODO_Comments">TODO Comments</h3>
+
+<div class="summary">
+<p>Use <code>TODO</code> comments for code that is temporary,
+a short-term solution, or good-enough but not perfect.</p>
+</div>
+
+<div class="stylebody">
+
+<p><code>TODO</code>s should include the string
+<code>TODO</code> in all caps, followed by the
+
+name, e-mail address, bug ID, or other
+identifier
+of the person or issue with the best context
+about the problem referenced by the <code>TODO</code>. The
+main purpose is to have a consistent <code>TODO</code> that
+can be searched to find out how to get more details upon
+request. A <code>TODO</code> is not a commitment that the
+person referenced will fix the problem. Thus when you create
+a <code>TODO</code> with a name, it is almost always your
+name that is given.</p>
+
+
+
+<div>
+<pre>// TODO(kl@gmail.com): Use a "*" here for concatenation operator.
+// TODO(Zeke) change this to use relations.
+// TODO(bug 12345): remove the "Last visitors" feature
+</pre>
+</div>
+
+<p>If your <code>TODO</code> is of the form "At a future
+date do something" make sure that you either include a
+very specific date ("Fix by November 2005") or a very
+specific event ("Remove this code when all clients can
+handle XML responses.").</p>
+
+</div>
+
+<h3 id="Deprecation_Comments">Deprecation Comments</h3>
+
+<div class="summary">
+<p>Mark deprecated interface points with <code>DEPRECATED</code>
+comments.</p>
+</div>
+
+<div class="stylebody">
+
+<p>You can mark an interface as deprecated by writing a
+comment containing the word <code>DEPRECATED</code> in
+all caps. The comment goes either before the declaration
+of the interface or on the same line as the
+declaration.</p>
+
+
+
+<p>After the word
+<code>DEPRECATED</code>, write your name, e-mail address,
+or other identifier in parentheses.</p>
+
+<p>A deprecation comment must include simple, clear
+directions for people to fix their callsites. In C++, you
+can implement a deprecated function as an inline function
+that calls the new interface point.</p>
+
+<p>Marking an interface point <code>DEPRECATED</code>
+will not magically cause any callsites to change. If you
+want people to actually stop using the deprecated
+facility, you will have to fix the callsites yourself or
+recruit a crew to help you.</p>
+
+<p>New code should not contain calls to deprecated
+interface points. Use the new interface point instead. If
+you cannot understand the directions, find the person who
+created the deprecation and ask them for help using the
+new interface point.</p>
+
+
+
+</div>
+
+<h2 id="Formatting">Formatting</h2>
+
+<p>Coding style and formatting are pretty arbitrary, but a
+
+project is much easier to follow
+if everyone uses the same style. Individuals may not agree with every
+aspect of the formatting rules, and some of the rules may take
+some getting used to, but it is important that all
+
+project contributors follow the
+style rules so that
+they can all read and understand
+everyone's code easily.</p>
+
+
+
+<p>To help you format code correctly, we've
+created a
+<a href="https://raw.githubusercontent.com/google/styleguide/gh-pages/google-c-style.el">
+settings file for emacs</a>.</p>
+
+<h3 id="Line_Length">Line Length</h3>
+
+<div class="summary">
+<p>Each line of text in your code should be at most 80
+characters long.</p>
+</div>
+
+<div class="stylebody">
+
+
+
+ <p>We recognize that this rule is
+controversial, but so much existing code already adheres
+to it, and we feel that consistency is important.</p>
+
+<div class="pros">
+<p>Those who favor this rule
+argue that it is rude to force them to resize
+their windows and there is no need for anything longer.
+Some folks are used to having several code windows
+side-by-side, and thus don't have room to widen their
+windows in any case. People set up their work environment
+assuming a particular maximum window width, and 80
+columns has been the traditional standard. Why change
+it?</p>
+</div>
+
+<div class="cons">
+<p>Proponents of change argue that a wider line can make
+code more readable. The 80-column limit is an hidebound
+throwback to 1960s mainframes; modern equipment has wide screens that
+can easily show longer lines.</p>
+</div>
+
+<div class="decision">
+<p> 80 characters is the maximum.</p>
+
+<p class="exception">Comment lines can be longer than 80
+characters if it is not feasible to split them without
+harming readability, ease of cut and paste or auto-linking
+-- e.g. if a line contains an example command or a literal
+URL longer than 80 characters.</p>
+
+<p class="exception">A raw-string literal may have content
+that exceeds 80 characters. Except for test code, such literals
+should appear near the top of a file.</p>
+
+<p class="exception">An <code>#include</code> statement with a
+long path may exceed 80 columns.</p>
+
+<p class="exception">You needn't be concerned about
+<a href="#The__define_Guard">header guards</a> that exceed
+the maximum length. </p>
+</div>
+
+</div>
+
+<h3 id="Non-ASCII_Characters">Non-ASCII Characters</h3>
+
+<div class="summary">
+<p>Non-ASCII characters should be rare, and must use UTF-8
+formatting.</p>
+</div>
+
+<div class="stylebody">
+
+<p>You shouldn't hard-code user-facing text in source,
+even English, so use of non-ASCII characters should be
+rare. However, in certain cases it is appropriate to
+include such words in your code. For example, if your
+code parses data files from foreign sources, it may be
+appropriate to hard-code the non-ASCII string(s) used in
+those data files as delimiters. More commonly, unittest
+code (which does not need to be localized) might
+contain non-ASCII strings. In such cases, you should use
+UTF-8, since that is an encoding
+understood by most tools able to handle more than just
+ASCII.</p>
+
+<p>Hex encoding is also OK, and encouraged where it
+enhances readability — for example,
+<code>"\xEF\xBB\xBF"</code>, or, even more simply,
+<code>u8"\uFEFF"</code>, is the Unicode zero-width
+no-break space character, which would be invisible if
+included in the source as straight UTF-8.</p>
+
+<p>Use the <code>u8</code> prefix
+to guarantee that a string literal containing
+<code>\uXXXX</code> escape sequences is encoded as UTF-8.
+Do not use it for strings containing non-ASCII characters
+encoded as UTF-8, because that will produce incorrect
+output if the compiler does not interpret the source file
+as UTF-8. </p>
+
+<p>You shouldn't use the C++11 <code>char16_t</code> and
+<code>char32_t</code> character types, since they're for
+non-UTF-8 text. For similar reasons you also shouldn't
+use <code>wchar_t</code> (unless you're writing code that
+interacts with the Windows API, which uses
+<code>wchar_t</code> extensively).</p>
+
+</div>
+
+<h3 id="Spaces_vs._Tabs">Spaces vs. Tabs</h3>
+
+<div class="summary">
+<p>Use only spaces, and indent 2 spaces at a time.</p>
+</div>
+
+<div class="stylebody">
+
+<p>We use spaces for indentation. Do not use tabs in your
+code. You should set your editor to emit spaces when you
+hit the tab key.</p>
+
+</div>
+
+<h3 id="Function_Declarations_and_Definitions">Function Declarations and Definitions</h3>
+
+<div class="summary">
+<p>Return type on the same line as function name, parameters
+on the same line if they fit. Wrap parameter lists which do
+not fit on a single line as you would wrap arguments in a
+<a href="#Function_Calls">function call</a>.</p>
+</div>
+
+<div class="stylebody">
+
+<p>Functions look like this:</p>
+
+
+<pre>ReturnType ClassName::FunctionName(Type par_name1, Type par_name2) {
+ DoSomething();
+ ...
+}
+</pre>
+
+<p>If you have too much text to fit on one line:</p>
+
+<pre>ReturnType ClassName::ReallyLongFunctionName(Type par_name1, Type par_name2,
+ Type par_name3) {
+ DoSomething();
+ ...
+}
+</pre>
+
+<p>or if you cannot fit even the first parameter:</p>
+
+<pre>ReturnType LongClassName::ReallyReallyReallyLongFunctionName(
+ Type par_name1, // 4 space indent
+ Type par_name2,
+ Type par_name3) {
+ DoSomething(); // 2 space indent
+ ...
+}
+</pre>
+
+<p>Some points to note:</p>
+
+<ul>
+ <li>Choose good parameter names.</li>
+
+ <li>Parameter names may be omitted only if the parameter is unused and its
+ purpose is obvious.</li>
+
+ <li>If you cannot fit the return type and the function
+ name on a single line, break between them.</li>
+
+ <li>If you break after the return type of a function
+ declaration or definition, do not indent.</li>
+
+ <li>The open parenthesis is always on the same line as
+ the function name.</li>
+
+ <li>There is never a space between the function name
+ and the open parenthesis.</li>
+
+ <li>There is never a space between the parentheses and
+ the parameters.</li>
+
+ <li>The open curly brace is always on the end of the last line of the function
+ declaration, not the start of the next line.</li>
+
+ <li>The close curly brace is either on the last line by
+ itself or on the same line as the open curly brace.</li>
+
+ <li>There should be a space between the close
+ parenthesis and the open curly brace.</li>
+
+ <li>All parameters should be aligned if possible.</li>
+
+ <li>Default indentation is 2 spaces.</li>
+
+ <li>Wrapped parameters have a 4 space indent.</li>
+</ul>
+
+<p>Unused parameters that are obvious from context may be omitted:</p>
+
+<pre>class Foo {
+ public:
+ Foo(Foo&&);
+ Foo(const Foo&);
+ Foo& operator=(Foo&&);
+ Foo& operator=(const Foo&);
+};
+</pre>
+
+<p>Unused parameters that might not be obvious should comment out the variable
+name in the function definition:</p>
+
+<pre>class Shape {
+ public:
+ virtual void Rotate(double radians) = 0;
+};
+
+class Circle : public Shape {
+ public:
+ void Rotate(double radians) override;
+};
+
+void Circle::Rotate(double /*radians*/) {}
+</pre>
+
+<pre class="badcode">// Bad - if someone wants to implement later, it's not clear what the
+// variable means.
+void Circle::Rotate(double) {}
+</pre>
+
+<p>Attributes, and macros that expand to attributes, appear at the very
+beginning of the function declaration or definition, before the
+return type:</p>
+<pre>MUST_USE_RESULT bool IsOK();
+</pre>
+
+</div>
+
+<h3 id="Formatting_Lambda_Expressions">Lambda Expressions</h3>
+
+<div class="summary">
+<p>Format parameters and bodies as for any other function, and capture
+lists like other comma-separated lists.</p>
+</div>
+
+<div class="stylebody">
+<p>For by-reference captures, do not leave a space between the
+ampersand (&) and the variable name.</p>
+<pre>int x = 0;
+auto x_plus_n = [&x](int n) -> int { return x + n; }
+</pre>
+<p>Short lambdas may be written inline as function arguments.</p>
+<pre>std::set<int> blacklist = {7, 8, 9};
+std::vector<int> digits = {3, 9, 1, 8, 4, 7, 1};
+digits.erase(std::remove_if(digits.begin(), digits.end(), [&blacklist](int i) {
+ return blacklist.find(i) != blacklist.end();
+ }),
+ digits.end());
+</pre>
+
+</div>
+
+<h3 id="Function_Calls">Function Calls</h3>
+
+<div class="summary">
+<p>Either write the call all on a single line, wrap the
+arguments at the parenthesis, or start the arguments on a new
+line indented by four spaces and continue at that 4 space
+indent. In the absence of other considerations, use the
+minimum number of lines, including placing multiple arguments
+on each line where appropriate.</p>
+</div>
+
+<div class="stylebody">
+
+<p>Function calls have the following format:</p>
+<pre>bool result = DoSomething(argument1, argument2, argument3);
+</pre>
+
+<p>If the arguments do not all fit on one line, they
+should be broken up onto multiple lines, with each
+subsequent line aligned with the first argument. Do not
+add spaces after the open paren or before the close
+paren:</p>
+<pre>bool result = DoSomething(averyveryveryverylongargument1,
+ argument2, argument3);
+</pre>
+
+<p>Arguments may optionally all be placed on subsequent
+lines with a four space indent:</p>
+<pre>if (...) {
+ ...
+ ...
+ if (...) {
+ bool result = DoSomething(
+ argument1, argument2, // 4 space indent
+ argument3, argument4);
+ ...
+ }
+</pre>
+
+<p>Put multiple arguments on a single line to reduce the
+number of lines necessary for calling a function unless
+there is a specific readability problem. Some find that
+formatting with strictly one argument on each line is
+more readable and simplifies editing of the arguments.
+However, we prioritize for the reader over the ease of
+editing arguments, and most readability problems are
+better addressed with the following techniques.</p>
+
+<p>If having multiple arguments in a single line decreases
+readability due to the complexity or confusing nature of the
+expressions that make up some arguments, try creating
+variables that capture those arguments in a descriptive name:</p>
+<pre>int my_heuristic = scores[x] * y + bases[x];
+bool result = DoSomething(my_heuristic, x, y, z);
+</pre>
+
+<p>Or put the confusing argument on its own line with
+an explanatory comment:</p>
+<pre>bool result = DoSomething(scores[x] * y + bases[x], // Score heuristic.
+ x, y, z);
+</pre>
+
+<p>If there is still a case where one argument is
+significantly more readable on its own line, then put it on
+its own line. The decision should be specific to the argument
+which is made more readable rather than a general policy.</p>
+
+<p>Sometimes arguments form a structure that is important
+for readability. In those cases, feel free to format the
+arguments according to that structure:</p>
+<pre>// Transform the widget by a 3x3 matrix.
+my_widget.Transform(x1, x2, x3,
+ y1, y2, y3,
+ z1, z2, z3);
+</pre>
+
+</div>
+
+<h3 id="Braced_Initializer_List_Format">Braced Initializer List Format</h3>
+
+<div class="summary">
+<p>Format a <a href="#Braced_Initializer_List">braced initializer list</a>
+exactly like you would format a function call in its place.</p>
+</div>
+
+<div class="stylebody">
+
+<p>If the braced list follows a name (e.g. a type or
+variable name), format as if the <code>{}</code> were the
+parentheses of a function call with that name. If there
+is no name, assume a zero-length name.</p>
+
+<pre>// Examples of braced init list on a single line.
+return {foo, bar};
+functioncall({foo, bar});
+std::pair<int, int> p{foo, bar};
+
+// When you have to wrap.
+SomeFunction(
+ {"assume a zero-length name before {"},
+ some_other_function_parameter);
+SomeType variable{
+ some, other, values,
+ {"assume a zero-length name before {"},
+ SomeOtherType{
+ "Very long string requiring the surrounding breaks.",
+ some, other values},
+ SomeOtherType{"Slightly shorter string",
+ some, other, values}};
+SomeType variable{
+ "This is too long to fit all in one line"};
+MyType m = { // Here, you could also break before {.
+ superlongvariablename1,
+ superlongvariablename2,
+ {short, interior, list},
+ {interiorwrappinglist,
+ interiorwrappinglist2}};
+</pre>
+
+</div>
+
+<h3 id="Conditionals">Conditionals</h3>
+
+<div class="summary">
+<p>Prefer no spaces inside parentheses. The <code>if</code>
+and <code>else</code> keywords belong on separate lines.</p>
+</div>
+
+<div class="stylebody">
+
+<p>There are two acceptable formats for a basic
+conditional statement. One includes spaces between the
+parentheses and the condition, and one does not.</p>
+
+<p>The most common form is without spaces. Either is
+fine, but <em>be consistent</em>. If you are modifying a
+file, use the format that is already present. If you are
+writing new code, use the format that the other files in
+that directory or project use. If in doubt and you have
+no personal preference, do not add the spaces.</p>
+
+<pre>if (condition) { // no spaces inside parentheses
+ ... // 2 space indent.
+} else if (...) { // The else goes on the same line as the closing brace.
+ ...
+} else {
+ ...
+}
+</pre>
+
+<p>If you prefer you may add spaces inside the
+parentheses:</p>
+
+<pre>if ( condition ) { // spaces inside parentheses - rare
+ ... // 2 space indent.
+} else { // The else goes on the same line as the closing brace.
+ ...
+}
+</pre>
+
+<p>Note that in all cases you must have a space between
+the <code>if</code> and the open parenthesis. You must
+also have a space between the close parenthesis and the
+curly brace, if you're using one.</p>
+
+<pre class="badcode">if(condition) { // Bad - space missing after IF.
+if (condition){ // Bad - space missing before {.
+if(condition){ // Doubly bad.
+</pre>
+
+<pre>if (condition) { // Good - proper space after IF and before {.
+</pre>
+
+<p>Short conditional statements may be written on one
+line if this enhances readability. You may use this only
+when the line is brief and the statement does not use the
+<code>else</code> clause.</p>
+
+<pre>if (x == kFoo) return new Foo();
+if (x == kBar) return new Bar();
+</pre>
+
+<p>This is not allowed when the if statement has an
+<code>else</code>:</p>
+
+<pre class="badcode">// Not allowed - IF statement on one line when there is an ELSE clause
+if (x) DoThis();
+else DoThat();
+</pre>
+
+<p>In general, curly braces are not required for
+single-line statements, but they are allowed if you like
+them; conditional or loop statements with complex
+conditions or statements may be more readable with curly
+braces. Some
+projects require that an
+<code>if</code> must always always have an accompanying
+brace.</p>
+
+<pre>if (condition)
+ DoSomething(); // 2 space indent.
+
+if (condition) {
+ DoSomething(); // 2 space indent.
+}
+</pre>
+
+<p>However, if one part of an
+<code>if</code>-<code>else</code> statement uses curly
+braces, the other part must too:</p>
+
+<pre class="badcode">// Not allowed - curly on IF but not ELSE
+if (condition) {
+ foo;
+} else
+ bar;
+
+// Not allowed - curly on ELSE but not IF
+if (condition)
+ foo;
+else {
+ bar;
+}
+</pre>
+
+<pre>// Curly braces around both IF and ELSE required because
+// one of the clauses used braces.
+if (condition) {
+ foo;
+} else {
+ bar;
+}
+</pre>
+
+</div>
+
+<h3 id="Loops_and_Switch_Statements">Loops and Switch Statements</h3>
+
+<div class="summary">
+<p>Switch statements may use braces for blocks. Annotate
+non-trivial fall-through between cases.
+Braces are optional for single-statement loops.
+Empty loop bodies should use empty braces or <code>continue</code>.</p>
+</div>
+
+<div class="stylebody">
+
+<p><code>case</code> blocks in <code>switch</code>
+statements can have curly braces or not, depending on
+your preference. If you do include curly braces they
+should be placed as shown below.</p>
+
+<p>If not conditional on an enumerated value, switch
+statements should always have a <code>default</code> case
+(in the case of an enumerated value, the compiler will
+warn you if any values are not handled). If the default
+case should never execute, simply
+<code>assert</code>:</p>
+
+
+
+<div>
+<pre>switch (var) {
+ case 0: { // 2 space indent
+ ... // 4 space indent
+ break;
+ }
+ case 1: {
+ ...
+ break;
+ }
+ default: {
+ assert(false);
+ }
+}
+</pre>
+</div>
+
+
+
+
+
+<p> Braces are optional for single-statement loops.</p>
+
+<pre>for (int i = 0; i < kSomeNumber; ++i)
+ printf("I love you\n");
+
+for (int i = 0; i < kSomeNumber; ++i) {
+ printf("I take it back\n");
+}
+</pre>
+
+
+<p>Empty loop bodies should use an empty pair of braces or <code>continue</code>,
+but not a single semicolon.</p>
+
+<pre>while (condition) {
+ // Repeat test until it returns false.
+}
+for (int i = 0; i < kSomeNumber; ++i) {} // Good - one newline is also OK.
+while (condition) continue; // Good - continue indicates no logic.
+</pre>
+
+<pre class="badcode">while (condition); // Bad - looks like part of do/while loop.
+</pre>
+
+</div>
+
+<h3 id="Pointer_and_Reference_Expressions">Pointer and Reference Expressions</h3>
+
+<div class="summary">
+<p>No spaces around period or arrow. Pointer operators do not
+have trailing spaces.</p>
+</div>
+
+<div class="stylebody">
+
+<p>The following are examples of correctly-formatted
+pointer and reference expressions:</p>
+
+<pre>x = *p;
+p = &x;
+x = r.y;
+x = r->y;
+</pre>
+
+<p>Note that:</p>
+
+<ul>
+ <li>There are no spaces around the period or arrow when
+ accessing a member.</li>
+
+ <li>Pointer operators have no space after the
+ <code>*</code> or <code>&</code>.</li>
+</ul>
+
+<p>When declaring a pointer variable or argument, you may
+place the asterisk adjacent to either the type or to the
+variable name:</p>
+
+<pre>// These are fine, space preceding.
+char *c;
+const string &str;
+
+// These are fine, space following.
+char* c;
+const string& str;
+</pre>
+
+It is allowed (if unusual) to declare multiple variables in the same
+declaration, but it is disallowed if any of those have pointer or
+reference decorations. Such declarations are easily misread.
+<pre>// Fine if helpful for readability.
+int x, y;
+</pre>
+<pre class="badcode">int x, *y; // Disallowed - no & or * in multiple declaration
+char * c; // Bad - spaces on both sides of *
+const string & str; // Bad - spaces on both sides of &
+</pre>
+
+<p>You should do this consistently within a single
+file,
+so, when modifying an existing file, use the style in
+that file.</p>
+
+</div>
+
+<h3 id="Boolean_Expressions">Boolean Expressions</h3>
+
+<div class="summary">
+<p>When you have a boolean expression that is longer than the
+<a href="#Line_Length">standard line length</a>, be
+consistent in how you break up the lines.</p>
+</div>
+
+<div class="stylebody">
+
+<p>In this example, the logical AND operator is always at
+the end of the lines:</p>
+
+<pre>if (this_one_thing > this_other_thing &&
+ a_third_thing == a_fourth_thing &&
+ yet_another && last_one) {
+ ...
+}
+</pre>
+
+<p>Note that when the code wraps in this example, both of
+the <code>&&</code> logical AND operators are at
+the end of the line. This is more common in Google code,
+though wrapping all operators at the beginning of the
+line is also allowed. Feel free to insert extra
+parentheses judiciously because they can be very helpful
+in increasing readability when used
+appropriately. Also note that you should always use
+the punctuation operators, such as
+<code>&&</code> and <code>~</code>, rather than
+the word operators, such as <code>and</code> and
+<code>compl</code>.</p>
+
+</div>
+
+<h3 id="Return_Values">Return Values</h3>
+
+<div class="summary">
+<p>Do not needlessly surround the <code>return</code>
+expression with parentheses.</p>
+</div>
+
+<div class="stylebody">
+
+<p>Use parentheses in <code>return expr;</code> only
+where you would use them in <code>x = expr;</code>.</p>
+
+<pre>return result; // No parentheses in the simple case.
+// Parentheses OK to make a complex expression more readable.
+return (some_long_condition &&
+ another_condition);
+</pre>
+
+<pre class="badcode">return (value); // You wouldn't write var = (value);
+return(result); // return is not a function!
+</pre>
+
+</div>
+
+
+
+<h3 id="Variable_and_Array_Initialization">Variable and Array Initialization</h3>
+
+<div class="summary">
+<p>Your choice of <code>=</code>, <code>()</code>, or
+<code>{}</code>.</p>
+</div>
+
+<div class="stylebody">
+
+<p>You may choose between <code>=</code>,
+<code>()</code>, and <code>{}</code>; the following are
+all correct:</p>
+
+<pre>int x = 3;
+int x(3);
+int x{3};
+string name = "Some Name";
+string name("Some Name");
+string name{"Some Name"};
+</pre>
+
+<p>Be careful when using a braced initialization list <code>{...}</code>
+on a type with an <code>std::initializer_list</code> constructor.
+A nonempty <i>braced-init-list</i> prefers the
+<code>std::initializer_list</code> constructor whenever
+possible. Note that empty braces <code>{}</code> are special, and
+will call a default constructor if available. To force the
+non-<code>std::initializer_list</code> constructor, use parentheses
+instead of braces.</p>
+
+<pre>std::vector<int> v(100, 1); // A vector of 100 1s.
+std::vector<int> v{100, 1}; // A vector of 100, 1.
+</pre>
+
+<p>Also, the brace form prevents narrowing of integral
+types. This can prevent some types of programming
+errors.</p>
+
+<pre>int pi(3.14); // OK -- pi == 3.
+int pi{3.14}; // Compile error: narrowing conversion.
+</pre>
+
+</div>
+
+<h3 id="Preprocessor_Directives">Preprocessor Directives</h3>
+
+<div class="summary">
+<p>The hash mark that starts a preprocessor directive should
+always be at the beginning of the line.</p>
+</div>
+
+<div class="stylebody">
+
+<p>Even when preprocessor directives are within the body
+of indented code, the directives should start at the
+beginning of the line.</p>
+
+<pre>// Good - directives at beginning of line
+ if (lopsided_score) {
+#if DISASTER_PENDING // Correct -- Starts at beginning of line
+ DropEverything();
+# if NOTIFY // OK but not required -- Spaces after #
+ NotifyClient();
+# endif
+#endif
+ BackToNormal();
+ }
+</pre>
+
+<pre class="badcode">// Bad - indented directives
+ if (lopsided_score) {
+ #if DISASTER_PENDING // Wrong! The "#if" should be at beginning of line
+ DropEverything();
+ #endif // Wrong! Do not indent "#endif"
+ BackToNormal();
+ }
+</pre>
+
+</div>
+
+<h3 id="Class_Format">Class Format</h3>
+
+<div class="summary">
+<p>Sections in <code>public</code>, <code>protected</code> and
+<code>private</code> order, each indented one space.</p>
+</div>
+
+<div class="stylebody">
+
+<p>The basic format for a class definition (lacking the
+comments, see <a href="#Class_Comments">Class
+Comments</a> for a discussion of what comments are
+needed) is:</p>
+
+<pre>class MyClass : public OtherClass {
+ public: // Note the 1 space indent!
+ MyClass(); // Regular 2 space indent.
+ explicit MyClass(int var);
+ ~MyClass() {}
+
+ void SomeFunction();
+ void SomeFunctionThatDoesNothing() {
+ }
+
+ void set_some_var(int var) { some_var_ = var; }
+ int some_var() const { return some_var_; }
+
+ private:
+ bool SomeInternalFunction();
+
+ int some_var_;
+ int some_other_var_;
+};
+</pre>
+
+<p>Things to note:</p>
+
+<ul>
+ <li>Any base class name should be on the same line as
+ the subclass name, subject to the 80-column limit.</li>
+
+ <li>The <code>public:</code>, <code>protected:</code>,
+ and <code>private:</code> keywords should be indented
+ one space.</li>
+
+ <li>Except for the first instance, these keywords
+ should be preceded by a blank line. This rule is
+ optional in small classes.</li>
+
+ <li>Do not leave a blank line after these
+ keywords.</li>
+
+ <li>The <code>public</code> section should be first,
+ followed by the <code>protected</code> and finally the
+ <code>private</code> section.</li>
+
+ <li>See <a href="#Declaration_Order">Declaration
+ Order</a> for rules on ordering declarations within
+ each of these sections.</li>
+</ul>
+
+</div>
+
+<h3 id="Constructor_Initializer_Lists">Constructor Initializer Lists</h3>
+
+<div class="summary">
+<p>Constructor initializer lists can be all on one line or
+with subsequent lines indented four spaces.</p>
+</div>
+
+<div class="stylebody">
+
+<p>The acceptable formats for initializer lists are:</p>
+
+<pre>// When everything fits on one line:
+MyClass::MyClass(int var) : some_var_(var) {
+ DoSomething();
+}
+
+// If the signature and initializer list are not all on one line,
+// you must wrap before the colon and indent 4 spaces:
+MyClass::MyClass(int var)
+ : some_var_(var), some_other_var_(var + 1) {
+ DoSomething();
+}
+
+// When the list spans multiple lines, put each member on its own line
+// and align them:
+MyClass::MyClass(int var)
+ : some_var_(var), // 4 space indent
+ some_other_var_(var + 1) { // lined up
+ DoSomething();
+}
+
+// As with any other code block, the close curly can be on the same
+// line as the open curly, if it fits.
+MyClass::MyClass(int var)
+ : some_var_(var) {}
+</pre>
+
+</div>
+
+<h3 id="Namespace_Formatting">Namespace Formatting</h3>
+
+<div class="summary">
+<p>The contents of namespaces are not indented.</p>
+</div>
+
+<div class="stylebody">
+
+<p><a href="#Namespaces">Namespaces</a> do not add an
+extra level of indentation. For example, use:</p>
+
+<pre>namespace {
+
+void foo() { // Correct. No extra indentation within namespace.
+ ...
+}
+
+} // namespace
+</pre>
+
+<p>Do not indent within a namespace:</p>
+
+<pre class="badcode">namespace {
+
+ // Wrong. Indented when it should not be.
+ void foo() {
+ ...
+ }
+
+} // namespace
+</pre>
+
+<p>When declaring nested namespaces, put each namespace
+on its own line.</p>
+
+<pre>namespace foo {
+namespace bar {
+</pre>
+
+</div>
+
+<h3 id="Horizontal_Whitespace">Horizontal Whitespace</h3>
+
+<div class="summary">
+<p>Use of horizontal whitespace depends on location. Never put
+trailing whitespace at the end of a line.</p>
+</div>
+
+<div class="stylebody">
+
+<h4 class="stylepoint_subsection">General</h4>
+
+<pre>void f(bool b) { // Open braces should always have a space before them.
+ ...
+int i = 0; // Semicolons usually have no space before them.
+// Spaces inside braces for braced-init-list are optional. If you use them,
+// put them on both sides!
+int x[] = { 0 };
+int x[] = {0};
+
+// Spaces around the colon in inheritance and initializer lists.
+class Foo : public Bar {
+ public:
+ // For inline function implementations, put spaces between the braces
+ // and the implementation itself.
+ Foo(int b) : Bar(), baz_(b) {} // No spaces inside empty braces.
+ void Reset() { baz_ = 0; } // Spaces separating braces from implementation.
+ ...
+</pre>
+
+<p>Adding trailing whitespace can cause extra work for
+others editing the same file, when they merge, as can
+removing existing trailing whitespace. So: Don't
+introduce trailing whitespace. Remove it if you're
+already changing that line, or do it in a separate
+clean-up
+operation (preferably when no-one
+else is working on the file).</p>
+
+<h4 class="stylepoint_subsection">Loops and Conditionals</h4>
+
+<pre>if (b) { // Space after the keyword in conditions and loops.
+} else { // Spaces around else.
+}
+while (test) {} // There is usually no space inside parentheses.
+switch (i) {
+for (int i = 0; i < 5; ++i) {
+// Loops and conditions may have spaces inside parentheses, but this
+// is rare. Be consistent.
+switch ( i ) {
+if ( test ) {
+for ( int i = 0; i < 5; ++i ) {
+// For loops always have a space after the semicolon. They may have a space
+// before the semicolon, but this is rare.
+for ( ; i < 5 ; ++i) {
+ ...
+
+// Range-based for loops always have a space before and after the colon.
+for (auto x : counts) {
+ ...
+}
+switch (i) {
+ case 1: // No space before colon in a switch case.
+ ...
+ case 2: break; // Use a space after a colon if there's code after it.
+</pre>
+
+<h4 class="stylepoint_subsection">Operators</h4>
+
+<pre>// Assignment operators always have spaces around them.
+x = 0;
+
+// Other binary operators usually have spaces around them, but it's
+// OK to remove spaces around factors. Parentheses should have no
+// internal padding.
+v = w * x + y / z;
+v = w*x + y/z;
+v = w * (x + z);
+
+// No spaces separating unary operators and their arguments.
+x = -5;
+++x;
+if (x && !y)
+ ...
+</pre>
+
+<h4 class="stylepoint_subsection">Templates and Casts</h4>
+
+<pre>// No spaces inside the angle brackets (< and >), before
+// <, or between >( in a cast
+std::vector<string> x;
+y = static_cast<char*>(x);
+
+// Spaces between type and pointer are OK, but be consistent.
+std::vector<char *> x;
+</pre>
+
+</div>
+
+<h3 id="Vertical_Whitespace">Vertical Whitespace</h3>
+
+<div class="summary">
+<p>Minimize use of vertical whitespace.</p>
+</div>
+
+<div class="stylebody">
+
+<p>This is more a principle than a rule: don't use blank
+lines when you don't have to. In particular, don't put
+more than one or two blank lines between functions,
+resist starting functions with a blank line, don't end
+functions with a blank line, and be discriminating with
+your use of blank lines inside functions.</p>
+
+<p>The basic principle is: The more code that fits on one
+screen, the easier it is to follow and understand the
+control flow of the program. Of course, readability can
+suffer from code being too dense as well as too spread
+out, so use your judgement. But in general, minimize use
+of vertical whitespace.</p>
+
+<p>Some rules of thumb to help when blank lines may be
+useful:</p>
+
+<ul>
+ <li>Blank lines at the beginning or end of a function
+ very rarely help readability.</li>
+
+ <li>Blank lines inside a chain of if-else blocks may
+ well help readability.</li>
+</ul>
+
+</div>
+
+<h2 id="Exceptions_to_the_Rules">Exceptions to the Rules</h2>
+
+<p>The coding conventions described above are mandatory.
+However, like all good rules, these sometimes have exceptions,
+which we discuss here.</p>
+
+
+
+<div>
+<h3 id="Existing_Non-conformant_Code">Existing Non-conformant Code</h3>
+
+<div class="summary">
+<p>You may diverge from the rules when dealing with code that
+does not conform to this style guide.</p>
+</div>
+
+<div class="stylebody">
+
+<p>If you find yourself modifying code that was written
+to specifications other than those presented by this
+guide, you may have to diverge from these rules in order
+to stay consistent with the local conventions in that
+code. If you are in doubt about how to do this, ask the
+original author or the person currently responsible for
+the code. Remember that <em>consistency</em> includes
+local consistency, too.</p>
+
+</div>
+</div>
+
+
+
+<h3 id="Windows_Code">Windows Code</h3>
+
+<div class="summary">
+<p> Windows
+programmers have developed their own set of coding
+conventions, mainly derived from the conventions in Windows
+headers and other Microsoft code. We want to make it easy
+for anyone to understand your code, so we have a single set
+of guidelines for everyone writing C++ on any platform.</p>
+</div>
+
+<div class="stylebody">
+<p>It is worth reiterating a few of the guidelines that
+you might forget if you are used to the prevalent Windows
+style:</p>
+
+<ul>
+ <li>Do not use Hungarian notation (for example, naming
+ an integer <code>iNum</code>). Use the Google naming
+ conventions, including the <code>.cc</code> extension
+ for source files.</li>
+
+ <li>Windows defines many of its own synonyms for
+ primitive types, such as <code>DWORD</code>,
+ <code>HANDLE</code>, etc. It is perfectly acceptable,
+ and encouraged, that you use these types when calling
+ Windows API functions. Even so, keep as close as you
+ can to the underlying C++ types. For example, use
+ <code>const TCHAR *</code> instead of
+ <code>LPCTSTR</code>.</li>
+
+ <li>When compiling with Microsoft Visual C++, set the
+ compiler to warning level 3 or higher, and treat all
+ warnings as errors.</li>
+
+ <li>Do not use <code>#pragma once</code>; instead use
+ the standard Google include guards. The path in the
+ include guards should be relative to the top of your
+ project tree.</li>
+
+ <li>In fact, do not use any nonstandard extensions,
+ like <code>#pragma</code> and <code>__declspec</code>,
+ unless you absolutely must. Using
+ <code>__declspec(dllimport)</code> and
+ <code>__declspec(dllexport)</code> is allowed; however,
+ you must use them through macros such as
+ <code>DLLIMPORT</code> and <code>DLLEXPORT</code>, so
+ that someone can easily disable the extensions if they
+ share the code.</li>
+</ul>
+
+<p>However, there are just a few rules that we
+occasionally need to break on Windows:</p>
+
+<ul>
+ <li>Normally we <a href="#Multiple_Inheritance">forbid
+ the use of multiple implementation inheritance</a>;
+ however, it is required when using COM and some ATL/WTL
+ classes. You may use multiple implementation
+ inheritance to implement COM or ATL/WTL classes and
+ interfaces.</li>
+
+ <li>Although you should not use exceptions in your own
+ code, they are used extensively in the ATL and some
+ STLs, including the one that comes with Visual C++.
+ When using the ATL, you should define
+ <code>_ATL_NO_EXCEPTIONS</code> to disable exceptions.
+ You should investigate whether you can also disable
+ exceptions in your STL, but if not, it is OK to turn on
+ exceptions in the compiler. (Note that this is only to
+ get the STL to compile. You should still not write
+ exception handling code yourself.)</li>
+
+ <li>The usual way of working with precompiled headers
+ is to include a header file at the top of each source
+ file, typically with a name like <code>StdAfx.h</code>
+ or <code>precompile.h</code>. To make your code easier
+ to share with other projects, avoid including this file
+ explicitly (except in <code>precompile.cc</code>), and
+ use the <code>/FI</code> compiler option to include the
+ file automatically.</li>
+
+ <li>Resource headers, which are usually named
+ <code>resource.h</code> and contain only macros, do not
+ need to conform to these style guidelines.</li>
+</ul>
+
+</div>
+
+<h2 class="ignoreLink">Parting Words</h2>
+
+<p>Use common sense and <em>BE CONSISTENT</em>.</p>
+
+<p>If you are editing code, take a few minutes to look at the
+code around you and determine its style. If they use spaces
+around their <code>if</code> clauses, you should, too. If their
+comments have little boxes of stars around them, make your
+comments have little boxes of stars around them too.</p>
+
+<p>The point of having style guidelines is to have a common
+vocabulary of coding so people can concentrate on what you are
+saying, rather than on how you are saying it. We present global
+style rules here so people know the vocabulary. But local style
+is also important. If code you add to a file looks drastically
+different from the existing code around it, the discontinuity
+throws readers out of their rhythm when they go to read it. Try
+to avoid this.</p>
+
+
+
+<p>OK, enough writing about writing code; the code itself is much
+more interesting. Have fun!</p>
+
+<hr>
+
+</div>
+</div>
+</body>
+</html>