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6 <title>A Few Coding Standards</title>
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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>
22 <li><a href="#scf_includes">#include Style</a></li>
23 <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>
48 <li><a href="#hl_preincrement">Prefer Preincrement</a></li>
Chris Lattner999cf092004-05-23 21:05:07 +000049 <li><a href="#hl_avoidendl">Avoid std::endl</a></li>
Misha Brukman75f258e2003-10-24 17:57:33 +000050 <li><a href="#hl_exploitcpp">Exploit C++ to its Fullest</a></li>
51 </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 Brukman1b36d672003-10-06 19:26:00 +0000114knows they should comment, so should you. :) Although we all should probably
115comment 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 Brukman75f258e2003-10-24 17:57:33 +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
197 comments. :)</li>
198 <li>When writing a header file that may be #included by a C source file.</li>
199 <li>When writing a source file that is used by a tool that only accepts C
200 style comments.</li>
201</ol>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000202
Misha Brukman75f258e2003-10-24 17:57:33 +0000203<p>To comment out a large block of code, use <tt>#if 0</tt> and <tt>#endif</tt>.
204These nest properly and are better behaved in general than C style comments.</p>
205
206</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000207
Chris Lattner2e5cbc22003-08-07 21:45:47 +0000208<!-- _______________________________________________________________________ -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000209<div class="doc_subsubsection">
210 <a name="scf_includes">#include Style</a>
211</div>
Chris Lattner2e5cbc22003-08-07 21:45:47 +0000212
Misha Brukman75f258e2003-10-24 17:57:33 +0000213<div class="doc_text">
214
215<p>Immediately after the <a href="#scf_commenting">header file comment</a> (and
Chris Lattner2e5cbc22003-08-07 21:45:47 +0000216include guards if working on a header file), the <a
Misha Brukmana8c760a2004-07-28 22:31:54 +0000217href="#hl_dontinclude">minimal</a> list of <tt>#include</tt>s required by the
218file should be listed. We prefer these <tt>#include</tt>s to be listed in this
219order:</p>
Chris Lattner2e5cbc22003-08-07 21:45:47 +0000220
221<ol>
Misha Brukman75f258e2003-10-24 17:57:33 +0000222 <li><a href="#mmheader">Main Module header</a></li>
223 <li><a href="#hl_privateheaders">Local/Private Headers</a></li>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000224 <li><tt>llvm/*</tt></li>
225 <li><tt>llvm/Analysis/*</tt></li>
226 <li><tt>llvm/Assembly/*</tt></li>
227 <li><tt>llvm/Bytecode/*</tt></li>
228 <li><tt>llvm/CodeGen/*</tt></li>
Misha Brukman75f258e2003-10-24 17:57:33 +0000229 <li>...</li>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000230 <li><tt>Support/*</tt></li>
231 <li><tt>Config/*</tt></li>
232 <li>System <tt>#includes</tt></li>
Chris Lattner2e5cbc22003-08-07 21:45:47 +0000233</ol>
234
Misha Brukman75f258e2003-10-24 17:57:33 +0000235<p>... and each catagory should be sorted by name.</p>
Chris Lattner2e5cbc22003-08-07 21:45:47 +0000236
Misha Brukman75f258e2003-10-24 17:57:33 +0000237<p><a name="mmheader">The "Main Module Header"</a> file applies to .cpp file
238which implement an interface defined by a .h file. This #include should always
239be included <b>first</b> regardless of where it lives on the file system. By
Chris Lattner2e5cbc22003-08-07 21:45:47 +0000240including a header file first in the .cpp files that implement the interfaces,
241we ensure that the header does not have any hidden dependencies which are not
242explicitly #included in the header, but should be. It is also a form of
243documentation in the .cpp file to indicate where the interfaces it implements
Misha Brukman75f258e2003-10-24 17:57:33 +0000244are defined.</p>
Chris Lattner2e5cbc22003-08-07 21:45:47 +0000245
Misha Brukman75f258e2003-10-24 17:57:33 +0000246</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000247
248<!-- _______________________________________________________________________ -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000249<div class="doc_subsubsection">
250 <a name="scf_codewidth">Source Code Width</a>
251</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000252
Misha Brukman75f258e2003-10-24 17:57:33 +0000253<div class="doc_text">
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000254
Misha Brukman75f258e2003-10-24 17:57:33 +0000255<p>Write your code to fit within 80 columns of text. This helps those of us who
256like to print out code and look at your code in an xterm without resizing
257it.</p>
258
259</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000260
261<!-- _______________________________________________________________________ -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000262<div class="doc_subsubsection">
263 <a name="scf_spacestabs">Use Spaces Instead of Tabs</a>
264</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000265
Misha Brukman75f258e2003-10-24 17:57:33 +0000266<div class="doc_text">
267
268<p>In all cases, prefer spaces to tabs in source files. People have different
Misha Brukman1b36d672003-10-06 19:26:00 +0000269prefered indentation levels, and different styles of indentation that they
270like... this is fine. What isn't is that different editors/viewers expand tabs
271out to different tab stops. This can cause your code to look completely
Misha Brukman75f258e2003-10-24 17:57:33 +0000272unreadable, and it is not worth dealing with.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000273
Misha Brukman75f258e2003-10-24 17:57:33 +0000274<p>As always, follow the <a href="#goldenrule">Golden Rule</a> above: follow the
Misha Brukman1b36d672003-10-06 19:26:00 +0000275style of existing code if your are modifying and extending it. If you like four
276spaces of indentation, <b>DO NOT</b> do that in the middle of a chunk of code
277with two spaces of indentation. Also, do not reindent a whole source file: it
Misha Brukman75f258e2003-10-24 17:57:33 +0000278makes for incredible diffs that are absolutely worthless.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000279
Misha Brukman75f258e2003-10-24 17:57:33 +0000280</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000281
282<!-- _______________________________________________________________________ -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000283<div class="doc_subsubsection">
284 <a name="scf_indentation">Indent Code Consistently</a>
285</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000286
Misha Brukman75f258e2003-10-24 17:57:33 +0000287<div class="doc_text">
288
289<p>Okay, your first year of programming you were told that indentation is
Misha Brukman1b36d672003-10-06 19:26:00 +0000290important. If you didn't believe and internalize this then, now is the time.
Misha Brukman75f258e2003-10-24 17:57:33 +0000291Just do it.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000292
Misha Brukman75f258e2003-10-24 17:57:33 +0000293</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000294
295
296<!-- ======================================================================= -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000297<div class="doc_subsection">
298 <a name="compilerissues">Compiler Issues</a>
299</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000300
301
302<!-- _______________________________________________________________________ -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000303<div class="doc_subsubsection">
304 <a name="ci_warningerrors">Treat Compiler Warnings Like Errors</a>
305</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000306
Misha Brukman75f258e2003-10-24 17:57:33 +0000307<div class="doc_text">
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000308
Misha Brukman75f258e2003-10-24 17:57:33 +0000309<p>If your code has compiler warnings in it, something is wrong: you aren't
310casting values correctly, your have "questionable" constructs in your code, or
311you are doing something legitimately wrong. Compiler warnings can cover up
312legitimate errors in output and make dealing with a translation unit
313difficult.</p>
314
315<p>It is not possible to prevent all warnings from all compilers, nor is it
Misha Brukman1b36d672003-10-06 19:26:00 +0000316desirable. Instead, pick a standard compiler (like <tt>gcc</tt>) that provides
317a good thorough set of warnings, and stick to them. At least in the case of
318<tt>gcc</tt>, it is possible to work around any spurious errors by changing the
319syntax of the code slightly. For example, an warning that annoys me occurs when
Misha Brukman75f258e2003-10-24 17:57:33 +0000320I write code like this:</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000321
Misha Brukmana8c760a2004-07-28 22:31:54 +0000322<div class="doc_code">
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000323<pre>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000324if (V = getValue()) {
325 ...
326}
Misha Brukman75f258e2003-10-24 17:57:33 +0000327</pre>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000328</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000329
Misha Brukman75f258e2003-10-24 17:57:33 +0000330<p><tt>gcc</tt> will warn me that I probably want to use the <tt>==</tt>
331operator, and that I probably mistyped it. In most cases, I haven't, and I
332really don't want the spurious errors. To fix this particular problem, I
333rewrite the code like this:</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000334
Misha Brukmana8c760a2004-07-28 22:31:54 +0000335<div class="doc_code">
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000336<pre>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000337if ((V = getValue())) {
338 ...
339}
Misha Brukman75f258e2003-10-24 17:57:33 +0000340</pre>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000341</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000342
Misha Brukman75f258e2003-10-24 17:57:33 +0000343<p>...which shuts <tt>gcc</tt> up. Any <tt>gcc</tt> warning that annoys you can
344be fixed by massaging the code appropriately.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000345
Misha Brukman75f258e2003-10-24 17:57:33 +0000346<p>These are the <tt>gcc</tt> warnings that I prefer to enable: <tt>-Wall
347-Winline -W -Wwrite-strings -Wno-unused</tt></p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000348
Misha Brukman75f258e2003-10-24 17:57:33 +0000349</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000350
351<!-- _______________________________________________________________________ -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000352<div class="doc_subsubsection">
Misha Brukman75f258e2003-10-24 17:57:33 +0000353 <a name="ci_portable_code">Write Portable Code</a>
354</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000355
Misha Brukman75f258e2003-10-24 17:57:33 +0000356<div class="doc_text">
357
358<p>In almost all cases, it is possible and within reason to write completely
Misha Brukman2d10d752003-07-28 21:57:18 +0000359portable code. If there are cases where it isn't possible to write portable
Misha Brukman75f258e2003-10-24 17:57:33 +0000360code, isolate it behind a well defined (and well documented) interface.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000361
Misha Brukman75f258e2003-10-24 17:57:33 +0000362<p>In practice, this means that you shouldn't assume much about the host
363compiler, including its support for "high tech" features like partial
364specialization of templates. In fact, Visual C++ 6 could be an important target
365for our work in the future, and we don't want to have to rewrite all of our code
366to support it.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000367
Misha Brukman75f258e2003-10-24 17:57:33 +0000368</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000369
Reid Spencerb40df562004-09-23 16:03:48 +0000370<!-- _______________________________________________________________________ -->
371<div class="doc_subsubsection">
372<a name="ci_class_struct">Use of <tt>class</tt> and <tt>struct</tt> Keywords</a>
373</div>
374<div class="doc_text">
375 <p>In C++ the <tt>class</tt> and <tt>struct</tt> keywoards can be used almost
376 interchangeably. The only difference is when they are used to declare a class:
377 <tt>class</tt> makes all members private by default while <tt>struct</tt>
378 makes all members public by defaut.<p>
379 <p>Unfortunately, not all compilers follow the rules and some will generate
380 different symbols based on whether <tt>class</tt> or <tt>struct</tt> was used
381 to declare the symbol. This can lead to problems at link time.</p>
382 <p>So, the rule for LLVM is this: "always use the <tt>class</tt> keyword,
383 unless <b>all</b> members are public in which case <tt>struct</tt> is
384 allowed.</p>
385</div>
386
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000387<!-- *********************************************************************** -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000388<div class="doc_section">
389 <a name="styleissues">Style Issues</a>
390</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000391<!-- *********************************************************************** -->
392
393
394<!-- ======================================================================= -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000395<div class="doc_subsection">
396 <a name="macro">The High Level Issues</a>
397</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000398
399
400<!-- _______________________________________________________________________ -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000401<div class="doc_subsubsection">
402 <a name="hl_module">A Public Header File <b>is</b> a Module</a>
403</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000404
Misha Brukman75f258e2003-10-24 17:57:33 +0000405<div class="doc_text">
406
407<p>C++ doesn't do too well in the modularity department. There is no real
Misha Brukman2d10d752003-07-28 21:57:18 +0000408encapsulation or data hiding (unless you use expensive protocol classes), but it
409is what we have to work with. When you write a public header file (in the LLVM
410source tree, they live in the top level "include" directory), you are defining a
Misha Brukman75f258e2003-10-24 17:57:33 +0000411module of functionality.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000412
Misha Brukman75f258e2003-10-24 17:57:33 +0000413<p>Ideally, modules should be completely independent of each other, and their
Misha Brukman2d10d752003-07-28 21:57:18 +0000414header files should only include the absolute minimum number of headers
415possible. A module is not just a class, a function, or a namespace: <a
416href="http://www.cuj.com/articles/2000/0002/0002c/0002c.htm">it's a collection
417of these</a> that defines an interface. This interface may be several
418functions, classes or data structures, but the important issue is how they work
Misha Brukman75f258e2003-10-24 17:57:33 +0000419together.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000420
Misha Brukman75f258e2003-10-24 17:57:33 +0000421<p>In general, a module should be implemented with one or more <tt>.cpp</tt>
422files. Each of these <tt>.cpp</tt> files should include the header that defines
423their interface first. This ensure that all of the dependences of the module
424header have been properly added to the module header itself, and are not
425implicit. System headers should be included after user headers for a
426translation unit.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000427
Misha Brukman75f258e2003-10-24 17:57:33 +0000428</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000429
430<!-- _______________________________________________________________________ -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000431<div class="doc_subsubsection">
432 <a name="hl_dontinclude">#include as Little as Possible</a>
433</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000434
Misha Brukman75f258e2003-10-24 17:57:33 +0000435<div class="doc_text">
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000436
Misha Brukman75f258e2003-10-24 17:57:33 +0000437<p><tt>#include</tt> hurts compile time performance. Don't do it unless you
438have to, especially in header files.</p>
439
440<p>But wait, sometimes you need to have the definition of a class to use it, or
441to inherit from it. In these cases go ahead and #include that header file. Be
Misha Brukman2d10d752003-07-28 21:57:18 +0000442aware however that there are many cases where you don't need to have the full
443definition of a class. If you are using a pointer or reference to a class, you
444don't need the header file. If you are simply returning a class instance from a
445prototyped function or method, you don't need it. In fact, for most cases, you
446simply don't need the definition of a class... and not <tt>#include</tt>'ing
Misha Brukman75f258e2003-10-24 17:57:33 +0000447speeds up compilation.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000448
Misha Brukman75f258e2003-10-24 17:57:33 +0000449<p>It is easy to try to go too overboard on this recommendation, however. You
Misha Brukman2d10d752003-07-28 21:57:18 +0000450<b>must</b> include all of the header files that you are using, either directly
451or indirectly (through another header file). To make sure that you don't
452accidently forget to include a header file in your module header, make sure to
453include your module header <b>first</b> in the implementation file (as mentioned
454above). This way there won't be any hidden dependencies that you'll find out
Misha Brukman75f258e2003-10-24 17:57:33 +0000455about later...</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000456
Misha Brukman75f258e2003-10-24 17:57:33 +0000457</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000458
459<!-- _______________________________________________________________________ -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000460<div class="doc_subsubsection">
461 <a name="hl_privateheaders">Keep "internal" Headers Private</a>
462</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000463
Misha Brukman75f258e2003-10-24 17:57:33 +0000464<div class="doc_text">
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000465
Misha Brukman75f258e2003-10-24 17:57:33 +0000466<p>Many modules have a complex implementation that causes them to use more than
467one implementation (<tt>.cpp</tt>) file. It is often tempting to put the
468internal communication interface (helper classes, extra functions, etc) in the
469public module header file. Don't do this. :)</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000470
Misha Brukman75f258e2003-10-24 17:57:33 +0000471<p>If you really need to do something like this, put a private header file in
472the same directory as the source files, and include it locally. This ensures
473that your private interface remains private and undisturbed by outsiders.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000474
Misha Brukman75f258e2003-10-24 17:57:33 +0000475<p>Note however, that it's okay to put extra implementation methods a public
476class itself... just make them private (or protected), and all is well.</p>
477
478</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000479
480<!-- ======================================================================= -->
Chris Lattner999cf092004-05-23 21:05:07 +0000481<div class="doc_subsection">
Misha Brukman75f258e2003-10-24 17:57:33 +0000482 <a name="micro">The Low Level Issues</a>
483</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000484
485
486<!-- _______________________________________________________________________ -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000487<div class="doc_subsubsection">
488 <a name="hl_assert">Assert Liberally</a>
489</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000490
Misha Brukman75f258e2003-10-24 17:57:33 +0000491<div class="doc_text">
492
493<p>Use the "<tt>assert</tt>" function to its fullest. Check all of your
Misha Brukman2d10d752003-07-28 21:57:18 +0000494preconditions and assumptions, you never know when a bug (not neccesarily even
495yours) might be caught early by an assertion, which reduces debugging time
496dramatically. The "<tt>&lt;cassert&gt;</tt>" header file is probably already
497included by the header files you are using, so it doesn't cost anything to use
Misha Brukman75f258e2003-10-24 17:57:33 +0000498it.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000499
Misha Brukman75f258e2003-10-24 17:57:33 +0000500<p>To further assist with debugging, make sure to put some kind of error message
501in the assertion statement (which is printed if the assertion is tripped). This
Misha Brukman2d10d752003-07-28 21:57:18 +0000502helps the poor debugging make sense of why an assertion is being made and
Misha Brukman75f258e2003-10-24 17:57:33 +0000503enforced, and hopefully what to do about it. Here is one complete example:</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000504
Misha Brukmana8c760a2004-07-28 22:31:54 +0000505<div class="doc_code">
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000506<pre>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000507inline Value *getOperand(unsigned i) {
508 assert(i &lt; Operands.size() &amp;&amp; "getOperand() out of range!");
509 return Operands[i];
510}
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000511</pre>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000512</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000513
Misha Brukman75f258e2003-10-24 17:57:33 +0000514<p>Here are some examples:</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000515
Misha Brukmana8c760a2004-07-28 22:31:54 +0000516<div class="doc_code">
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000517<pre>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000518assert(Ty-&gt;isPointerType() &amp;&amp; "Can't allocate a non pointer type!");
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000519
Misha Brukmana8c760a2004-07-28 22:31:54 +0000520assert((Opcode == Shl || Opcode == Shr) &amp;&amp; "ShiftInst Opcode invalid!");
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000521
Misha Brukmana8c760a2004-07-28 22:31:54 +0000522assert(idx &lt; getNumSuccessors() &amp;&amp; "Successor # out of range!");
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000523
Misha Brukmana8c760a2004-07-28 22:31:54 +0000524assert(V1.getType() == V2.getType() &amp;&amp; "Constant types must be identical!");
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000525
Misha Brukmana8c760a2004-07-28 22:31:54 +0000526assert(isa&lt;PHINode&gt;(Succ-&gt;front()) &amp;&amp; "Only works on PHId BBs!");
Misha Brukman75f258e2003-10-24 17:57:33 +0000527</pre>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000528</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000529
Misha Brukman75f258e2003-10-24 17:57:33 +0000530<p>You get the idea...</p>
531
532</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000533
534
535<!-- _______________________________________________________________________ -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000536<div class="doc_subsubsection">
537 <a name="hl_preincrement">Prefer Preincrement</a>
538</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000539
Misha Brukman75f258e2003-10-24 17:57:33 +0000540<div class="doc_text">
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000541
Misha Brukmana8c760a2004-07-28 22:31:54 +0000542<p>Hard fast rule: Preincrement (<tt>++X</tt>) may be no slower than
543postincrement (<tt>X++</tt>) and could very well be a lot faster than it. Use
544preincrementation whenever possible.</p>
Misha Brukman75f258e2003-10-24 17:57:33 +0000545
546<p>The semantics of postincrement include making a copy of the value being
Misha Brukman2d10d752003-07-28 21:57:18 +0000547incremented, returning it, and then preincrementing the "work value". For
548primitive types, this isn't a big deal... but for iterators, it can be a huge
549issue (for example, some iterators contains stack and set objects in them...
550copying an iterator could invoke the copy ctor's of these as well). In general,
Misha Brukman75f258e2003-10-24 17:57:33 +0000551get in the habit of always using preincrement, and you won't have a problem.</p>
552
553</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000554
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000555<!-- _______________________________________________________________________ -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000556<div class="doc_subsubsection">
Chris Lattner999cf092004-05-23 21:05:07 +0000557 <a name="hl_avoidendl">Avoid std::endl</a>
Misha Brukman75f258e2003-10-24 17:57:33 +0000558</div>
Chris Lattner13c46592002-01-20 19:01:26 +0000559
Misha Brukman75f258e2003-10-24 17:57:33 +0000560<div class="doc_text">
561
Chris Lattner999cf092004-05-23 21:05:07 +0000562<p>The <tt>std::endl</tt> modifier, when used with iostreams outputs a newline
563to the output stream specified. In addition to doing this, however, it also
564flushes the output stream. In other words, these are equivalent:</p>
Chris Lattner13c46592002-01-20 19:01:26 +0000565
Misha Brukmana8c760a2004-07-28 22:31:54 +0000566<div class="doc_code">
Chris Lattner13c46592002-01-20 19:01:26 +0000567<pre>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000568std::cout &lt;&lt; std::endl;
569std::cout &lt;&lt; '\n' &lt;&lt; std::flush;
Chris Lattner13c46592002-01-20 19:01:26 +0000570</pre>
Misha Brukmana8c760a2004-07-28 22:31:54 +0000571</div>
Chris Lattner13c46592002-01-20 19:01:26 +0000572
Misha Brukman75f258e2003-10-24 17:57:33 +0000573<p>Most of the time, you probably have no reason to flush the output stream, so
Misha Brukmana8c760a2004-07-28 22:31:54 +0000574it's better to use a literal <tt>'\n'</tt>.</p>
Chris Lattner13c46592002-01-20 19:01:26 +0000575
Misha Brukman75f258e2003-10-24 17:57:33 +0000576</div>
Chris Lattner13c46592002-01-20 19:01:26 +0000577
578<!-- _______________________________________________________________________ -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000579<div class="doc_subsubsection">
580 <a name="hl_exploitcpp">Exploit C++ to its Fullest</a>
581</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000582
Misha Brukman75f258e2003-10-24 17:57:33 +0000583<div class="doc_text">
584
Misha Brukmana8c760a2004-07-28 22:31:54 +0000585<p>C++ is a powerful language. With a firm grasp on its capabilities, you can
586make write effective, consise, readable and maintainable code all at the same
587time. By staying consistent, you reduce the amount of special cases that need
588to be remembered. Reducing the total number of lines of code you write is a
589good way to avoid documentation, and avoid giving bugs a place to hide.</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000590
Misha Brukman75f258e2003-10-24 17:57:33 +0000591<p>For these reasons, come to know and love the contents of your local
Misha Brukman2d10d752003-07-28 21:57:18 +0000592&lt;algorithm&gt; header file. Know about &lt;functional&gt; and what it can do
Misha Brukman75f258e2003-10-24 17:57:33 +0000593for you. C++ is just a tool that wants you to master it. :)</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000594
Misha Brukman75f258e2003-10-24 17:57:33 +0000595</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000596
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000597<!-- *********************************************************************** -->
Misha Brukman75f258e2003-10-24 17:57:33 +0000598<div class="doc_section">
599 <a name="seealso">See Also</a>
600</div>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000601<!-- *********************************************************************** -->
602
Misha Brukman75f258e2003-10-24 17:57:33 +0000603<div class="doc_text">
604
605<p>A lot of these comments and recommendations have been culled for other
606sources. Two particularly important books for our work are:</p>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000607
608<ol>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000609
Tanya Lattner09cf73c2004-06-22 04:24:55 +0000610<li><a href="http://www.aw-bc.com/catalog/academic/product/0,1144,0201310155,00.html">Effective
Misha Brukman75f258e2003-10-24 17:57:33 +0000611C++</a> by Scott Meyers. There is an online version of the book (only some
612chapters though) <a
Chris Lattner999cf092004-05-23 21:05:07 +0000613href="http://www.awlonline.com/cseng/meyerscddemo/">available as well</a>. Also
614interesting and useful are "More Effective C++" and "Effective STL" by the same
615author.</li>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000616
Misha Brukman75f258e2003-10-24 17:57:33 +0000617<li><a href="http://cseng.aw.com/book/0,3828,0201633620,00.html">Large-Scale C++
618Software Design</a> by John Lakos</li>
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000619
Misha Brukman75f258e2003-10-24 17:57:33 +0000620</ol>
621
622<p>If you get some free time, and you haven't read them: do so, you might learn
623something. :)</p>
624
625</div>
626
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000627<!-- *********************************************************************** -->
628
629<hr>
Misha Brukman6c16c4b2004-05-12 18:37:22 +0000630<address>
631 <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
632 src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a>
633 <a href="http://validator.w3.org/check/referer"><img
634 src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01!"></a>
Misha Brukman75f258e2003-10-24 17:57:33 +0000635
Misha Brukman6c16c4b2004-05-12 18:37:22 +0000636 <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
637 <a href="http://llvm.cs.uiuc.edu">LLVM Compiler Infrastructure</a><br>
Misha Brukman75f258e2003-10-24 17:57:33 +0000638 Last modified: $Date$
Misha Brukman6c16c4b2004-05-12 18:37:22 +0000639</address>
Misha Brukman75f258e2003-10-24 17:57:33 +0000640
641</body>
642</html>