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6 <title>A Few Coding Standards</title>
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8<body>
Chris Lattnerd80c43c2001-07-09 03:27:08 +00009
Misha Brukman75f258e2003-10-24 17:57:33 +000010<div class="doc_title">
11 A Few Coding Standards
12</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +000013
14<ol>
Misha Brukman75f258e2003-10-24 17:57:33 +000015 <li><a href="#introduction">Introduction</a></li>
Chris Lattner7ae36bb2001-07-23 20:40:41 +000016 <li><a href="#mechanicalissues">Mechanical Source Issues</a>
Chris Lattnerd80c43c2001-07-09 03:27:08 +000017 <ol>
18 <li><a href="#sourceformating">Source Code Formatting</a>
Misha Brukman75f258e2003-10-24 17:57:33 +000019 <ol>
20 <li><a href="#scf_commenting">Commenting</a></li>
21 <li><a href="#scf_commentformat">Comment Formatting</a></li>
Misha Brukman91d64af2004-10-26 16:18:43 +000022 <li><a href="#scf_includes"><tt>#include</tt> Style</a></li>
Misha Brukman75f258e2003-10-24 17:57:33 +000023 <li><a href="#scf_codewidth">Source Code Width</a></li>
24 <li><a href="#scf_spacestabs">Use Spaces Instead of Tabs</a></li>
25 <li><a href="#scf_indentation">Indent Code Consistently</a></li>
26 </ol></li>
Chris Lattnerd80c43c2001-07-09 03:27:08 +000027 <li><a href="#compilerissues">Compiler Issues</a>
28 <ol>
Misha Brukman75f258e2003-10-24 17:57:33 +000029 <li><a href="#ci_warningerrors">Treat Compiler Warnings Like
30 Errors</a></li>
Misha Brukman75f258e2003-10-24 17:57:33 +000031 <li><a href="#ci_portable_code">Write Portable Code</a></li>
Reid Spencerb40df562004-09-23 16:03:48 +000032 <li><a href="#ci_class_struct">Use of class/struct Keywords</a></li>
Misha Brukman75f258e2003-10-24 17:57:33 +000033 </ol></li>
34 </ol></li>
Chris Lattnerd80c43c2001-07-09 03:27:08 +000035 <li><a href="#styleissues">Style Issues</a>
36 <ol>
37 <li><a href="#macro">The High Level Issues</a>
38 <ol>
Misha Brukman75f258e2003-10-24 17:57:33 +000039 <li><a href="#hl_module">A Public Header File <b>is</b> a
40 Module</a></li>
41 <li><a href="#hl_dontinclude">#include as Little as Possible</a></li>
42 <li><a href="#hl_privateheaders">Keep "internal" Headers
43 Private</a></li>
44 </ol></li>
Chris Lattnerd80c43c2001-07-09 03:27:08 +000045 <li><a href="#micro">The Low Level Issues</a>
46 <ol>
Misha Brukman75f258e2003-10-24 17:57:33 +000047 <li><a href="#hl_assert">Assert Liberally</a></li>
Chris Lattnerb0bff9e2006-01-01 21:59:22 +000048 <li><a href="#hl_ns_std">Do not use 'using namespace std'</a></li>
Misha Brukman75f258e2003-10-24 17:57:33 +000049 <li><a href="#hl_preincrement">Prefer Preincrement</a></li>
Misha Brukman91d64af2004-10-26 16:18:43 +000050 <li><a href="#hl_avoidendl">Avoid <tt>std::endl</tt></a></li>
Misha Brukman75f258e2003-10-24 17:57:33 +000051 </ol></li>
Misha Brukman75f258e2003-10-24 17:57:33 +000052 </ol></li>
53 <li><a href="#seealso">See Also</a></li>
54</ol>
Chris Lattnerd80c43c2001-07-09 03:27:08 +000055
Chris Lattner999cf092004-05-23 21:05:07 +000056<div class="doc_author">
57 <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p>
58</div>
59
Chris Lattnerd80c43c2001-07-09 03:27:08 +000060
61<!-- *********************************************************************** -->
Misha Brukman75f258e2003-10-24 17:57:33 +000062<div class="doc_section">
63 <a name="introduction">Introduction</a>
64</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +000065<!-- *********************************************************************** -->
66
Misha Brukman75f258e2003-10-24 17:57:33 +000067<div class="doc_text">
68
69<p>This document attempts to describe a few coding standards that are being used
70in the LLVM source tree. Although no coding standards should be regarded as
Misha Brukman1b36d672003-10-06 19:26:00 +000071absolute requirements to be followed in all instances, coding standards can be
Misha Brukman75f258e2003-10-24 17:57:33 +000072useful.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +000073
Misha Brukman75f258e2003-10-24 17:57:33 +000074<p>This document intentionally does not prescribe fixed standards for religious
Misha Brukman1b36d672003-10-06 19:26:00 +000075issues such as brace placement and space usage. For issues like this, follow
Misha Brukman75f258e2003-10-24 17:57:33 +000076the golden rule:</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +000077
Misha Brukman75f258e2003-10-24 17:57:33 +000078<blockquote>
Chris Lattnerd80c43c2001-07-09 03:27:08 +000079
Misha Brukman75f258e2003-10-24 17:57:33 +000080<p><b><a name="goldenrule">If you are adding a significant body of source to a
81project, feel free to use whatever style you are most comfortable with. If you
82are extending, enhancing, or bug fixing already implemented code, use the style
83that is already being used so that the source is uniform and easy to
84follow.</a></b></p>
85
86</blockquote>
87
88<p>The ultimate goal of these guidelines is the increase readability and
Misha Brukman1b36d672003-10-06 19:26:00 +000089maintainability of our common source base. If you have suggestions for topics to
Misha Brukman75f258e2003-10-24 17:57:33 +000090be included, please mail them to <a
91href="mailto:sabre@nondot.org">Chris</a>.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +000092
Misha Brukman75f258e2003-10-24 17:57:33 +000093</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +000094
95<!-- *********************************************************************** -->
Misha Brukman75f258e2003-10-24 17:57:33 +000096<div class="doc_section">
97 <a name="mechanicalissues">Mechanical Source Issues</a>
98</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +000099<!-- *********************************************************************** -->
100
101<!-- ======================================================================= -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000102<div class="doc_subsection">
103 <a name="sourceformating">Source Code Formatting</a>
104</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000105
106<!-- _______________________________________________________________________ -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000107<div class="doc_subsubsection">
108 <a name="scf_commenting">Commenting</a>
109</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000110
Misha Brukman75f258e2003-10-24 17:57:33 +0000111<div class="doc_text">
112
113<p>Comments are one critical part of readability and maintainability. Everyone
Misha Brukman3b53a262004-12-04 00:32:12 +0000114knows they should comment, so should you. Although we all should probably
Misha Brukman1b36d672003-10-06 19:26:00 +0000115comment our code more than we do, there are a few very critical places that
Misha Brukman75f258e2003-10-24 17:57:33 +0000116documentation is very useful:</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000117
Chris Lattner999cf092004-05-23 21:05:07 +0000118<b>File Headers</b>
Misha Brukman75f258e2003-10-24 17:57:33 +0000119
120<p>Every source file should have a header on it that
121describes the basic purpose of the file. If a file does not have a header, it
122should not be checked into CVS. Most source trees will probably have a standard
123file header format. The standard format for the LLVM source tree looks like
124this:</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000125
Misha Brukmana8c760a2004-07-28 22:31:54 +0000126<div class="doc_code">
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000127<pre>
Chris Lattnerc49fcd12003-10-13 14:58:11 +0000128//===-- llvm/Instruction.h - Instruction class definition -------*- C++ -*-===//
Chris Lattner999cf092004-05-23 21:05:07 +0000129//
130// The LLVM Compiler Infrastructure
131//
132// This file was developed by the LLVM research group and is distributed under
133// the University of Illinois Open Source License. See LICENSE.TXT for details.
134//
135//===----------------------------------------------------------------------===//
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000136//
137// This file contains the declaration of the Instruction class, which is the
138// base class for all of the VM instructions.
139//
140//===----------------------------------------------------------------------===//
141</pre>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000142</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000143
Chris Lattner999cf092004-05-23 21:05:07 +0000144<p>A few things to note about this particular format: The "<tt>-*- C++
Misha Brukman75f258e2003-10-24 17:57:33 +0000145-*-</tt>" string on the first line is there to tell Emacs that the source file
Misha Brukman69c58892004-07-28 22:37:57 +0000146is a C++ file, not a C file (Emacs assumes .h files are C files by default).
147Note that this tag is not necessary in .cpp files. The name of the file is also
148on the first line, along with a very short description of the purpose of the
149file. This is important when printing out code and flipping though lots of
150pages.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000151
Chris Lattner999cf092004-05-23 21:05:07 +0000152<p>The next section in the file is a concise note that defines the license that
153the file is released under. This makes it perfectly clear what terms the source
154code can be distributed under.</p>
155
Misha Brukman75f258e2003-10-24 17:57:33 +0000156<p>The main body of the description does not have to be very long in most cases.
Misha Brukman1b36d672003-10-06 19:26:00 +0000157Here it's only two lines. If an algorithm is being implemented or something
158tricky is going on, a reference to the paper where it is published should be
Misha Brukman75f258e2003-10-24 17:57:33 +0000159included, as well as any notes or "gotchas" in the code to watch out for.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000160
Chris Lattner999cf092004-05-23 21:05:07 +0000161<b>Class overviews</b>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000162
Misha Brukman75f258e2003-10-24 17:57:33 +0000163<p>Classes are one fundemental part of a good object oriented design. As such,
164a class definition should have a comment block that explains what the class is
Misha Brukman1b36d672003-10-06 19:26:00 +0000165used for... if it's not obvious. If it's so completely obvious your grandma
166could figure it out, it's probably safe to leave it out. Naming classes
Misha Brukman3b53a262004-12-04 00:32:12 +0000167something sane goes a long ways towards avoiding writing documentation.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000168
169
Chris Lattner999cf092004-05-23 21:05:07 +0000170<b>Method information</b>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000171
Misha Brukman75f258e2003-10-24 17:57:33 +0000172<p>Methods defined in a class (as well as any global functions) should also be
Misha Brukman1b36d672003-10-06 19:26:00 +0000173documented properly. A quick note about what it does any a description of the
174borderline behaviour is all that is necessary here (unless something
175particularly tricky or insideous is going on). The hope is that people can
176figure out how to use your interfaces without reading the code itself... that is
Misha Brukman75f258e2003-10-24 17:57:33 +0000177the goal metric.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000178
Misha Brukman75f258e2003-10-24 17:57:33 +0000179<p>Good things to talk about here are what happens when something unexpected
180happens: does the method return null? Abort? Format your hard disk?</p>
181
Misha Brukman75f258e2003-10-24 17:57:33 +0000182</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000183
184<!-- _______________________________________________________________________ -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000185<div class="doc_subsubsection">
186 <a name="scf_commentformat">Comment Formatting</a>
187</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000188
Misha Brukman75f258e2003-10-24 17:57:33 +0000189<div class="doc_text">
190
191<p>In general, prefer C++ style (<tt>//</tt>) comments. They take less space,
Misha Brukman1b36d672003-10-06 19:26:00 +0000192require less typing, don't have nesting problems, etc. There are a few cases
Misha Brukman75f258e2003-10-24 17:57:33 +0000193when it is useful to use C style (<tt>/* */</tt>) comments however:</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000194
195<ol>
Misha Brukman75f258e2003-10-24 17:57:33 +0000196 <li>When writing a C code: Obviously if you are writing C code, use C style
Misha Brukman3b53a262004-12-04 00:32:12 +0000197 comments.</li>
Misha Brukman91d64af2004-10-26 16:18:43 +0000198 <li>When writing a header file that may be <tt>#include</tt>d by a C source
199 file.</li>
Misha Brukman75f258e2003-10-24 17:57:33 +0000200 <li>When writing a source file that is used by a tool that only accepts C
201 style comments.</li>
202</ol>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000203
Misha Brukman75f258e2003-10-24 17:57:33 +0000204<p>To comment out a large block of code, use <tt>#if 0</tt> and <tt>#endif</tt>.
205These nest properly and are better behaved in general than C style comments.</p>
206
207</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000208
Chris Lattner2e5cbc22003-08-07 21:45:47 +0000209<!-- _______________________________________________________________________ -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000210<div class="doc_subsubsection">
Misha Brukman91d64af2004-10-26 16:18:43 +0000211 <a name="scf_includes"><tt>#include</tt> Style</a>
Misha Brukman75f258e2003-10-24 17:57:33 +0000212</div>
Chris Lattner2e5cbc22003-08-07 21:45:47 +0000213
Misha Brukman75f258e2003-10-24 17:57:33 +0000214<div class="doc_text">
215
216<p>Immediately after the <a href="#scf_commenting">header file comment</a> (and
Chris Lattner2e5cbc22003-08-07 21:45:47 +0000217include guards if working on a header file), the <a
Misha Brukmana8c760a2004-07-28 22:31:54 +0000218href="#hl_dontinclude">minimal</a> list of <tt>#include</tt>s required by the
219file should be listed. We prefer these <tt>#include</tt>s to be listed in this
220order:</p>
Chris Lattner2e5cbc22003-08-07 21:45:47 +0000221
222<ol>
Misha Brukman75f258e2003-10-24 17:57:33 +0000223 <li><a href="#mmheader">Main Module header</a></li>
224 <li><a href="#hl_privateheaders">Local/Private Headers</a></li>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000225 <li><tt>llvm/*</tt></li>
226 <li><tt>llvm/Analysis/*</tt></li>
227 <li><tt>llvm/Assembly/*</tt></li>
228 <li><tt>llvm/Bytecode/*</tt></li>
229 <li><tt>llvm/CodeGen/*</tt></li>
Misha Brukman75f258e2003-10-24 17:57:33 +0000230 <li>...</li>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000231 <li><tt>Support/*</tt></li>
232 <li><tt>Config/*</tt></li>
233 <li>System <tt>#includes</tt></li>
Chris Lattner2e5cbc22003-08-07 21:45:47 +0000234</ol>
235
Misha Brukman75f258e2003-10-24 17:57:33 +0000236<p>... and each catagory should be sorted by name.</p>
Chris Lattner2e5cbc22003-08-07 21:45:47 +0000237
Misha Brukman75f258e2003-10-24 17:57:33 +0000238<p><a name="mmheader">The "Main Module Header"</a> file applies to .cpp file
Misha Brukman91d64af2004-10-26 16:18:43 +0000239which implement an interface defined by a .h file. This <tt>#include</tt>
240should always be included <b>first</b> regardless of where it lives on the file
241system. By including a header file first in the .cpp files that implement the
242interfaces, we ensure that the header does not have any hidden dependencies
243which are not explicitly #included in the header, but should be. It is also a
244form of documentation in the .cpp file to indicate where the interfaces it
245implements are defined.</p>
Chris Lattner2e5cbc22003-08-07 21:45:47 +0000246
Misha Brukman75f258e2003-10-24 17:57:33 +0000247</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000248
249<!-- _______________________________________________________________________ -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000250<div class="doc_subsubsection">
251 <a name="scf_codewidth">Source Code Width</a>
252</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000253
Misha Brukman75f258e2003-10-24 17:57:33 +0000254<div class="doc_text">
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000255
Misha Brukman75f258e2003-10-24 17:57:33 +0000256<p>Write your code to fit within 80 columns of text. This helps those of us who
257like to print out code and look at your code in an xterm without resizing
258it.</p>
259
260</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000261
262<!-- _______________________________________________________________________ -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000263<div class="doc_subsubsection">
264 <a name="scf_spacestabs">Use Spaces Instead of Tabs</a>
265</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000266
Misha Brukman75f258e2003-10-24 17:57:33 +0000267<div class="doc_text">
268
269<p>In all cases, prefer spaces to tabs in source files. People have different
Misha Brukman1b36d672003-10-06 19:26:00 +0000270prefered indentation levels, and different styles of indentation that they
271like... this is fine. What isn't is that different editors/viewers expand tabs
272out to different tab stops. This can cause your code to look completely
Misha Brukman75f258e2003-10-24 17:57:33 +0000273unreadable, and it is not worth dealing with.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000274
Misha Brukman75f258e2003-10-24 17:57:33 +0000275<p>As always, follow the <a href="#goldenrule">Golden Rule</a> above: follow the
Misha Brukman1b36d672003-10-06 19:26:00 +0000276style of existing code if your are modifying and extending it. If you like four
277spaces of indentation, <b>DO NOT</b> do that in the middle of a chunk of code
278with two spaces of indentation. Also, do not reindent a whole source file: it
Misha Brukman75f258e2003-10-24 17:57:33 +0000279makes for incredible diffs that are absolutely worthless.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000280
Misha Brukman75f258e2003-10-24 17:57:33 +0000281</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000282
283<!-- _______________________________________________________________________ -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000284<div class="doc_subsubsection">
285 <a name="scf_indentation">Indent Code Consistently</a>
286</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000287
Misha Brukman75f258e2003-10-24 17:57:33 +0000288<div class="doc_text">
289
290<p>Okay, your first year of programming you were told that indentation is
Misha Brukman1b36d672003-10-06 19:26:00 +0000291important. If you didn't believe and internalize this then, now is the time.
Misha Brukman75f258e2003-10-24 17:57:33 +0000292Just do it.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000293
Misha Brukman75f258e2003-10-24 17:57:33 +0000294</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000295
296
297<!-- ======================================================================= -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000298<div class="doc_subsection">
299 <a name="compilerissues">Compiler Issues</a>
300</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000301
302
303<!-- _______________________________________________________________________ -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000304<div class="doc_subsubsection">
305 <a name="ci_warningerrors">Treat Compiler Warnings Like Errors</a>
306</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000307
Misha Brukman75f258e2003-10-24 17:57:33 +0000308<div class="doc_text">
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000309
Misha Brukman75f258e2003-10-24 17:57:33 +0000310<p>If your code has compiler warnings in it, something is wrong: you aren't
311casting values correctly, your have "questionable" constructs in your code, or
312you are doing something legitimately wrong. Compiler warnings can cover up
313legitimate errors in output and make dealing with a translation unit
314difficult.</p>
315
316<p>It is not possible to prevent all warnings from all compilers, nor is it
Misha Brukman1b36d672003-10-06 19:26:00 +0000317desirable. Instead, pick a standard compiler (like <tt>gcc</tt>) that provides
318a good thorough set of warnings, and stick to them. At least in the case of
319<tt>gcc</tt>, it is possible to work around any spurious errors by changing the
320syntax of the code slightly. For example, an warning that annoys me occurs when
Misha Brukman75f258e2003-10-24 17:57:33 +0000321I write code like this:</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000322
Misha Brukmana8c760a2004-07-28 22:31:54 +0000323<div class="doc_code">
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000324<pre>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000325if (V = getValue()) {
326 ...
327}
Misha Brukman75f258e2003-10-24 17:57:33 +0000328</pre>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000329</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000330
Misha Brukman75f258e2003-10-24 17:57:33 +0000331<p><tt>gcc</tt> will warn me that I probably want to use the <tt>==</tt>
332operator, and that I probably mistyped it. In most cases, I haven't, and I
333really don't want the spurious errors. To fix this particular problem, I
334rewrite the code like this:</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000335
Misha Brukmana8c760a2004-07-28 22:31:54 +0000336<div class="doc_code">
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000337<pre>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000338if ((V = getValue())) {
339 ...
340}
Misha Brukman75f258e2003-10-24 17:57:33 +0000341</pre>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000342</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000343
Misha Brukman75f258e2003-10-24 17:57:33 +0000344<p>...which shuts <tt>gcc</tt> up. Any <tt>gcc</tt> warning that annoys you can
345be fixed by massaging the code appropriately.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000346
Misha Brukman75f258e2003-10-24 17:57:33 +0000347<p>These are the <tt>gcc</tt> warnings that I prefer to enable: <tt>-Wall
348-Winline -W -Wwrite-strings -Wno-unused</tt></p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000349
Misha Brukman75f258e2003-10-24 17:57:33 +0000350</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000351
352<!-- _______________________________________________________________________ -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000353<div class="doc_subsubsection">
Misha Brukman75f258e2003-10-24 17:57:33 +0000354 <a name="ci_portable_code">Write Portable Code</a>
355</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000356
Misha Brukman75f258e2003-10-24 17:57:33 +0000357<div class="doc_text">
358
359<p>In almost all cases, it is possible and within reason to write completely
Misha Brukman2d10d752003-07-28 21:57:18 +0000360portable code. If there are cases where it isn't possible to write portable
Misha Brukman75f258e2003-10-24 17:57:33 +0000361code, isolate it behind a well defined (and well documented) interface.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000362
Misha Brukman75f258e2003-10-24 17:57:33 +0000363<p>In practice, this means that you shouldn't assume much about the host
364compiler, including its support for "high tech" features like partial
365specialization of templates. In fact, Visual C++ 6 could be an important target
366for our work in the future, and we don't want to have to rewrite all of our code
367to support it.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000368
Misha Brukman75f258e2003-10-24 17:57:33 +0000369</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000370
Reid Spencerb40df562004-09-23 16:03:48 +0000371<!-- _______________________________________________________________________ -->
372<div class="doc_subsubsection">
373<a name="ci_class_struct">Use of <tt>class</tt> and <tt>struct</tt> Keywords</a>
374</div>
375<div class="doc_text">
Misha Brukmanedcc78a2004-10-26 15:45:13 +0000376
377<p>In C++, the <tt>class</tt> and <tt>struct</tt> keywords can be used almost
378interchangeably. The only difference is when they are used to declare a class:
379<tt>class</tt> makes all members private by default while <tt>struct</tt> makes
380all members public by default.</p>
381
382<p>Unfortunately, not all compilers follow the rules and some will generate
383different symbols based on whether <tt>class</tt> or <tt>struct</tt> was used to
384declare the symbol. This can lead to problems at link time.</p>
385
386<p>So, the rule for LLVM is to always use the <tt>class</tt> keyword, unless
387<b>all</b> members are public, in which case <tt>struct</tt> is allowed.</p>
388
Reid Spencerb40df562004-09-23 16:03:48 +0000389</div>
390
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000391<!-- *********************************************************************** -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000392<div class="doc_section">
393 <a name="styleissues">Style Issues</a>
394</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000395<!-- *********************************************************************** -->
396
397
398<!-- ======================================================================= -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000399<div class="doc_subsection">
400 <a name="macro">The High Level Issues</a>
401</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000402
403
404<!-- _______________________________________________________________________ -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000405<div class="doc_subsubsection">
406 <a name="hl_module">A Public Header File <b>is</b> a Module</a>
407</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000408
Misha Brukman75f258e2003-10-24 17:57:33 +0000409<div class="doc_text">
410
411<p>C++ doesn't do too well in the modularity department. There is no real
Misha Brukman2d10d752003-07-28 21:57:18 +0000412encapsulation or data hiding (unless you use expensive protocol classes), but it
413is what we have to work with. When you write a public header file (in the LLVM
414source tree, they live in the top level "include" directory), you are defining a
Misha Brukman75f258e2003-10-24 17:57:33 +0000415module of functionality.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000416
Misha Brukman75f258e2003-10-24 17:57:33 +0000417<p>Ideally, modules should be completely independent of each other, and their
Misha Brukman2d10d752003-07-28 21:57:18 +0000418header files should only include the absolute minimum number of headers
419possible. A module is not just a class, a function, or a namespace: <a
420href="http://www.cuj.com/articles/2000/0002/0002c/0002c.htm">it's a collection
421of these</a> that defines an interface. This interface may be several
422functions, classes or data structures, but the important issue is how they work
Misha Brukman75f258e2003-10-24 17:57:33 +0000423together.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000424
Misha Brukman75f258e2003-10-24 17:57:33 +0000425<p>In general, a module should be implemented with one or more <tt>.cpp</tt>
426files. Each of these <tt>.cpp</tt> files should include the header that defines
427their interface first. This ensure that all of the dependences of the module
428header have been properly added to the module header itself, and are not
429implicit. System headers should be included after user headers for a
430translation unit.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000431
Misha Brukman75f258e2003-10-24 17:57:33 +0000432</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000433
434<!-- _______________________________________________________________________ -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000435<div class="doc_subsubsection">
Misha Brukman91d64af2004-10-26 16:18:43 +0000436 <a name="hl_dontinclude"><tt>#include</tt> as Little as Possible</a>
Misha Brukman75f258e2003-10-24 17:57:33 +0000437</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000438
Misha Brukman75f258e2003-10-24 17:57:33 +0000439<div class="doc_text">
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000440
Misha Brukman75f258e2003-10-24 17:57:33 +0000441<p><tt>#include</tt> hurts compile time performance. Don't do it unless you
442have to, especially in header files.</p>
443
444<p>But wait, sometimes you need to have the definition of a class to use it, or
Misha Brukman91d64af2004-10-26 16:18:43 +0000445to inherit from it. In these cases go ahead and <tt>#include</tt> that header
446file. Be aware however that there are many cases where you don't need to have
447the full definition of a class. If you are using a pointer or reference to a
448class, you don't need the header file. If you are simply returning a class
449instance from a prototyped function or method, you don't need it. In fact, for
450most cases, you simply don't need the definition of a class... and not
451<tt>#include</tt>'ing speeds up compilation.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000452
Misha Brukman75f258e2003-10-24 17:57:33 +0000453<p>It is easy to try to go too overboard on this recommendation, however. You
Misha Brukman2d10d752003-07-28 21:57:18 +0000454<b>must</b> include all of the header files that you are using, either directly
455or indirectly (through another header file). To make sure that you don't
456accidently forget to include a header file in your module header, make sure to
457include your module header <b>first</b> in the implementation file (as mentioned
458above). This way there won't be any hidden dependencies that you'll find out
Misha Brukman75f258e2003-10-24 17:57:33 +0000459about later...</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000460
Misha Brukman75f258e2003-10-24 17:57:33 +0000461</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000462
463<!-- _______________________________________________________________________ -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000464<div class="doc_subsubsection">
465 <a name="hl_privateheaders">Keep "internal" Headers Private</a>
466</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000467
Misha Brukman75f258e2003-10-24 17:57:33 +0000468<div class="doc_text">
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000469
Misha Brukman75f258e2003-10-24 17:57:33 +0000470<p>Many modules have a complex implementation that causes them to use more than
471one implementation (<tt>.cpp</tt>) file. It is often tempting to put the
472internal communication interface (helper classes, extra functions, etc) in the
Misha Brukman3b53a262004-12-04 00:32:12 +0000473public module header file. Don't do this.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000474
Misha Brukman75f258e2003-10-24 17:57:33 +0000475<p>If you really need to do something like this, put a private header file in
476the same directory as the source files, and include it locally. This ensures
477that your private interface remains private and undisturbed by outsiders.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000478
Misha Brukman75f258e2003-10-24 17:57:33 +0000479<p>Note however, that it's okay to put extra implementation methods a public
480class itself... just make them private (or protected), and all is well.</p>
481
482</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000483
484<!-- ======================================================================= -->
Chris Lattner999cf092004-05-23 21:05:07 +0000485<div class="doc_subsection">
Misha Brukman75f258e2003-10-24 17:57:33 +0000486 <a name="micro">The Low Level Issues</a>
487</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000488
489
490<!-- _______________________________________________________________________ -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000491<div class="doc_subsubsection">
492 <a name="hl_assert">Assert Liberally</a>
493</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000494
Misha Brukman75f258e2003-10-24 17:57:33 +0000495<div class="doc_text">
496
497<p>Use the "<tt>assert</tt>" function to its fullest. Check all of your
Misha Brukman2d10d752003-07-28 21:57:18 +0000498preconditions and assumptions, you never know when a bug (not neccesarily even
499yours) might be caught early by an assertion, which reduces debugging time
500dramatically. The "<tt>&lt;cassert&gt;</tt>" header file is probably already
501included by the header files you are using, so it doesn't cost anything to use
Misha Brukman75f258e2003-10-24 17:57:33 +0000502it.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000503
Misha Brukman75f258e2003-10-24 17:57:33 +0000504<p>To further assist with debugging, make sure to put some kind of error message
505in the assertion statement (which is printed if the assertion is tripped). This
Misha Brukman2d10d752003-07-28 21:57:18 +0000506helps the poor debugging make sense of why an assertion is being made and
Misha Brukman75f258e2003-10-24 17:57:33 +0000507enforced, and hopefully what to do about it. Here is one complete example:</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000508
Misha Brukmana8c760a2004-07-28 22:31:54 +0000509<div class="doc_code">
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000510<pre>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000511inline Value *getOperand(unsigned i) {
512 assert(i &lt; Operands.size() &amp;&amp; "getOperand() out of range!");
513 return Operands[i];
514}
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000515</pre>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000516</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000517
Misha Brukman75f258e2003-10-24 17:57:33 +0000518<p>Here are some examples:</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000519
Misha Brukmana8c760a2004-07-28 22:31:54 +0000520<div class="doc_code">
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000521<pre>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000522assert(Ty-&gt;isPointerType() &amp;&amp; "Can't allocate a non pointer type!");
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000523
Misha Brukmana8c760a2004-07-28 22:31:54 +0000524assert((Opcode == Shl || Opcode == Shr) &amp;&amp; "ShiftInst Opcode invalid!");
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000525
Misha Brukmana8c760a2004-07-28 22:31:54 +0000526assert(idx &lt; getNumSuccessors() &amp;&amp; "Successor # out of range!");
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000527
Misha Brukmana8c760a2004-07-28 22:31:54 +0000528assert(V1.getType() == V2.getType() &amp;&amp; "Constant types must be identical!");
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000529
Misha Brukmana8c760a2004-07-28 22:31:54 +0000530assert(isa&lt;PHINode&gt;(Succ-&gt;front()) &amp;&amp; "Only works on PHId BBs!");
Misha Brukman75f258e2003-10-24 17:57:33 +0000531</pre>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000532</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000533
Misha Brukman75f258e2003-10-24 17:57:33 +0000534<p>You get the idea...</p>
535
536</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000537
Chris Lattnerb0bff9e2006-01-01 21:59:22 +0000538<!-- _______________________________________________________________________ -->
539<div class="doc_subsubsection">
540 <a name="hl_ns_std">Do not use 'using namespace std'</a>
541</div>
542
543<div class="doc_text">
544<p>In LLVM, we prefer to explicitly prefix all identifiers from the standard
545namespace with an "std::" prefix, rather than rely on "using namespace std;".
546</p>
547
548<p> In header files, adding a 'using namespace XXX' directive pollutes the
549namespace of any source file that includes the header. This is clearly a bad
550thing.</p>
551
552<p>In implementation files (e.g. .cpp files) the rule is more of a stylistic
553rule, but is still important. Basically, using explicit namespace prefixes
554makes
555the code <b>more clear</b> - because it is immediately obvious what facilities
556are being used and where they are coming from - and <b>more portable</b> -
557because namespace clashes cannot occur between LLVM code and other namespaces.
558The portability rule is important because different standard library
559implementations expose different symbols (potentially ones they shouldn't) and
560future revisions to the C++ standard will add more symbols to the std
561namespace. As such, we never 'using namespace std;' in LLVM.</p>
562
563<p>The exception to the general rule (i.e. it's not an exception for the std
564namespace) is for implementation files. For example, all of the code in the
565LLVM project implements code that lives in the 'llvm' namespace. As such, it
566is ok, and actually more clear, for the .cpp files to have a 'using namespace
567llvm' directive at their top, after the #includes. The general form of this
568rule is that any .cpp file that implements code in any namespace may use that
569namespace (and its parents), but should not use any others.</p>
570
571</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000572
573<!-- _______________________________________________________________________ -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000574<div class="doc_subsubsection">
575 <a name="hl_preincrement">Prefer Preincrement</a>
576</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000577
Misha Brukman75f258e2003-10-24 17:57:33 +0000578<div class="doc_text">
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000579
Misha Brukmana8c760a2004-07-28 22:31:54 +0000580<p>Hard fast rule: Preincrement (<tt>++X</tt>) may be no slower than
581postincrement (<tt>X++</tt>) and could very well be a lot faster than it. Use
582preincrementation whenever possible.</p>
Misha Brukman75f258e2003-10-24 17:57:33 +0000583
584<p>The semantics of postincrement include making a copy of the value being
Misha Brukman2d10d752003-07-28 21:57:18 +0000585incremented, returning it, and then preincrementing the "work value". For
586primitive types, this isn't a big deal... but for iterators, it can be a huge
587issue (for example, some iterators contains stack and set objects in them...
588copying an iterator could invoke the copy ctor's of these as well). In general,
Misha Brukman75f258e2003-10-24 17:57:33 +0000589get in the habit of always using preincrement, and you won't have a problem.</p>
590
591</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000592
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000593<!-- _______________________________________________________________________ -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000594<div class="doc_subsubsection">
Misha Brukman91d64af2004-10-26 16:18:43 +0000595 <a name="hl_avoidendl">Avoid <tt>std::endl</tt></a>
Misha Brukman75f258e2003-10-24 17:57:33 +0000596</div>
Chris Lattner13c46592002-01-20 19:01:26 +0000597
Misha Brukman75f258e2003-10-24 17:57:33 +0000598<div class="doc_text">
599
Chris Lattner999cf092004-05-23 21:05:07 +0000600<p>The <tt>std::endl</tt> modifier, when used with iostreams outputs a newline
601to the output stream specified. In addition to doing this, however, it also
602flushes the output stream. In other words, these are equivalent:</p>
Chris Lattner13c46592002-01-20 19:01:26 +0000603
Misha Brukmana8c760a2004-07-28 22:31:54 +0000604<div class="doc_code">
Chris Lattner13c46592002-01-20 19:01:26 +0000605<pre>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000606std::cout &lt;&lt; std::endl;
607std::cout &lt;&lt; '\n' &lt;&lt; std::flush;
Chris Lattner13c46592002-01-20 19:01:26 +0000608</pre>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000609</div>
Chris Lattner13c46592002-01-20 19:01:26 +0000610
Misha Brukman75f258e2003-10-24 17:57:33 +0000611<p>Most of the time, you probably have no reason to flush the output stream, so
Misha Brukmana8c760a2004-07-28 22:31:54 +0000612it's better to use a literal <tt>'\n'</tt>.</p>
Chris Lattner13c46592002-01-20 19:01:26 +0000613
Misha Brukman75f258e2003-10-24 17:57:33 +0000614</div>
Chris Lattner13c46592002-01-20 19:01:26 +0000615
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000616<!-- *********************************************************************** -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000617<div class="doc_section">
618 <a name="seealso">See Also</a>
619</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000620<!-- *********************************************************************** -->
621
Misha Brukman75f258e2003-10-24 17:57:33 +0000622<div class="doc_text">
623
624<p>A lot of these comments and recommendations have been culled for other
625sources. Two particularly important books for our work are:</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000626
627<ol>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000628
Tanya Lattner09cf73c2004-06-22 04:24:55 +0000629<li><a href="http://www.aw-bc.com/catalog/academic/product/0,1144,0201310155,00.html">Effective
Misha Brukman75f258e2003-10-24 17:57:33 +0000630C++</a> by Scott Meyers. There is an online version of the book (only some
631chapters though) <a
Chris Lattner999cf092004-05-23 21:05:07 +0000632href="http://www.awlonline.com/cseng/meyerscddemo/">available as well</a>. Also
633interesting and useful are "More Effective C++" and "Effective STL" by the same
634author.</li>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000635
Misha Brukman75f258e2003-10-24 17:57:33 +0000636<li><a href="http://cseng.aw.com/book/0,3828,0201633620,00.html">Large-Scale C++
637Software Design</a> by John Lakos</li>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000638
Misha Brukman75f258e2003-10-24 17:57:33 +0000639</ol>
640
641<p>If you get some free time, and you haven't read them: do so, you might learn
Misha Brukman3b53a262004-12-04 00:32:12 +0000642something.</p>
Misha Brukman75f258e2003-10-24 17:57:33 +0000643
644</div>
645
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000646<!-- *********************************************************************** -->
647
648<hr>
Misha Brukman6c16c4b2004-05-12 18:37:22 +0000649<address>
650 <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
651 src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a>
652 <a href="http://validator.w3.org/check/referer"><img
653 src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01!"></a>
Misha Brukman75f258e2003-10-24 17:57:33 +0000654
Misha Brukman6c16c4b2004-05-12 18:37:22 +0000655 <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
656 <a href="http://llvm.cs.uiuc.edu">LLVM Compiler Infrastructure</a><br>
Misha Brukman75f258e2003-10-24 17:57:33 +0000657 Last modified: $Date$
Misha Brukman6c16c4b2004-05-12 18:37:22 +0000658</address>
Misha Brukman75f258e2003-10-24 17:57:33 +0000659
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