1 ===================== 2 LLVM Coding Standards 3 ===================== 4 5 .. contents:: 6 :local: 7 8 Introduction 9 ============ 10 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 15 design (like LLVM). 16 17 This document intentionally does not prescribe fixed standards for religious 18 issues such as brace placement and space usage. For issues like this, follow 19 the golden rule: 20 21 .. _Golden Rule: 22 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 25 easy to follow.** 26 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. 32 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. 41 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 (a] nondot.org>`_. 45 46 Mechanical Source Issues 47 ======================== 48 49 Source Code Formatting 50 ---------------------- 51 52 Commenting 53 ^^^^^^^^^^ 54 55 Comments are one critical part of readability and maintainability. Everyone 56 knows they should comment their code, and so should you. When writing comments, 57 write them as English prose, which means they should use proper capitalization, 58 punctuation, etc. Aim to describe what the code is trying to do and why, not 59 *how* it does it at a micro level. Here are a few critical things to document: 60 61 .. _header file comment: 62 63 File Headers 64 """""""""""" 65 66 Every source file should have a header on it that describes the basic purpose of 67 the file. If a file does not have a header, it should not be checked into the 68 tree. The standard header looks like this: 69 70 .. code-block:: c++ 71 72 //===-- llvm/Instruction.h - Instruction class definition -------*- C++ -*-===// 73 // 74 // The LLVM Compiler Infrastructure 75 // 76 // This file is distributed under the University of Illinois Open Source 77 // License. See LICENSE.TXT for details. 78 // 79 //===----------------------------------------------------------------------===// 80 /// 81 /// \file 82 /// \brief This file contains the declaration of the Instruction class, which is 83 /// the base class for all of the VM instructions. 84 /// 85 //===----------------------------------------------------------------------===// 86 87 A few things to note about this particular format: The "``-*- C++ -*-``" string 88 on the first line is there to tell Emacs that the source file is a C++ file, not 89 a C file (Emacs assumes ``.h`` files are C files by default). 90 91 .. note:: 92 93 This tag is not necessary in ``.cpp`` files. The name of the file is also 94 on the first line, along with a very short description of the purpose of the 95 file. This is important when printing out code and flipping though lots of 96 pages. 97 98 The next section in the file is a concise note that defines the license that the 99 file is released under. This makes it perfectly clear what terms the source 100 code can be distributed under and should not be modified in any way. 101 102 The main body is a ``doxygen`` comment describing the purpose of the file. It 103 should have a ``\brief`` command that describes the file in one or two 104 sentences. Any additional information should be separated by a blank line. If 105 an algorithm is being implemented or something tricky is going on, a reference 106 to the paper where it is published should be included, as well as any notes or 107 *gotchas* in the code to watch out for. 108 109 Class overviews 110 """"""""""""""" 111 112 Classes are one fundamental part of a good object oriented design. As such, a 113 class definition should have a comment block that explains what the class is 114 used for and how it works. Every non-trivial class is expected to have a 115 ``doxygen`` comment block. 116 117 Method information 118 """""""""""""""""" 119 120 Methods defined in a class (as well as any global functions) should also be 121 documented properly. A quick note about what it does and a description of the 122 borderline behaviour is all that is necessary here (unless something 123 particularly tricky or insidious is going on). The hope is that people can 124 figure out how to use your interfaces without reading the code itself. 125 126 Good things to talk about here are what happens when something unexpected 127 happens: does the method return null? Abort? Format your hard disk? 128 129 Comment Formatting 130 ^^^^^^^^^^^^^^^^^^ 131 132 In general, prefer C++ style (``//``) comments. They take less space, require 133 less typing, don't have nesting problems, etc. There are a few cases when it is 134 useful to use C style (``/* */``) comments however: 135 136 #. When writing C code: Obviously if you are writing C code, use C style 137 comments. 138 139 #. When writing a header file that may be ``#include``\d by a C source file. 140 141 #. When writing a source file that is used by a tool that only accepts C style 142 comments. 143 144 To comment out a large block of code, use ``#if 0`` and ``#endif``. These nest 145 properly and are better behaved in general than C style comments. 146 147 Doxygen Use in Documentation Comments 148 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 149 150 Use the ``\file`` command to turn the standard file header into a file-level 151 comment. 152 153 Include descriptive ``\brief`` paragraphs for all public interfaces (public 154 classes, member and non-member functions). Explain API use and purpose in 155 ``\brief`` paragraphs, don't just restate the information that can be inferred 156 from the API name. Put detailed discussion into separate paragraphs. 157 158 To refer to parameter names inside a paragraph, use the ``\p name`` command. 159 Don't use the ``\arg name`` command since it starts a new paragraph that 160 contains documentation for the parameter. 161 162 Wrap non-inline code examples in ``\code ... \endcode``. 163 164 To document a function parameter, start a new paragraph with the 165 ``\param name`` command. If the parameter is used as an out or an in/out 166 parameter, use the ``\param [out] name`` or ``\param [in,out] name`` command, 167 respectively. 168 169 To describe function return value, start a new paragraph with the ``\returns`` 170 command. 171 172 A minimal documentation comment: 173 174 .. code-block:: c++ 175 176 /// \brief Does foo and bar. 177 void fooBar(bool Baz); 178 179 A documentation comment that uses all Doxygen features in a preferred way: 180 181 .. code-block:: c++ 182 183 /// \brief Does foo and bar. 184 /// 185 /// Does not do foo the usual way if \p Baz is true. 186 /// 187 /// Typical usage: 188 /// \code 189 /// fooBar(false, "quux", Res); 190 /// \endcode 191 /// 192 /// \param Quux kind of foo to do. 193 /// \param [out] Result filled with bar sequence on foo success. 194 /// 195 /// \returns true on success. 196 bool fooBar(bool Baz, StringRef Quux, std::vector<int> &Result); 197 198 Don't duplicate the documentation comment in the header file and in the 199 implementation file. Put the documentation comments for public APIs into the 200 header file. Documentation comments for private APIs can go to the 201 implementation file. In any case, implementation files can include additional 202 comments (not necessarily in Doxygen markup) to explain implementation details 203 as needed. 204 205 Don't duplicate function or class name at the beginning of the comment. 206 For humans it is obvious which function or class is being documented; 207 automatic documentation processing tools are smart enough to bind the comment 208 to the correct declaration. 209 210 Wrong: 211 212 .. code-block:: c++ 213 214 // In Something.h: 215 216 /// Something - An abstraction for some complicated thing. 217 class Something { 218 public: 219 /// fooBar - Does foo and bar. 220 void fooBar(); 221 }; 222 223 // In Something.cpp: 224 225 /// fooBar - Does foo and bar. 226 void Something::fooBar() { ... } 227 228 Correct: 229 230 .. code-block:: c++ 231 232 // In Something.h: 233 234 /// \brief An abstraction for some complicated thing. 235 class Something { 236 public: 237 /// \brief Does foo and bar. 238 void fooBar(); 239 }; 240 241 // In Something.cpp: 242 243 // Builds a B-tree in order to do foo. See paper by... 244 void Something::fooBar() { ... } 245 246 It is not required to use additional Doxygen features, but sometimes it might 247 be a good idea to do so. 248 249 Consider: 250 251 * adding comments to any narrow namespace containing a collection of 252 related functions or types; 253 254 * using top-level groups to organize a collection of related functions at 255 namespace scope where the grouping is smaller than the namespace; 256 257 * using member groups and additional comments attached to member 258 groups to organize within a class. 259 260 For example: 261 262 .. code-block:: c++ 263 264 class Something { 265 /// \name Functions that do Foo. 266 /// @{ 267 void fooBar(); 268 void fooBaz(); 269 /// @} 270 ... 271 }; 272 273 ``#include`` Style 274 ^^^^^^^^^^^^^^^^^^ 275 276 Immediately after the `header file comment`_ (and include guards if working on a 277 header file), the `minimal list of #includes`_ required by the file should be 278 listed. We prefer these ``#include``\s to be listed in this order: 279 280 .. _Main Module Header: 281 .. _Local/Private Headers: 282 283 #. Main Module Header 284 #. Local/Private Headers 285 #. ``llvm/...`` 286 #. System ``#include``\s 287 288 and each category should be sorted lexicographically by the full path. 289 290 The `Main Module Header`_ file applies to ``.cpp`` files which implement an 291 interface defined by a ``.h`` file. This ``#include`` should always be included 292 **first** regardless of where it lives on the file system. By including a 293 header file first in the ``.cpp`` files that implement the interfaces, we ensure 294 that the header does not have any hidden dependencies which are not explicitly 295 ``#include``\d in the header, but should be. It is also a form of documentation 296 in the ``.cpp`` file to indicate where the interfaces it implements are defined. 297 298 .. _fit into 80 columns: 299 300 Source Code Width 301 ^^^^^^^^^^^^^^^^^ 302 303 Write your code to fit within 80 columns of text. This helps those of us who 304 like to print out code and look at your code in an ``xterm`` without resizing 305 it. 306 307 The longer answer is that there must be some limit to the width of the code in 308 order to reasonably allow developers to have multiple files side-by-side in 309 windows on a modest display. If you are going to pick a width limit, it is 310 somewhat arbitrary but you might as well pick something standard. Going with 90 311 columns (for example) instead of 80 columns wouldn't add any significant value 312 and would be detrimental to printing out code. Also many other projects have 313 standardized on 80 columns, so some people have already configured their editors 314 for it (vs something else, like 90 columns). 315 316 This is one of many contentious issues in coding standards, but it is not up for 317 debate. 318 319 Use Spaces Instead of Tabs 320 ^^^^^^^^^^^^^^^^^^^^^^^^^^ 321 322 In all cases, prefer spaces to tabs in source files. People have different 323 preferred indentation levels, and different styles of indentation that they 324 like; this is fine. What isn't fine is that different editors/viewers expand 325 tabs out to different tab stops. This can cause your code to look completely 326 unreadable, and it is not worth dealing with. 327 328 As always, follow the `Golden Rule`_ above: follow the style of 329 existing code if you are modifying and extending it. If you like four spaces of 330 indentation, **DO NOT** do that in the middle of a chunk of code with two spaces 331 of indentation. Also, do not reindent a whole source file: it makes for 332 incredible diffs that are absolutely worthless. 333 334 Indent Code Consistently 335 ^^^^^^^^^^^^^^^^^^^^^^^^ 336 337 Okay, in your first year of programming you were told that indentation is 338 important. If you didn't believe and internalize this then, now is the time. 339 Just do it. 340 341 Compiler Issues 342 --------------- 343 344 Treat Compiler Warnings Like Errors 345 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 346 347 If your code has compiler warnings in it, something is wrong --- you aren't 348 casting values correctly, you have "questionable" constructs in your code, or 349 you are doing something legitimately wrong. Compiler warnings can cover up 350 legitimate errors in output and make dealing with a translation unit difficult. 351 352 It is not possible to prevent all warnings from all compilers, nor is it 353 desirable. Instead, pick a standard compiler (like ``gcc``) that provides a 354 good thorough set of warnings, and stick to it. At least in the case of 355 ``gcc``, it is possible to work around any spurious errors by changing the 356 syntax of the code slightly. For example, a warning that annoys me occurs when 357 I write code like this: 358 359 .. code-block:: c++ 360 361 if (V = getValue()) { 362 ... 363 } 364 365 ``gcc`` will warn me that I probably want to use the ``==`` operator, and that I 366 probably mistyped it. In most cases, I haven't, and I really don't want the 367 spurious errors. To fix this particular problem, I rewrite the code like 368 this: 369 370 .. code-block:: c++ 371 372 if ((V = getValue())) { 373 ... 374 } 375 376 which shuts ``gcc`` up. Any ``gcc`` warning that annoys you can be fixed by 377 massaging the code appropriately. 378 379 Write Portable Code 380 ^^^^^^^^^^^^^^^^^^^ 381 382 In almost all cases, it is possible and within reason to write completely 383 portable code. If there are cases where it isn't possible to write portable 384 code, isolate it behind a well defined (and well documented) interface. 385 386 In practice, this means that you shouldn't assume much about the host compiler 387 (and Visual Studio tends to be the lowest common denominator). If advanced 388 features are used, they should only be an implementation detail of a library 389 which has a simple exposed API, and preferably be buried in ``libSystem``. 390 391 Do not use RTTI or Exceptions 392 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 393 394 In an effort to reduce code and executable size, LLVM does not use RTTI 395 (e.g. ``dynamic_cast<>;``) or exceptions. These two language features violate 396 the general C++ principle of *"you only pay for what you use"*, causing 397 executable bloat even if exceptions are never used in the code base, or if RTTI 398 is never used for a class. Because of this, we turn them off globally in the 399 code. 400 401 That said, LLVM does make extensive use of a hand-rolled form of RTTI that use 402 templates like `isa<>, cast<>, and dyn_cast<> <ProgrammersManual.html#isa>`_. 403 This form of RTTI is opt-in and can be 404 :doc:`added to any class <HowToSetUpLLVMStyleRTTI>`. It is also 405 substantially more efficient than ``dynamic_cast<>``. 406 407 .. _static constructor: 408 409 Do not use Static Constructors 410 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 411 412 Static constructors and destructors (e.g. global variables whose types have a 413 constructor or destructor) should not be added to the code base, and should be 414 removed wherever possible. Besides `well known problems 415 <http://yosefk.com/c++fqa/ctors.html#fqa-10.12>`_ where the order of 416 initialization is undefined between globals in different source files, the 417 entire concept of static constructors is at odds with the common use case of 418 LLVM as a library linked into a larger application. 419 420 Consider the use of LLVM as a JIT linked into another application (perhaps for 421 `OpenGL, custom languages <http://llvm.org/Users.html>`_, `shaders in movies 422 <http://llvm.org/devmtg/2010-11/Gritz-OpenShadingLang.pdf>`_, etc). Due to the 423 design of static constructors, they must be executed at startup time of the 424 entire application, regardless of whether or how LLVM is used in that larger 425 application. There are two problems with this: 426 427 * The time to run the static constructors impacts startup time of applications 428 --- a critical time for GUI apps, among others. 429 430 * The static constructors cause the app to pull many extra pages of memory off 431 the disk: both the code for the constructor in each ``.o`` file and the small 432 amount of data that gets touched. In addition, touched/dirty pages put more 433 pressure on the VM system on low-memory machines. 434 435 We would really like for there to be zero cost for linking in an additional LLVM 436 target or other library into an application, but static constructors violate 437 this goal. 438 439 That said, LLVM unfortunately does contain static constructors. It would be a 440 `great project <http://llvm.org/PR11944>`_ for someone to purge all static 441 constructors from LLVM, and then enable the ``-Wglobal-constructors`` warning 442 flag (when building with Clang) to ensure we do not regress in the future. 443 444 Use of ``class`` and ``struct`` Keywords 445 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 446 447 In C++, the ``class`` and ``struct`` keywords can be used almost 448 interchangeably. The only difference is when they are used to declare a class: 449 ``class`` makes all members private by default while ``struct`` makes all 450 members public by default. 451 452 Unfortunately, not all compilers follow the rules and some will generate 453 different symbols based on whether ``class`` or ``struct`` was used to declare 454 the symbol. This can lead to problems at link time. 455 456 So, the rule for LLVM is to always use the ``class`` keyword, unless **all** 457 members are public and the type is a C++ `POD 458 <http://en.wikipedia.org/wiki/Plain_old_data_structure>`_ type, in which case 459 ``struct`` is allowed. 460 461 Style Issues 462 ============ 463 464 The High-Level Issues 465 --------------------- 466 467 A Public Header File **is** a Module 468 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 469 470 C++ doesn't do too well in the modularity department. There is no real 471 encapsulation or data hiding (unless you use expensive protocol classes), but it 472 is what we have to work with. When you write a public header file (in the LLVM 473 source tree, they live in the top level "``include``" directory), you are 474 defining a module of functionality. 475 476 Ideally, modules should be completely independent of each other, and their 477 header files should only ``#include`` the absolute minimum number of headers 478 possible. A module is not just a class, a function, or a namespace: it's a 479 collection of these that defines an interface. This interface may be several 480 functions, classes, or data structures, but the important issue is how they work 481 together. 482 483 In general, a module should be implemented by one or more ``.cpp`` files. Each 484 of these ``.cpp`` files should include the header that defines their interface 485 first. This ensures that all of the dependences of the module header have been 486 properly added to the module header itself, and are not implicit. System 487 headers should be included after user headers for a translation unit. 488 489 .. _minimal list of #includes: 490 491 ``#include`` as Little as Possible 492 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 493 494 ``#include`` hurts compile time performance. Don't do it unless you have to, 495 especially in header files. 496 497 But wait! Sometimes you need to have the definition of a class to use it, or to 498 inherit from it. In these cases go ahead and ``#include`` that header file. Be 499 aware however that there are many cases where you don't need to have the full 500 definition of a class. If you are using a pointer or reference to a class, you 501 don't need the header file. If you are simply returning a class instance from a 502 prototyped function or method, you don't need it. In fact, for most cases, you 503 simply don't need the definition of a class. And not ``#include``\ing speeds up 504 compilation. 505 506 It is easy to try to go too overboard on this recommendation, however. You 507 **must** include all of the header files that you are using --- you can include 508 them either directly or indirectly through another header file. To make sure 509 that you don't accidentally forget to include a header file in your module 510 header, make sure to include your module header **first** in the implementation 511 file (as mentioned above). This way there won't be any hidden dependencies that 512 you'll find out about later. 513 514 Keep "Internal" Headers Private 515 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 516 517 Many modules have a complex implementation that causes them to use more than one 518 implementation (``.cpp``) file. It is often tempting to put the internal 519 communication interface (helper classes, extra functions, etc) in the public 520 module header file. Don't do this! 521 522 If you really need to do something like this, put a private header file in the 523 same directory as the source files, and include it locally. This ensures that 524 your private interface remains private and undisturbed by outsiders. 525 526 .. note:: 527 528 It's okay to put extra implementation methods in a public class itself. Just 529 make them private (or protected) and all is well. 530 531 .. _early exits: 532 533 Use Early Exits and ``continue`` to Simplify Code 534 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 535 536 When reading code, keep in mind how much state and how many previous decisions 537 have to be remembered by the reader to understand a block of code. Aim to 538 reduce indentation where possible when it doesn't make it more difficult to 539 understand the code. One great way to do this is by making use of early exits 540 and the ``continue`` keyword in long loops. As an example of using an early 541 exit from a function, consider this "bad" code: 542 543 .. code-block:: c++ 544 545 Value *doSomething(Instruction *I) { 546 if (!isa<TerminatorInst>(I) && 547 I->hasOneUse() && doOtherThing(I)) { 548 ... some long code .... 549 } 550 551 return 0; 552 } 553 554 This code has several problems if the body of the ``'if'`` is large. When 555 you're looking at the top of the function, it isn't immediately clear that this 556 *only* does interesting things with non-terminator instructions, and only 557 applies to things with the other predicates. Second, it is relatively difficult 558 to describe (in comments) why these predicates are important because the ``if`` 559 statement makes it difficult to lay out the comments. Third, when you're deep 560 within the body of the code, it is indented an extra level. Finally, when 561 reading the top of the function, it isn't clear what the result is if the 562 predicate isn't true; you have to read to the end of the function to know that 563 it returns null. 564 565 It is much preferred to format the code like this: 566 567 .. code-block:: c++ 568 569 Value *doSomething(Instruction *I) { 570 // Terminators never need 'something' done to them because ... 571 if (isa<TerminatorInst>(I)) 572 return 0; 573 574 // We conservatively avoid transforming instructions with multiple uses 575 // because goats like cheese. 576 if (!I->hasOneUse()) 577 return 0; 578 579 // This is really just here for example. 580 if (!doOtherThing(I)) 581 return 0; 582 583 ... some long code .... 584 } 585 586 This fixes these problems. A similar problem frequently happens in ``for`` 587 loops. A silly example is something like this: 588 589 .. code-block:: c++ 590 591 for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ++II) { 592 if (BinaryOperator *BO = dyn_cast<BinaryOperator>(II)) { 593 Value *LHS = BO->getOperand(0); 594 Value *RHS = BO->getOperand(1); 595 if (LHS != RHS) { 596 ... 597 } 598 } 599 } 600 601 When you have very, very small loops, this sort of structure is fine. But if it 602 exceeds more than 10-15 lines, it becomes difficult for people to read and 603 understand at a glance. The problem with this sort of code is that it gets very 604 nested very quickly. Meaning that the reader of the code has to keep a lot of 605 context in their brain to remember what is going immediately on in the loop, 606 because they don't know if/when the ``if`` conditions will have ``else``\s etc. 607 It is strongly preferred to structure the loop like this: 608 609 .. code-block:: c++ 610 611 for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ++II) { 612 BinaryOperator *BO = dyn_cast<BinaryOperator>(II); 613 if (!BO) continue; 614 615 Value *LHS = BO->getOperand(0); 616 Value *RHS = BO->getOperand(1); 617 if (LHS == RHS) continue; 618 619 ... 620 } 621 622 This has all the benefits of using early exits for functions: it reduces nesting 623 of the loop, it makes it easier to describe why the conditions are true, and it 624 makes it obvious to the reader that there is no ``else`` coming up that they 625 have to push context into their brain for. If a loop is large, this can be a 626 big understandability win. 627 628 Don't use ``else`` after a ``return`` 629 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 630 631 For similar reasons above (reduction of indentation and easier reading), please 632 do not use ``'else'`` or ``'else if'`` after something that interrupts control 633 flow --- like ``return``, ``break``, ``continue``, ``goto``, etc. For 634 example, this is *bad*: 635 636 .. code-block:: c++ 637 638 case 'J': { 639 if (Signed) { 640 Type = Context.getsigjmp_bufType(); 641 if (Type.isNull()) { 642 Error = ASTContext::GE_Missing_sigjmp_buf; 643 return QualType(); 644 } else { 645 break; 646 } 647 } else { 648 Type = Context.getjmp_bufType(); 649 if (Type.isNull()) { 650 Error = ASTContext::GE_Missing_jmp_buf; 651 return QualType(); 652 } else { 653 break; 654 } 655 } 656 } 657 658 It is better to write it like this: 659 660 .. code-block:: c++ 661 662 case 'J': 663 if (Signed) { 664 Type = Context.getsigjmp_bufType(); 665 if (Type.isNull()) { 666 Error = ASTContext::GE_Missing_sigjmp_buf; 667 return QualType(); 668 } 669 } else { 670 Type = Context.getjmp_bufType(); 671 if (Type.isNull()) { 672 Error = ASTContext::GE_Missing_jmp_buf; 673 return QualType(); 674 } 675 } 676 break; 677 678 Or better yet (in this case) as: 679 680 .. code-block:: c++ 681 682 case 'J': 683 if (Signed) 684 Type = Context.getsigjmp_bufType(); 685 else 686 Type = Context.getjmp_bufType(); 687 688 if (Type.isNull()) { 689 Error = Signed ? ASTContext::GE_Missing_sigjmp_buf : 690 ASTContext::GE_Missing_jmp_buf; 691 return QualType(); 692 } 693 break; 694 695 The idea is to reduce indentation and the amount of code you have to keep track 696 of when reading the code. 697 698 Turn Predicate Loops into Predicate Functions 699 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 700 701 It is very common to write small loops that just compute a boolean value. There 702 are a number of ways that people commonly write these, but an example of this 703 sort of thing is: 704 705 .. code-block:: c++ 706 707 bool FoundFoo = false; 708 for (unsigned I = 0, E = BarList.size(); I != E; ++I) 709 if (BarList[I]->isFoo()) { 710 FoundFoo = true; 711 break; 712 } 713 714 if (FoundFoo) { 715 ... 716 } 717 718 This sort of code is awkward to write, and is almost always a bad sign. Instead 719 of this sort of loop, we strongly prefer to use a predicate function (which may 720 be `static`_) that uses `early exits`_ to compute the predicate. We prefer the 721 code to be structured like this: 722 723 .. code-block:: c++ 724 725 /// \returns true if the specified list has an element that is a foo. 726 static bool containsFoo(const std::vector<Bar*> &List) { 727 for (unsigned I = 0, E = List.size(); I != E; ++I) 728 if (List[I]->isFoo()) 729 return true; 730 return false; 731 } 732 ... 733 734 if (containsFoo(BarList)) { 735 ... 736 } 737 738 There are many reasons for doing this: it reduces indentation and factors out 739 code which can often be shared by other code that checks for the same predicate. 740 More importantly, it *forces you to pick a name* for the function, and forces 741 you to write a comment for it. In this silly example, this doesn't add much 742 value. However, if the condition is complex, this can make it a lot easier for 743 the reader to understand the code that queries for this predicate. Instead of 744 being faced with the in-line details of how we check to see if the BarList 745 contains a foo, we can trust the function name and continue reading with better 746 locality. 747 748 The Low-Level Issues 749 -------------------- 750 751 Name Types, Functions, Variables, and Enumerators Properly 752 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 753 754 Poorly-chosen names can mislead the reader and cause bugs. We cannot stress 755 enough how important it is to use *descriptive* names. Pick names that match 756 the semantics and role of the underlying entities, within reason. Avoid 757 abbreviations unless they are well known. After picking a good name, make sure 758 to use consistent capitalization for the name, as inconsistency requires clients 759 to either memorize the APIs or to look it up to find the exact spelling. 760 761 In general, names should be in camel case (e.g. ``TextFileReader`` and 762 ``isLValue()``). Different kinds of declarations have different rules: 763 764 * **Type names** (including classes, structs, enums, typedefs, etc) should be 765 nouns and start with an upper-case letter (e.g. ``TextFileReader``). 766 767 * **Variable names** should be nouns (as they represent state). The name should 768 be camel case, and start with an upper case letter (e.g. ``Leader`` or 769 ``Boats``). 770 771 * **Function names** should be verb phrases (as they represent actions), and 772 command-like function should be imperative. The name should be camel case, 773 and start with a lower case letter (e.g. ``openFile()`` or ``isFoo()``). 774 775 * **Enum declarations** (e.g. ``enum Foo {...}``) are types, so they should 776 follow the naming conventions for types. A common use for enums is as a 777 discriminator for a union, or an indicator of a subclass. When an enum is 778 used for something like this, it should have a ``Kind`` suffix 779 (e.g. ``ValueKind``). 780 781 * **Enumerators** (e.g. ``enum { Foo, Bar }``) and **public member variables** 782 should start with an upper-case letter, just like types. Unless the 783 enumerators are defined in their own small namespace or inside a class, 784 enumerators should have a prefix corresponding to the enum declaration name. 785 For example, ``enum ValueKind { ... };`` may contain enumerators like 786 ``VK_Argument``, ``VK_BasicBlock``, etc. Enumerators that are just 787 convenience constants are exempt from the requirement for a prefix. For 788 instance: 789 790 .. code-block:: c++ 791 792 enum { 793 MaxSize = 42, 794 Density = 12 795 }; 796 797 As an exception, classes that mimic STL classes can have member names in STL's 798 style of lower-case words separated by underscores (e.g. ``begin()``, 799 ``push_back()``, and ``empty()``). 800 801 Here are some examples of good and bad names: 802 803 .. code-block:: c++ 804 805 class VehicleMaker { 806 ... 807 Factory<Tire> F; // Bad -- abbreviation and non-descriptive. 808 Factory<Tire> Factory; // Better. 809 Factory<Tire> TireFactory; // Even better -- if VehicleMaker has more than one 810 // kind of factories. 811 }; 812 813 Vehicle MakeVehicle(VehicleType Type) { 814 VehicleMaker M; // Might be OK if having a short life-span. 815 Tire Tmp1 = M.makeTire(); // Bad -- 'Tmp1' provides no information. 816 Light Headlight = M.makeLight("head"); // Good -- descriptive. 817 ... 818 } 819 820 Assert Liberally 821 ^^^^^^^^^^^^^^^^ 822 823 Use the "``assert``" macro to its fullest. Check all of your preconditions and 824 assumptions, you never know when a bug (not necessarily even yours) might be 825 caught early by an assertion, which reduces debugging time dramatically. The 826 "``<cassert>``" header file is probably already included by the header files you 827 are using, so it doesn't cost anything to use it. 828 829 To further assist with debugging, make sure to put some kind of error message in 830 the assertion statement, which is printed if the assertion is tripped. This 831 helps the poor debugger make sense of why an assertion is being made and 832 enforced, and hopefully what to do about it. Here is one complete example: 833 834 .. code-block:: c++ 835 836 inline Value *getOperand(unsigned I) { 837 assert(I < Operands.size() && "getOperand() out of range!"); 838 return Operands[I]; 839 } 840 841 Here are more examples: 842 843 .. code-block:: c++ 844 845 assert(Ty->isPointerType() && "Can't allocate a non pointer type!"); 846 847 assert((Opcode == Shl || Opcode == Shr) && "ShiftInst Opcode invalid!"); 848 849 assert(idx < getNumSuccessors() && "Successor # out of range!"); 850 851 assert(V1.getType() == V2.getType() && "Constant types must be identical!"); 852 853 assert(isa<PHINode>(Succ->front()) && "Only works on PHId BBs!"); 854 855 You get the idea. 856 857 In the past, asserts were used to indicate a piece of code that should not be 858 reached. These were typically of the form: 859 860 .. code-block:: c++ 861 862 assert(0 && "Invalid radix for integer literal"); 863 864 This has a few issues, the main one being that some compilers might not 865 understand the assertion, or warn about a missing return in builds where 866 assertions are compiled out. 867 868 Today, we have something much better: ``llvm_unreachable``: 869 870 .. code-block:: c++ 871 872 llvm_unreachable("Invalid radix for integer literal"); 873 874 When assertions are enabled, this will print the message if it's ever reached 875 and then exit the program. When assertions are disabled (i.e. in release 876 builds), ``llvm_unreachable`` becomes a hint to compilers to skip generating 877 code for this branch. If the compiler does not support this, it will fall back 878 to the "abort" implementation. 879 880 Another issue is that values used only by assertions will produce an "unused 881 value" warning when assertions are disabled. For example, this code will warn: 882 883 .. code-block:: c++ 884 885 unsigned Size = V.size(); 886 assert(Size > 42 && "Vector smaller than it should be"); 887 888 bool NewToSet = Myset.insert(Value); 889 assert(NewToSet && "The value shouldn't be in the set yet"); 890 891 These are two interesting different cases. In the first case, the call to 892 ``V.size()`` is only useful for the assert, and we don't want it executed when 893 assertions are disabled. Code like this should move the call into the assert 894 itself. In the second case, the side effects of the call must happen whether 895 the assert is enabled or not. In this case, the value should be cast to void to 896 disable the warning. To be specific, it is preferred to write the code like 897 this: 898 899 .. code-block:: c++ 900 901 assert(V.size() > 42 && "Vector smaller than it should be"); 902 903 bool NewToSet = Myset.insert(Value); (void)NewToSet; 904 assert(NewToSet && "The value shouldn't be in the set yet"); 905 906 Do Not Use ``using namespace std`` 907 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 908 909 In LLVM, we prefer to explicitly prefix all identifiers from the standard 910 namespace with an "``std::``" prefix, rather than rely on "``using namespace 911 std;``". 912 913 In header files, adding a ``'using namespace XXX'`` directive pollutes the 914 namespace of any source file that ``#include``\s the header. This is clearly a 915 bad thing. 916 917 In implementation files (e.g. ``.cpp`` files), the rule is more of a stylistic 918 rule, but is still important. Basically, using explicit namespace prefixes 919 makes the code **clearer**, because it is immediately obvious what facilities 920 are being used and where they are coming from. And **more portable**, because 921 namespace clashes cannot occur between LLVM code and other namespaces. The 922 portability rule is important because different standard library implementations 923 expose different symbols (potentially ones they shouldn't), and future revisions 924 to the C++ standard will add more symbols to the ``std`` namespace. As such, we 925 never use ``'using namespace std;'`` in LLVM. 926 927 The exception to the general rule (i.e. it's not an exception for the ``std`` 928 namespace) is for implementation files. For example, all of the code in the 929 LLVM project implements code that lives in the 'llvm' namespace. As such, it is 930 ok, and actually clearer, for the ``.cpp`` files to have a ``'using namespace 931 llvm;'`` directive at the top, after the ``#include``\s. This reduces 932 indentation in the body of the file for source editors that indent based on 933 braces, and keeps the conceptual context cleaner. The general form of this rule 934 is that any ``.cpp`` file that implements code in any namespace may use that 935 namespace (and its parents'), but should not use any others. 936 937 Provide a Virtual Method Anchor for Classes in Headers 938 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 939 940 If a class is defined in a header file and has a vtable (either it has virtual 941 methods or it derives from classes with virtual methods), it must always have at 942 least one out-of-line virtual method in the class. Without this, the compiler 943 will copy the vtable and RTTI into every ``.o`` file that ``#include``\s the 944 header, bloating ``.o`` file sizes and increasing link times. 945 946 Don't use default labels in fully covered switches over enumerations 947 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 948 949 ``-Wswitch`` warns if a switch, without a default label, over an enumeration 950 does not cover every enumeration value. If you write a default label on a fully 951 covered switch over an enumeration then the ``-Wswitch`` warning won't fire 952 when new elements are added to that enumeration. To help avoid adding these 953 kinds of defaults, Clang has the warning ``-Wcovered-switch-default`` which is 954 off by default but turned on when building LLVM with a version of Clang that 955 supports the warning. 956 957 A knock-on effect of this stylistic requirement is that when building LLVM with 958 GCC you may get warnings related to "control may reach end of non-void function" 959 if you return from each case of a covered switch-over-enum because GCC assumes 960 that the enum expression may take any representable value, not just those of 961 individual enumerators. To suppress this warning, use ``llvm_unreachable`` after 962 the switch. 963 964 Use ``LLVM_DELETED_FUNCTION`` to mark uncallable methods 965 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 966 967 Prior to C++11, a common pattern to make a class uncopyable was to declare an 968 unimplemented copy constructor and copy assignment operator and make them 969 private. This would give a compiler error for accessing a private method or a 970 linker error because it wasn't implemented. 971 972 With C++11, we can mark methods that won't be implemented with ``= delete``. 973 This will trigger a much better error message and tell the compiler that the 974 method will never be implemented. This enables other checks like 975 ``-Wunused-private-field`` to run correctly on classes that contain these 976 methods. 977 978 To maintain compatibility with C++03, ``LLVM_DELETED_FUNCTION`` should be used 979 which will expand to ``= delete`` if the compiler supports it. These methods 980 should still be declared private. Example of the uncopyable pattern: 981 982 .. code-block:: c++ 983 984 class DontCopy { 985 private: 986 DontCopy(const DontCopy&) LLVM_DELETED_FUNCTION; 987 DontCopy &operator =(const DontCopy&) LLVM_DELETED_FUNCTION; 988 public: 989 ... 990 }; 991 992 Don't evaluate ``end()`` every time through a loop 993 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 994 995 Because C++ doesn't have a standard "``foreach``" loop (though it can be 996 emulated with macros and may be coming in C++'0x) we end up writing a lot of 997 loops that manually iterate from begin to end on a variety of containers or 998 through other data structures. One common mistake is to write a loop in this 999 style: 1000 1001 .. code-block:: c++ 1002 1003 BasicBlock *BB = ... 1004 for (BasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) 1005 ... use I ... 1006 1007 The problem with this construct is that it evaluates "``BB->end()``" every time 1008 through the loop. Instead of writing the loop like this, we strongly prefer 1009 loops to be written so that they evaluate it once before the loop starts. A 1010 convenient way to do this is like so: 1011 1012 .. code-block:: c++ 1013 1014 BasicBlock *BB = ... 1015 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) 1016 ... use I ... 1017 1018 The observant may quickly point out that these two loops may have different 1019 semantics: if the container (a basic block in this case) is being mutated, then 1020 "``BB->end()``" may change its value every time through the loop and the second 1021 loop may not in fact be correct. If you actually do depend on this behavior, 1022 please write the loop in the first form and add a comment indicating that you 1023 did it intentionally. 1024 1025 Why do we prefer the second form (when correct)? Writing the loop in the first 1026 form has two problems. First it may be less efficient than evaluating it at the 1027 start of the loop. In this case, the cost is probably minor --- a few extra 1028 loads every time through the loop. However, if the base expression is more 1029 complex, then the cost can rise quickly. I've seen loops where the end 1030 expression was actually something like: "``SomeMap[X]->end()``" and map lookups 1031 really aren't cheap. By writing it in the second form consistently, you 1032 eliminate the issue entirely and don't even have to think about it. 1033 1034 The second (even bigger) issue is that writing the loop in the first form hints 1035 to the reader that the loop is mutating the container (a fact that a comment 1036 would handily confirm!). If you write the loop in the second form, it is 1037 immediately obvious without even looking at the body of the loop that the 1038 container isn't being modified, which makes it easier to read the code and 1039 understand what it does. 1040 1041 While the second form of the loop is a few extra keystrokes, we do strongly 1042 prefer it. 1043 1044 ``#include <iostream>`` is Forbidden 1045 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 1046 1047 The use of ``#include <iostream>`` in library files is hereby **forbidden**, 1048 because many common implementations transparently inject a `static constructor`_ 1049 into every translation unit that includes it. 1050 1051 Note that using the other stream headers (``<sstream>`` for example) is not 1052 problematic in this regard --- just ``<iostream>``. However, ``raw_ostream`` 1053 provides various APIs that are better performing for almost every use than 1054 ``std::ostream`` style APIs. 1055 1056 .. note:: 1057 1058 New code should always use `raw_ostream`_ for writing, or the 1059 ``llvm::MemoryBuffer`` API for reading files. 1060 1061 .. _raw_ostream: 1062 1063 Use ``raw_ostream`` 1064 ^^^^^^^^^^^^^^^^^^^ 1065 1066 LLVM includes a lightweight, simple, and efficient stream implementation in 1067 ``llvm/Support/raw_ostream.h``, which provides all of the common features of 1068 ``std::ostream``. All new code should use ``raw_ostream`` instead of 1069 ``ostream``. 1070 1071 Unlike ``std::ostream``, ``raw_ostream`` is not a template and can be forward 1072 declared as ``class raw_ostream``. Public headers should generally not include 1073 the ``raw_ostream`` header, but use forward declarations and constant references 1074 to ``raw_ostream`` instances. 1075 1076 Avoid ``std::endl`` 1077 ^^^^^^^^^^^^^^^^^^^ 1078 1079 The ``std::endl`` modifier, when used with ``iostreams`` outputs a newline to 1080 the output stream specified. In addition to doing this, however, it also 1081 flushes the output stream. In other words, these are equivalent: 1082 1083 .. code-block:: c++ 1084 1085 std::cout << std::endl; 1086 std::cout << '\n' << std::flush; 1087 1088 Most of the time, you probably have no reason to flush the output stream, so 1089 it's better to use a literal ``'\n'``. 1090 1091 Don't use ``inline`` when defining a function in a class definition 1092 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 1093 1094 A member function defined in a class definition is implicitly inline, so don't 1095 put the ``inline`` keyword in this case. 1096 1097 Don't: 1098 1099 .. code-block:: c++ 1100 1101 class Foo { 1102 public: 1103 inline void bar() { 1104 // ... 1105 } 1106 }; 1107 1108 Do: 1109 1110 .. code-block:: c++ 1111 1112 class Foo { 1113 public: 1114 void bar() { 1115 // ... 1116 } 1117 }; 1118 1119 Microscopic Details 1120 ------------------- 1121 1122 This section describes preferred low-level formatting guidelines along with 1123 reasoning on why we prefer them. 1124 1125 Spaces Before Parentheses 1126 ^^^^^^^^^^^^^^^^^^^^^^^^^ 1127 1128 We prefer to put a space before an open parenthesis only in control flow 1129 statements, but not in normal function call expressions and function-like 1130 macros. For example, this is good: 1131 1132 .. code-block:: c++ 1133 1134 if (X) ... 1135 for (I = 0; I != 100; ++I) ... 1136 while (LLVMRocks) ... 1137 1138 somefunc(42); 1139 assert(3 != 4 && "laws of math are failing me"); 1140 1141 A = foo(42, 92) + bar(X); 1142 1143 and this is bad: 1144 1145 .. code-block:: c++ 1146 1147 if(X) ... 1148 for(I = 0; I != 100; ++I) ... 1149 while(LLVMRocks) ... 1150 1151 somefunc (42); 1152 assert (3 != 4 && "laws of math are failing me"); 1153 1154 A = foo (42, 92) + bar (X); 1155 1156 The reason for doing this is not completely arbitrary. This style makes control 1157 flow operators stand out more, and makes expressions flow better. The function 1158 call operator binds very tightly as a postfix operator. Putting a space after a 1159 function name (as in the last example) makes it appear that the code might bind 1160 the arguments of the left-hand-side of a binary operator with the argument list 1161 of a function and the name of the right side. More specifically, it is easy to 1162 misread the "``A``" example as: 1163 1164 .. code-block:: c++ 1165 1166 A = foo ((42, 92) + bar) (X); 1167 1168 when skimming through the code. By avoiding a space in a function, we avoid 1169 this misinterpretation. 1170 1171 Prefer Preincrement 1172 ^^^^^^^^^^^^^^^^^^^ 1173 1174 Hard fast rule: Preincrement (``++X``) may be no slower than postincrement 1175 (``X++``) and could very well be a lot faster than it. Use preincrementation 1176 whenever possible. 1177 1178 The semantics of postincrement include making a copy of the value being 1179 incremented, returning it, and then preincrementing the "work value". For 1180 primitive types, this isn't a big deal. But for iterators, it can be a huge 1181 issue (for example, some iterators contains stack and set objects in them... 1182 copying an iterator could invoke the copy ctor's of these as well). In general, 1183 get in the habit of always using preincrement, and you won't have a problem. 1184 1185 1186 Namespace Indentation 1187 ^^^^^^^^^^^^^^^^^^^^^ 1188 1189 In general, we strive to reduce indentation wherever possible. This is useful 1190 because we want code to `fit into 80 columns`_ without wrapping horribly, but 1191 also because it makes it easier to understand the code. Namespaces are a funny 1192 thing: they are often large, and we often desire to put lots of stuff into them 1193 (so they can be large). Other times they are tiny, because they just hold an 1194 enum or something similar. In order to balance this, we use different 1195 approaches for small versus large namespaces. 1196 1197 If a namespace definition is small and *easily* fits on a screen (say, less than 1198 35 lines of code), then you should indent its body. Here's an example: 1199 1200 .. code-block:: c++ 1201 1202 namespace llvm { 1203 namespace X86 { 1204 /// \brief An enum for the x86 relocation codes. Note that 1205 /// the terminology here doesn't follow x86 convention - word means 1206 /// 32-bit and dword means 64-bit. 1207 enum RelocationType { 1208 /// \brief PC relative relocation, add the relocated value to 1209 /// the value already in memory, after we adjust it for where the PC is. 1210 reloc_pcrel_word = 0, 1211 1212 /// \brief PIC base relative relocation, add the relocated value to 1213 /// the value already in memory, after we adjust it for where the 1214 /// PIC base is. 1215 reloc_picrel_word = 1, 1216 1217 /// \brief Absolute relocation, just add the relocated value to the 1218 /// value already in memory. 1219 reloc_absolute_word = 2, 1220 reloc_absolute_dword = 3 1221 }; 1222 } 1223 } 1224 1225 Since the body is small, indenting adds value because it makes it very clear 1226 where the namespace starts and ends, and it is easy to take the whole thing in 1227 in one "gulp" when reading the code. If the blob of code in the namespace is 1228 larger (as it typically is in a header in the ``llvm`` or ``clang`` namespaces), 1229 do not indent the code, and add a comment indicating what namespace is being 1230 closed. For example: 1231 1232 .. code-block:: c++ 1233 1234 namespace llvm { 1235 namespace knowledge { 1236 1237 /// This class represents things that Smith can have an intimate 1238 /// understanding of and contains the data associated with it. 1239 class Grokable { 1240 ... 1241 public: 1242 explicit Grokable() { ... } 1243 virtual ~Grokable() = 0; 1244 1245 ... 1246 1247 }; 1248 1249 } // end namespace knowledge 1250 } // end namespace llvm 1251 1252 Because the class is large, we don't expect that the reader can easily 1253 understand the entire concept in a glance, and the end of the file (where the 1254 namespaces end) may be a long ways away from the place they open. As such, 1255 indenting the contents of the namespace doesn't add any value, and detracts from 1256 the readability of the class. In these cases it is best to *not* indent the 1257 contents of the namespace. 1258 1259 .. _static: 1260 1261 Anonymous Namespaces 1262 ^^^^^^^^^^^^^^^^^^^^ 1263 1264 After talking about namespaces in general, you may be wondering about anonymous 1265 namespaces in particular. Anonymous namespaces are a great language feature 1266 that tells the C++ compiler that the contents of the namespace are only visible 1267 within the current translation unit, allowing more aggressive optimization and 1268 eliminating the possibility of symbol name collisions. Anonymous namespaces are 1269 to C++ as "static" is to C functions and global variables. While "``static``" 1270 is available in C++, anonymous namespaces are more general: they can make entire 1271 classes private to a file. 1272 1273 The problem with anonymous namespaces is that they naturally want to encourage 1274 indentation of their body, and they reduce locality of reference: if you see a 1275 random function definition in a C++ file, it is easy to see if it is marked 1276 static, but seeing if it is in an anonymous namespace requires scanning a big 1277 chunk of the file. 1278 1279 Because of this, we have a simple guideline: make anonymous namespaces as small 1280 as possible, and only use them for class declarations. For example, this is 1281 good: 1282 1283 .. code-block:: c++ 1284 1285 namespace { 1286 class StringSort { 1287 ... 1288 public: 1289 StringSort(...) 1290 bool operator<(const char *RHS) const; 1291 }; 1292 } // end anonymous namespace 1293 1294 static void runHelper() { 1295 ... 1296 } 1297 1298 bool StringSort::operator<(const char *RHS) const { 1299 ... 1300 } 1301 1302 This is bad: 1303 1304 .. code-block:: c++ 1305 1306 namespace { 1307 class StringSort { 1308 ... 1309 public: 1310 StringSort(...) 1311 bool operator<(const char *RHS) const; 1312 }; 1313 1314 void runHelper() { 1315 ... 1316 } 1317 1318 bool StringSort::operator<(const char *RHS) const { 1319 ... 1320 } 1321 1322 } // end anonymous namespace 1323 1324 This is bad specifically because if you're looking at "``runHelper``" in the middle 1325 of a large C++ file, that you have no immediate way to tell if it is local to 1326 the file. When it is marked static explicitly, this is immediately obvious. 1327 Also, there is no reason to enclose the definition of "``operator<``" in the 1328 namespace just because it was declared there. 1329 1330 See Also 1331 ======== 1332 1333 A lot of these comments and recommendations have been culled from other sources. 1334 Two particularly important books for our work are: 1335 1336 #. `Effective C++ 1337 <http://www.amazon.com/Effective-Specific-Addison-Wesley-Professional-Computing/dp/0321334876>`_ 1338 by Scott Meyers. Also interesting and useful are "More Effective C++" and 1339 "Effective STL" by the same author. 1340 1341 #. `Large-Scale C++ Software Design 1342 <http://www.amazon.com/Large-Scale-Software-Design-John-Lakos/dp/0201633620/ref=sr_1_1>`_ 1343 by John Lakos 1344 1345 If you get some free time, and you haven't read them: do so, you might learn 1346 something. 1347