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3 <html>
4 <head>
7 </head>
8 <body>
11 LLVM Coding Standards
12 </div>
14 <ol>
17 <ol>
19 <ol>
26 </ol></li>
28 <ol>
30 Errors</a></li>
33 </ol></li>
34 </ol></li>
36 <ol>
38 <ol>
40 Module</a></li>
43 Private</a></li>
45 Code</a></li>
47 return</a></li>
49 Functions</a></li>
50 </ol></li>
52 <ol>
56 classes in headers</a></li>
58 loop</a></li>
63 </ol></li>
66 <ol>
71 </ol></li>
74 </ol></li>
76 </ol>
80 </div>
83 <!-- *********************************************************************** -->
86 </div>
87 <!-- *********************************************************************** -->
91 <p>This document attempts to describe a few coding standards that are being used
92 in the LLVM source tree. Although no coding standards should be regarded as
93 absolute requirements to be followed in all instances, coding standards can be
94 useful.</p>
96 <p>This document intentionally does not prescribe fixed standards for religious
97 issues such as brace placement and space usage. For issues like this, follow
98 the golden rule:</p>
100 <blockquote>
103 project, feel free to use whatever style you are most comfortable with. If you
104 are extending, enhancing, or bug fixing already implemented code, use the style
105 that is already being used so that the source is uniform and easy to
106 follow.</a></b></p>
108 </blockquote>
110 <p>The ultimate goal of these guidelines is the increase readability and
111 maintainability of our common source base. If you have suggestions for topics to
112 be included, please mail them to <a
115 </div>
117 <!-- *********************************************************************** -->
120 </div>
121 <!-- *********************************************************************** -->
123 <!-- ======================================================================= -->
126 </div>
128 <!-- _______________________________________________________________________ -->
131 </div>
135 <p>Comments are one critical part of readability and maintainability. Everyone
136 knows they should comment, so should you. When writing comments, write them as
137 English prose, which means they should use proper capitalization, punctuation,
138 etc. Although we all should probably
139 comment our code more than we do, there are a few very critical places that
140 documentation is very useful:</p>
144 <p>Every source file should have a header on it that describes the basic
145 purpose of the file. If a file does not have a header, it should not be
146 checked into Subversion. Most source trees will probably have a standard
147 file header format. The standard format for the LLVM source tree looks like
148 this:</p>
151 <pre>
152 //===-- llvm/Instruction.h - Instruction class definition -------*- C++ -*-===//
153 //
154 // The LLVM Compiler Infrastructure
155 //
156 // This file is distributed under the University of Illinois Open Source
157 // License. See LICENSE.TXT for details.
158 //
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>
169 -*-</tt>" string on the first line is there to tell Emacs that the source file
170 is a C++ file, not a C file (Emacs assumes .h files are C files by default).
171 Note that this tag is not necessary in .cpp files. The name of the file is also
172 on the first line, along with a very short description of the purpose of the
173 file. This is important when printing out code and flipping though lots of
174 pages.</p>
176 <p>The next section in the file is a concise note that defines the license
177 that the file is released under. This makes it perfectly clear what terms the
178 source code can be distributed under and should not be modified in any way.</p>
180 <p>The main body of the description does not have to be very long in most cases.
181 Here it's only two lines. If an algorithm is being implemented or something
182 tricky is going on, a reference to the paper where it is published should be
187 <p>Classes are one fundamental part of a good object oriented design. As such,
188 a class definition should have a comment block that explains what the class is
189 used for... if it's not obvious. If it's so completely obvious your grandma
190 could figure it out, it's probably safe to leave it out. Naming classes
191 something sane goes a long ways towards avoiding writing documentation.</p>
196 <p>Methods defined in a class (as well as any global functions) should also be
197 documented properly. A quick note about what it does and a description of the
198 borderline behaviour is all that is necessary here (unless something
199 particularly tricky or insidious is going on). The hope is that people can
200 figure out how to use your interfaces without reading the code itself... that is
201 the goal metric.</p>
203 <p>Good things to talk about here are what happens when something unexpected
204 happens: does the method return null? Abort? Format your hard disk?</p>
206 </div>
208 <!-- _______________________________________________________________________ -->
211 </div>
216 require less typing, don't have nesting problems, etc. There are a few cases
219 <ol>
220 <li>When writing a C code: Obviously if you are writing C code, use C style
221 comments.</li>
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>
229 These nest properly and are better behaved in general than C style comments.</p>
231 </div>
233 <!-- _______________________________________________________________________ -->
236 </div>
241 include guards if working on a header file), the <a
244 order:</p>
246 <ol>
258 </ol>
265 system. By including a header file first in the .cpp files that implement the
266 interfaces, we ensure that the header does not have any hidden dependencies
267 which are not explicitly #included in the header, but should be. It is also a
268 form of documentation in the .cpp file to indicate where the interfaces it
269 implements are defined.</p>
271 </div>
273 <!-- _______________________________________________________________________ -->
276 </div>
281 like to print out code and look at your code in an xterm without resizing
282 it.</p>
284 <p>The longer answer is that there must be some limit to the width of the code
285 in order to reasonably allow developers to have multiple files side-by-side in
286 windows on a modest display. If you are going to pick a width limit, it is
287 somewhat arbitrary but you might as well pick something standard. Going with
289 value and would be detrimental to printing out code. Also many other projects
290 have standardized on 80 columns, so some people have already configured their
293 <p>This is one of many contentious issues in coding standards, but is not up
294 for debate.</p>
296 </div>
298 <!-- _______________________________________________________________________ -->
301 </div>
305 <p>In all cases, prefer spaces to tabs in source files. People have different
306 preferred indentation levels, and different styles of indentation that they
307 like... this is fine. What isn't is that different editors/viewers expand tabs
308 out to different tab stops. This can cause your code to look completely
309 unreadable, and it is not worth dealing with.</p>
312 style of existing code if your are modifying and extending it. If you like four
314 with two spaces of indentation. Also, do not reindent a whole source file: it
315 makes for incredible diffs that are absolutely worthless.</p>
317 </div>
319 <!-- _______________________________________________________________________ -->
322 </div>
326 <p>Okay, your first year of programming you were told that indentation is
327 important. If you didn't believe and internalize this then, now is the time.
328 Just do it.</p>
330 </div>
333 <!-- ======================================================================= -->
336 </div>
339 <!-- _______________________________________________________________________ -->
342 </div>
346 <p>If your code has compiler warnings in it, something is wrong: you aren't
347 casting values correctly, your have "questionable" constructs in your code, or
348 you are doing something legitimately wrong. Compiler warnings can cover up
349 legitimate errors in output and make dealing with a translation unit
350 difficult.</p>
352 <p>It is not possible to prevent all warnings from all compilers, nor is it
354 a good thorough set of warnings, and stick to them. At least in the case of
356 syntax of the code slightly. For example, an warning that annoys me occurs when
357 I write code like this:</p>
360 <pre>
361 if (V = getValue()) {
362 ...
363 }
364 </pre>
365 </div>
368 operator, and that I probably mistyped it. In most cases, I haven't, and I
369 really don't want the spurious errors. To fix this particular problem, I
370 rewrite the code like this:</p>
373 <pre>
374 if ((V = getValue())) {
375 ...
376 }
377 </pre>
378 </div>
381 be fixed by massaging the code appropriately.</p>
384 -Winline -W -Wwrite-strings -Wno-unused</tt></p>
386 </div>
388 <!-- _______________________________________________________________________ -->
391 </div>
395 <p>In almost all cases, it is possible and within reason to write completely
396 portable code. If there are cases where it isn't possible to write portable
397 code, isolate it behind a well defined (and well documented) interface.</p>
399 <p>In practice, this means that you shouldn't assume much about the host
400 compiler, including its support for "high tech" features like partial
401 specialization of templates. If these features are used, they should only be
402 an implementation detail of a library which has a simple exposed API.</p>
404 </div>
406 <!-- _______________________________________________________________________ -->
409 </div>
413 interchangeably. The only difference is when they are used to declare a class:
415 all members public by default.</p>
417 <p>Unfortunately, not all compilers follow the rules and some will generate
419 declare the symbol. This can lead to problems at link time.</p>
425 </div>
427 <!-- *********************************************************************** -->
430 </div>
431 <!-- *********************************************************************** -->
434 <!-- ======================================================================= -->
437 </div>
438 <!-- ======================================================================= -->
441 <!-- _______________________________________________________________________ -->
444 </div>
448 <p>C++ doesn't do too well in the modularity department. There is no real
449 encapsulation or data hiding (unless you use expensive protocol classes), but it
450 is what we have to work with. When you write a public header file (in the LLVM
451 source tree, they live in the top level "include" directory), you are defining a
452 module of functionality.</p>
454 <p>Ideally, modules should be completely independent of each other, and their
455 header files should only include the absolute minimum number of headers
456 possible. A module is not just a class, a function, or a namespace: <a
458 of these</a> that defines an interface. This interface may be several
459 functions, classes or data structures, but the important issue is how they work
460 together.</p>
464 their interface first. This ensure that all of the dependences of the module
465 header have been properly added to the module header itself, and are not
466 implicit. System headers should be included after user headers for a
467 translation unit.</p>
469 </div>
471 <!-- _______________________________________________________________________ -->
474 </div>
479 have to, especially in header files.</p>
481 <p>But wait, sometimes you need to have the definition of a class to use it, or
483 file. Be aware however that there are many cases where you don't need to have
484 the full definition of a class. If you are using a pointer or reference to a
485 class, you don't need the header file. If you are simply returning a class
486 instance from a prototyped function or method, you don't need it. In fact, for
487 most cases, you simply don't need the definition of a class... and not
490 <p>It is easy to try to go too overboard on this recommendation, however. You
492 include them either directly
493 or indirectly (through another header file). To make sure that you don't
494 accidentally forget to include a header file in your module header, make sure to
496 above). This way there won't be any hidden dependencies that you'll find out
497 about later...</p>
499 </div>
501 <!-- _______________________________________________________________________ -->
504 </div>
508 <p>Many modules have a complex implementation that causes them to use more than
510 internal communication interface (helper classes, extra functions, etc) in the
511 public module header file. Don't do this.</p>
513 <p>If you really need to do something like this, put a private header file in
514 the same directory as the source files, and include it locally. This ensures
515 that your private interface remains private and undisturbed by outsiders.</p>
517 <p>Note however, that it's okay to put extra implementation methods a public
518 class itself... just make them private (or protected), and all is well.</p>
520 </div>
522 <!-- _______________________________________________________________________ -->
525 </div>
529 <p>When reading code, keep in mind how much state and how many previous
530 decisions have to be remembered by the reader to understand a block of code.
531 Aim to reduce indentation where possible when it doesn't make it more difficult
532 to understand the code. One great way to do this is by making use of early
533 exits and the 'continue' keyword in long loops. As an example of using an early
537 <pre>
538 Value *DoSomething(Instruction *I) {
541 ... some long code ....
542 }
544 return 0;
545 }
546 </pre>
547 </div>
549 <p>This code has several problems if the body of the 'if' is large. When you're
550 looking at the top of the function, it isn't immediately clear that this
552 applies to things with the other predicates. Second, it is relatively difficult
553 to describe (in comments) why these predicates are important because the if
554 statement makes it difficult to lay out the comments. Third, when you're deep
555 within the body of the code, it is indented an extra level. Finally, when
556 reading the top of the function, it isn't clear what the result is if the
557 predicate isn't true, you have to read to the end of the function to know that
558 it returns null.</p>
563 <pre>
564 Value *DoSomething(Instruction *I) {
565 // Terminators never need 'something' done to them because, ...
567 return 0;
569 // We conservatively avoid transforming instructions with multiple uses
570 // because goats like cheese.
571 if (!I->hasOneUse())
572 return 0;
574 // This is really just here for example.
575 if (!SomeOtherThing(I))
576 return 0;
578 ... some long code ....
579 }
580 </pre>
581 </div>
583 <p>This fixes these problems. A similar problem frequently happens in for
584 loops. A silly example is something like this:</p>
587 <pre>
592 if (LHS != RHS) {
593 ...
594 }
595 }
596 }
597 </pre>
598 </div>
600 <p>When you have very very small loops, this sort of structure is fine, but if
602 understand at a glance.
603 The problem with this sort of code is that it gets very nested very quickly,
604 meaning that the reader of the code has to keep a lot of context in their brain
605 to remember what is going immediately on in the loop, because they don't know
606 if/when the if conditions will have elses etc. It is strongly preferred to
607 structure the loop like this:</p>
610 <pre>
613 if (!BO) continue;
617 if (LHS == RHS) continue;
618 }
619 </pre>
620 </div>
622 <p>This has all the benefits of using early exits from functions: it reduces
623 nesting of the loop, it makes it easier to describe why the conditions are true,
624 and it makes it obvious to the reader that there is no "else" coming up that
625 they have to push context into their brain for. If a loop is large, this can
626 be a big understandability win.</p>
628 </div>
630 <!-- _______________________________________________________________________ -->
633 </div>
637 <p>For similar reasons above (reduction of indentation and easier reading),
639 control flow like return, break, continue, goto, etc. For example, this is
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>
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>
692 <pre>
693 case 'J':
694 if (Signed)
695 Type = Context.getsigjmp_bufType();
696 else
697 Type = Context.getjmp_bufType();
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>
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>
711 </div>
713 <!-- _______________________________________________________________________ -->
716 </div>
720 <p>It is very common to write small loops that just compute a boolean
721 value. There are a number of ways that people commonly write these, but an
722 example of this sort of thing is:</p>
725 <pre>
727 for (unsigned i = 0, e = BarList.size(); i != e; ++i)
728 if (BarList[i]->isFoo()) {
730 break;
731 }
734 ...
735 }
736 </pre>
737 </div>
739 <p>This sort of code is awkward to write, and is almost always a bad sign.
740 Instead of this sort of loop, we strongly prefer to use a predicate function
743 the code to be structured like this:
744 </p>
748 <pre>
749 /// ListContainsFoo - Return true if the specified list has an element that is
750 /// a foo.
752 for (unsigned i = 0, e = List.size(); i != e; ++i)
753 if (List[i]->isFoo())
754 return true;
755 return false;
756 }
757 ...
760 ...
761 }
762 </pre>
763 </div>
765 <p>There are many reasons for doing this: it reduces indentation and factors out
766 code which can often be shared by other code that checks for the same predicate.
768 forces you to write a comment for it. In this silly example, this doesn't add
769 much value. However, if the condition is complex, this can make it a lot easier
770 for the reader to understand the code that queries for this predicate. Instead
771 of being faced with the in-line details of how we check to see if the BarList
772 contains a foo, we can trust the function name and continue reading with better
773 locality.</p>
775 </div>
778 <!-- ======================================================================= -->
781 </div>
782 <!-- ======================================================================= -->
785 <!-- _______________________________________________________________________ -->
788 </div>
793 preconditions and assumptions, you never know when a bug (not necessarily even
794 yours) might be caught early by an assertion, which reduces debugging time
795 dramatically. The "<tt><cassert></tt>" header file is probably already
796 included by the header files you are using, so it doesn't cost anything to use
797 it.</p>
799 <p>To further assist with debugging, make sure to put some kind of error message
800 in the assertion statement (which is printed if the assertion is tripped). This
801 helps the poor debugging make sense of why an assertion is being made and
802 enforced, and hopefully what to do about it. Here is one complete example:</p>
805 <pre>
806 inline Value *getOperand(unsigned i) {
808 return Operands[i];
809 }
810 </pre>
811 </div>
816 <pre>
826 </pre>
827 </div>
831 <p>Please be aware when adding assert statements that not all compilers are aware of
832 the semantics of the assert. In some places, asserts are used to indicate a piece of
833 code that should not be reached. These are typically of the form:</p>
836 <pre>
838 </pre>
839 </div>
841 <p>When used in a function that returns a value, they should be followed with a return
842 statement and a comment indicating that this line is never reached. This will prevent
843 a compiler which is unable to deduce that the assert statement never returns from
844 generating a warning.</p>
847 <pre>
849 // Not reached
850 return 0;
851 </pre>
852 </div>
854 </div>
856 <!-- _______________________________________________________________________ -->
859 </div>
862 <p>In LLVM, we prefer to explicitly prefix all identifiers from the standard
863 namespace with an "<tt>std::</tt>" prefix, rather than rely on
868 clearly a bad thing.</p>
870 <p>In implementation files (e.g. .cpp files), the rule is more of a stylistic
871 rule, but is still important. Basically, using explicit namespace prefixes
874 namespace clashes cannot occur between LLVM code and other namespaces. The
875 portability rule is important because different standard library implementations
876 expose different symbols (potentially ones they shouldn't), and future revisions
880 <p>The exception to the general rule (i.e. it's not an exception for
882 the code in the LLVM project implements code that lives in the 'llvm' namespace.
883 As such, it is ok, and actually clearer, for the .cpp files to have a '<tt>using
885 general form of this rule is that any .cpp file that implements code in any
886 namespace may use that namespace (and its parents'), but should not use any
887 others.</p>
889 </div>
891 <!-- _______________________________________________________________________ -->
894 in headers</a>
895 </div>
899 <p>If a class is defined in a header file and has a v-table (either it has
900 virtual methods or it derives from classes with virtual methods), it must
901 always have at least one out-of-line virtual method in the class. Without
904 increasing link times.</p>
906 </div>
908 <!-- _______________________________________________________________________ -->
911 </div>
916 with macros and may be coming in C++'0x) we end up writing a lot of loops that
917 manually iterate from begin to end on a variety of containers or through other
918 data structures. One common mistake is to write a loop in this style:</p>
921 <pre>
922 BasicBlock *BB = ...
924 ... use I ...
925 </pre>
926 </div>
929 every time through the loop. Instead of writing the loop like this, we strongly
930 prefer loops to be written so that they evaluate it once before the loop starts.
931 A convenient way to do this is like so:</p>
934 <pre>
935 BasicBlock *BB = ...
937 ... use I ...
938 </pre>
939 </div>
941 <p>The observant may quickly point out that these two loops may have different
942 semantics: 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
944 second loop may not in fact be correct. If you actually do depend on this
945 behavior, please write the loop in the first form and add a comment indicating
946 that you did it intentionally.</p>
948 <p>Why do we prefer the second form (when correct)? Writing the loop in the
949 first form has two problems: First it may be less efficient than evaluating it
950 at the start of the loop. In this case, the cost is probably minor: a few extra
951 loads every time through the loop. However, if the base expression is more
952 complex, then the cost can rise quickly. I've seen loops where the end
953 expression was actually something like: "<tt>SomeMap[x]->end()</tt>" and map
954 lookups really aren't cheap. By writing it in the second form consistently, you
955 eliminate the issue entirely and don't even have to think about it.</p>
957 <p>The second (even bigger) issue is that writing the loop in the first form
958 hints to the reader that the loop is mutating the container (a fact that a
959 comment would handily confirm!). If you write the loop in the second form, it
960 is immediately obvious without even looking at the body of the loop that the
961 container isn't being modified, which makes it easier to read the code and
962 understand what it does.</p>
964 <p>While the second form of the loop is a few extra keystrokes, we do strongly
965 prefer it.</p>
967 </div>
969 <!-- _______________________________________________________________________ -->
972 </div>
978 support clients using LLVM libraries as part of larger systems. In particular,
979 we statically link LLVM into some dynamic libraries. Even if LLVM isn't used,
980 the static c'tors are run whenever an application start up that uses the dynamic
981 library. There are two problems with this:</p>
983 <ol>
984 <li>The time to run the static c'tors impacts startup time of
986 <li>The static c'tors cause the app to pull many extra pages of memory off 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>
994 However, raw_ostream provides various APIs that are better performing for almost
995 every use than std::ostream style APIs, so you should just use it for new
996 code.</p>
998 <p><b>New code should always
1002 </div>
1005 <!-- _______________________________________________________________________ -->
1008 </div>
1013 to the output stream specified. In addition to doing this, however, it also
1014 flushes the output stream. In other words, these are equivalent:</p>
1017 <pre>
1018 std::cout << std::endl;
1020 </pre>
1021 </div>
1023 <p>Most of the time, you probably have no reason to flush the output stream, so
1026 </div>
1029 <!-- _______________________________________________________________________ -->
1032 </div>
1036 <p>LLVM includes a lightweight, simple, and efficient stream implementation
1046 </div>
1049 <!-- ======================================================================= -->
1052 </div>
1053 <!-- ======================================================================= -->
1055 <p>This section describes preferred low-level formatting guidelines along with
1056 reasoning on why we prefer them.</p>
1058 <!-- _______________________________________________________________________ -->
1061 </div>
1065 <p>We prefer to put a space before a parentheses only in control flow
1066 statements, but not in normal function call expressions and function-like
1067 macros. For example, this is good:</p>
1070 <pre>
1079 </pre>
1080 </div>
1085 <pre>
1094 </pre>
1095 </div>
1097 <p>The reason for doing this is not completely arbitrary. This style makes
1098 control flow operators stand out more, and makes expressions flow better. The
1099 function call operator binds very tightly as a postfix operator. Putting
1100 a space after a function name (as in the last example) makes it appear that
1101 the code might bind the arguments of the left-hand-side of a binary operator
1102 with the argument list of a function and the name of the right side. More
1106 <pre>
1108 </pre>
1109 </div>
1111 <p>... when skimming through the code. By avoiding a space in a function, we
1112 avoid this misinterpretation.</p>
1114 </div>
1116 <!-- _______________________________________________________________________ -->
1119 </div>
1125 preincrementation whenever possible.</p>
1127 <p>The semantics of postincrement include making a copy of the value being
1128 incremented, returning it, and then preincrementing the "work value". For
1129 primitive types, this isn't a big deal... but for iterators, it can be a huge
1130 issue (for example, some iterators contains stack and set objects in them...
1131 copying an iterator could invoke the copy ctor's of these as well). In general,
1132 get in the habit of always using preincrement, and you won't have a problem.</p>
1134 </div>
1136 <!-- _______________________________________________________________________ -->
1139 </div>
1143 <p>
1144 In general, we strive to reduce indentation where ever possible. This is useful
1146 wrapping horribly, but also because it makes it easier to understand the code.
1147 Namespaces are a funny thing: they are often large, and we often desire to put
1148 lots of stuff into them (so they can be large). Other times they are tiny,
1149 because they just hold an enum or something similar. In order to balance this,
1150 we use different approaches for small versus large namespaces.
1151 </p>
1153 <p>
1155 less than 35 lines of code), then you should indent its body. Here's an
1156 example:
1157 </p>
1160 <pre>
1161 namespace llvm {
1162 namespace X86 {
1163 /// RelocationType - An enum for the x86 relocation codes. Note that
1164 /// the terminology here doesn't follow x86 convention - word means
1166 enum RelocationType {
1167 /// reloc_pcrel_word - PC relative relocation, add the relocated value to
1168 /// the value already in memory, after we adjust it for where the PC is.
1169 reloc_pcrel_word = 0,
1171 /// reloc_picrel_word - PIC base relative relocation, add the relocated
1172 /// value to the value already in memory, after we adjust it for where the
1173 /// PIC base is.
1174 reloc_picrel_word = 1,
1176 /// reloc_absolute_word, reloc_absolute_dword - Absolute relocation, just
1177 /// add the relocated value to the value already in memory.
1178 reloc_absolute_word = 2,
1179 reloc_absolute_dword = 3
1180 };
1181 }
1182 }
1183 </pre>
1184 </div>
1186 <p>Since the body is small, indenting adds value because it makes it very clear
1187 where the namespace starts and ends, and it is easy to take the whole thing in
1188 in one "gulp" when reading the code. If the blob of code in the namespace is
1189 larger (as it typically is in a header in the llvm or clang namespaces), do not
1190 indent the code, and add a comment indicating what namespace is being closed.
1191 For example:</p>
1194 <pre>
1195 namespace llvm {
1196 namespace knowledge {
1198 /// Grokable - This class represents things that Smith can have an intimate
1199 /// understanding of and contains the data associated with it.
1200 class Grokable {
1201 ...
1202 public:
1203 explicit Grokable() { ... }
1204 virtual ~Grokable() = 0;
1206 ...
1208 };
1210 } // end namespace knowledge
1211 } // end namespace llvm
1212 </pre>
1213 </div>
1215 <p>Because the class is large, we don't expect that the reader can easily
1216 understand the entire concept in a glance, and the end of the file (where the
1217 namespaces end) may be a long ways away from the place they open. As such,
1218 indenting the contents of the namespace doesn't add any value, and detracts from
1220 the contents of the namespace.</p>
1222 </div>
1224 <!-- _______________________________________________________________________ -->
1227 </div>
1231 <p>After talking about namespaces in general, you may be wondering about
1232 anonymous namespaces in particular.
1233 Anonymous namespaces are a great language feature that tells the C++ compiler
1234 that the contents of the namespace are only visible within the current
1235 translation unit, allowing more aggressive optimization and eliminating the
1236 possibility of symbol name collisions. Anonymous namespaces are to C++ as
1238 in C++, anonymous namespaces are more general: they can make entire classes
1239 private to a file.</p>
1241 <p>The problem with anonymous namespaces is that they naturally want to
1242 encourage indentation of their body, and they reduce locality of reference: if
1243 you see a random function definition in a C++ file, it is easy to see if it is
1244 marked static, but seeing if it is in an anonymous namespace requires scanning
1245 a big chunk of the file.</p>
1247 <p>Because of this, we have a simple guideline: make anonymous namespaces as
1248 small as possible, and only use them for class declarations. For example, this
1249 is good:</p>
1252 <pre>
1254 class StringSort {
1255 ...
1256 public:
1257 StringSort(...)
1258 bool operator<(const char *RHS) const;
1259 };
1262 static void Helper() {
1263 ...
1264 }
1266 bool StringSort::operator<(const char *RHS) const {
1267 ...
1268 }
1270 </pre>
1271 </div>
1277 <pre>
1279 class StringSort {
1280 ...
1281 public:
1282 StringSort(...)
1283 bool operator<(const char *RHS) const;
1284 };
1286 void Helper() {
1287 ...
1288 }
1290 bool StringSort::operator<(const char *RHS) const {
1291 ...
1292 }
1296 </pre>
1297 </div>
1301 of a large C++ file, that you have no immediate way to tell if it is local to
1302 the file. When it is marked static explicitly, this is immediately obvious.
1303 Also, there is no reason to enclose the definition of "operator<" in the
1304 namespace just because it was declared there.
1305 </p>
1307 </div>
1311 <!-- *********************************************************************** -->
1314 </div>
1315 <!-- *********************************************************************** -->
1319 <p>A lot of these comments and recommendations have been culled for other
1320 sources. Two particularly important books for our work are:</p>
1322 <ol>
1324 <li><a href="http://www.amazon.com/Effective-Specific-Addison-Wesley-Professional-Computing/dp/0321334876">Effective
1325 C++</a> by Scott Meyers. Also
1327 author.</li>
1331 </ol>
1333 <p>If you get some free time, and you haven't read them: do so, you might learn
1334 something.</p>
1336 </div>
1338 <!-- *********************************************************************** -->
1340 <hr>
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