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Misha Brukman4dbc9d52008-12-10 23:07:02 +0000[diff] [blame]6 <title>LLVM Coding Standards</title>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]7</head>
8<body>
9
10<div class="doc_title">
Misha Brukman4dbc9d52008-12-10 23:07:02 +0000[diff] [blame]11 LLVM Coding Standards
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]12</div>
13
14<ol>
15 <li><a href="#introduction">Introduction</a></li>
16 <li><a href="#mechanicalissues">Mechanical Source Issues</a>
17 <ol>
18 <li><a href="#sourceformating">Source Code Formatting</a>
19 <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"><tt>#include</tt> 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>
27 <li><a href="#compilerissues">Compiler Issues</a>
28 <ol>
29 <li><a href="#ci_warningerrors">Treat Compiler Warnings Like
30 Errors</a></li>
31 <li><a href="#ci_portable_code">Write Portable Code</a></li>
32 <li><a href="#ci_class_struct">Use of class/struct Keywords</a></li>
33 </ol></li>
34 </ol></li>
35 <li><a href="#styleissues">Style Issues</a>
36 <ol>
37 <li><a href="#macro">The High Level Issues</a>
38 <ol>
39 <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>
Chris Lattnereb1e8c32009-07-22 05:40:54 +0000[diff] [blame]44 <li><a href="#hl_earlyexit">Use Early Exits and 'continue' to Simplify
45 Code</a></li>
Chris Lattner598fcfd2009-07-28 22:54:04 +0000[diff] [blame]46 <li><a href="#hl_else_after_return">Don't use "else" after a
47 return</a></li>
Chris Lattnereb1e8c32009-07-22 05:40:54 +0000[diff] [blame]48 <li><a href="#hl_predicateloops">Turn Predicate Loops into Predicate
49 Functions</a></li>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]50 </ol></li>
51 <li><a href="#micro">The Low Level Issues</a>
52 <ol>
53 <li><a href="#ll_assert">Assert Liberally</a></li>
54 <li><a href="#ll_ns_std">Do not use 'using namespace std'</a></li>
55 <li><a href="#ll_virtual_anch">Provide a virtual method anchor for
56 classes in headers</a></li>
Chris Lattner01b8af32009-06-30 06:13:23 +0000[diff] [blame]57 <li><a href="#ll_end">Don't evaluate end() every time through a
58 loop</a></li>
Chris Lattnereb1e8c32009-07-22 05:40:54 +0000[diff] [blame]59 <li><a href="#ll_iostream"><tt>#include &lt;iostream&gt;</tt> is
60 <em>forbidden</em></a></li>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]61 <li><a href="#ll_avoidendl">Avoid <tt>std::endl</tt></a></li>
Daniel Dunbar2f099eb2009-07-24 23:04:51 +0000[diff] [blame]62 <li><a href="#ll_raw_ostream">Use <tt>raw_ostream</tt></a</li>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]63 </ol></li>
Chris Lattnereb1e8c32009-07-22 05:40:54 +0000[diff] [blame]64
65 <li><a href="#nano">Microscopic Details</a>
66 <ol>
67 <li><a href="#micro_spaceparen">Spaces Before Parentheses</a></li>
68 <li><a href="#micro_preincrement">Prefer Preincrement</a></li>
69 <li><a href="#micro_namespaceindent">Namespace Indentation</a></li>
70 <li><a href="#micro_anonns">Anonymous Namespaces</a></li>
71 </ol></li>
72
73
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]74 </ol></li>
75 <li><a href="#seealso">See Also</a></li>
76</ol>
77
78<div class="doc_author">
Chris Lattnere7346a92009-07-22 05:43:01 +0000[diff] [blame]79 <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]80</div>
81
82
83<!-- *********************************************************************** -->
84<div class="doc_section">
85 <a name="introduction">Introduction</a>
86</div>
87<!-- *********************************************************************** -->
88
89<div class="doc_text">
90
91<p>This document attempts to describe a few coding standards that are being used
92in the LLVM source tree. Although no coding standards should be regarded as
93absolute requirements to be followed in all instances, coding standards can be
94useful.</p>
95
96<p>This document intentionally does not prescribe fixed standards for religious
97issues such as brace placement and space usage. For issues like this, follow
98the golden rule:</p>
99
100<blockquote>
101
102<p><b><a name="goldenrule">If you are adding a significant body of source to a
103project, feel free to use whatever style you are most comfortable with. If you
104are extending, enhancing, or bug fixing already implemented code, use the style
105that is already being used so that the source is uniform and easy to
106follow.</a></b></p>
107
108</blockquote>
109
110<p>The ultimate goal of these guidelines is the increase readability and
111maintainability of our common source base. If you have suggestions for topics to
112be included, please mail them to <a
113href="mailto:sabre@nondot.org">Chris</a>.</p>
114
115</div>
116
117<!-- *********************************************************************** -->
118<div class="doc_section">
119 <a name="mechanicalissues">Mechanical Source Issues</a>
120</div>
121<!-- *********************************************************************** -->
122
123<!-- ======================================================================= -->
124<div class="doc_subsection">
125 <a name="sourceformating">Source Code Formatting</a>
126</div>
127
128<!-- _______________________________________________________________________ -->
129<div class="doc_subsubsection">
130 <a name="scf_commenting">Commenting</a>
131</div>
132
133<div class="doc_text">
134
135<p>Comments are one critical part of readability and maintainability. Everyone
Chris Lattner48386692009-07-12 00:10:24 +0000[diff] [blame]136knows they should comment, so should you. When writing comments, write them as
137English prose, which means they should use proper capitalization, punctuation,
138etc. Although we all should probably
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]139comment our code more than we do, there are a few very critical places that
140documentation is very useful:</p>
141
142<b>File Headers</b>
143
144<p>Every source file should have a header on it that describes the basic
145purpose of the file. If a file does not have a header, it should not be
146checked into Subversion. Most source trees will probably have a standard
147file header format. The standard format for the LLVM source tree looks like
148this:</p>
149
150<div class="doc_code">
151<pre>
152//===-- llvm/Instruction.h - Instruction class definition -------*- C++ -*-===//
Misha Brukman0fd5f3e2009-01-02 16:58:42 +0000[diff] [blame]153//
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]154// The LLVM Compiler Infrastructure
155//
Chris Lattnerfb5959f2007-12-29 19:56:08 +0000[diff] [blame]156// This file is distributed under the University of Illinois Open Source
157// License. See LICENSE.TXT for details.
Misha Brukman0fd5f3e2009-01-02 16:58:42 +0000[diff] [blame]158//
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]159//===----------------------------------------------------------------------===//
160//
161// This file contains the declaration of the Instruction class, which is the
162// base class for all of the VM instructions.
163//
164//===----------------------------------------------------------------------===//
165</pre>
166</div>
167
Chris Lattnerfb5959f2007-12-29 19:56:08 +0000[diff] [blame]168<p>A few things to note about this particular format: The "<tt>-*- C++
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]169-*-</tt>" string on the first line is there to tell Emacs that the source file
170is a C++ file, not a C file (Emacs assumes .h files are C files by default).
171Note that this tag is not necessary in .cpp files. The name of the file is also
172on the first line, along with a very short description of the purpose of the
173file. This is important when printing out code and flipping though lots of
174pages.</p>
175
Chris Lattnerfb5959f2007-12-29 19:56:08 +0000[diff] [blame]176<p>The next section in the file is a concise note that defines the license
177that the file is released under. This makes it perfectly clear what terms the
178source code can be distributed under and should not be modified in any way.</p>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]179
180<p>The main body of the description does not have to be very long in most cases.
181Here it's only two lines. If an algorithm is being implemented or something
182tricky is going on, a reference to the paper where it is published should be
183included, as well as any notes or "gotchas" in the code to watch out for.</p>
184
185<b>Class overviews</b>
186
187<p>Classes are one fundamental part of a good object oriented design. As such,
188a class definition should have a comment block that explains what the class is
189used for... if it's not obvious. If it's so completely obvious your grandma
190could figure it out, it's probably safe to leave it out. Naming classes
191something sane goes a long ways towards avoiding writing documentation.</p>
192
193
194<b>Method information</b>
195
196<p>Methods defined in a class (as well as any global functions) should also be
197documented properly. A quick note about what it does any a description of the
198borderline behaviour is all that is necessary here (unless something
199particularly tricky or insideous is going on). The hope is that people can
200figure out how to use your interfaces without reading the code itself... that is
201the goal metric.</p>
202
203<p>Good things to talk about here are what happens when something unexpected
204happens: does the method return null? Abort? Format your hard disk?</p>
205
206</div>
207
208<!-- _______________________________________________________________________ -->
209<div class="doc_subsubsection">
210 <a name="scf_commentformat">Comment Formatting</a>
211</div>
212
213<div class="doc_text">
214
215<p>In general, prefer C++ style (<tt>//</tt>) comments. They take less space,
216require less typing, don't have nesting problems, etc. There are a few cases
217when it is useful to use C style (<tt>/* */</tt>) comments however:</p>
218
219<ol>
220 <li>When writing a C code: Obviously if you are writing C code, use C style
221 comments.</li>
222 <li>When writing a header file that may be <tt>#include</tt>d by a C source
223 file.</li>
224 <li>When writing a source file that is used by a tool that only accepts C
225 style comments.</li>
226</ol>
227
228<p>To comment out a large block of code, use <tt>#if 0</tt> and <tt>#endif</tt>.
229These nest properly and are better behaved in general than C style comments.</p>
230
231</div>
232
233<!-- _______________________________________________________________________ -->
234<div class="doc_subsubsection">
235 <a name="scf_includes"><tt>#include</tt> Style</a>
236</div>
237
238<div class="doc_text">
239
240<p>Immediately after the <a href="#scf_commenting">header file comment</a> (and
241include guards if working on a header file), the <a
242href="#hl_dontinclude">minimal</a> list of <tt>#include</tt>s required by the
243file should be listed. We prefer these <tt>#include</tt>s to be listed in this
244order:</p>
245
246<ol>
247 <li><a href="#mmheader">Main Module header</a></li>
248 <li><a href="#hl_privateheaders">Local/Private Headers</a></li>
249 <li><tt>llvm/*</tt></li>
250 <li><tt>llvm/Analysis/*</tt></li>
251 <li><tt>llvm/Assembly/*</tt></li>
252 <li><tt>llvm/Bytecode/*</tt></li>
253 <li><tt>llvm/CodeGen/*</tt></li>
254 <li>...</li>
255 <li><tt>Support/*</tt></li>
256 <li><tt>Config/*</tt></li>
257 <li>System <tt>#includes</tt></li>
258</ol>
259
Nick Lewycky61819f72008-11-29 20:13:25 +0000[diff] [blame]260<p>... and each category should be sorted by name.</p>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]261
262<p><a name="mmheader">The "Main Module Header"</a> file applies to .cpp file
263which implement an interface defined by a .h file. This <tt>#include</tt>
264should always be included <b>first</b> regardless of where it lives on the file
265system. By including a header file first in the .cpp files that implement the
266interfaces, we ensure that the header does not have any hidden dependencies
267which are not explicitly #included in the header, but should be. It is also a
268form of documentation in the .cpp file to indicate where the interfaces it
269implements are defined.</p>
270
271</div>
272
273<!-- _______________________________________________________________________ -->
274<div class="doc_subsubsection">
275 <a name="scf_codewidth">Source Code Width</a>
276</div>
277
278<div class="doc_text">
279
280<p>Write your code to fit within 80 columns of text. This helps those of us who
281like to print out code and look at your code in an xterm without resizing
282it.</p>
283
Chris Lattnerb5daf702008-07-08 05:12:37 +0000[diff] [blame]284<p>The longer answer is that there must be some limit to the width of the code
285in order to reasonably allow developers to have multiple files side-by-side in
286windows on a modest display. If you are going to pick a width limit, it is
287somewhat arbitrary but you might as well pick something standard. Going with
28890 columns (for example) instead of 80 columns wouldn't add any significant
289value and would be detrimental to printing out code. Also many other projects
290have standardized on 80 columns, so some people have already configured their
291editors for it (vs something else, like 90 columns).</p>
292
293<p>This is one of many contentious issues in coding standards, but is not up
294for debate.</p>
295
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]296</div>
297
298<!-- _______________________________________________________________________ -->
299<div class="doc_subsubsection">
300 <a name="scf_spacestabs">Use Spaces Instead of Tabs</a>
301</div>
302
303<div class="doc_text">
304
305<p>In all cases, prefer spaces to tabs in source files. People have different
306prefered indentation levels, and different styles of indentation that they
307like... this is fine. What isn't is that different editors/viewers expand tabs
308out to different tab stops. This can cause your code to look completely
309unreadable, and it is not worth dealing with.</p>
310
311<p>As always, follow the <a href="#goldenrule">Golden Rule</a> above: follow the
312style of existing code if your are modifying and extending it. If you like four
313spaces of indentation, <b>DO NOT</b> do that in the middle of a chunk of code
314with two spaces of indentation. Also, do not reindent a whole source file: it
315makes for incredible diffs that are absolutely worthless.</p>
316
317</div>
318
319<!-- _______________________________________________________________________ -->
320<div class="doc_subsubsection">
321 <a name="scf_indentation">Indent Code Consistently</a>
322</div>
323
324<div class="doc_text">
325
326<p>Okay, your first year of programming you were told that indentation is
327important. If you didn't believe and internalize this then, now is the time.
328Just do it.</p>
329
330</div>
331
332
333<!-- ======================================================================= -->
334<div class="doc_subsection">
335 <a name="compilerissues">Compiler Issues</a>
336</div>
337
338
339<!-- _______________________________________________________________________ -->
340<div class="doc_subsubsection">
341 <a name="ci_warningerrors">Treat Compiler Warnings Like Errors</a>
342</div>
343
344<div class="doc_text">
345
346<p>If your code has compiler warnings in it, something is wrong: you aren't
347casting values correctly, your have "questionable" constructs in your code, or
348you are doing something legitimately wrong. Compiler warnings can cover up
349legitimate errors in output and make dealing with a translation unit
350difficult.</p>
351
352<p>It is not possible to prevent all warnings from all compilers, nor is it
353desirable. Instead, pick a standard compiler (like <tt>gcc</tt>) that provides
354a good thorough set of warnings, and stick to them. At least in the case of
355<tt>gcc</tt>, it is possible to work around any spurious errors by changing the
356syntax of the code slightly. For example, an warning that annoys me occurs when
357I write code like this:</p>
358
359<div class="doc_code">
360<pre>
361if (V = getValue()) {
362 ...
363}
364</pre>
365</div>
366
367<p><tt>gcc</tt> will warn me that I probably want to use the <tt>==</tt>
368operator, and that I probably mistyped it. In most cases, I haven't, and I
369really don't want the spurious errors. To fix this particular problem, I
370rewrite the code like this:</p>
371
372<div class="doc_code">
373<pre>
374if ((V = getValue())) {
375 ...
376}
377</pre>
378</div>
379
380<p>...which shuts <tt>gcc</tt> up. Any <tt>gcc</tt> warning that annoys you can
381be fixed by massaging the code appropriately.</p>
382
383<p>These are the <tt>gcc</tt> warnings that I prefer to enable: <tt>-Wall
384-Winline -W -Wwrite-strings -Wno-unused</tt></p>
385
386</div>
387
388<!-- _______________________________________________________________________ -->
389<div class="doc_subsubsection">
390 <a name="ci_portable_code">Write Portable Code</a>
391</div>
392
393<div class="doc_text">
394
395<p>In almost all cases, it is possible and within reason to write completely
396portable code. If there are cases where it isn't possible to write portable
397code, isolate it behind a well defined (and well documented) interface.</p>
398
399<p>In practice, this means that you shouldn't assume much about the host
400compiler, including its support for "high tech" features like partial
Chris Lattnerafec0252009-03-23 04:52:53 +0000[diff] [blame]401specialization of templates. If these features are used, they should only be
402an implementation detail of a library which has a simple exposed API.</p>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]403
404</div>
405
406<!-- _______________________________________________________________________ -->
407<div class="doc_subsubsection">
408<a name="ci_class_struct">Use of <tt>class</tt> and <tt>struct</tt> Keywords</a>
409</div>
410<div class="doc_text">
411
412<p>In C++, the <tt>class</tt> and <tt>struct</tt> keywords can be used almost
413interchangeably. The only difference is when they are used to declare a class:
414<tt>class</tt> makes all members private by default while <tt>struct</tt> makes
415all members public by default.</p>
416
417<p>Unfortunately, not all compilers follow the rules and some will generate
418different symbols based on whether <tt>class</tt> or <tt>struct</tt> was used to
419declare the symbol. This can lead to problems at link time.</p>
420
421<p>So, the rule for LLVM is to always use the <tt>class</tt> keyword, unless
Chris Lattnerd3ee8372009-08-11 17:38:15 +0000[diff] [blame]422<b>all</b> members are public and the type is a C++ "POD" type, in which case
423<tt>struct</tt> is allowed.</p>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]424
425</div>
426
427<!-- *********************************************************************** -->
428<div class="doc_section">
429 <a name="styleissues">Style Issues</a>
430</div>
431<!-- *********************************************************************** -->
432
433
434<!-- ======================================================================= -->
435<div class="doc_subsection">
436 <a name="macro">The High Level Issues</a>
437</div>
Chris Lattnereb1e8c32009-07-22 05:40:54 +0000[diff] [blame]438<!-- ======================================================================= -->
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]439
440
441<!-- _______________________________________________________________________ -->
442<div class="doc_subsubsection">
443 <a name="hl_module">A Public Header File <b>is</b> a Module</a>
444</div>
445
446<div class="doc_text">
447
448<p>C++ doesn't do too well in the modularity department. There is no real
449encapsulation or data hiding (unless you use expensive protocol classes), but it
450is what we have to work with. When you write a public header file (in the LLVM
451source tree, they live in the top level "include" directory), you are defining a
452module of functionality.</p>
453
454<p>Ideally, modules should be completely independent of each other, and their
455header files should only include the absolute minimum number of headers
456possible. A module is not just a class, a function, or a namespace: <a
457href="http://www.cuj.com/articles/2000/0002/0002c/0002c.htm">it's a collection
458of these</a> that defines an interface. This interface may be several
459functions, classes or data structures, but the important issue is how they work
460together.</p>
461
462<p>In general, a module should be implemented with one or more <tt>.cpp</tt>
463files. Each of these <tt>.cpp</tt> files should include the header that defines
464their interface first. This ensure that all of the dependences of the module
465header have been properly added to the module header itself, and are not
466implicit. System headers should be included after user headers for a
467translation unit.</p>
468
469</div>
470
471<!-- _______________________________________________________________________ -->
472<div class="doc_subsubsection">
473 <a name="hl_dontinclude"><tt>#include</tt> as Little as Possible</a>
474</div>
475
476<div class="doc_text">
477
478<p><tt>#include</tt> hurts compile time performance. Don't do it unless you
479have to, especially in header files.</p>
480
481<p>But wait, sometimes you need to have the definition of a class to use it, or
482to inherit from it. In these cases go ahead and <tt>#include</tt> that header
483file. Be aware however that there are many cases where you don't need to have
484the full definition of a class. If you are using a pointer or reference to a
485class, you don't need the header file. If you are simply returning a class
486instance from a prototyped function or method, you don't need it. In fact, for
487most cases, you simply don't need the definition of a class... and not
488<tt>#include</tt>'ing speeds up compilation.</p>
489
490<p>It is easy to try to go too overboard on this recommendation, however. You
491<b>must</b> include all of the header files that you are using -- you can
492include them either directly
493or indirectly (through another header file). To make sure that you don't
494accidently forget to include a header file in your module header, make sure to
495include your module header <b>first</b> in the implementation file (as mentioned
496above). This way there won't be any hidden dependencies that you'll find out
497about later...</p>
498
499</div>
500
501<!-- _______________________________________________________________________ -->
502<div class="doc_subsubsection">
503 <a name="hl_privateheaders">Keep "internal" Headers Private</a>
504</div>
505
506<div class="doc_text">
507
508<p>Many modules have a complex implementation that causes them to use more than
509one implementation (<tt>.cpp</tt>) file. It is often tempting to put the
510internal communication interface (helper classes, extra functions, etc) in the
511public module header file. Don't do this.</p>
512
513<p>If you really need to do something like this, put a private header file in
514the same directory as the source files, and include it locally. This ensures
515that your private interface remains private and undisturbed by outsiders.</p>
516
517<p>Note however, that it's okay to put extra implementation methods a public
518class itself... just make them private (or protected), and all is well.</p>
519
520</div>
521
522<!-- _______________________________________________________________________ -->
523<div class="doc_subsubsection">
Chris Lattnereb1e8c32009-07-22 05:40:54 +0000[diff] [blame]524 <a name="hl_earlyexit">Use Early Exits and 'continue' to Simplify Code</a>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]525</div>
526
527<div class="doc_text">
528
Chris Lattnereb1e8c32009-07-22 05:40:54 +0000[diff] [blame]529<p>When reading code, keep in mind how much state and how many previous
530decisions have to be remembered by the reader to understand a block of code.
531Aim to reduce indentation where possible when it doesn't make it more difficult
532to understand the code. One great way to do this is by making use of early
533exits and the 'continue' keyword in long loops. As an example of using an early
534exit from a function, consider this "bad" code:</p>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]535
Chris Lattnereb1e8c32009-07-22 05:40:54 +0000[diff] [blame]536<div class="doc_code">
537<pre>
538Value *DoSomething(Instruction *I) {
539 if (!isa&lt;TerminatorInst&gt;(I) &amp;&amp;
540 I-&gt;hasOneUse() &amp;&amp; SomeOtherThing(I)) {
541 ... some long code ....
542 }
543
544 return 0;
545}
546</pre>
547</div>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]548
Chris Lattnereb1e8c32009-07-22 05:40:54 +0000[diff] [blame]549<p>This code has several problems if the body of the 'if' is large. When you're
550looking at the top of the function, it isn't immediately clear that this
551<em>only</em> does interesting things with non-terminator instructions, and only
552applies to things with the other predicates. Second, it is relatively difficult
553to describe (in comments) why these predicates are important because the if
554statement makes it difficult to lay out the comments. Third, when you're deep
555within the body of the code, it is indented an extra level. Finally, when
556reading the top of the function, it isn't clear what the result is if the
557predicate isn't true, you have to read to the end of the function to know that
558it returns null.</p>
Matthijs Kooijmanc4c0af22008-07-30 12:14:10 +0000[diff] [blame]559
Chris Lattnereb1e8c32009-07-22 05:40:54 +0000[diff] [blame]560<p>It is much preferred to format the code like this:</p>
561
562<div class="doc_code">
563<pre>
564Value *DoSomething(Instruction *I) {
565 // Terminators never need 'something' done to them because, ...
566 if (isa&lt;TerminatorInst&gt;(I))
567 return 0;
568
569 // We conservatively avoid transforming instructions with multiple uses
570 // because goats like cheese.
571 if (!I-&gt;hasOneUse())
572 return 0;
573
574 // This is really just here for example.
575 if (!SomeOtherThing(I))
576 return 0;
577
578 ... some long code ....
579}
580</pre>
581</div>
582
583<p>This fixes these problems. A similar problem frequently happens in for
584loops. A silly example is something like this:</p>
585
586<div class="doc_code">
587<pre>
588 for (BasicBlock::iterator II = BB-&gt;begin(), E = BB-&gt;end(); II != E; ++II) {
589 if (BinaryOperator *BO = dyn_cast&lt;BinaryOperator&gt;(II)) {
590 Value *LHS = BO-&gt;getOperand(0);
591 Value *RHS = BO-&gt;getOperand(1);
592 if (LHS != RHS) {
593 ...
594 }
595 }
596 }
597</pre>
598</div>
599
600<p>When you have very very small loops, this sort of structure is fine, but if
601it exceeds more than 10-15 lines, it becomes difficult for people to read and
602understand at a glance.
603The problem with this sort of code is that it gets very nested very quickly,
604meaning that the reader of the code has to keep a lot of context in their brain
605to remember what is going immediately on in the loop, because they don't know
606if/when the if conditions will have elses etc. It is strongly preferred to
607structure the loop like this:</p>
608
609<div class="doc_code">
610<pre>
611 for (BasicBlock::iterator II = BB-&gt;begin(), E = BB-&gt;end(); II != E; ++II) {
612 BinaryOperator *BO = dyn_cast&lt;BinaryOperator&gt;(II);
613 if (!BO) continue;
614
615 Value *LHS = BO-&gt;getOperand(0);
616 Value *RHS = BO-&gt;getOperand(1);
617 if (LHS == RHS) continue;
618 }
619</pre>
620</div>
621
622<p>This has all the benefits of using early exits from functions: it reduces
623nesting of the loop, it makes it easier to describe why the conditions are true,
624and it makes it obvious to the reader that there is no "else" coming up that
625they have to push context into their brain for. If a loop is large, this can
626be a big understandability win.</p>
627
628</div>
629
Chris Lattner598fcfd2009-07-28 22:54:04 +0000[diff] [blame]630<!-- _______________________________________________________________________ -->
631<div class="doc_subsubsection">
632 <a name="hl_else_after_return">Don't use "else" after a return</a>
633</div>
634
635<div class="doc_text">
636
637<p>For similar reasons above (reduction of indentation and easier reading),
638 please do not use "else" or "else if" after something that interrupts
639 control flow like return, break, continue, goto, etc. For example, this is
640 "bad":</p>
641
642<div class="doc_code">
643<pre>
644 case 'J': {
645 if (Signed) {
646 Type = Context.getsigjmp_bufType();
647 if (Type.isNull()) {
648 Error = ASTContext::GE_Missing_sigjmp_buf;
649 return QualType();
650 } else {
651 break;
652 }
653 } else {
654 Type = Context.getjmp_bufType();
655 if (Type.isNull()) {
656 Error = ASTContext::GE_Missing_jmp_buf;
657 return QualType();
658 } else {
659 break;
660 }
661 }
662 }
663 }
664</pre>
665</div>
666
667<p>It is better to write this something like:</p>
668
669<div class="doc_code">
670<pre>
671 case 'J':
672 if (Signed) {
673 Type = Context.getsigjmp_bufType();
674 if (Type.isNull()) {
675 Error = ASTContext::GE_Missing_sigjmp_buf;
676 return QualType();
677 }
678 } else {
679 Type = Context.getjmp_bufType();
680 if (Type.isNull()) {
681 Error = ASTContext::GE_Missing_jmp_buf;
682 return QualType();
683 }
684 }
685 break;
686</pre>
687</div>
688
689<p>Or better yet (in this case), as:</p>
690
691<div class="doc_code">
692<pre>
693 case 'J':
694 if (Signed)
695 Type = Context.getsigjmp_bufType();
696 else
697 Type = Context.getjmp_bufType();
698
699 if (Type.isNull()) {
700 Error = Signed ? ASTContext::GE_Missing_sigjmp_buf :
701 ASTContext::GE_Missing_jmp_buf;
702 return QualType();
703 }
704 break;
705</pre>
706</div>
707
708<p>The idea is to reduce indentation and the amount of code you have to keep
709 track of when reading the code.</p>
710
711</div>
Chris Lattnereb1e8c32009-07-22 05:40:54 +0000[diff] [blame]712
713<!-- _______________________________________________________________________ -->
714<div class="doc_subsubsection">
715 <a name="hl_predicateloops">Turn Predicate Loops into Predicate Functions</a>
716</div>
717
718<div class="doc_text">
719
Chris Lattner5af04d22009-07-22 16:54:14 +0000[diff] [blame]720<p>It is very common to write small loops that just compute a boolean
Chris Lattnereb1e8c32009-07-22 05:40:54 +0000[diff] [blame]721 value. There are a number of ways that people commonly write these, but an
722 example of this sort of thing is:</p>
723
724<div class="doc_code">
725<pre>
726 <b>bool FoundFoo = false;</b>
727 for (unsigned i = 0, e = BarList.size(); i != e; ++i)
728 if (BarList[i]-&gt;isFoo()) {
729 <b>FoundFoo = true;</b>
730 break;
731 }
732
733 <b>if (FoundFoo) {</b>
734 ...
735 }
736</pre>
737</div>
738
739<p>This sort of code is awkward to write, and is almost always a bad sign.
740Instead of this sort of loop, we strongly prefer to use a predicate function
741(which may be <a href="#micro_anonns">static</a>) that uses
Chris Lattner5af04d22009-07-22 16:54:14 +0000[diff] [blame]742<a href="#hl_earlyexit">early exits</a> to compute the predicate. We prefer
743the code to be structured like this:
Chris Lattnereb1e8c32009-07-22 05:40:54 +0000[diff] [blame]744</p>
745
746
747<div class="doc_code">
748<pre>
749/// ListContainsFoo - Return true if the specified list has an element that is
750/// a foo.
751static bool ListContainsFoo(const std::vector&lt;Bar*&gt; &amp;List) {
752 for (unsigned i = 0, e = List.size(); i != e; ++i)
753 if (List[i]-&gt;isFoo())
754 return true;
755 return false;
756}
757...
758
759 <b>if (ListContainsFoo(BarList)) {</b>
760 ...
761 }
762</pre>
763</div>
764
765<p>There are many reasons for doing this: it reduces indentation and factors out
766code which can often be shared by other code that checks for the same predicate.
767More importantly, it <em>forces you to pick a name</em> for the function, and
768forces you to write a comment for it. In this silly example, this doesn't add
769much value. However, if the condition is complex, this can make it a lot easier
770for the reader to understand the code that queries for this predicate. Instead
Chris Lattner6ead3272009-07-22 16:30:39 +0000[diff] [blame]771of being faced with the in-line details of how we check to see if the BarList
Chris Lattnereb1e8c32009-07-22 05:40:54 +0000[diff] [blame]772contains a foo, we can trust the function name and continue reading with better
773locality.</p>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]774
775</div>
776
777
778<!-- ======================================================================= -->
779<div class="doc_subsection">
780 <a name="micro">The Low Level Issues</a>
781</div>
Chris Lattnereb1e8c32009-07-22 05:40:54 +0000[diff] [blame]782<!-- ======================================================================= -->
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]783
784
785<!-- _______________________________________________________________________ -->
786<div class="doc_subsubsection">
787 <a name="ll_assert">Assert Liberally</a>
788</div>
789
790<div class="doc_text">
791
792<p>Use the "<tt>assert</tt>" function to its fullest. Check all of your
793preconditions and assumptions, you never know when a bug (not neccesarily even
794yours) might be caught early by an assertion, which reduces debugging time
795dramatically. The "<tt>&lt;cassert&gt;</tt>" header file is probably already
796included by the header files you are using, so it doesn't cost anything to use
797it.</p>
798
799<p>To further assist with debugging, make sure to put some kind of error message
800in the assertion statement (which is printed if the assertion is tripped). This
801helps the poor debugging make sense of why an assertion is being made and
802enforced, and hopefully what to do about it. Here is one complete example:</p>
803
804<div class="doc_code">
805<pre>
806inline Value *getOperand(unsigned i) {
807 assert(i &lt; Operands.size() &amp;&amp; "getOperand() out of range!");
808 return Operands[i];
809}
810</pre>
811</div>
812
813<p>Here are some examples:</p>
814
815<div class="doc_code">
816<pre>
817assert(Ty-&gt;isPointerType() &amp;&amp; "Can't allocate a non pointer type!");
818
819assert((Opcode == Shl || Opcode == Shr) &amp;&amp; "ShiftInst Opcode invalid!");
820
821assert(idx &lt; getNumSuccessors() &amp;&amp; "Successor # out of range!");
822
823assert(V1.getType() == V2.getType() &amp;&amp; "Constant types must be identical!");
824
825assert(isa&lt;PHINode&gt;(Succ-&gt;front()) &amp;&amp; "Only works on PHId BBs!");
826</pre>
827</div>
828
829<p>You get the idea...</p>
830
Nick Lewycky009216a2008-05-31 23:54:55 +0000[diff] [blame]831<p>Please be aware when adding assert statements that not all compilers are aware of
832the semantics of the assert. In some places, asserts are used to indicate a piece of
833code that should not be reached. These are typically of the form:</p>
834
835<div class="doc_code">
836<pre>
Dan Gohman4dfac702008-11-24 17:18:39 +0000[diff] [blame]837assert(0 &amp;&amp; "Some helpful error message");
Nick Lewycky009216a2008-05-31 23:54:55 +0000[diff] [blame]838</pre>
839</div>
840
841<p>When used in a function that returns a value, they should be followed with a return
842statement and a comment indicating that this line is never reached. This will prevent
843a compiler which is unable to deduce that the assert statement never returns from
844generating a warning.</p>
845
846<div class="doc_code">
847<pre>
Dan Gohman4dfac702008-11-24 17:18:39 +0000[diff] [blame]848assert(0 &amp;&amp; "Some helpful error message");
Nick Lewycky009216a2008-05-31 23:54:55 +0000[diff] [blame]849// Not reached
850return 0;
851</pre>
852</div>
853
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]854</div>
855
856<!-- _______________________________________________________________________ -->
857<div class="doc_subsubsection">
858 <a name="ll_ns_std">Do not use '<tt>using namespace std</tt>'</a>
859</div>
860
861<div class="doc_text">
862<p>In LLVM, we prefer to explicitly prefix all identifiers from the standard
863namespace with an "<tt>std::</tt>" prefix, rather than rely on
864"<tt>using namespace std;</tt>".</p>
865
866<p> In header files, adding a '<tt>using namespace XXX</tt>' directive pollutes
Misha Brukmanbcc53732008-12-11 19:37:04 +0000[diff] [blame]867the namespace of any source file that <tt>#include</tt>s the header. This is
868clearly a bad thing.</p>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]869
870<p>In implementation files (e.g. .cpp files), the rule is more of a stylistic
871rule, but is still important. Basically, using explicit namespace prefixes
872makes the code <b>clearer</b>, because it is immediately obvious what facilities
873are being used and where they are coming from, and <b>more portable</b>, because
874namespace clashes cannot occur between LLVM code and other namespaces. The
875portability rule is important because different standard library implementations
876expose different symbols (potentially ones they shouldn't), and future revisions
877to the C++ standard will add more symbols to the <tt>std</tt> namespace. As
878such, we never use '<tt>using namespace std;</tt>' in LLVM.</p>
879
880<p>The exception to the general rule (i.e. it's not an exception for
881the <tt>std</tt> namespace) is for implementation files. For example, all of
882the code in the LLVM project implements code that lives in the 'llvm' namespace.
883As such, it is ok, and actually clearer, for the .cpp files to have a '<tt>using
884namespace llvm</tt>' directive at their top, after the <tt>#include</tt>s. The
885general form of this rule is that any .cpp file that implements code in any
886namespace may use that namespace (and its parents'), but should not use any
887others.</p>
888
889</div>
890
891<!-- _______________________________________________________________________ -->
892<div class="doc_subsubsection">
893 <a name="ll_virtual_anch">Provide a virtual method anchor for classes
894 in headers</a>
895</div>
896
897<div class="doc_text">
898
899<p>If a class is defined in a header file and has a v-table (either it has
900virtual methods or it derives from classes with virtual methods), it must
901always have at least one out-of-line virtual method in the class. Without
Misha Brukmanbcc53732008-12-11 19:37:04 +0000[diff] [blame]902this, the compiler will copy the vtable and RTTI into every <tt>.o</tt> file
903that <tt>#include</tt>s the header, bloating <tt>.o</tt> file sizes and
904increasing link times.</p>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]905
906</div>
907
Chris Lattner01b8af32009-06-30 06:13:23 +0000[diff] [blame]908<!-- _______________________________________________________________________ -->
909<div class="doc_subsubsection">
910 <a name="ll_end">Don't evaluate end() every time through a loop</a>
911</div>
912
913<div class="doc_text">
914
915<p>Because C++ doesn't have a standard "foreach" loop (though it can be emulated
916with macros and may be coming in C++'0x) we end up writing a lot of loops that
917manually iterate from begin to end on a variety of containers or through other
918data structures. One common mistake is to write a loop in this style:</p>
919
920<div class="doc_code">
921<pre>
922 BasicBlock *BB = ...
923 for (BasicBlock::iterator I = BB->begin(); I != <b>BB->end()</b>; ++I)
924 ... use I ...
925</pre>
926</div>
927
928<p>The problem with this construct is that it evaluates "<tt>BB->end()</tt>"
929every time through the loop. Instead of writing the loop like this, we strongly
930prefer loops to be written so that they evaluate it once before the loop starts.
931A convenient way to do this is like so:</p>
932
933<div class="doc_code">
934<pre>
935 BasicBlock *BB = ...
936 for (BasicBlock::iterator I = BB->begin(), E = <b>BB->end()</b>; I != E; ++I)
937 ... use I ...
938</pre>
939</div>
940
941<p>The observant may quickly point out that these two loops may have different
942semantics: if the container (a basic block in this case) is being mutated, then
943"<tt>BB->end()</tt>" may change its value every time through the loop and the
944second loop may not in fact be correct. If you actually do depend on this
Chris Lattnerdc43b342009-06-30 06:20:03 +0000[diff] [blame]945behavior, please write the loop in the first form and add a comment indicating
Chris Lattner01b8af32009-06-30 06:13:23 +0000[diff] [blame]946that you did it intentionally.</p>
947
948<p>Why do we prefer the second form (when correct)? Writing the loop in the
949first form has two problems: First it may be less efficient than evaluating it
950at the start of the loop. In this case, the cost is probably minor: a few extra
951loads every time through the loop. However, if the base expression is more
952complex, then the cost can rise quickly. I've seen loops where the end
953expression was actually something like: "<tt>SomeMap[x]->end()</tt>" and map
Chris Lattner088affa2009-06-30 06:27:54 +0000[diff] [blame]954lookups really aren't cheap. By writing it in the second form consistently, you
Chris Lattner01b8af32009-06-30 06:13:23 +0000[diff] [blame]955eliminate the issue entirely and don't even have to think about it.</p>
956
Chris Lattnerdc43b342009-06-30 06:20:03 +0000[diff] [blame]957<p>The second (even bigger) issue is that writing the loop in the first form
Chris Lattner01b8af32009-06-30 06:13:23 +0000[diff] [blame]958hints to the reader that the loop is mutating the container (a fact that a
959comment would handily confirm!). If you write the loop in the second form, it
960is immediately obvious without even looking at the body of the loop that the
961container isn't being modified, which makes it easier to read the code and
962understand what it does.</p>
963
964<p>While the second form of the loop is a few extra keystrokes, we do strongly
965prefer it.</p>
966
967</div>
968
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]969<!-- _______________________________________________________________________ -->
970<div class="doc_subsubsection">
Chris Lattnereb1e8c32009-07-22 05:40:54 +0000[diff] [blame]971 <a name="ll_iostream"><tt>#include &lt;iostream&gt;</tt> is forbidden</a>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]972</div>
973
974<div class="doc_text">
975
Chris Lattnereb1e8c32009-07-22 05:40:54 +0000[diff] [blame]976<p>The use of <tt>#include &lt;iostream&gt;</tt> in library files is
977hereby <b><em>forbidden</em></b>. The primary reason for doing this is to
978support clients using LLVM libraries as part of larger systems. In particular,
979we statically link LLVM into some dynamic libraries. Even if LLVM isn't used,
980the static c'tors are run whenever an application start up that uses the dynamic
981library. There are two problems with this:</p>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]982
Chris Lattnereb1e8c32009-07-22 05:40:54 +0000[diff] [blame]983<ol>
984 <li>The time to run the static c'tors impacts startup time of
985 applications&mdash;a critical time for GUI apps.</li>
986 <li>The static c'tors cause the app to pull many extra pages of memory off the
987 disk: both the code for the static c'tors in each <tt>.o</tt> file and the
988 small amount of data that gets touched. In addition, touched/dirty pages
989 put more pressure on the VM system on low-memory machines.</li>
990</ol>
991
992<p>Note that using the other stream headers (<tt>&lt;sstream&gt;</tt> for
993example) is allowed normally, it is just <tt>&lt;iostream&gt;</tt> that is
994causing problems.</p>
995
Daniel Dunbar2f099eb2009-07-24 23:04:51 +0000[diff] [blame]996<p>In addition, new code should always
997use <a href="#ll_raw_ostream"><tt>raw_ostream</tt></a> or
998the <tt>llvm::MemoryBuffer</tt> API (for reading in files).</p>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]999
1000</div>
1001
Chris Lattnereb1e8c32009-07-22 05:40:54 +0000[diff] [blame]1002
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]1003<!-- _______________________________________________________________________ -->
1004<div class="doc_subsubsection">
1005 <a name="ll_avoidendl">Avoid <tt>std::endl</tt></a>
1006</div>
1007
1008<div class="doc_text">
1009
1010<p>The <tt>std::endl</tt> modifier, when used with iostreams outputs a newline
1011to the output stream specified. In addition to doing this, however, it also
1012flushes the output stream. In other words, these are equivalent:</p>
1013
1014<div class="doc_code">
1015<pre>
1016std::cout &lt;&lt; std::endl;
1017std::cout &lt;&lt; '\n' &lt;&lt; std::flush;
1018</pre>
1019</div>
1020
1021<p>Most of the time, you probably have no reason to flush the output stream, so
1022it's better to use a literal <tt>'\n'</tt>.</p>
1023
1024</div>
1025
1026
Daniel Dunbar2f099eb2009-07-24 23:04:51 +0000[diff] [blame]1027<!-- _______________________________________________________________________ -->
1028<div class="doc_subsubsection">
1029 <a name="ll_raw_ostream">Use <tt>raw_ostream</tt></a>
1030</div>
1031
1032<div class="doc_text">
1033
1034<p>LLVM includes a lightweight, simple, and efficient stream implementation
1035in <tt>llvm/Support/raw_ostream.h</tt> which provides all of the common features
Daniel Dunbared0da882009-07-24 23:54:34 +0000[diff] [blame]1036of <tt>std::ostream</tt>. All new code should use <tt>raw_ostream</tt> instead
1037of <tt>ostream</tt>.</p>
Daniel Dunbar2f099eb2009-07-24 23:04:51 +0000[diff] [blame]1038
Daniel Dunbared0da882009-07-24 23:54:34 +0000[diff] [blame]1039<p>Unlike <tt>std::ostream</tt>, <tt>raw_ostream</tt> is not a template and can
Daniel Dunbar2f099eb2009-07-24 23:04:51 +0000[diff] [blame]1040be forward declared as <tt>class raw_ostream</tt>. Public headers should
1041generally not include the <tt>raw_ostream</tt> header, but use forward
1042declarations and constant references to <tt>raw_ostream</tt> instances.</p>
1043
1044</div>
1045
1046
Chris Lattnereb1e8c32009-07-22 05:40:54 +0000[diff] [blame]1047<!-- ======================================================================= -->
1048<div class="doc_subsection">
1049 <a name="nano">Microscopic Details</a>
1050</div>
1051<!-- ======================================================================= -->
1052
1053<p>This section describes preferred low-level formatting guidelines along with
1054reasoning on why we prefer them.</p>
1055
1056<!-- _______________________________________________________________________ -->
1057<div class="doc_subsubsection">
1058 <a name="micro_spaceparen">Spaces Before Parentheses</a>
1059</div>
1060
1061<div class="doc_text">
1062
1063<p>We prefer to put a space before a parentheses only in control flow
1064statements, but not in normal function call expressions and function-like
1065macros. For example, this is good:</p>
1066
1067<div class="doc_code">
1068<pre>
1069 <b>if (</b>x) ...
1070 <b>for (</b>i = 0; i != 100; ++i) ...
1071 <b>while (</b>llvm_rocks) ...
1072
1073 <b>somefunc(</b>42);
1074 <b><a href="#ll_assert">assert</a>(</b>3 != 4 &amp;&amp; "laws of math are failing me");
1075
1076 a = <b>foo(</b>42, 92) + <b>bar(</b>x);
1077 </pre>
1078</div>
1079
1080<p>... and this is bad:</p>
1081
1082<div class="doc_code">
1083<pre>
1084 <b>if(</b>x) ...
1085 <b>for(</b>i = 0; i != 100; ++i) ...
1086 <b>while(</b>llvm_rocks) ...
1087
1088 <b>somefunc (</b>42);
1089 <b><a href="#ll_assert">assert</a> (</b>3 != 4 &amp;&amp; "laws of math are failing me");
1090
1091 a = <b>foo (</b>42, 92) + <b>bar (</b>x);
1092</pre>
1093</div>
1094
1095<p>The reason for doing this is not completely arbitrary. This style makes
1096 control flow operators stand out more, and makes expressions flow better. The
1097 function call operator binds very tightly as a postfix operator. Putting
1098 a space after a function name (as in the last example) makes it appear that
1099 the code might bind the arguments of the left-hand-side of a binary operator
1100 with the argument list of a function and the name of the right side. More
1101 specifically, it is easy to misread the "a" example as:</p>
1102
1103<div class="doc_code">
1104<pre>
1105 a = foo <b>(</b>(42, 92) + bar<b>)</b> (x);
1106</pre>
1107</div>
1108
1109<p>... when skimming through the code. By avoiding a space in a function, we
1110avoid this misinterpretation.</p>
1111
1112</div>
1113
1114<!-- _______________________________________________________________________ -->
1115<div class="doc_subsubsection">
1116 <a name="micro_preincrement">Prefer Preincrement</a>
1117</div>
1118
1119<div class="doc_text">
1120
1121<p>Hard fast rule: Preincrement (<tt>++X</tt>) may be no slower than
1122postincrement (<tt>X++</tt>) and could very well be a lot faster than it. Use
1123preincrementation whenever possible.</p>
1124
1125<p>The semantics of postincrement include making a copy of the value being
1126incremented, returning it, and then preincrementing the "work value". For
1127primitive types, this isn't a big deal... but for iterators, it can be a huge
1128issue (for example, some iterators contains stack and set objects in them...
1129copying an iterator could invoke the copy ctor's of these as well). In general,
1130get in the habit of always using preincrement, and you won't have a problem.</p>
1131
1132</div>
1133
1134<!-- _______________________________________________________________________ -->
1135<div class="doc_subsubsection">
1136 <a name="micro_namespaceindent">Namespace Indentation</a>
1137</div>
1138
1139<div class="doc_text">
1140
1141<p>
1142In general, we strive to reduce indentation where ever possible. This is useful
1143because we want code to <a href="#scf_codewidth">fit into 80 columns</a> without
1144wrapping horribly, but also because it makes it easier to understand the code.
1145Namespaces are a funny thing: they are often large, and we often desire to put
1146lots of stuff into them (so they can be large). Other times they are tiny,
1147because they just hold an enum or something similar. In order to balance this,
1148we use different approaches for small versus large namespaces.
1149</p>
1150
1151<p>
1152If a namespace definition is small and <em>easily</em> fits on a screen (say,
1153less than 35 lines of code), then you should indent its body. Here's an
1154example:
1155</p>
1156
1157<div class="doc_code">
1158<pre>
Chris Lattner5af04d22009-07-22 16:54:14 +0000[diff] [blame]1159namespace llvm {
1160 namespace X86 {
1161 /// RelocationType - An enum for the x86 relocation codes. Note that
1162 /// the terminology here doesn't follow x86 convention - word means
1163 /// 32-bit and dword means 64-bit.
1164 enum RelocationType {
1165 /// reloc_pcrel_word - PC relative relocation, add the relocated value to
1166 /// the value already in memory, after we adjust it for where the PC is.
1167 reloc_pcrel_word = 0,
1168
1169 /// reloc_picrel_word - PIC base relative relocation, add the relocated
1170 /// value to the value already in memory, after we adjust it for where the
1171 /// PIC base is.
1172 reloc_picrel_word = 1,
1173
1174 /// reloc_absolute_word, reloc_absolute_dword - Absolute relocation, just
1175 /// add the relocated value to the value already in memory.
1176 reloc_absolute_word = 2,
1177 reloc_absolute_dword = 3
1178 };
1179 }
Chris Lattnereb1e8c32009-07-22 05:40:54 +0000[diff] [blame]1180}
1181</pre>
1182</div>
1183
1184<p>Since the body is small, indenting adds value because it makes it very clear
1185where the namespace starts and ends, and it is easy to take the whole thing in
1186in one "gulp" when reading the code. If the blob of code in the namespace is
1187larger (as it typically is in a header in the llvm or clang namespaces), do not
1188indent the code, and add a comment indicating what namespace is being closed.
1189For example:</p>
1190
1191<div class="doc_code">
1192<pre>
1193namespace llvm {
1194namespace knowledge {
1195
1196/// Grokable - This class represents things that Smith can have an intimate
1197/// understanding of and contains the data associated with it.
1198class Grokable {
1199...
1200public:
1201 explicit Grokable() { ... }
1202 virtual ~Grokable() = 0;
1203
1204 ...
1205
1206};
1207
1208} // end namespace knowledge
1209} // end namespace llvm
1210</pre>
1211</div>
1212
1213<p>Because the class is large, we don't expect that the reader can easily
1214understand the entire concept in a glance, and the end of the file (where the
1215namespaces end) may be a long ways away from the place they open. As such,
1216indenting the contents of the namespace doesn't add any value, and detracts from
1217the readability of the class. In these cases it is best to <em>not</em> indent
1218the contents of the namespace.</p>
1219
1220</div>
1221
1222<!-- _______________________________________________________________________ -->
1223<div class="doc_subsubsection">
1224 <a name="micro_anonns">Anonymous Namespaces</a>
1225</div>
1226
1227<div class="doc_text">
1228
Chris Lattner5af04d22009-07-22 16:54:14 +0000[diff] [blame]1229<p>After talking about namespaces in general, you may be wondering about
1230anonymous namespaces in particular.
Chris Lattnereb1e8c32009-07-22 05:40:54 +0000[diff] [blame]1231Anonymous namespaces are a great language feature that tells the C++ compiler
1232that the contents of the namespace are only visible within the current
1233translation unit, allowing more aggressive optimization and eliminating the
1234possibility of symbol name collisions. Anonymous namespaces are to C++ as
1235"static" is to C functions and global variables. While "static" is available
1236in C++, anonymous namespaces are more general: they can make entire classes
1237private to a file.</p>
1238
1239<p>The problem with anonymous namespaces is that they naturally want to
1240encourage indentation of their body, and they reduce locality of reference: if
1241you see a random function definition in a C++ file, it is easy to see if it is
1242marked static, but seeing if it is in an anonymous namespace requires scanning
1243a big chunk of the file.</p>
1244
1245<p>Because of this, we have a simple guideline: make anonymous namespaces as
1246small as possible, and only use them for class declarations. For example, this
1247is good:</p>
1248
1249<div class="doc_code">
1250<pre>
1251<b>namespace {</b>
1252 class StringSort {
1253 ...
1254 public:
1255 StringSort(...)
1256 bool operator&lt;(const char *RHS) const;
1257 };
1258<b>} // end anonymous namespace</b>
1259
1260static void Helper() {
1261 ...
1262}
1263
1264bool StringSort::operator&lt;(const char *RHS) const {
1265 ...
1266}
1267
1268</pre>
1269</div>
1270
1271<p>This is bad:</p>
1272
1273
1274<div class="doc_code">
1275<pre>
1276<b>namespace {</b>
1277class StringSort {
1278...
1279public:
1280 StringSort(...)
1281 bool operator&lt;(const char *RHS) const;
1282};
1283
1284void Helper() {
1285 ...
1286}
1287
1288bool StringSort::operator&lt;(const char *RHS) const {
1289 ...
1290}
1291
1292<b>} // end anonymous namespace</b>
1293
1294</pre>
1295</div>
1296
1297
1298<p>This is bad specifically because if you're looking at "Helper" in the middle
1299of a large C++ file, that you have no immediate way to tell if it is local to
1300the file. When it is marked static explicitly, this is immediately obvious.
1301Also, there is no reason to enclose the definition of "operator&lt;" in the
Chris Lattner5af04d22009-07-22 16:54:14 +0000[diff] [blame]1302namespace just because it was declared there.
Chris Lattnereb1e8c32009-07-22 05:40:54 +0000[diff] [blame]1303</p>
1304
1305</div>
1306
1307
1308
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]1309<!-- *********************************************************************** -->
1310<div class="doc_section">
1311 <a name="seealso">See Also</a>
1312</div>
1313<!-- *********************************************************************** -->
1314
1315<div class="doc_text">
1316
1317<p>A lot of these comments and recommendations have been culled for other
1318sources. Two particularly important books for our work are:</p>
1319
1320<ol>
1321
Chris Lattnerb5955b72007-11-09 21:49:08 +0000[diff] [blame]1322<li><a href="http://www.amazon.com/Effective-Specific-Addison-Wesley-Professional-Computing/dp/0321334876">Effective
1323C++</a> by Scott Meyers. Also
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]1324interesting and useful are "More Effective C++" and "Effective STL" by the same
1325author.</li>
1326
Chris Lattnerb5955b72007-11-09 21:49:08 +0000[diff] [blame]1327<li>Large-Scale C++ Software Design by John Lakos</li>
Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]1328
1329</ol>
1330
1331<p>If you get some free time, and you haven't read them: do so, you might learn
1332something.</p>
1333
1334</div>
1335
1336<!-- *********************************************************************** -->
1337
1338<hr>
1339<address>
1340 <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
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Dan Gohmanf17a25c2007-07-18 16:29:46 +0000[diff] [blame]1344
1345 <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
1346 <a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
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