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6 <title>LLVM Coding Standards</title>
10 <div class="doc_title">
15 <li><a href="#introduction">Introduction</a></li>
16 <li><a href="#mechanicalissues">Mechanical Source Issues</a>
18 <li><a href="#sourceformating">Source Code Formatting</a>
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>
27 <li><a href="#compilerissues">Compiler Issues</a>
29 <li><a href="#ci_warningerrors">Treat Compiler Warnings Like
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>
35 <li><a href="#styleissues">Style Issues</a>
37 <li><a href="#macro">The High Level Issues</a>
39 <li><a href="#hl_module">A Public Header File <b>is</b> a
41 <li><a href="#hl_dontinclude">#include as Little as Possible</a></li>
42 <li><a href="#hl_privateheaders">Keep "internal" Headers
44 <li><a href="#ll_iostream"><tt>#include <iostream></tt> is
45 <em>forbidden</em></a></li>
47 <li><a href="#micro">The Low Level Issues</a>
49 <li><a href="#ll_assert">Assert Liberally</a></li>
50 <li><a href="#ll_ns_std">Do not use 'using namespace std'</a></li>
51 <li><a href="#ll_virtual_anch">Provide a virtual method anchor for
52 classes in headers</a></li>
53 <li><a href="#ll_end">Don't evaluate end() every time through a
55 <li><a href="#ll_preincrement">Prefer Preincrement</a></li>
56 <li><a href="#ll_avoidendl">Avoid <tt>std::endl</tt></a></li>
59 <li><a href="#seealso">See Also</a></li>
62 <div class="doc_author">
63 <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a> and
64 <a href="mailto:void@nondot.org">Bill Wendling</a></p>
68 <!-- *********************************************************************** -->
69 <div class="doc_section">
70 <a name="introduction">Introduction</a>
72 <!-- *********************************************************************** -->
74 <div class="doc_text">
76 <p>This document attempts to describe a few coding standards that are being used
77 in the LLVM source tree. Although no coding standards should be regarded as
78 absolute requirements to be followed in all instances, coding standards can be
81 <p>This document intentionally does not prescribe fixed standards for religious
82 issues such as brace placement and space usage. For issues like this, follow
87 <p><b><a name="goldenrule">If you are adding a significant body of source to a
88 project, feel free to use whatever style you are most comfortable with. If you
89 are extending, enhancing, or bug fixing already implemented code, use the style
90 that is already being used so that the source is uniform and easy to
95 <p>The ultimate goal of these guidelines is the increase readability and
96 maintainability of our common source base. If you have suggestions for topics to
97 be included, please mail them to <a
98 href="mailto:sabre@nondot.org">Chris</a>.</p>
102 <!-- *********************************************************************** -->
103 <div class="doc_section">
104 <a name="mechanicalissues">Mechanical Source Issues</a>
106 <!-- *********************************************************************** -->
108 <!-- ======================================================================= -->
109 <div class="doc_subsection">
110 <a name="sourceformating">Source Code Formatting</a>
113 <!-- _______________________________________________________________________ -->
114 <div class="doc_subsubsection">
115 <a name="scf_commenting">Commenting</a>
118 <div class="doc_text">
120 <p>Comments are one critical part of readability and maintainability. Everyone
121 knows they should comment, so should you. When writing comments, write them as
122 English prose, which means they should use proper capitalization, punctuation,
123 etc. Although we all should probably
124 comment our code more than we do, there are a few very critical places that
125 documentation is very useful:</p>
129 <p>Every source file should have a header on it that describes the basic
130 purpose of the file. If a file does not have a header, it should not be
131 checked into Subversion. Most source trees will probably have a standard
132 file header format. The standard format for the LLVM source tree looks like
135 <div class="doc_code">
137 //===-- llvm/Instruction.h - Instruction class definition -------*- C++ -*-===//
139 // The LLVM Compiler Infrastructure
141 // This file is distributed under the University of Illinois Open Source
142 // License. See LICENSE.TXT for details.
144 //===----------------------------------------------------------------------===//
146 // This file contains the declaration of the Instruction class, which is the
147 // base class for all of the VM instructions.
149 //===----------------------------------------------------------------------===//
153 <p>A few things to note about this particular format: The "<tt>-*- C++
154 -*-</tt>" string on the first line is there to tell Emacs that the source file
155 is a C++ file, not a C file (Emacs assumes .h files are C files by default).
156 Note that this tag is not necessary in .cpp files. The name of the file is also
157 on the first line, along with a very short description of the purpose of the
158 file. This is important when printing out code and flipping though lots of
161 <p>The next section in the file is a concise note that defines the license
162 that the file is released under. This makes it perfectly clear what terms the
163 source code can be distributed under and should not be modified in any way.</p>
165 <p>The main body of the description does not have to be very long in most cases.
166 Here it's only two lines. If an algorithm is being implemented or something
167 tricky is going on, a reference to the paper where it is published should be
168 included, as well as any notes or "gotchas" in the code to watch out for.</p>
170 <b>Class overviews</b>
172 <p>Classes are one fundamental part of a good object oriented design. As such,
173 a class definition should have a comment block that explains what the class is
174 used for... if it's not obvious. If it's so completely obvious your grandma
175 could figure it out, it's probably safe to leave it out. Naming classes
176 something sane goes a long ways towards avoiding writing documentation.</p>
179 <b>Method information</b>
181 <p>Methods defined in a class (as well as any global functions) should also be
182 documented properly. A quick note about what it does any a description of the
183 borderline behaviour is all that is necessary here (unless something
184 particularly tricky or insideous is going on). The hope is that people can
185 figure out how to use your interfaces without reading the code itself... that is
188 <p>Good things to talk about here are what happens when something unexpected
189 happens: does the method return null? Abort? Format your hard disk?</p>
193 <!-- _______________________________________________________________________ -->
194 <div class="doc_subsubsection">
195 <a name="scf_commentformat">Comment Formatting</a>
198 <div class="doc_text">
200 <p>In general, prefer C++ style (<tt>//</tt>) comments. They take less space,
201 require less typing, don't have nesting problems, etc. There are a few cases
202 when it is useful to use C style (<tt>/* */</tt>) comments however:</p>
205 <li>When writing a C code: Obviously if you are writing C code, use C style
207 <li>When writing a header file that may be <tt>#include</tt>d by a C source
209 <li>When writing a source file that is used by a tool that only accepts C
213 <p>To comment out a large block of code, use <tt>#if 0</tt> and <tt>#endif</tt>.
214 These nest properly and are better behaved in general than C style comments.</p>
218 <!-- _______________________________________________________________________ -->
219 <div class="doc_subsubsection">
220 <a name="scf_includes"><tt>#include</tt> Style</a>
223 <div class="doc_text">
225 <p>Immediately after the <a href="#scf_commenting">header file comment</a> (and
226 include guards if working on a header file), the <a
227 href="#hl_dontinclude">minimal</a> list of <tt>#include</tt>s required by the
228 file should be listed. We prefer these <tt>#include</tt>s to be listed in this
232 <li><a href="#mmheader">Main Module header</a></li>
233 <li><a href="#hl_privateheaders">Local/Private Headers</a></li>
234 <li><tt>llvm/*</tt></li>
235 <li><tt>llvm/Analysis/*</tt></li>
236 <li><tt>llvm/Assembly/*</tt></li>
237 <li><tt>llvm/Bytecode/*</tt></li>
238 <li><tt>llvm/CodeGen/*</tt></li>
240 <li><tt>Support/*</tt></li>
241 <li><tt>Config/*</tt></li>
242 <li>System <tt>#includes</tt></li>
245 <p>... and each category should be sorted by name.</p>
247 <p><a name="mmheader">The "Main Module Header"</a> file applies to .cpp file
248 which implement an interface defined by a .h file. This <tt>#include</tt>
249 should always be included <b>first</b> regardless of where it lives on the file
250 system. By including a header file first in the .cpp files that implement the
251 interfaces, we ensure that the header does not have any hidden dependencies
252 which are not explicitly #included in the header, but should be. It is also a
253 form of documentation in the .cpp file to indicate where the interfaces it
254 implements are defined.</p>
258 <!-- _______________________________________________________________________ -->
259 <div class="doc_subsubsection">
260 <a name="scf_codewidth">Source Code Width</a>
263 <div class="doc_text">
265 <p>Write your code to fit within 80 columns of text. This helps those of us who
266 like to print out code and look at your code in an xterm without resizing
269 <p>The longer answer is that there must be some limit to the width of the code
270 in order to reasonably allow developers to have multiple files side-by-side in
271 windows on a modest display. If you are going to pick a width limit, it is
272 somewhat arbitrary but you might as well pick something standard. Going with
273 90 columns (for example) instead of 80 columns wouldn't add any significant
274 value and would be detrimental to printing out code. Also many other projects
275 have standardized on 80 columns, so some people have already configured their
276 editors for it (vs something else, like 90 columns).</p>
278 <p>This is one of many contentious issues in coding standards, but is not up
283 <!-- _______________________________________________________________________ -->
284 <div class="doc_subsubsection">
285 <a name="scf_spacestabs">Use Spaces Instead of Tabs</a>
288 <div class="doc_text">
290 <p>In all cases, prefer spaces to tabs in source files. People have different
291 prefered indentation levels, and different styles of indentation that they
292 like... this is fine. What isn't is that different editors/viewers expand tabs
293 out to different tab stops. This can cause your code to look completely
294 unreadable, and it is not worth dealing with.</p>
296 <p>As always, follow the <a href="#goldenrule">Golden Rule</a> above: follow the
297 style of existing code if your are modifying and extending it. If you like four
298 spaces of indentation, <b>DO NOT</b> do that in the middle of a chunk of code
299 with two spaces of indentation. Also, do not reindent a whole source file: it
300 makes for incredible diffs that are absolutely worthless.</p>
304 <!-- _______________________________________________________________________ -->
305 <div class="doc_subsubsection">
306 <a name="scf_indentation">Indent Code Consistently</a>
309 <div class="doc_text">
311 <p>Okay, your first year of programming you were told that indentation is
312 important. If you didn't believe and internalize this then, now is the time.
318 <!-- ======================================================================= -->
319 <div class="doc_subsection">
320 <a name="compilerissues">Compiler Issues</a>
324 <!-- _______________________________________________________________________ -->
325 <div class="doc_subsubsection">
326 <a name="ci_warningerrors">Treat Compiler Warnings Like Errors</a>
329 <div class="doc_text">
331 <p>If your code has compiler warnings in it, something is wrong: you aren't
332 casting values correctly, your have "questionable" constructs in your code, or
333 you are doing something legitimately wrong. Compiler warnings can cover up
334 legitimate errors in output and make dealing with a translation unit
337 <p>It is not possible to prevent all warnings from all compilers, nor is it
338 desirable. Instead, pick a standard compiler (like <tt>gcc</tt>) that provides
339 a good thorough set of warnings, and stick to them. At least in the case of
340 <tt>gcc</tt>, it is possible to work around any spurious errors by changing the
341 syntax of the code slightly. For example, an warning that annoys me occurs when
342 I write code like this:</p>
344 <div class="doc_code">
346 if (V = getValue()) {
352 <p><tt>gcc</tt> will warn me that I probably want to use the <tt>==</tt>
353 operator, and that I probably mistyped it. In most cases, I haven't, and I
354 really don't want the spurious errors. To fix this particular problem, I
355 rewrite the code like this:</p>
357 <div class="doc_code">
359 if ((V = getValue())) {
365 <p>...which shuts <tt>gcc</tt> up. Any <tt>gcc</tt> warning that annoys you can
366 be fixed by massaging the code appropriately.</p>
368 <p>These are the <tt>gcc</tt> warnings that I prefer to enable: <tt>-Wall
369 -Winline -W -Wwrite-strings -Wno-unused</tt></p>
373 <!-- _______________________________________________________________________ -->
374 <div class="doc_subsubsection">
375 <a name="ci_portable_code">Write Portable Code</a>
378 <div class="doc_text">
380 <p>In almost all cases, it is possible and within reason to write completely
381 portable code. If there are cases where it isn't possible to write portable
382 code, isolate it behind a well defined (and well documented) interface.</p>
384 <p>In practice, this means that you shouldn't assume much about the host
385 compiler, including its support for "high tech" features like partial
386 specialization of templates. If these features are used, they should only be
387 an implementation detail of a library which has a simple exposed API.</p>
391 <!-- _______________________________________________________________________ -->
392 <div class="doc_subsubsection">
393 <a name="ci_class_struct">Use of <tt>class</tt> and <tt>struct</tt> Keywords</a>
395 <div class="doc_text">
397 <p>In C++, the <tt>class</tt> and <tt>struct</tt> keywords can be used almost
398 interchangeably. The only difference is when they are used to declare a class:
399 <tt>class</tt> makes all members private by default while <tt>struct</tt> makes
400 all members public by default.</p>
402 <p>Unfortunately, not all compilers follow the rules and some will generate
403 different symbols based on whether <tt>class</tt> or <tt>struct</tt> was used to
404 declare the symbol. This can lead to problems at link time.</p>
406 <p>So, the rule for LLVM is to always use the <tt>class</tt> keyword, unless
407 <b>all</b> members are public, in which case <tt>struct</tt> is allowed.</p>
411 <!-- *********************************************************************** -->
412 <div class="doc_section">
413 <a name="styleissues">Style Issues</a>
415 <!-- *********************************************************************** -->
418 <!-- ======================================================================= -->
419 <div class="doc_subsection">
420 <a name="macro">The High Level Issues</a>
424 <!-- _______________________________________________________________________ -->
425 <div class="doc_subsubsection">
426 <a name="hl_module">A Public Header File <b>is</b> a Module</a>
429 <div class="doc_text">
431 <p>C++ doesn't do too well in the modularity department. There is no real
432 encapsulation or data hiding (unless you use expensive protocol classes), but it
433 is what we have to work with. When you write a public header file (in the LLVM
434 source tree, they live in the top level "include" directory), you are defining a
435 module of functionality.</p>
437 <p>Ideally, modules should be completely independent of each other, and their
438 header files should only include the absolute minimum number of headers
439 possible. A module is not just a class, a function, or a namespace: <a
440 href="http://www.cuj.com/articles/2000/0002/0002c/0002c.htm">it's a collection
441 of these</a> that defines an interface. This interface may be several
442 functions, classes or data structures, but the important issue is how they work
445 <p>In general, a module should be implemented with one or more <tt>.cpp</tt>
446 files. Each of these <tt>.cpp</tt> files should include the header that defines
447 their interface first. This ensure that all of the dependences of the module
448 header have been properly added to the module header itself, and are not
449 implicit. System headers should be included after user headers for a
450 translation unit.</p>
454 <!-- _______________________________________________________________________ -->
455 <div class="doc_subsubsection">
456 <a name="hl_dontinclude"><tt>#include</tt> as Little as Possible</a>
459 <div class="doc_text">
461 <p><tt>#include</tt> hurts compile time performance. Don't do it unless you
462 have to, especially in header files.</p>
464 <p>But wait, sometimes you need to have the definition of a class to use it, or
465 to inherit from it. In these cases go ahead and <tt>#include</tt> that header
466 file. Be aware however that there are many cases where you don't need to have
467 the full definition of a class. If you are using a pointer or reference to a
468 class, you don't need the header file. If you are simply returning a class
469 instance from a prototyped function or method, you don't need it. In fact, for
470 most cases, you simply don't need the definition of a class... and not
471 <tt>#include</tt>'ing speeds up compilation.</p>
473 <p>It is easy to try to go too overboard on this recommendation, however. You
474 <b>must</b> include all of the header files that you are using -- you can
475 include them either directly
476 or indirectly (through another header file). To make sure that you don't
477 accidently forget to include a header file in your module header, make sure to
478 include your module header <b>first</b> in the implementation file (as mentioned
479 above). This way there won't be any hidden dependencies that you'll find out
484 <!-- _______________________________________________________________________ -->
485 <div class="doc_subsubsection">
486 <a name="hl_privateheaders">Keep "internal" Headers Private</a>
489 <div class="doc_text">
491 <p>Many modules have a complex implementation that causes them to use more than
492 one implementation (<tt>.cpp</tt>) file. It is often tempting to put the
493 internal communication interface (helper classes, extra functions, etc) in the
494 public module header file. Don't do this.</p>
496 <p>If you really need to do something like this, put a private header file in
497 the same directory as the source files, and include it locally. This ensures
498 that your private interface remains private and undisturbed by outsiders.</p>
500 <p>Note however, that it's okay to put extra implementation methods a public
501 class itself... just make them private (or protected), and all is well.</p>
505 <!-- _______________________________________________________________________ -->
506 <div class="doc_subsubsection">
507 <a name="ll_iostream"><tt>#include <iostream></tt> is forbidden</a>
510 <div class="doc_text">
512 <p>The use of <tt>#include <iostream></tt> in library files is
513 hereby <b><em>forbidden</em></b>. The primary reason for doing this is to
514 support clients using LLVM libraries as part of larger systems. In particular,
515 we statically link LLVM into some dynamic libraries. Even if LLVM isn't used,
516 the static c'tors are run whenever an application start up that uses the dynamic
517 library. There are two problems with this:</p>
520 <li>The time to run the static c'tors impacts startup time of
521 applications—a critical time for GUI apps.</li>
522 <li>The static c'tors cause the app to pull many extra pages of memory off the
523 disk: both the code for the static c'tors in each <tt>.o</tt> file and the
524 small amount of data that gets touched. In addition, touched/dirty pages
525 put more pressure on the VM system on low-memory machines.</li>
528 <p>Note that using the other stream headers (<tt><sstream></tt> for
529 example) is allowed normally, it is just <tt><iostream></tt> that is
530 causing problems.</p>
532 <p>The preferred replacement for stream functionality is the
533 <tt>llvm::raw_ostream</tt> class (for writing to output streams of various
534 sorts) and the <tt>llvm::MemoryBuffer</tt> API (for reading in files).</p>
539 <!-- ======================================================================= -->
540 <div class="doc_subsection">
541 <a name="micro">The Low Level Issues</a>
545 <!-- _______________________________________________________________________ -->
546 <div class="doc_subsubsection">
547 <a name="ll_assert">Assert Liberally</a>
550 <div class="doc_text">
552 <p>Use the "<tt>assert</tt>" function to its fullest. Check all of your
553 preconditions and assumptions, you never know when a bug (not neccesarily even
554 yours) might be caught early by an assertion, which reduces debugging time
555 dramatically. The "<tt><cassert></tt>" header file is probably already
556 included by the header files you are using, so it doesn't cost anything to use
559 <p>To further assist with debugging, make sure to put some kind of error message
560 in the assertion statement (which is printed if the assertion is tripped). This
561 helps the poor debugging make sense of why an assertion is being made and
562 enforced, and hopefully what to do about it. Here is one complete example:</p>
564 <div class="doc_code">
566 inline Value *getOperand(unsigned i) {
567 assert(i < Operands.size() && "getOperand() out of range!");
573 <p>Here are some examples:</p>
575 <div class="doc_code">
577 assert(Ty->isPointerType() && "Can't allocate a non pointer type!");
579 assert((Opcode == Shl || Opcode == Shr) && "ShiftInst Opcode invalid!");
581 assert(idx < getNumSuccessors() && "Successor # out of range!");
583 assert(V1.getType() == V2.getType() && "Constant types must be identical!");
585 assert(isa<PHINode>(Succ->front()) && "Only works on PHId BBs!");
589 <p>You get the idea...</p>
591 <p>Please be aware when adding assert statements that not all compilers are aware of
592 the semantics of the assert. In some places, asserts are used to indicate a piece of
593 code that should not be reached. These are typically of the form:</p>
595 <div class="doc_code">
597 assert(0 && "Some helpful error message");
601 <p>When used in a function that returns a value, they should be followed with a return
602 statement and a comment indicating that this line is never reached. This will prevent
603 a compiler which is unable to deduce that the assert statement never returns from
604 generating a warning.</p>
606 <div class="doc_code">
608 assert(0 && "Some helpful error message");
616 <!-- _______________________________________________________________________ -->
617 <div class="doc_subsubsection">
618 <a name="ll_ns_std">Do not use '<tt>using namespace std</tt>'</a>
621 <div class="doc_text">
622 <p>In LLVM, we prefer to explicitly prefix all identifiers from the standard
623 namespace with an "<tt>std::</tt>" prefix, rather than rely on
624 "<tt>using namespace std;</tt>".</p>
626 <p> In header files, adding a '<tt>using namespace XXX</tt>' directive pollutes
627 the namespace of any source file that <tt>#include</tt>s the header. This is
628 clearly a bad thing.</p>
630 <p>In implementation files (e.g. .cpp files), the rule is more of a stylistic
631 rule, but is still important. Basically, using explicit namespace prefixes
632 makes the code <b>clearer</b>, because it is immediately obvious what facilities
633 are being used and where they are coming from, and <b>more portable</b>, because
634 namespace clashes cannot occur between LLVM code and other namespaces. The
635 portability rule is important because different standard library implementations
636 expose different symbols (potentially ones they shouldn't), and future revisions
637 to the C++ standard will add more symbols to the <tt>std</tt> namespace. As
638 such, we never use '<tt>using namespace std;</tt>' in LLVM.</p>
640 <p>The exception to the general rule (i.e. it's not an exception for
641 the <tt>std</tt> namespace) is for implementation files. For example, all of
642 the code in the LLVM project implements code that lives in the 'llvm' namespace.
643 As such, it is ok, and actually clearer, for the .cpp files to have a '<tt>using
644 namespace llvm</tt>' directive at their top, after the <tt>#include</tt>s. The
645 general form of this rule is that any .cpp file that implements code in any
646 namespace may use that namespace (and its parents'), but should not use any
651 <!-- _______________________________________________________________________ -->
652 <div class="doc_subsubsection">
653 <a name="ll_virtual_anch">Provide a virtual method anchor for classes
657 <div class="doc_text">
659 <p>If a class is defined in a header file and has a v-table (either it has
660 virtual methods or it derives from classes with virtual methods), it must
661 always have at least one out-of-line virtual method in the class. Without
662 this, the compiler will copy the vtable and RTTI into every <tt>.o</tt> file
663 that <tt>#include</tt>s the header, bloating <tt>.o</tt> file sizes and
664 increasing link times.</p>
668 <!-- _______________________________________________________________________ -->
669 <div class="doc_subsubsection">
670 <a name="ll_end">Don't evaluate end() every time through a loop</a>
673 <div class="doc_text">
675 <p>Because C++ doesn't have a standard "foreach" loop (though it can be emulated
676 with macros and may be coming in C++'0x) we end up writing a lot of loops that
677 manually iterate from begin to end on a variety of containers or through other
678 data structures. One common mistake is to write a loop in this style:</p>
680 <div class="doc_code">
683 for (BasicBlock::iterator I = BB->begin(); I != <b>BB->end()</b>; ++I)
688 <p>The problem with this construct is that it evaluates "<tt>BB->end()</tt>"
689 every time through the loop. Instead of writing the loop like this, we strongly
690 prefer loops to be written so that they evaluate it once before the loop starts.
691 A convenient way to do this is like so:</p>
693 <div class="doc_code">
696 for (BasicBlock::iterator I = BB->begin(), E = <b>BB->end()</b>; I != E; ++I)
701 <p>The observant may quickly point out that these two loops may have different
702 semantics: if the container (a basic block in this case) is being mutated, then
703 "<tt>BB->end()</tt>" may change its value every time through the loop and the
704 second loop may not in fact be correct. If you actually do depend on this
705 behavior, please write the loop in the first form and add a comment indicating
706 that you did it intentionally.</p>
708 <p>Why do we prefer the second form (when correct)? Writing the loop in the
709 first form has two problems: First it may be less efficient than evaluating it
710 at the start of the loop. In this case, the cost is probably minor: a few extra
711 loads every time through the loop. However, if the base expression is more
712 complex, then the cost can rise quickly. I've seen loops where the end
713 expression was actually something like: "<tt>SomeMap[x]->end()</tt>" and map
714 lookups really aren't cheap. By writing it in the second form consistently, you
715 eliminate the issue entirely and don't even have to think about it.</p>
717 <p>The second (even bigger) issue is that writing the loop in the first form
718 hints to the reader that the loop is mutating the container (a fact that a
719 comment would handily confirm!). If you write the loop in the second form, it
720 is immediately obvious without even looking at the body of the loop that the
721 container isn't being modified, which makes it easier to read the code and
722 understand what it does.</p>
724 <p>While the second form of the loop is a few extra keystrokes, we do strongly
730 <!-- _______________________________________________________________________ -->
731 <div class="doc_subsubsection">
732 <a name="ll_preincrement">Prefer Preincrement</a>
735 <div class="doc_text">
737 <p>Hard fast rule: Preincrement (<tt>++X</tt>) may be no slower than
738 postincrement (<tt>X++</tt>) and could very well be a lot faster than it. Use
739 preincrementation whenever possible.</p>
741 <p>The semantics of postincrement include making a copy of the value being
742 incremented, returning it, and then preincrementing the "work value". For
743 primitive types, this isn't a big deal... but for iterators, it can be a huge
744 issue (for example, some iterators contains stack and set objects in them...
745 copying an iterator could invoke the copy ctor's of these as well). In general,
746 get in the habit of always using preincrement, and you won't have a problem.</p>
750 <!-- _______________________________________________________________________ -->
751 <div class="doc_subsubsection">
752 <a name="ll_avoidendl">Avoid <tt>std::endl</tt></a>
755 <div class="doc_text">
757 <p>The <tt>std::endl</tt> modifier, when used with iostreams outputs a newline
758 to the output stream specified. In addition to doing this, however, it also
759 flushes the output stream. In other words, these are equivalent:</p>
761 <div class="doc_code">
763 std::cout << std::endl;
764 std::cout << '\n' << std::flush;
768 <p>Most of the time, you probably have no reason to flush the output stream, so
769 it's better to use a literal <tt>'\n'</tt>.</p>
774 <!-- *********************************************************************** -->
775 <div class="doc_section">
776 <a name="seealso">See Also</a>
778 <!-- *********************************************************************** -->
780 <div class="doc_text">
782 <p>A lot of these comments and recommendations have been culled for other
783 sources. Two particularly important books for our work are:</p>
787 <li><a href="http://www.amazon.com/Effective-Specific-Addison-Wesley-Professional-Computing/dp/0321334876">Effective
788 C++</a> by Scott Meyers. Also
789 interesting and useful are "More Effective C++" and "Effective STL" by the same
792 <li>Large-Scale C++ Software Design by John Lakos</li>
796 <p>If you get some free time, and you haven't read them: do so, you might learn
801 <!-- *********************************************************************** -->
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810 <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
811 <a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
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