www/cxx_compatibility.html - platform/external/clang - Gitiles

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 <!-- ======================================================================= -->
 <h1>Clang's C++ Compatibility</h1>
 <!-- ======================================================================= -->

 <ul>
 <li><a href="#intro">Introduction</a></li>
 <li><a href="#vla">Variable-length arrays</a></li>
 <li><a href="#init_static_const">Initialization of non-integral static const data members within a class definition</a></li>
 <li><a href="#dep_lookup">Unqualified lookup in templates</a></li>
 <li><a href="#dep_lookup_bases">Unqualified lookup into dependent bases of class templates</a></li>
 <li><a href="#bad_templates">Templates with no valid instantiations</a></li>
 <li><a href="#default_init_const">Default initialization of const variable of a class type requires user-defined default constructor</a></li>
 </ul>

 <!-- ======================================================================= -->
 <h2 id="intro">Introduction</h2>
 <!-- ======================================================================= -->

 <p>Clang strives to strictly conform to the C++ standard.  That means
 it will reject invalid C++ code that another compiler may accept.
 This page helps you decide whether a Clang error message means a
 C++-conformance bug in your code and how you can fix it.</p>

 <!-- ======================================================================= -->
 <h2 id="vla">Variable-length arrays</h2>
 <!-- ======================================================================= -->

 <p>GCC allows an array's size to be determined at run time. This,
 however, is not standard C++. Furthermore, it is a potential security
 hole as an incorrect array size may overflow the stack. If Clang tells
 you <tt>"variable length arrays are not permitted in C++"</tt>, here
 are some ways in which you can fix it:</p>

 <ol>
 <li>replace it with a fixed-size array if you can determine a
     reasonable upper bound at compile time; sometimes this is as
     simple as changing <tt>int size = ...;</tt> to <tt>const int size
     = ...;</tt> (if the definition of <tt>size</tt> is a compile-time
     integral constant);</li>
 <li>use an <tt>std::string</tt> instead of a <tt>char []</tt>;</li>
 <li>use <tt>std::vector</tt> or some other suitable container type;
     or</li>
 <li>allocate the array on the heap instead using <tt>new Type[]</tt> -
     just remember to <tt>delete[]</tt> it.</li>
 </ol>

 <!-- ======================================================================= -->
 <h2 id="init_static_const">Initialization of non-integral static const data members within a class definition</h2>
 <!-- ======================================================================= -->

 The following code is ill-formed in C++'03:

 <pre>
 class SomeClass {
  public:
   static const double SomeConstant = 0.5;
 };

 const double SomeClass::SomeConstant;
 </pre>

 Clang errors with something similar to:

 <pre>
 .../your_file.h:42:42: error: 'SomeConstant' can only be initialized if it is a static const integral data member
   static const double SomeConstant = 0.5;
                       ^              ~~~
 </pre>

 Only <i>integral</i> constant expressions are allowed as initializers
 within the class definition. See C++'03 [class.static.data] p4 for the
 details of this restriction.  The fix here is straightforward: move
 the initializer to the definition of the static data member, which
 must exist outside of the class definition:

 <pre>
 class SomeClass {
  public:
   static const double SomeConstant;
 };

 const double SomeClass::SomeConstant<b> = 0.5</b>;
 </pre>

 Note that the forthcoming C++0x standard will allow this.

 <!-- ======================================================================= -->
 <h2 id="dep_lookup">Unqualified lookup in templates</h2>
 <!-- ======================================================================= -->

 Some versions of GCC accept the following invalid code:

 <pre>
 template &lt;typename T&gt; struct Foo {
   void Work(T x) {
     func(x);
   }
 };
 ...
 void func(int x);
 ...
 template struct Foo&lt;int&gt;; // or anything else that instantiates Foo&lt;int&gt;::Work
 </pre>

 The standard says that unqualified names like <tt>func</tt> are looked up
 when the template is defined, not when it's instantiated.  Since
 <tt>void func(int)</tt> was not declared yet when <tt>Foo</tt> was
 defined, it's not considered.  The fix is usually to
 declare <tt>func</tt> before <tt>Foo</tt>.

 <p>This is complicated by <i>argument-dependent lookup</i> (ADL),
 which is done when unqualified names are called as functions,
 like <tt>func(x)</tt> above.  The standard says that ADL is performed
 in both places if any of the arguments are type-dependent, like
 <tt>x</tt> is in this example.  However, ADL does nothing for builtin
 types like <tt>int</tt>, so the example is still invalid.  See
 [basic.lookup.argdep] for more information.

 <!-- ======================================================================= -->
 <h2 id="dep_lookup_bases">Unqualified lookup into dependent bases of class templates</h2>
 <!-- ======================================================================= -->

 Some versions of GCC accept the following invalid code:

 <pre>
 template &lt;typename T&gt; struct Base {
   void DoThis(T x) {}
   static void DoThat(T x) {}
 };

 template &lt;typename T&gt; struct Derived : public Base&lt;T&gt; {
   void Work(T x) {
     DoThis(x);  // Invalid!
     DoThat(x);  // Invalid!
   }
 };
 </pre>

 Clang correctly rejects it with the following errors
 (when <tt>Derived</tt> is eventually instantiated):

 <pre>
 my_file.cpp:8:5: error: use of undeclared identifier 'DoThis'
     DoThis(x);
     ^
     this-&gt;
 my_file.cpp:2:8: note: must qualify identifier to find this declaration in dependent base class
   void DoThis(T x) {}
        ^
 my_file.cpp:9:5: error: use of undeclared identifier 'DoThat'
     DoThat(x);
     ^
     this-&gt;
 my_file.cpp:3:15: note: must qualify identifier to find this declaration in dependent base class
   static void DoThat(T x) {}
 </pre>

 Like we said <a href="#dep_lookup">above</a>, unqualified names like
 <tt>DoThis</tt> and <tt>DoThat</tt> are looked up when the template
 <tt>Derived</tt> is defined, not when it's instantiated.  When we look
 up a name used in a class, we usually look into the base classes.
 However, we can't look into the base class <tt>Base&lt;T&gt;</tt>
 because its type depends on the template argument <tt>T</tt>, so the
 standard says we should just ignore it.  See [temp.dep]p3 for details.

 <p>The fix, as Clang tells you, is to tell the compiler that we want a
 class member by prefixing the calls with <tt>this-&gt;</tt>:

 <pre>
   void Work(T x) {
     <b>this-&gt;</b>DoThis(x);
     <b>this-&gt;</b>DoThat(x);
   }
 </pre>

 Alternatively, you can tell the compiler exactly where to look:

 <pre>
   void Work(T x) {
     <b>Base&lt;T&gt;</b>::DoThis(x);
     <b>Base&lt;T&gt;</b>::DoThat(x);
   }
 </pre>

 This works whether the methods are static or not, but be careful:
 if <tt>DoThis</tt> is virtual, calling it this way will bypass virtual
 dispatch!

 <!-- ======================================================================= -->
 <h2 id="bad_templates">Templates with no valid instantiations</h2>
 <!-- ======================================================================= -->

 The following code contains a typo: the programmer
 meant <tt>init()</tt> but wrote <tt>innit()</tt> instead.

 <pre>
   template &lt;class T&gt; class Processor {
     ...
     void init();
     ...
   };
   ...
   template &lt;class T&gt; void process() {
     Processor&lt;T&gt; processor;
     processor.innit();       // <-- should be 'init()'
     ...
   }
 </pre>

 Unfortunately, we can't flag this mistake as soon as we see it: inside
 a template, we're not allowed to make assumptions about "dependent
 types" like <tt>Processor&lt;T&gt;</tt>.  Suppose that later on in
 this file the programmer adds an explicit specialization
 of <tt>Processor</tt>, like so:

 <pre>
   template &lt;&gt; class Processor&lt;char*&gt; {
     void innit();
   };
 </pre>

 Now the program will work &mdash; as long as the programmer only ever
 instantiates <tt>process()</tt> with <tt>T = char*</tt>!  This is why
 it's hard, and sometimes impossible, to diagnose mistakes in a
 template definition before it's instantiated.

 <p>The standard says that a template with no valid instantiations is
 ill-formed.  Clang tries to do as much checking as possible at
 definition-time instead of instantiation-time: not only does this
 produce clearer diagnostics, but it also substantially improves
 compile times when using pre-compiled headers.  The downside to this
 philosophy is that Clang sometimes fails to process files because they
 contain broken templates that are no longer used.  The solution is
 simple: since the code is unused, just remove it.

 <!-- ======================================================================= -->
 <h2 id="default_init_const">Default initialization of const variable of a class type requires user-defined default constructor</h2>
 <!-- ======================================================================= -->

 If a <tt>class</tt> or <tt>struct</tt> has no user-defined default
 constructor, C++ doesn't allow you to default construct a <tt>const</tt>
 instance of it like this ([dcl.init], p9):

 <pre>
 class Foo {
  public:
   // The compiler-supplied default constructor works fine, so we
   // don't bother with defining one.
   ...
 };

 void Bar() {
   const Foo foo;  // Error!
   ...
 }
 </pre>

 To fix this, you can define a default constructor for the class:

 <pre>
 class Foo {
  public:
   Foo() {}
   ...
 };

 void Bar() {
   const Foo foo;  // Now the compiler is happy.
   ...
 }
 </pre>

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	<title>Clang - C++ Compatibility</title>
	<link type="text/css" rel="stylesheet" href="menu.css" />
	<link type="text/css" rel="stylesheet" href="content.css" />
	<style type="text/css">
	</style>
	</head>
	<body>

	<!--#include virtual="menu.html.incl"-->

	<div id="content">

	<!-- ======================================================================= -->
	<h1>Clang's C++ Compatibility</h1>
	<!-- ======================================================================= -->

	<ul>
	<li><a href="#intro">Introduction</a></li>
	<li><a href="#vla">Variable-length arrays</a></li>
	<li><a href="#init_static_const">Initialization of non-integral static const data members within a class definition</a></li>
	<li><a href="#dep_lookup">Unqualified lookup in templates</a></li>
	<li><a href="#dep_lookup_bases">Unqualified lookup into dependent bases of class templates</a></li>
	<li><a href="#bad_templates">Templates with no valid instantiations</a></li>
	<li><a href="#default_init_const">Default initialization of const variable of a class type requires user-defined default constructor</a></li>
	</ul>

	<!-- ======================================================================= -->
	<h2 id="intro">Introduction</h2>
	<!-- ======================================================================= -->

	<p>Clang strives to strictly conform to the C++ standard. That means
	it will reject invalid C++ code that another compiler may accept.
	This page helps you decide whether a Clang error message means a
	C++-conformance bug in your code and how you can fix it.</p>

	<!-- ======================================================================= -->
	<h2 id="vla">Variable-length arrays</h2>
	<!-- ======================================================================= -->

	<p>GCC allows an array's size to be determined at run time. This,
	however, is not standard C++. Furthermore, it is a potential security
	hole as an incorrect array size may overflow the stack. If Clang tells
	you <tt>"variable length arrays are not permitted in C++"</tt>, here
	are some ways in which you can fix it:</p>

	<ol>
	<li>replace it with a fixed-size array if you can determine a
	reasonable upper bound at compile time; sometimes this is as
	simple as changing <tt>int size = ...;</tt> to <tt>const int size
	= ...;</tt> (if the definition of <tt>size</tt> is a compile-time
	integral constant);</li>
	<li>use an <tt>std::string</tt> instead of a <tt>char []</tt>;</li>
	<li>use <tt>std::vector</tt> or some other suitable container type;
	or</li>
	<li>allocate the array on the heap instead using <tt>new Type[]</tt> -
	just remember to <tt>delete[]</tt> it.</li>
	</ol>

	<!-- ======================================================================= -->
	<h2 id="init_static_const">Initialization of non-integral static const data members within a class definition</h2>
	<!-- ======================================================================= -->

	The following code is ill-formed in C++'03:

	<pre>
	class SomeClass {
	public:
	static const double SomeConstant = 0.5;
	};

	const double SomeClass::SomeConstant;
	</pre>

	Clang errors with something similar to:

	<pre>
	.../your_file.h:42:42: error: 'SomeConstant' can only be initialized if it is a static const integral data member
	static const double SomeConstant = 0.5;
	^ ~~~
	</pre>

	Only <i>integral</i> constant expressions are allowed as initializers
	within the class definition. See C++'03 [class.static.data] p4 for the
	details of this restriction. The fix here is straightforward: move
	the initializer to the definition of the static data member, which
	must exist outside of the class definition:

	<pre>
	class SomeClass {
	public:
	static const double SomeConstant;
	};

	const double SomeClass::SomeConstant<b> = 0.5</b>;
	</pre>

	Note that the forthcoming C++0x standard will allow this.

	<!-- ======================================================================= -->
	<h2 id="dep_lookup">Unqualified lookup in templates</h2>
	<!-- ======================================================================= -->

	Some versions of GCC accept the following invalid code:

	<pre>
	template <typename T> struct Foo {
	void Work(T x) {
	func(x);
	}
	};
	...
	void func(int x);
	...
	template struct Foo<int>; // or anything else that instantiates Foo<int>::Work
	</pre>

	The standard says that unqualified names like <tt>func</tt> are looked up
	when the template is defined, not when it's instantiated. Since
	<tt>void func(int)</tt> was not declared yet when <tt>Foo</tt> was
	defined, it's not considered. The fix is usually to
	declare <tt>func</tt> before <tt>Foo</tt>.

	<p>This is complicated by <i>argument-dependent lookup</i> (ADL),
	which is done when unqualified names are called as functions,
	like <tt>func(x)</tt> above. The standard says that ADL is performed
	in both places if any of the arguments are type-dependent, like
	<tt>x</tt> is in this example. However, ADL does nothing for builtin
	types like <tt>int</tt>, so the example is still invalid. See
	[basic.lookup.argdep] for more information.

	<!-- ======================================================================= -->
	<h2 id="dep_lookup_bases">Unqualified lookup into dependent bases of class templates</h2>
	<!-- ======================================================================= -->

	Some versions of GCC accept the following invalid code:

	<pre>
	template <typename T> struct Base {
	void DoThis(T x) {}
	static void DoThat(T x) {}
	};

	template <typename T> struct Derived : public Base<T> {
	void Work(T x) {
	DoThis(x); // Invalid!
	DoThat(x); // Invalid!
	}
	};
	</pre>

	Clang correctly rejects it with the following errors
	(when <tt>Derived</tt> is eventually instantiated):

	<pre>
	my_file.cpp:8:5: error: use of undeclared identifier 'DoThis'
	DoThis(x);
	^
	this->
	my_file.cpp:2:8: note: must qualify identifier to find this declaration in dependent base class
	void DoThis(T x) {}
	^
	my_file.cpp:9:5: error: use of undeclared identifier 'DoThat'
	DoThat(x);
	^
	this->
	my_file.cpp:3:15: note: must qualify identifier to find this declaration in dependent base class
	static void DoThat(T x) {}
	</pre>

	Like we said <a href="#dep_lookup">above</a>, unqualified names like
	<tt>DoThis</tt> and <tt>DoThat</tt> are looked up when the template
	<tt>Derived</tt> is defined, not when it's instantiated. When we look
	up a name used in a class, we usually look into the base classes.
	However, we can't look into the base class <tt>Base<T></tt>
	because its type depends on the template argument <tt>T</tt>, so the
	standard says we should just ignore it. See [temp.dep]p3 for details.

	<p>The fix, as Clang tells you, is to tell the compiler that we want a
	class member by prefixing the calls with <tt>this-></tt>:

	<pre>
	void Work(T x) {
	<b>this-></b>DoThis(x);
	<b>this-></b>DoThat(x);
	}
	</pre>

	Alternatively, you can tell the compiler exactly where to look:

	<pre>
	void Work(T x) {
	<b>Base<T></b>::DoThis(x);
	<b>Base<T></b>::DoThat(x);
	}
	</pre>

	This works whether the methods are static or not, but be careful:
	if <tt>DoThis</tt> is virtual, calling it this way will bypass virtual
	dispatch!

	<!-- ======================================================================= -->
	<h2 id="bad_templates">Templates with no valid instantiations</h2>
	<!-- ======================================================================= -->

	The following code contains a typo: the programmer
	meant <tt>init()</tt> but wrote <tt>innit()</tt> instead.

	<pre>
	template <class T> class Processor {
	...
	void init();
	...
	};
	...
	template <class T> void process() {
	Processor<T> processor;
	processor.innit(); // <-- should be 'init()'
	...
	}
	</pre>

	Unfortunately, we can't flag this mistake as soon as we see it: inside
	a template, we're not allowed to make assumptions about "dependent
	types" like <tt>Processor<T></tt>. Suppose that later on in
	this file the programmer adds an explicit specialization
	of <tt>Processor</tt>, like so:

	<pre>
	template <> class Processor<char*> {
	void innit();
	};
	</pre>

	Now the program will work — as long as the programmer only ever
	instantiates <tt>process()</tt> with <tt>T = char*</tt>! This is why
	it's hard, and sometimes impossible, to diagnose mistakes in a
	template definition before it's instantiated.

	<p>The standard says that a template with no valid instantiations is
	ill-formed. Clang tries to do as much checking as possible at
	definition-time instead of instantiation-time: not only does this
	produce clearer diagnostics, but it also substantially improves
	compile times when using pre-compiled headers. The downside to this
	philosophy is that Clang sometimes fails to process files because they
	contain broken templates that are no longer used. The solution is
	simple: since the code is unused, just remove it.

	<!-- ======================================================================= -->
	<h2 id="default_init_const">Default initialization of const variable of a class type requires user-defined default constructor</h2>
	<!-- ======================================================================= -->

	If a <tt>class</tt> or <tt>struct</tt> has no user-defined default
	constructor, C++ doesn't allow you to default construct a <tt>const</tt>
	instance of it like this ([dcl.init], p9):

	<pre>
	class Foo {
	public:
	// The compiler-supplied default constructor works fine, so we
	// don't bother with defining one.
	...
	};

	void Bar() {
	const Foo foo; // Error!
	...
	}
	</pre>

	To fix this, you can define a default constructor for the class:

	<pre>
	class Foo {
	public:
	Foo() {}
	...
	};

	void Bar() {
	const Foo foo; // Now the compiler is happy.
	...
	}
	</pre>

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