11 This document attempts to describe a few coding standards that are being used in
12 the LLVM source tree. Although no coding standards should be regarded as
13 absolute requirements to be followed in all instances, coding standards are
14 particularly important for large-scale code bases that follow a library-based
17 While this document may provide guidance for some mechanical formatting issues,
18 whitespace, or other "microscopic details", these are not fixed standards.
19 Always follow the golden rule:
23 **If you are extending, enhancing, or bug fixing already implemented code,
24 use the style that is already being used so that the source is uniform and
27 Note that some code bases (e.g. ``libc++``) have really good reasons to deviate
28 from the coding standards. In the case of ``libc++``, this is because the
29 naming and other conventions are dictated by the C++ standard. If you think
30 there is a specific good reason to deviate from the standards here, please bring
31 it up on the LLVMdev mailing list.
33 There are some conventions that are not uniformly followed in the code base
34 (e.g. the naming convention). This is because they are relatively new, and a
35 lot of code was written before they were put in place. Our long term goal is
36 for the entire codebase to follow the convention, but we explicitly *do not*
37 want patches that do large-scale reformating of existing code. On the other
38 hand, it is reasonable to rename the methods of a class if you're about to
39 change it in some other way. Just do the reformating as a separate commit from
40 the functionality change.
42 The ultimate goal of these guidelines is the increase readability and
43 maintainability of our common source base. If you have suggestions for topics to
44 be included, please mail them to `Chris <mailto:sabre@nondot.org>`_.
46 Languages, Libraries, and Standards
47 ===================================
49 Most source code in LLVM and other LLVM projects using these coding standards
50 is C++ code. There are some places where C code is used either due to
51 environment restrictions, historical restrictions, or due to third-party source
52 code imported into the tree. Generally, our preference is for standards
53 conforming, modern, and portable C++ code as the implementation language of
59 LLVM, Clang, and LLD are currently written using C++11 conforming code,
60 although we restrict ourselves to features which are available in the major
61 toolchains supported as host compilers. The LLDB project is even more
62 aggressive in the set of host compilers supported and thus uses still more
63 features. Regardless of the supported features, code is expected to (when
64 reasonable) be standard, portable, and modern C++11 code. We avoid unnecessary
65 vendor-specific extensions, etc.
70 Use the C++ standard library facilities whenever they are available for
71 a particular task. LLVM and related projects emphasize and rely on the standard
72 library facilities for as much as possible. Common support libraries providing
73 functionality missing from the standard library for which there are standard
74 interfaces or active work on adding standard interfaces will often be
75 implemented in the LLVM namespace following the expected standard interface.
77 There are some exceptions such as the standard I/O streams library which are
78 avoided. Also, there is much more detailed information on these subjects in the
79 `Programmer's Manual`_.
81 .. _Programmer's Manual:
82 http://llvm.org/docs/ProgrammersManual.html
84 Supported C++11 Language and Library Features
85 ---------------------------------------------
87 While LLVM, Clang, and LLD use C++11, not all features are available in all of
88 the toolchains which we support. The set of features supported for use in LLVM
89 is the intersection of those supported in MSVC 2012, GCC 4.7, and Clang 3.1.
90 The ultimate definition of this set is what build bots with those respective
91 toolchains accept. Don't argue with the build bots. However, we have some
92 guidance below to help you know what to expect.
94 Each toolchain provides a good reference for what it accepts:
96 * Clang: http://clang.llvm.org/cxx_status.html
97 * GCC: http://gcc.gnu.org/projects/cxx0x.html
98 * MSVC: http://msdn.microsoft.com/en-us/library/hh567368.aspx
100 In most cases, the MSVC list will be the dominating factor. Here is a summary
101 of the features that are expected to work. Features not on this list are
102 unlikely to be supported by our host compilers.
104 * Rvalue references: N2118_
106 * But *not* Rvalue references for ``*this`` or member qualifiers (N2439_)
108 * Static assert: N1720_
109 * ``auto`` type deduction: N1984_, N1737_
110 * Trailing return types: N2541_
112 * ``decltype``: N2343_
113 * Nested closing right angle brackets: N1757_
114 * Extern templates: N1987_
115 * ``nullptr``: N2431_
116 * Strongly-typed and forward declarable enums: N2347_, N2764_
117 * Local and unnamed types as template arguments: N2657_
118 * Range-based for-loop: N2930_
119 * ``override`` and ``final``: N2928_, N3206_, N3272_
120 * Atomic operations and the C++11 memory model: N2429_
122 .. _N2118: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n2118.html
123 .. _N2439: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2439.htm
124 .. _N1720: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2004/n1720.html
125 .. _N1984: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n1984.pdf
126 .. _N1737: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2004/n1737.pdf
127 .. _N2541: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2541.htm
128 .. _N2927: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2009/n2927.pdf
129 .. _N2343: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2343.pdf
130 .. _N1757: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2005/n1757.html
131 .. _N1987: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n1987.htm
132 .. _N2431: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2431.pdf
133 .. _N2347: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2347.pdf
134 .. _N2764: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2764.pdf
135 .. _N2657: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2657.htm
136 .. _N2930: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2009/n2930.html
137 .. _N2928: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2009/n2928.htm
138 .. _N3206: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2010/n3206.htm
139 .. _N3272: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2011/n3272.htm
140 .. _N2429: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2429.htm
142 The supported features in the C++11 standard libraries are less well tracked,
143 but also much greater. Most of the standard libraries implement most of C++11's
144 library. The most likely lowest common denominator is Linux support. For
145 libc++, the support is just poorly tested and undocumented but expected to be
146 largely complete. YMMV. For libstdc++, the support is documented in detail in
147 `the libstdc++ manual`_. There are some very minor missing facilities that are
148 unlikely to be common problems, and there are a few larger gaps that are worth
151 * Not all of the type traits are implemented
152 * No regular expression library.
153 * While most of the atomics library is well implemented, the fences are
154 missing. Fortunately, they are rarely needed.
155 * The locale support is incomplete.
157 Other than these areas you should assume the standard library is available and
158 working as expected until some build bot tells you otherwise. If you're in an
159 uncertain area of one of the above points, but you cannot test on a Linux
160 system, your best approach is to minimize your use of these features, and watch
161 the Linux build bots to find out if your usage triggered a bug. For example, if
162 you hit a type trait which doesn't work we can then add support to LLVM's
163 traits header to emulate it.
165 .. _the libstdc++ manual:
166 http://gcc.gnu.org/onlinedocs/gcc-4.7.3/libstdc++/manual/manual/status.html#status.iso.2011
168 Mechanical Source Issues
169 ========================
171 Source Code Formatting
172 ----------------------
177 Comments are one critical part of readability and maintainability. Everyone
178 knows they should comment their code, and so should you. When writing comments,
179 write them as English prose, which means they should use proper capitalization,
180 punctuation, etc. Aim to describe what the code is trying to do and why, not
181 *how* it does it at a micro level. Here are a few critical things to document:
183 .. _header file comment:
188 Every source file should have a header on it that describes the basic purpose of
189 the file. If a file does not have a header, it should not be checked into the
190 tree. The standard header looks like this:
194 //===-- llvm/Instruction.h - Instruction class definition -------*- C++ -*-===//
196 // The LLVM Compiler Infrastructure
198 // This file is distributed under the University of Illinois Open Source
199 // License. See LICENSE.TXT for details.
201 //===----------------------------------------------------------------------===//
204 /// \brief This file contains the declaration of the Instruction class, which is
205 /// the base class for all of the VM instructions.
207 //===----------------------------------------------------------------------===//
209 A few things to note about this particular format: The "``-*- C++ -*-``" string
210 on the first line is there to tell Emacs that the source file is a C++ file, not
211 a C file (Emacs assumes ``.h`` files are C files by default).
215 This tag is not necessary in ``.cpp`` files. The name of the file is also
216 on the first line, along with a very short description of the purpose of the
217 file. This is important when printing out code and flipping though lots of
220 The next section in the file is a concise note that defines the license that the
221 file is released under. This makes it perfectly clear what terms the source
222 code can be distributed under and should not be modified in any way.
224 The main body is a ``doxygen`` comment describing the purpose of the file. It
225 should have a ``\brief`` command that describes the file in one or two
226 sentences. Any additional information should be separated by a blank line. If
227 an algorithm is being implemented or something tricky is going on, a reference
228 to the paper where it is published should be included, as well as any notes or
229 *gotchas* in the code to watch out for.
234 Classes are one fundamental part of a good object oriented design. As such, a
235 class definition should have a comment block that explains what the class is
236 used for and how it works. Every non-trivial class is expected to have a
237 ``doxygen`` comment block.
242 Methods defined in a class (as well as any global functions) should also be
243 documented properly. A quick note about what it does and a description of the
244 borderline behaviour is all that is necessary here (unless something
245 particularly tricky or insidious is going on). The hope is that people can
246 figure out how to use your interfaces without reading the code itself.
248 Good things to talk about here are what happens when something unexpected
249 happens: does the method return null? Abort? Format your hard disk?
254 In general, prefer C++ style (``//``) comments. They take less space, require
255 less typing, don't have nesting problems, etc. There are a few cases when it is
256 useful to use C style (``/* */``) comments however:
258 #. When writing C code: Obviously if you are writing C code, use C style
261 #. When writing a header file that may be ``#include``\d by a C source file.
263 #. When writing a source file that is used by a tool that only accepts C style
266 To comment out a large block of code, use ``#if 0`` and ``#endif``. These nest
267 properly and are better behaved in general than C style comments.
269 Doxygen Use in Documentation Comments
270 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
272 Use the ``\file`` command to turn the standard file header into a file-level
275 Include descriptive ``\brief`` paragraphs for all public interfaces (public
276 classes, member and non-member functions). Explain API use and purpose in
277 ``\brief`` paragraphs, don't just restate the information that can be inferred
278 from the API name. Put detailed discussion into separate paragraphs.
280 To refer to parameter names inside a paragraph, use the ``\p name`` command.
281 Don't use the ``\arg name`` command since it starts a new paragraph that
282 contains documentation for the parameter.
284 Wrap non-inline code examples in ``\code ... \endcode``.
286 To document a function parameter, start a new paragraph with the
287 ``\param name`` command. If the parameter is used as an out or an in/out
288 parameter, use the ``\param [out] name`` or ``\param [in,out] name`` command,
291 To describe function return value, start a new paragraph with the ``\returns``
294 A minimal documentation comment:
298 /// \brief Does foo and bar.
299 void fooBar(bool Baz);
301 A documentation comment that uses all Doxygen features in a preferred way:
305 /// \brief Does foo and bar.
307 /// Does not do foo the usual way if \p Baz is true.
311 /// fooBar(false, "quux", Res);
314 /// \param Quux kind of foo to do.
315 /// \param [out] Result filled with bar sequence on foo success.
317 /// \returns true on success.
318 bool fooBar(bool Baz, StringRef Quux, std::vector<int> &Result);
320 Don't duplicate the documentation comment in the header file and in the
321 implementation file. Put the documentation comments for public APIs into the
322 header file. Documentation comments for private APIs can go to the
323 implementation file. In any case, implementation files can include additional
324 comments (not necessarily in Doxygen markup) to explain implementation details
327 Don't duplicate function or class name at the beginning of the comment.
328 For humans it is obvious which function or class is being documented;
329 automatic documentation processing tools are smart enough to bind the comment
330 to the correct declaration.
338 /// Something - An abstraction for some complicated thing.
341 /// fooBar - Does foo and bar.
347 /// fooBar - Does foo and bar.
348 void Something::fooBar() { ... }
356 /// \brief An abstraction for some complicated thing.
359 /// \brief Does foo and bar.
365 // Builds a B-tree in order to do foo. See paper by...
366 void Something::fooBar() { ... }
368 It is not required to use additional Doxygen features, but sometimes it might
369 be a good idea to do so.
373 * adding comments to any narrow namespace containing a collection of
374 related functions or types;
376 * using top-level groups to organize a collection of related functions at
377 namespace scope where the grouping is smaller than the namespace;
379 * using member groups and additional comments attached to member
380 groups to organize within a class.
387 /// \name Functions that do Foo.
398 Immediately after the `header file comment`_ (and include guards if working on a
399 header file), the `minimal list of #includes`_ required by the file should be
400 listed. We prefer these ``#include``\s to be listed in this order:
402 .. _Main Module Header:
403 .. _Local/Private Headers:
405 #. Main Module Header
406 #. Local/Private Headers
408 #. System ``#include``\s
410 and each category should be sorted lexicographically by the full path.
412 The `Main Module Header`_ file applies to ``.cpp`` files which implement an
413 interface defined by a ``.h`` file. This ``#include`` should always be included
414 **first** regardless of where it lives on the file system. By including a
415 header file first in the ``.cpp`` files that implement the interfaces, we ensure
416 that the header does not have any hidden dependencies which are not explicitly
417 ``#include``\d in the header, but should be. It is also a form of documentation
418 in the ``.cpp`` file to indicate where the interfaces it implements are defined.
420 .. _fit into 80 columns:
425 Write your code to fit within 80 columns of text. This helps those of us who
426 like to print out code and look at your code in an ``xterm`` without resizing
429 The longer answer is that there must be some limit to the width of the code in
430 order to reasonably allow developers to have multiple files side-by-side in
431 windows on a modest display. If you are going to pick a width limit, it is
432 somewhat arbitrary but you might as well pick something standard. Going with 90
433 columns (for example) instead of 80 columns wouldn't add any significant value
434 and would be detrimental to printing out code. Also many other projects have
435 standardized on 80 columns, so some people have already configured their editors
436 for it (vs something else, like 90 columns).
438 This is one of many contentious issues in coding standards, but it is not up for
441 Use Spaces Instead of Tabs
442 ^^^^^^^^^^^^^^^^^^^^^^^^^^
444 In all cases, prefer spaces to tabs in source files. People have different
445 preferred indentation levels, and different styles of indentation that they
446 like; this is fine. What isn't fine is that different editors/viewers expand
447 tabs out to different tab stops. This can cause your code to look completely
448 unreadable, and it is not worth dealing with.
450 As always, follow the `Golden Rule`_ above: follow the style of
451 existing code if you are modifying and extending it. If you like four spaces of
452 indentation, **DO NOT** do that in the middle of a chunk of code with two spaces
453 of indentation. Also, do not reindent a whole source file: it makes for
454 incredible diffs that are absolutely worthless.
456 Indent Code Consistently
457 ^^^^^^^^^^^^^^^^^^^^^^^^
459 Okay, in your first year of programming you were told that indentation is
460 important. If you didn't believe and internalize this then, now is the time.
461 Just do it. With the introduction of C++11, there are some new formatting
462 challenges that merit some suggestions to help have consistent, maintainable,
463 and tool-friendly formatting and indentation.
465 Format Lambdas Like Blocks Of Code
466 """"""""""""""""""""""""""""""""""
468 When formatting a multi-line lambda, format it like a block of code, that's
469 what it is. If there is only one multi-line lambda in a statement, and there
470 are no expressions lexically after it in the statement, drop the indent to the
471 standard two space indent for a block of code, as if it were an if-block opened
472 by the preceding part of the statement:
476 std::sort(foo.begin(), foo.end(), [&](Foo a, Foo b) -> bool {
481 return a.bam < b.bam;
484 To take best advantage of this formatting, if you are designing an API which
485 accepts a continuation or single callable argument (be it a functor, or
486 a ``std::function``), it should be the last argument if at all possible.
488 If there are multiple multi-line lambdas in a statement, or there is anything
489 interesting after the lambda in the statement, indent the block two spaces from
490 the indent of the ``[]``:
494 dyn_switch(V->stripPointerCasts(),
498 [] (SelectInst *SI) {
499 // process selects...
504 [] (AllocaInst *AI) {
505 // process allocas...
508 Braced Initializer Lists
509 """"""""""""""""""""""""
511 With C++11, there are significantly more uses of braced lists to perform
512 initialization. These allow you to easily construct aggregate temporaries in
513 expressions among other niceness. They now have a natural way of ending up
514 nested within each other and within function calls in order to build up
515 aggregates (such as option structs) from local variables. To make matters
516 worse, we also have many more uses of braces in an expression context that are
517 *not* performing initialization.
519 The historically common formatting of braced initialization of aggregate
520 variables does not mix cleanly with deep nesting, general expression contexts,
521 function arguments, and lambdas. We suggest new code use a simple rule for
522 formatting braced initialization lists: act as-if the braces were parentheses
523 in a function call. The formatting rules exactly match those already well
524 understood for formatting nested function calls. Examples:
528 foo({a, b, c}, {1, 2, 3});
530 llvm::Constant *Mask[] = {
531 llvm::ConstantInt::get(llvm::Type::getInt32Ty(getLLVMContext()), 0),
532 llvm::ConstantInt::get(llvm::Type::getInt32Ty(getLLVMContext()), 1),
533 llvm::ConstantInt::get(llvm::Type::getInt32Ty(getLLVMContext()), 2)};
535 This formatting scheme also makes it particularly easy to get predictable,
536 consistent, and automatic formatting with tools like `Clang Format`_.
538 .. _Clang Format: http://clang.llvm.org/docs/ClangFormat.html
540 Language and Compiler Issues
541 ----------------------------
543 Treat Compiler Warnings Like Errors
544 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
546 If your code has compiler warnings in it, something is wrong --- you aren't
547 casting values correctly, you have "questionable" constructs in your code, or
548 you are doing something legitimately wrong. Compiler warnings can cover up
549 legitimate errors in output and make dealing with a translation unit difficult.
551 It is not possible to prevent all warnings from all compilers, nor is it
552 desirable. Instead, pick a standard compiler (like ``gcc``) that provides a
553 good thorough set of warnings, and stick to it. At least in the case of
554 ``gcc``, it is possible to work around any spurious errors by changing the
555 syntax of the code slightly. For example, a warning that annoys me occurs when
556 I write code like this:
560 if (V = getValue()) {
564 ``gcc`` will warn me that I probably want to use the ``==`` operator, and that I
565 probably mistyped it. In most cases, I haven't, and I really don't want the
566 spurious errors. To fix this particular problem, I rewrite the code like
571 if ((V = getValue())) {
575 which shuts ``gcc`` up. Any ``gcc`` warning that annoys you can be fixed by
576 massaging the code appropriately.
581 In almost all cases, it is possible and within reason to write completely
582 portable code. If there are cases where it isn't possible to write portable
583 code, isolate it behind a well defined (and well documented) interface.
585 In practice, this means that you shouldn't assume much about the host compiler
586 (and Visual Studio tends to be the lowest common denominator). If advanced
587 features are used, they should only be an implementation detail of a library
588 which has a simple exposed API, and preferably be buried in ``libSystem``.
590 Do not use RTTI or Exceptions
591 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
593 In an effort to reduce code and executable size, LLVM does not use RTTI
594 (e.g. ``dynamic_cast<>;``) or exceptions. These two language features violate
595 the general C++ principle of *"you only pay for what you use"*, causing
596 executable bloat even if exceptions are never used in the code base, or if RTTI
597 is never used for a class. Because of this, we turn them off globally in the
600 That said, LLVM does make extensive use of a hand-rolled form of RTTI that use
601 templates like `isa<>, cast<>, and dyn_cast<> <ProgrammersManual.html#isa>`_.
602 This form of RTTI is opt-in and can be
603 :doc:`added to any class <HowToSetUpLLVMStyleRTTI>`. It is also
604 substantially more efficient than ``dynamic_cast<>``.
606 .. _static constructor:
608 Do not use Static Constructors
609 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
611 Static constructors and destructors (e.g. global variables whose types have a
612 constructor or destructor) should not be added to the code base, and should be
613 removed wherever possible. Besides `well known problems
614 <http://yosefk.com/c++fqa/ctors.html#fqa-10.12>`_ where the order of
615 initialization is undefined between globals in different source files, the
616 entire concept of static constructors is at odds with the common use case of
617 LLVM as a library linked into a larger application.
619 Consider the use of LLVM as a JIT linked into another application (perhaps for
620 `OpenGL, custom languages <http://llvm.org/Users.html>`_, `shaders in movies
621 <http://llvm.org/devmtg/2010-11/Gritz-OpenShadingLang.pdf>`_, etc). Due to the
622 design of static constructors, they must be executed at startup time of the
623 entire application, regardless of whether or how LLVM is used in that larger
624 application. There are two problems with this:
626 * The time to run the static constructors impacts startup time of applications
627 --- a critical time for GUI apps, among others.
629 * The static constructors cause the app to pull many extra pages of memory off
630 the disk: both the code for the constructor in each ``.o`` file and the small
631 amount of data that gets touched. In addition, touched/dirty pages put more
632 pressure on the VM system on low-memory machines.
634 We would really like for there to be zero cost for linking in an additional LLVM
635 target or other library into an application, but static constructors violate
638 That said, LLVM unfortunately does contain static constructors. It would be a
639 `great project <http://llvm.org/PR11944>`_ for someone to purge all static
640 constructors from LLVM, and then enable the ``-Wglobal-constructors`` warning
641 flag (when building with Clang) to ensure we do not regress in the future.
643 Use of ``class`` and ``struct`` Keywords
644 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
646 In C++, the ``class`` and ``struct`` keywords can be used almost
647 interchangeably. The only difference is when they are used to declare a class:
648 ``class`` makes all members private by default while ``struct`` makes all
649 members public by default.
651 Unfortunately, not all compilers follow the rules and some will generate
652 different symbols based on whether ``class`` or ``struct`` was used to declare
653 the symbol (e.g., MSVC). This can lead to problems at link time.
655 * All declarations and definitions of a given ``class`` or ``struct`` must use
656 the same keyword. For example:
662 // Breaks mangling in MSVC.
663 struct Foo { int Data; };
665 * As a rule of thumb, ``struct`` should be kept to structures where *all*
666 members are declared public.
670 // Foo feels like a class... this is strange.
676 int getData() const { return Data; }
677 void setData(int D) { Data = D; }
680 // Bar isn't POD, but it does look like a struct.
686 Do not use Braced Initializer Lists to Call a Constructor
687 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
689 In C++11 there is a "generalized initialization syntax" which allows calling
690 constructors using braced initializer lists. Do not use these to call
691 constructors with any interesting logic or if you care that you're calling some
692 *particular* constructor. Those should look like function calls using
693 parentheses rather than like aggregate initialization. Similarly, if you need
694 to explicitly name the type and call its constructor to create a temporary,
695 don't use a braced initializer list. Instead, use a braced initializer list
696 (without any type for temporaries) when doing aggregate initialization or
697 something notionally equivalent. Examples:
703 // Construct a Foo by reading data from the disk in the whizbang format, ...
704 Foo(std::string filename);
706 // Construct a Foo by looking up the Nth element of some global data ...
712 // The Foo constructor call is very deliberate, no braces.
713 std::fill(foo.begin(), foo.end(), Foo("name"));
715 // The pair is just being constructed like an aggregate, use braces.
716 bar_map.insert({my_key, my_value});
718 If you use a braced initializer list when initializing a variable, use an equals before the open curly brace:
722 int data[] = {0, 1, 2, 3};
724 Use ``auto`` Type Deduction to Make Code More Readable
725 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
727 Some are advocating a policy of "almost always ``auto``" in C++11, however LLVM
728 uses a more moderate stance. Use ``auto`` if and only if it makes the code more
729 readable or easier to maintain. Don't "almost always" use ``auto``, but do use
730 ``auto`` with initializers like ``cast<Foo>(...)`` or other places where the
731 type is already obvious from the context. Another time when ``auto`` works well
732 for these purposes is when the type would have been abstracted away anyways,
733 often behind a container's typedef such as ``std::vector<T>::iterator``.
738 The High-Level Issues
739 ---------------------
741 A Public Header File **is** a Module
742 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
744 C++ doesn't do too well in the modularity department. There is no real
745 encapsulation or data hiding (unless you use expensive protocol classes), but it
746 is what we have to work with. When you write a public header file (in the LLVM
747 source tree, they live in the top level "``include``" directory), you are
748 defining a module of functionality.
750 Ideally, modules should be completely independent of each other, and their
751 header files should only ``#include`` the absolute minimum number of headers
752 possible. A module is not just a class, a function, or a namespace: it's a
753 collection of these that defines an interface. This interface may be several
754 functions, classes, or data structures, but the important issue is how they work
757 In general, a module should be implemented by one or more ``.cpp`` files. Each
758 of these ``.cpp`` files should include the header that defines their interface
759 first. This ensures that all of the dependences of the module header have been
760 properly added to the module header itself, and are not implicit. System
761 headers should be included after user headers for a translation unit.
763 .. _minimal list of #includes:
765 ``#include`` as Little as Possible
766 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
768 ``#include`` hurts compile time performance. Don't do it unless you have to,
769 especially in header files.
771 But wait! Sometimes you need to have the definition of a class to use it, or to
772 inherit from it. In these cases go ahead and ``#include`` that header file. Be
773 aware however that there are many cases where you don't need to have the full
774 definition of a class. If you are using a pointer or reference to a class, you
775 don't need the header file. If you are simply returning a class instance from a
776 prototyped function or method, you don't need it. In fact, for most cases, you
777 simply don't need the definition of a class. And not ``#include``\ing speeds up
780 It is easy to try to go too overboard on this recommendation, however. You
781 **must** include all of the header files that you are using --- you can include
782 them either directly or indirectly through another header file. To make sure
783 that you don't accidentally forget to include a header file in your module
784 header, make sure to include your module header **first** in the implementation
785 file (as mentioned above). This way there won't be any hidden dependencies that
786 you'll find out about later.
788 Keep "Internal" Headers Private
789 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
791 Many modules have a complex implementation that causes them to use more than one
792 implementation (``.cpp``) file. It is often tempting to put the internal
793 communication interface (helper classes, extra functions, etc) in the public
794 module header file. Don't do this!
796 If you really need to do something like this, put a private header file in the
797 same directory as the source files, and include it locally. This ensures that
798 your private interface remains private and undisturbed by outsiders.
802 It's okay to put extra implementation methods in a public class itself. Just
803 make them private (or protected) and all is well.
807 Use Early Exits and ``continue`` to Simplify Code
808 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
810 When reading code, keep in mind how much state and how many previous decisions
811 have to be remembered by the reader to understand a block of code. Aim to
812 reduce indentation where possible when it doesn't make it more difficult to
813 understand the code. One great way to do this is by making use of early exits
814 and the ``continue`` keyword in long loops. As an example of using an early
815 exit from a function, consider this "bad" code:
819 Value *doSomething(Instruction *I) {
820 if (!isa<TerminatorInst>(I) &&
821 I->hasOneUse() && doOtherThing(I)) {
822 ... some long code ....
828 This code has several problems if the body of the ``'if'`` is large. When
829 you're looking at the top of the function, it isn't immediately clear that this
830 *only* does interesting things with non-terminator instructions, and only
831 applies to things with the other predicates. Second, it is relatively difficult
832 to describe (in comments) why these predicates are important because the ``if``
833 statement makes it difficult to lay out the comments. Third, when you're deep
834 within the body of the code, it is indented an extra level. Finally, when
835 reading the top of the function, it isn't clear what the result is if the
836 predicate isn't true; you have to read to the end of the function to know that
839 It is much preferred to format the code like this:
843 Value *doSomething(Instruction *I) {
844 // Terminators never need 'something' done to them because ...
845 if (isa<TerminatorInst>(I))
848 // We conservatively avoid transforming instructions with multiple uses
849 // because goats like cheese.
853 // This is really just here for example.
854 if (!doOtherThing(I))
857 ... some long code ....
860 This fixes these problems. A similar problem frequently happens in ``for``
861 loops. A silly example is something like this:
865 for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ++II) {
866 if (BinaryOperator *BO = dyn_cast<BinaryOperator>(II)) {
867 Value *LHS = BO->getOperand(0);
868 Value *RHS = BO->getOperand(1);
875 When you have very, very small loops, this sort of structure is fine. But if it
876 exceeds more than 10-15 lines, it becomes difficult for people to read and
877 understand at a glance. The problem with this sort of code is that it gets very
878 nested very quickly. Meaning that the reader of the code has to keep a lot of
879 context in their brain to remember what is going immediately on in the loop,
880 because they don't know if/when the ``if`` conditions will have ``else``\s etc.
881 It is strongly preferred to structure the loop like this:
885 for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ++II) {
886 BinaryOperator *BO = dyn_cast<BinaryOperator>(II);
889 Value *LHS = BO->getOperand(0);
890 Value *RHS = BO->getOperand(1);
891 if (LHS == RHS) continue;
896 This has all the benefits of using early exits for functions: it reduces nesting
897 of the loop, it makes it easier to describe why the conditions are true, and it
898 makes it obvious to the reader that there is no ``else`` coming up that they
899 have to push context into their brain for. If a loop is large, this can be a
900 big understandability win.
902 Don't use ``else`` after a ``return``
903 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
905 For similar reasons above (reduction of indentation and easier reading), please
906 do not use ``'else'`` or ``'else if'`` after something that interrupts control
907 flow --- like ``return``, ``break``, ``continue``, ``goto``, etc. For
908 example, this is *bad*:
914 Type = Context.getsigjmp_bufType();
916 Error = ASTContext::GE_Missing_sigjmp_buf;
922 Type = Context.getjmp_bufType();
924 Error = ASTContext::GE_Missing_jmp_buf;
932 It is better to write it like this:
938 Type = Context.getsigjmp_bufType();
940 Error = ASTContext::GE_Missing_sigjmp_buf;
944 Type = Context.getjmp_bufType();
946 Error = ASTContext::GE_Missing_jmp_buf;
952 Or better yet (in this case) as:
958 Type = Context.getsigjmp_bufType();
960 Type = Context.getjmp_bufType();
963 Error = Signed ? ASTContext::GE_Missing_sigjmp_buf :
964 ASTContext::GE_Missing_jmp_buf;
969 The idea is to reduce indentation and the amount of code you have to keep track
970 of when reading the code.
972 Turn Predicate Loops into Predicate Functions
973 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
975 It is very common to write small loops that just compute a boolean value. There
976 are a number of ways that people commonly write these, but an example of this
981 bool FoundFoo = false;
982 for (unsigned I = 0, E = BarList.size(); I != E; ++I)
983 if (BarList[I]->isFoo()) {
992 This sort of code is awkward to write, and is almost always a bad sign. Instead
993 of this sort of loop, we strongly prefer to use a predicate function (which may
994 be `static`_) that uses `early exits`_ to compute the predicate. We prefer the
995 code to be structured like this:
999 /// \returns true if the specified list has an element that is a foo.
1000 static bool containsFoo(const std::vector<Bar*> &List) {
1001 for (unsigned I = 0, E = List.size(); I != E; ++I)
1002 if (List[I]->isFoo())
1008 if (containsFoo(BarList)) {
1012 There are many reasons for doing this: it reduces indentation and factors out
1013 code which can often be shared by other code that checks for the same predicate.
1014 More importantly, it *forces you to pick a name* for the function, and forces
1015 you to write a comment for it. In this silly example, this doesn't add much
1016 value. However, if the condition is complex, this can make it a lot easier for
1017 the reader to understand the code that queries for this predicate. Instead of
1018 being faced with the in-line details of how we check to see if the BarList
1019 contains a foo, we can trust the function name and continue reading with better
1022 The Low-Level Issues
1023 --------------------
1025 Name Types, Functions, Variables, and Enumerators Properly
1026 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
1028 Poorly-chosen names can mislead the reader and cause bugs. We cannot stress
1029 enough how important it is to use *descriptive* names. Pick names that match
1030 the semantics and role of the underlying entities, within reason. Avoid
1031 abbreviations unless they are well known. After picking a good name, make sure
1032 to use consistent capitalization for the name, as inconsistency requires clients
1033 to either memorize the APIs or to look it up to find the exact spelling.
1035 In general, names should be in camel case (e.g. ``TextFileReader`` and
1036 ``isLValue()``). Different kinds of declarations have different rules:
1038 * **Type names** (including classes, structs, enums, typedefs, etc) should be
1039 nouns and start with an upper-case letter (e.g. ``TextFileReader``).
1041 * **Variable names** should be nouns (as they represent state). The name should
1042 be camel case, and start with an upper case letter (e.g. ``Leader`` or
1045 * **Function names** should be verb phrases (as they represent actions), and
1046 command-like function should be imperative. The name should be camel case,
1047 and start with a lower case letter (e.g. ``openFile()`` or ``isFoo()``).
1049 * **Enum declarations** (e.g. ``enum Foo {...}``) are types, so they should
1050 follow the naming conventions for types. A common use for enums is as a
1051 discriminator for a union, or an indicator of a subclass. When an enum is
1052 used for something like this, it should have a ``Kind`` suffix
1053 (e.g. ``ValueKind``).
1055 * **Enumerators** (e.g. ``enum { Foo, Bar }``) and **public member variables**
1056 should start with an upper-case letter, just like types. Unless the
1057 enumerators are defined in their own small namespace or inside a class,
1058 enumerators should have a prefix corresponding to the enum declaration name.
1059 For example, ``enum ValueKind { ... };`` may contain enumerators like
1060 ``VK_Argument``, ``VK_BasicBlock``, etc. Enumerators that are just
1061 convenience constants are exempt from the requirement for a prefix. For
1071 As an exception, classes that mimic STL classes can have member names in STL's
1072 style of lower-case words separated by underscores (e.g. ``begin()``,
1073 ``push_back()``, and ``empty()``). Classes that provide multiple
1074 iterators should add a singular prefix to ``begin()`` and ``end()``
1075 (e.g. ``global_begin()`` and ``use_begin()``).
1077 Here are some examples of good and bad names:
1081 class VehicleMaker {
1083 Factory<Tire> F; // Bad -- abbreviation and non-descriptive.
1084 Factory<Tire> Factory; // Better.
1085 Factory<Tire> TireFactory; // Even better -- if VehicleMaker has more than one
1086 // kind of factories.
1089 Vehicle MakeVehicle(VehicleType Type) {
1090 VehicleMaker M; // Might be OK if having a short life-span.
1091 Tire Tmp1 = M.makeTire(); // Bad -- 'Tmp1' provides no information.
1092 Light Headlight = M.makeLight("head"); // Good -- descriptive.
1099 Use the "``assert``" macro to its fullest. Check all of your preconditions and
1100 assumptions, you never know when a bug (not necessarily even yours) might be
1101 caught early by an assertion, which reduces debugging time dramatically. The
1102 "``<cassert>``" header file is probably already included by the header files you
1103 are using, so it doesn't cost anything to use it.
1105 To further assist with debugging, make sure to put some kind of error message in
1106 the assertion statement, which is printed if the assertion is tripped. This
1107 helps the poor debugger make sense of why an assertion is being made and
1108 enforced, and hopefully what to do about it. Here is one complete example:
1112 inline Value *getOperand(unsigned I) {
1113 assert(I < Operands.size() && "getOperand() out of range!");
1117 Here are more examples:
1121 assert(Ty->isPointerType() && "Can't allocate a non-pointer type!");
1123 assert((Opcode == Shl || Opcode == Shr) && "ShiftInst Opcode invalid!");
1125 assert(idx < getNumSuccessors() && "Successor # out of range!");
1127 assert(V1.getType() == V2.getType() && "Constant types must be identical!");
1129 assert(isa<PHINode>(Succ->front()) && "Only works on PHId BBs!");
1133 In the past, asserts were used to indicate a piece of code that should not be
1134 reached. These were typically of the form:
1138 assert(0 && "Invalid radix for integer literal");
1140 This has a few issues, the main one being that some compilers might not
1141 understand the assertion, or warn about a missing return in builds where
1142 assertions are compiled out.
1144 Today, we have something much better: ``llvm_unreachable``:
1148 llvm_unreachable("Invalid radix for integer literal");
1150 When assertions are enabled, this will print the message if it's ever reached
1151 and then exit the program. When assertions are disabled (i.e. in release
1152 builds), ``llvm_unreachable`` becomes a hint to compilers to skip generating
1153 code for this branch. If the compiler does not support this, it will fall back
1154 to the "abort" implementation.
1156 Another issue is that values used only by assertions will produce an "unused
1157 value" warning when assertions are disabled. For example, this code will warn:
1161 unsigned Size = V.size();
1162 assert(Size > 42 && "Vector smaller than it should be");
1164 bool NewToSet = Myset.insert(Value);
1165 assert(NewToSet && "The value shouldn't be in the set yet");
1167 These are two interesting different cases. In the first case, the call to
1168 ``V.size()`` is only useful for the assert, and we don't want it executed when
1169 assertions are disabled. Code like this should move the call into the assert
1170 itself. In the second case, the side effects of the call must happen whether
1171 the assert is enabled or not. In this case, the value should be cast to void to
1172 disable the warning. To be specific, it is preferred to write the code like
1177 assert(V.size() > 42 && "Vector smaller than it should be");
1179 bool NewToSet = Myset.insert(Value); (void)NewToSet;
1180 assert(NewToSet && "The value shouldn't be in the set yet");
1182 Do Not Use ``using namespace std``
1183 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
1185 In LLVM, we prefer to explicitly prefix all identifiers from the standard
1186 namespace with an "``std::``" prefix, rather than rely on "``using namespace
1189 In header files, adding a ``'using namespace XXX'`` directive pollutes the
1190 namespace of any source file that ``#include``\s the header. This is clearly a
1193 In implementation files (e.g. ``.cpp`` files), the rule is more of a stylistic
1194 rule, but is still important. Basically, using explicit namespace prefixes
1195 makes the code **clearer**, because it is immediately obvious what facilities
1196 are being used and where they are coming from. And **more portable**, because
1197 namespace clashes cannot occur between LLVM code and other namespaces. The
1198 portability rule is important because different standard library implementations
1199 expose different symbols (potentially ones they shouldn't), and future revisions
1200 to the C++ standard will add more symbols to the ``std`` namespace. As such, we
1201 never use ``'using namespace std;'`` in LLVM.
1203 The exception to the general rule (i.e. it's not an exception for the ``std``
1204 namespace) is for implementation files. For example, all of the code in the
1205 LLVM project implements code that lives in the 'llvm' namespace. As such, it is
1206 ok, and actually clearer, for the ``.cpp`` files to have a ``'using namespace
1207 llvm;'`` directive at the top, after the ``#include``\s. This reduces
1208 indentation in the body of the file for source editors that indent based on
1209 braces, and keeps the conceptual context cleaner. The general form of this rule
1210 is that any ``.cpp`` file that implements code in any namespace may use that
1211 namespace (and its parents'), but should not use any others.
1213 Provide a Virtual Method Anchor for Classes in Headers
1214 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
1216 If a class is defined in a header file and has a vtable (either it has virtual
1217 methods or it derives from classes with virtual methods), it must always have at
1218 least one out-of-line virtual method in the class. Without this, the compiler
1219 will copy the vtable and RTTI into every ``.o`` file that ``#include``\s the
1220 header, bloating ``.o`` file sizes and increasing link times.
1222 Don't use default labels in fully covered switches over enumerations
1223 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
1225 ``-Wswitch`` warns if a switch, without a default label, over an enumeration
1226 does not cover every enumeration value. If you write a default label on a fully
1227 covered switch over an enumeration then the ``-Wswitch`` warning won't fire
1228 when new elements are added to that enumeration. To help avoid adding these
1229 kinds of defaults, Clang has the warning ``-Wcovered-switch-default`` which is
1230 off by default but turned on when building LLVM with a version of Clang that
1231 supports the warning.
1233 A knock-on effect of this stylistic requirement is that when building LLVM with
1234 GCC you may get warnings related to "control may reach end of non-void function"
1235 if you return from each case of a covered switch-over-enum because GCC assumes
1236 that the enum expression may take any representable value, not just those of
1237 individual enumerators. To suppress this warning, use ``llvm_unreachable`` after
1240 Use ``LLVM_DELETED_FUNCTION`` to mark uncallable methods
1241 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
1243 Prior to C++11, a common pattern to make a class uncopyable was to declare an
1244 unimplemented copy constructor and copy assignment operator and make them
1245 private. This would give a compiler error for accessing a private method or a
1246 linker error because it wasn't implemented.
1248 With C++11, we can mark methods that won't be implemented with ``= delete``.
1249 This will trigger a much better error message and tell the compiler that the
1250 method will never be implemented. This enables other checks like
1251 ``-Wunused-private-field`` to run correctly on classes that contain these
1254 To maintain compatibility with C++03, ``LLVM_DELETED_FUNCTION`` should be used
1255 which will expand to ``= delete`` if the compiler supports it. These methods
1256 should still be declared private. Example of the uncopyable pattern:
1262 DontCopy(const DontCopy&) LLVM_DELETED_FUNCTION;
1263 DontCopy &operator =(const DontCopy&) LLVM_DELETED_FUNCTION;
1268 Don't evaluate ``end()`` every time through a loop
1269 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
1271 Because C++ doesn't have a standard "``foreach``" loop (though it can be
1272 emulated with macros and may be coming in C++'0x) we end up writing a lot of
1273 loops that manually iterate from begin to end on a variety of containers or
1274 through other data structures. One common mistake is to write a loop in this
1279 BasicBlock *BB = ...
1280 for (BasicBlock::iterator I = BB->begin(); I != BB->end(); ++I)
1283 The problem with this construct is that it evaluates "``BB->end()``" every time
1284 through the loop. Instead of writing the loop like this, we strongly prefer
1285 loops to be written so that they evaluate it once before the loop starts. A
1286 convenient way to do this is like so:
1290 BasicBlock *BB = ...
1291 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
1294 The observant may quickly point out that these two loops may have different
1295 semantics: if the container (a basic block in this case) is being mutated, then
1296 "``BB->end()``" may change its value every time through the loop and the second
1297 loop may not in fact be correct. If you actually do depend on this behavior,
1298 please write the loop in the first form and add a comment indicating that you
1299 did it intentionally.
1301 Why do we prefer the second form (when correct)? Writing the loop in the first
1302 form has two problems. First it may be less efficient than evaluating it at the
1303 start of the loop. In this case, the cost is probably minor --- a few extra
1304 loads every time through the loop. However, if the base expression is more
1305 complex, then the cost can rise quickly. I've seen loops where the end
1306 expression was actually something like: "``SomeMap[X]->end()``" and map lookups
1307 really aren't cheap. By writing it in the second form consistently, you
1308 eliminate the issue entirely and don't even have to think about it.
1310 The second (even bigger) issue is that writing the loop in the first form hints
1311 to the reader that the loop is mutating the container (a fact that a comment
1312 would handily confirm!). If you write the loop in the second form, it is
1313 immediately obvious without even looking at the body of the loop that the
1314 container isn't being modified, which makes it easier to read the code and
1315 understand what it does.
1317 While the second form of the loop is a few extra keystrokes, we do strongly
1320 ``#include <iostream>`` is Forbidden
1321 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
1323 The use of ``#include <iostream>`` in library files is hereby **forbidden**,
1324 because many common implementations transparently inject a `static constructor`_
1325 into every translation unit that includes it.
1327 Note that using the other stream headers (``<sstream>`` for example) is not
1328 problematic in this regard --- just ``<iostream>``. However, ``raw_ostream``
1329 provides various APIs that are better performing for almost every use than
1330 ``std::ostream`` style APIs.
1334 New code should always use `raw_ostream`_ for writing, or the
1335 ``llvm::MemoryBuffer`` API for reading files.
1342 LLVM includes a lightweight, simple, and efficient stream implementation in
1343 ``llvm/Support/raw_ostream.h``, which provides all of the common features of
1344 ``std::ostream``. All new code should use ``raw_ostream`` instead of
1347 Unlike ``std::ostream``, ``raw_ostream`` is not a template and can be forward
1348 declared as ``class raw_ostream``. Public headers should generally not include
1349 the ``raw_ostream`` header, but use forward declarations and constant references
1350 to ``raw_ostream`` instances.
1355 The ``std::endl`` modifier, when used with ``iostreams`` outputs a newline to
1356 the output stream specified. In addition to doing this, however, it also
1357 flushes the output stream. In other words, these are equivalent:
1361 std::cout << std::endl;
1362 std::cout << '\n' << std::flush;
1364 Most of the time, you probably have no reason to flush the output stream, so
1365 it's better to use a literal ``'\n'``.
1367 Don't use ``inline`` when defining a function in a class definition
1368 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
1370 A member function defined in a class definition is implicitly inline, so don't
1371 put the ``inline`` keyword in this case.
1398 This section describes preferred low-level formatting guidelines along with
1399 reasoning on why we prefer them.
1401 Spaces Before Parentheses
1402 ^^^^^^^^^^^^^^^^^^^^^^^^^
1404 We prefer to put a space before an open parenthesis only in control flow
1405 statements, but not in normal function call expressions and function-like
1406 macros. For example, this is good:
1411 for (I = 0; I != 100; ++I) ...
1412 while (LLVMRocks) ...
1415 assert(3 != 4 && "laws of math are failing me");
1417 A = foo(42, 92) + bar(X);
1424 for(I = 0; I != 100; ++I) ...
1425 while(LLVMRocks) ...
1428 assert (3 != 4 && "laws of math are failing me");
1430 A = foo (42, 92) + bar (X);
1432 The reason for doing this is not completely arbitrary. This style makes control
1433 flow operators stand out more, and makes expressions flow better. The function
1434 call operator binds very tightly as a postfix operator. Putting a space after a
1435 function name (as in the last example) makes it appear that the code might bind
1436 the arguments of the left-hand-side of a binary operator with the argument list
1437 of a function and the name of the right side. More specifically, it is easy to
1438 misread the "``A``" example as:
1442 A = foo ((42, 92) + bar) (X);
1444 when skimming through the code. By avoiding a space in a function, we avoid
1445 this misinterpretation.
1450 Hard fast rule: Preincrement (``++X``) may be no slower than postincrement
1451 (``X++``) and could very well be a lot faster than it. Use preincrementation
1454 The semantics of postincrement include making a copy of the value being
1455 incremented, returning it, and then preincrementing the "work value". For
1456 primitive types, this isn't a big deal. But for iterators, it can be a huge
1457 issue (for example, some iterators contains stack and set objects in them...
1458 copying an iterator could invoke the copy ctor's of these as well). In general,
1459 get in the habit of always using preincrement, and you won't have a problem.
1462 Namespace Indentation
1463 ^^^^^^^^^^^^^^^^^^^^^
1465 In general, we strive to reduce indentation wherever possible. This is useful
1466 because we want code to `fit into 80 columns`_ without wrapping horribly, but
1467 also because it makes it easier to understand the code. To facilitate this and
1468 avoid some insanely deep nesting on occasion, don't indent namespaces. If it
1469 helps readability, feel free to add a comment indicating what namespace is
1470 being closed by a ``}``. For example:
1475 namespace knowledge {
1477 /// This class represents things that Smith can have an intimate
1478 /// understanding of and contains the data associated with it.
1482 explicit Grokable() { ... }
1483 virtual ~Grokable() = 0;
1489 } // end namespace knowledge
1490 } // end namespace llvm
1493 Feel free to skip the closing comment when the namespace being closed is
1494 obvious for any reason. For example, the outer-most namespace in a header file
1495 is rarely a source of confusion. But namespaces both anonymous and named in
1496 source files that are being closed half way through the file probably could use
1501 Anonymous Namespaces
1502 ^^^^^^^^^^^^^^^^^^^^
1504 After talking about namespaces in general, you may be wondering about anonymous
1505 namespaces in particular. Anonymous namespaces are a great language feature
1506 that tells the C++ compiler that the contents of the namespace are only visible
1507 within the current translation unit, allowing more aggressive optimization and
1508 eliminating the possibility of symbol name collisions. Anonymous namespaces are
1509 to C++ as "static" is to C functions and global variables. While "``static``"
1510 is available in C++, anonymous namespaces are more general: they can make entire
1511 classes private to a file.
1513 The problem with anonymous namespaces is that they naturally want to encourage
1514 indentation of their body, and they reduce locality of reference: if you see a
1515 random function definition in a C++ file, it is easy to see if it is marked
1516 static, but seeing if it is in an anonymous namespace requires scanning a big
1519 Because of this, we have a simple guideline: make anonymous namespaces as small
1520 as possible, and only use them for class declarations. For example, this is
1530 bool operator<(const char *RHS) const;
1532 } // end anonymous namespace
1534 static void runHelper() {
1538 bool StringSort::operator<(const char *RHS) const {
1552 bool operator<(const char *RHS) const;
1559 bool StringSort::operator<(const char *RHS) const {
1563 } // end anonymous namespace
1565 This is bad specifically because if you're looking at "``runHelper``" in the middle
1566 of a large C++ file, that you have no immediate way to tell if it is local to
1567 the file. When it is marked static explicitly, this is immediately obvious.
1568 Also, there is no reason to enclose the definition of "``operator<``" in the
1569 namespace just because it was declared there.
1574 A lot of these comments and recommendations have been culled from other sources.
1575 Two particularly important books for our work are:
1578 <http://www.amazon.com/Effective-Specific-Addison-Wesley-Professional-Computing/dp/0321334876>`_
1579 by Scott Meyers. Also interesting and useful are "More Effective C++" and
1580 "Effective STL" by the same author.
1582 #. `Large-Scale C++ Software Design
1583 <http://www.amazon.com/Large-Scale-Software-Design-John-Lakos/dp/0201633620/ref=sr_1_1>`_
1586 If you get some free time, and you haven't read them: do so, you might learn