1 <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" 2 "http://www.w3.org/TR/html4/strict.dtd"> 3 <html> 4 <head> 5 <link rel="stylesheet" href="llvm.css" type="text/css"> 6 <title>LLVM Coding Standards</title> 7 </head> 8 <body> 9 10 <h1> 11 LLVM Coding Standards 12 </h1> 13 14 <ol> 15 <li><a href="#introduction">Introduction</a></li> 16 <li><a href="#mechanicalissues">Mechanical Source Issues</a> 17 <ol> 18 <li><a href="#sourceformating">Source Code Formatting</a> 19 <ol> 20 <li><a href="#scf_commenting">Commenting</a></li> 21 <li><a href="#scf_commentformat">Comment Formatting</a></li> 22 <li><a href="#scf_includes"><tt>#include</tt> Style</a></li> 23 <li><a href="#scf_codewidth">Source Code Width</a></li> 24 <li><a href="#scf_spacestabs">Use Spaces Instead of Tabs</a></li> 25 <li><a href="#scf_indentation">Indent Code Consistently</a></li> 26 </ol></li> 27 <li><a href="#compilerissues">Compiler Issues</a> 28 <ol> 29 <li><a href="#ci_warningerrors">Treat Compiler Warnings Like 30 Errors</a></li> 31 <li><a href="#ci_portable_code">Write Portable Code</a></li> 32 <li><a href="#ci_rtti_exceptions">Do not use RTTI or Exceptions</a></li> 33 <li><a href="#ci_class_struct">Use of <tt>class</tt>/<tt>struct</tt> Keywords</a></li> 34 </ol></li> 35 </ol></li> 36 <li><a href="#styleissues">Style Issues</a> 37 <ol> 38 <li><a href="#macro">The High-Level Issues</a> 39 <ol> 40 <li><a href="#hl_module">A Public Header File <b>is</b> a 41 Module</a></li> 42 <li><a href="#hl_dontinclude"><tt>#include</tt> as Little as Possible</a></li> 43 <li><a href="#hl_privateheaders">Keep "internal" Headers 44 Private</a></li> 45 <li><a href="#hl_earlyexit">Use Early Exits and <tt>continue</tt> to Simplify 46 Code</a></li> 47 <li><a href="#hl_else_after_return">Don't use <tt>else</tt> after a 48 <tt>return</tt></a></li> 49 <li><a href="#hl_predicateloops">Turn Predicate Loops into Predicate 50 Functions</a></li> 51 </ol></li> 52 <li><a href="#micro">The Low-Level Issues</a> 53 <ol> 54 <li><a href="#ll_naming">Name Types, Functions, Variables, and Enumerators Properly</a></li> 55 <li><a href="#ll_assert">Assert Liberally</a></li> 56 <li><a href="#ll_ns_std">Do not use '<tt>using namespace std</tt>'</a></li> 57 <li><a href="#ll_virtual_anch">Provide a virtual method anchor for 58 classes in headers</a></li> 59 <li><a href="#ll_end">Don't evaluate <tt>end()</tt> every time through a 60 loop</a></li> 61 <li><a href="#ll_iostream"><tt>#include <iostream></tt> is 62 <em>forbidden</em></a></li> 63 <li><a href="#ll_raw_ostream">Use <tt>raw_ostream</tt></a></li> 64 <li><a href="#ll_avoidendl">Avoid <tt>std::endl</tt></a></li> 65 </ol></li> 66 67 <li><a href="#nano">Microscopic Details</a> 68 <ol> 69 <li><a href="#micro_spaceparen">Spaces Before Parentheses</a></li> 70 <li><a href="#micro_preincrement">Prefer Preincrement</a></li> 71 <li><a href="#micro_namespaceindent">Namespace Indentation</a></li> 72 <li><a href="#micro_anonns">Anonymous Namespaces</a></li> 73 </ol></li> 74 75 76 </ol></li> 77 <li><a href="#seealso">See Also</a></li> 78 </ol> 79 80 <div class="doc_author"> 81 <p>Written by <a href="mailto:sabre (a] nondot.org">Chris Lattner</a></p> 82 </div> 83 84 85 <!-- *********************************************************************** --> 86 <h2> 87 <a name="introduction">Introduction</a> 88 </h2> 89 <!-- *********************************************************************** --> 90 91 <div> 92 93 <p>This document attempts to describe a few coding standards that are being used 94 in the LLVM source tree. Although no coding standards should be regarded as 95 absolute requirements to be followed in all instances, coding standards can be 96 useful.</p> 97 98 <p>This document intentionally does not prescribe fixed standards for religious 99 issues such as brace placement and space usage. For issues like this, follow 100 the golden rule:</p> 101 102 <blockquote> 103 104 <p><b><a name="goldenrule">If you are adding a significant body of source to a 105 project, feel free to use whatever style you are most comfortable with. If you 106 are extending, enhancing, or bug fixing already implemented code, use the style 107 that is already being used so that the source is uniform and easy to 108 follow.</a></b></p> 109 110 </blockquote> 111 112 <p>The ultimate goal of these guidelines is the increase readability and 113 maintainability of our common source base. If you have suggestions for topics to 114 be included, please mail them to <a 115 href="mailto:sabre (a] nondot.org">Chris</a>.</p> 116 117 </div> 118 119 <!-- *********************************************************************** --> 120 <h2> 121 <a name="mechanicalissues">Mechanical Source Issues</a> 122 </h2> 123 <!-- *********************************************************************** --> 124 125 <div> 126 127 <!-- ======================================================================= --> 128 <h3> 129 <a name="sourceformating">Source Code Formatting</a> 130 </h3> 131 132 <div> 133 134 <!-- _______________________________________________________________________ --> 135 <h4> 136 <a name="scf_commenting">Commenting</a> 137 </h4> 138 139 <div> 140 141 <p>Comments are one critical part of readability and maintainability. Everyone 142 knows they should comment, so should you. When writing comments, write them as 143 English prose, which means they should use proper capitalization, punctuation, 144 etc. Although we all should probably 145 comment our code more than we do, there are a few very critical places that 146 documentation is very useful:</p> 147 148 <h5>File Headers</h5> 149 150 <div> 151 152 <p>Every source file should have a header on it that describes the basic 153 purpose of the file. If a file does not have a header, it should not be 154 checked into Subversion. Most source trees will probably have a standard 155 file header format. The standard format for the LLVM source tree looks like 156 this:</p> 157 158 <div class="doc_code"> 159 <pre> 160 //===-- llvm/Instruction.h - Instruction class definition -------*- C++ -*-===// 161 // 162 // The LLVM Compiler Infrastructure 163 // 164 // This file is distributed under the University of Illinois Open Source 165 // License. See LICENSE.TXT for details. 166 // 167 //===----------------------------------------------------------------------===// 168 // 169 // This file contains the declaration of the Instruction class, which is the 170 // base class for all of the VM instructions. 171 // 172 //===----------------------------------------------------------------------===// 173 </pre> 174 </div> 175 176 <p>A few things to note about this particular format: The "<tt>-*- C++ 177 -*-</tt>" string on the first line is there to tell Emacs that the source file 178 is a C++ file, not a C file (Emacs assumes <tt>.h</tt> files are C files by default). 179 Note that this tag is not necessary in <tt>.cpp</tt> files. The name of the file is also 180 on the first line, along with a very short description of the purpose of the 181 file. This is important when printing out code and flipping though lots of 182 pages.</p> 183 184 <p>The next section in the file is a concise note that defines the license 185 that the file is released under. This makes it perfectly clear what terms the 186 source code can be distributed under and should not be modified in any way.</p> 187 188 <p>The main body of the description does not have to be very long in most cases. 189 Here it's only two lines. If an algorithm is being implemented or something 190 tricky is going on, a reference to the paper where it is published should be 191 included, as well as any notes or "gotchas" in the code to watch out for.</p> 192 193 </div> 194 195 <h5>Class overviews</h5> 196 197 <p>Classes are one fundamental part of a good object oriented design. As such, 198 a class definition should have a comment block that explains what the class is 199 used for... if it's not obvious. If it's so completely obvious your grandma 200 could figure it out, it's probably safe to leave it out. Naming classes 201 something sane goes a long ways towards avoiding writing documentation.</p> 202 203 204 <h5>Method information</h5> 205 206 <div> 207 208 <p>Methods defined in a class (as well as any global functions) should also be 209 documented properly. A quick note about what it does and a description of the 210 borderline behaviour is all that is necessary here (unless something 211 particularly tricky or insidious is going on). The hope is that people can 212 figure out how to use your interfaces without reading the code itself... that is 213 the goal metric.</p> 214 215 <p>Good things to talk about here are what happens when something unexpected 216 happens: does the method return null? Abort? Format your hard disk?</p> 217 218 </div> 219 220 </div> 221 222 <!-- _______________________________________________________________________ --> 223 <h4> 224 <a name="scf_commentformat">Comment Formatting</a> 225 </h4> 226 227 <div> 228 229 <p>In general, prefer C++ style (<tt>//</tt>) comments. They take less space, 230 require less typing, don't have nesting problems, etc. There are a few cases 231 when it is useful to use C style (<tt>/* */</tt>) comments however:</p> 232 233 <ol> 234 <li>When writing C code: Obviously if you are writing C code, use C style 235 comments.</li> 236 <li>When writing a header file that may be <tt>#include</tt>d by a C source 237 file.</li> 238 <li>When writing a source file that is used by a tool that only accepts C 239 style comments.</li> 240 </ol> 241 242 <p>To comment out a large block of code, use <tt>#if 0</tt> and <tt>#endif</tt>. 243 These nest properly and are better behaved in general than C style comments.</p> 244 245 </div> 246 247 <!-- _______________________________________________________________________ --> 248 <h4> 249 <a name="scf_includes"><tt>#include</tt> Style</a> 250 </h4> 251 252 <div> 253 254 <p>Immediately after the <a href="#scf_commenting">header file comment</a> (and 255 include guards if working on a header file), the <a 256 href="#hl_dontinclude">minimal</a> list of <tt>#include</tt>s required by the 257 file should be listed. We prefer these <tt>#include</tt>s to be listed in this 258 order:</p> 259 260 <ol> 261 <li><a href="#mmheader">Main Module Header</a></li> 262 <li><a href="#hl_privateheaders">Local/Private Headers</a></li> 263 <li><tt>llvm/*</tt></li> 264 <li><tt>llvm/Analysis/*</tt></li> 265 <li><tt>llvm/Assembly/*</tt></li> 266 <li><tt>llvm/Bitcode/*</tt></li> 267 <li><tt>llvm/CodeGen/*</tt></li> 268 <li>...</li> 269 <li><tt>Support/*</tt></li> 270 <li><tt>Config/*</tt></li> 271 <li>System <tt>#includes</tt></li> 272 </ol> 273 274 <p>and each category should be sorted by name.</p> 275 276 <p><a name="mmheader">The "Main Module Header"</a> file applies to <tt>.cpp</tt> files 277 which implement an interface defined by a <tt>.h</tt> file. This <tt>#include</tt> 278 should always be included <b>first</b> regardless of where it lives on the file 279 system. By including a header file first in the <tt>.cpp</tt> files that implement the 280 interfaces, we ensure that the header does not have any hidden dependencies 281 which are not explicitly #included in the header, but should be. It is also a 282 form of documentation in the <tt>.cpp</tt> file to indicate where the interfaces it 283 implements are defined.</p> 284 285 </div> 286 287 <!-- _______________________________________________________________________ --> 288 <h4> 289 <a name="scf_codewidth">Source Code Width</a> 290 </h4> 291 292 <div> 293 294 <p>Write your code to fit within 80 columns of text. This helps those of us who 295 like to print out code and look at your code in an xterm without resizing 296 it.</p> 297 298 <p>The longer answer is that there must be some limit to the width of the code 299 in order to reasonably allow developers to have multiple files side-by-side in 300 windows on a modest display. If you are going to pick a width limit, it is 301 somewhat arbitrary but you might as well pick something standard. Going with 302 90 columns (for example) instead of 80 columns wouldn't add any significant 303 value and would be detrimental to printing out code. Also many other projects 304 have standardized on 80 columns, so some people have already configured their 305 editors for it (vs something else, like 90 columns).</p> 306 307 <p>This is one of many contentious issues in coding standards, but it is not up 308 for debate.</p> 309 310 </div> 311 312 <!-- _______________________________________________________________________ --> 313 <h4> 314 <a name="scf_spacestabs">Use Spaces Instead of Tabs</a> 315 </h4> 316 317 <div> 318 319 <p>In all cases, prefer spaces to tabs in source files. People have different 320 preferred indentation levels, and different styles of indentation that they 321 like; this is fine. What isn't fine is that different editors/viewers expand 322 tabs out to different tab stops. This can cause your code to look completely 323 unreadable, and it is not worth dealing with.</p> 324 325 <p>As always, follow the <a href="#goldenrule">Golden Rule</a> above: follow the 326 style of existing code if you are modifying and extending it. If you like four 327 spaces of indentation, <b>DO NOT</b> do that in the middle of a chunk of code 328 with two spaces of indentation. Also, do not reindent a whole source file: it 329 makes for incredible diffs that are absolutely worthless.</p> 330 331 </div> 332 333 <!-- _______________________________________________________________________ --> 334 <h4> 335 <a name="scf_indentation">Indent Code Consistently</a> 336 </h4> 337 338 <div> 339 340 <p>Okay, in your first year of programming you were told that indentation is 341 important. If you didn't believe and internalize this then, now is the time. 342 Just do it.</p> 343 344 </div> 345 346 </div> 347 348 <!-- ======================================================================= --> 349 <h3> 350 <a name="compilerissues">Compiler Issues</a> 351 </h3> 352 353 <div> 354 355 <!-- _______________________________________________________________________ --> 356 <h4> 357 <a name="ci_warningerrors">Treat Compiler Warnings Like Errors</a> 358 </h4> 359 360 <div> 361 362 <p>If your code has compiler warnings in it, something is wrong — you 363 aren't casting values correctly, your have "questionable" constructs in your 364 code, or you are doing something legitimately wrong. Compiler warnings can 365 cover up legitimate errors in output and make dealing with a translation unit 366 difficult.</p> 367 368 <p>It is not possible to prevent all warnings from all compilers, nor is it 369 desirable. Instead, pick a standard compiler (like <tt>gcc</tt>) that provides 370 a good thorough set of warnings, and stick to it. At least in the case of 371 <tt>gcc</tt>, it is possible to work around any spurious errors by changing the 372 syntax of the code slightly. For example, a warning that annoys me occurs when 373 I write code like this:</p> 374 375 <div class="doc_code"> 376 <pre> 377 if (V = getValue()) { 378 ... 379 } 380 </pre> 381 </div> 382 383 <p><tt>gcc</tt> will warn me that I probably want to use the <tt>==</tt> 384 operator, and that I probably mistyped it. In most cases, I haven't, and I 385 really don't want the spurious errors. To fix this particular problem, I 386 rewrite the code like this:</p> 387 388 <div class="doc_code"> 389 <pre> 390 if ((V = getValue())) { 391 ... 392 } 393 </pre> 394 </div> 395 396 <p>which shuts <tt>gcc</tt> up. Any <tt>gcc</tt> warning that annoys you can 397 be fixed by massaging the code appropriately.</p> 398 399 <p>These are the <tt>gcc</tt> warnings that I prefer to enable:</p> 400 401 <div class="doc_code"> 402 <pre> 403 -Wall -Winline -W -Wwrite-strings -Wno-unused 404 </pre> 405 </div> 406 407 </div> 408 409 <!-- _______________________________________________________________________ --> 410 <h4> 411 <a name="ci_portable_code">Write Portable Code</a> 412 </h4> 413 414 <div> 415 416 <p>In almost all cases, it is possible and within reason to write completely 417 portable code. If there are cases where it isn't possible to write portable 418 code, isolate it behind a well defined (and well documented) interface.</p> 419 420 <p>In practice, this means that you shouldn't assume much about the host 421 compiler, and Visual Studio tends to be the lowest common denominator. 422 If advanced features are used, they should only be an implementation detail of 423 a library which has a simple exposed API, and preferably be buried in 424 libSystem.</p> 425 426 </div> 427 428 <!-- _______________________________________________________________________ --> 429 <h4> 430 <a name="ci_rtti_exceptions">Do not use RTTI or Exceptions</a> 431 </h4> 432 <div> 433 434 <p>In an effort to reduce code and executable size, LLVM does not use RTTI 435 (e.g. <tt>dynamic_cast<></tt>) or exceptions. These two language features 436 violate the general C++ principle of <i>"you only pay for what you use"</i>, 437 causing executable bloat even if exceptions are never used in the code base, or 438 if RTTI is never used for a class. Because of this, we turn them off globally 439 in the code.</p> 440 441 <p>That said, LLVM does make extensive use of a hand-rolled form of RTTI that 442 use templates like <a href="ProgrammersManual.html#isa"><tt>isa<></tt>, 443 <tt>cast<></tt>, and <tt>dyn_cast<></tt></a>. This form of RTTI is 444 opt-in and can be added to any class. It is also substantially more efficient 445 than <tt>dynamic_cast<></tt>.</p> 446 447 </div> 448 449 <!-- _______________________________________________________________________ --> 450 <h4> 451 <a name="ci_class_struct">Use of <tt>class</tt> and <tt>struct</tt> Keywords</a> 452 </h4> 453 <div> 454 455 <p>In C++, the <tt>class</tt> and <tt>struct</tt> keywords can be used almost 456 interchangeably. The only difference is when they are used to declare a class: 457 <tt>class</tt> makes all members private by default while <tt>struct</tt> makes 458 all members public by default.</p> 459 460 <p>Unfortunately, not all compilers follow the rules and some will generate 461 different symbols based on whether <tt>class</tt> or <tt>struct</tt> was used to 462 declare the symbol. This can lead to problems at link time.</p> 463 464 <p>So, the rule for LLVM is to always use the <tt>class</tt> keyword, unless 465 <b>all</b> members are public and the type is a C++ 466 <a href="http://en.wikipedia.org/wiki/Plain_old_data_structure">POD</a> type, in 467 which case <tt>struct</tt> is allowed.</p> 468 469 </div> 470 471 </div> 472 473 </div> 474 475 <!-- *********************************************************************** --> 476 <h2> 477 <a name="styleissues">Style Issues</a> 478 </h2> 479 <!-- *********************************************************************** --> 480 481 <div> 482 483 <!-- ======================================================================= --> 484 <h3> 485 <a name="macro">The High-Level Issues</a> 486 </h3> 487 <!-- ======================================================================= --> 488 489 <div> 490 491 <!-- _______________________________________________________________________ --> 492 <h4> 493 <a name="hl_module">A Public Header File <b>is</b> a Module</a> 494 </h4> 495 496 <div> 497 498 <p>C++ doesn't do too well in the modularity department. There is no real 499 encapsulation or data hiding (unless you use expensive protocol classes), but it 500 is what we have to work with. When you write a public header file (in the LLVM 501 source tree, they live in the top level "<tt>include</tt>" directory), you are 502 defining a module of functionality.</p> 503 504 <p>Ideally, modules should be completely independent of each other, and their 505 header files should only <tt>#include</tt> the absolute minimum number of 506 headers possible. A module is not just a class, a function, or a 507 namespace: <a href="http://www.cuj.com/articles/2000/0002/0002c/0002c.htm">it's 508 a collection of these</a> that defines an interface. This interface may be 509 several functions, classes, or data structures, but the important issue is how 510 they work together.</p> 511 512 <p>In general, a module should be implemented by one or more <tt>.cpp</tt> 513 files. Each of these <tt>.cpp</tt> files should include the header that defines 514 their interface first. This ensures that all of the dependences of the module 515 header have been properly added to the module header itself, and are not 516 implicit. System headers should be included after user headers for a 517 translation unit.</p> 518 519 </div> 520 521 <!-- _______________________________________________________________________ --> 522 <h4> 523 <a name="hl_dontinclude"><tt>#include</tt> as Little as Possible</a> 524 </h4> 525 526 <div> 527 528 <p><tt>#include</tt> hurts compile time performance. Don't do it unless you 529 have to, especially in header files.</p> 530 531 <p>But wait! Sometimes you need to have the definition of a class to use it, or 532 to inherit from it. In these cases go ahead and <tt>#include</tt> that header 533 file. Be aware however that there are many cases where you don't need to have 534 the full definition of a class. If you are using a pointer or reference to a 535 class, you don't need the header file. If you are simply returning a class 536 instance from a prototyped function or method, you don't need it. In fact, for 537 most cases, you simply don't need the definition of a class. And not 538 <tt>#include</tt>'ing speeds up compilation.</p> 539 540 <p>It is easy to try to go too overboard on this recommendation, however. You 541 <b>must</b> include all of the header files that you are using — you can 542 include them either directly or indirectly (through another header file). To 543 make sure that you don't accidentally forget to include a header file in your 544 module header, make sure to include your module header <b>first</b> in the 545 implementation file (as mentioned above). This way there won't be any hidden 546 dependencies that you'll find out about later.</p> 547 548 </div> 549 550 <!-- _______________________________________________________________________ --> 551 <h4> 552 <a name="hl_privateheaders">Keep "Internal" Headers Private</a> 553 </h4> 554 555 <div> 556 557 <p>Many modules have a complex implementation that causes them to use more than 558 one implementation (<tt>.cpp</tt>) file. It is often tempting to put the 559 internal communication interface (helper classes, extra functions, etc) in the 560 public module header file. Don't do this!</p> 561 562 <p>If you really need to do something like this, put a private header file in 563 the same directory as the source files, and include it locally. This ensures 564 that your private interface remains private and undisturbed by outsiders.</p> 565 566 <p>Note however, that it's okay to put extra implementation methods in a public 567 class itself. Just make them private (or protected) and all is well.</p> 568 569 </div> 570 571 <!-- _______________________________________________________________________ --> 572 <h4> 573 <a name="hl_earlyexit">Use Early Exits and <tt>continue</tt> to Simplify Code</a> 574 </h4> 575 576 <div> 577 578 <p>When reading code, keep in mind how much state and how many previous 579 decisions have to be remembered by the reader to understand a block of code. 580 Aim to reduce indentation where possible when it doesn't make it more difficult 581 to understand the code. One great way to do this is by making use of early 582 exits and the <tt>continue</tt> keyword in long loops. As an example of using 583 an early exit from a function, consider this "bad" code:</p> 584 585 <div class="doc_code"> 586 <pre> 587 Value *DoSomething(Instruction *I) { 588 if (!isa<TerminatorInst>(I) && 589 I->hasOneUse() && SomeOtherThing(I)) { 590 ... some long code .... 591 } 592 593 return 0; 594 } 595 </pre> 596 </div> 597 598 <p>This code has several problems if the body of the '<tt>if</tt>' is large. 599 When you're looking at the top of the function, it isn't immediately clear that 600 this <em>only</em> does interesting things with non-terminator instructions, and 601 only applies to things with the other predicates. Second, it is relatively 602 difficult to describe (in comments) why these predicates are important because 603 the <tt>if</tt> statement makes it difficult to lay out the comments. Third, 604 when you're deep within the body of the code, it is indented an extra level. 605 Finally, when reading the top of the function, it isn't clear what the result is 606 if the predicate isn't true; you have to read to the end of the function to know 607 that it returns null.</p> 608 609 <p>It is much preferred to format the code like this:</p> 610 611 <div class="doc_code"> 612 <pre> 613 Value *DoSomething(Instruction *I) { 614 // Terminators never need 'something' done to them because ... 615 if (isa<TerminatorInst>(I)) 616 return 0; 617 618 // We conservatively avoid transforming instructions with multiple uses 619 // because goats like cheese. 620 if (!I->hasOneUse()) 621 return 0; 622 623 // This is really just here for example. 624 if (!SomeOtherThing(I)) 625 return 0; 626 627 ... some long code .... 628 } 629 </pre> 630 </div> 631 632 <p>This fixes these problems. A similar problem frequently happens in <tt>for</tt> 633 loops. A silly example is something like this:</p> 634 635 <div class="doc_code"> 636 <pre> 637 for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ++II) { 638 if (BinaryOperator *BO = dyn_cast<BinaryOperator>(II)) { 639 Value *LHS = BO->getOperand(0); 640 Value *RHS = BO->getOperand(1); 641 if (LHS != RHS) { 642 ... 643 } 644 } 645 } 646 </pre> 647 </div> 648 649 <p>When you have very, very small loops, this sort of structure is fine. But if 650 it exceeds more than 10-15 lines, it becomes difficult for people to read and 651 understand at a glance. The problem with this sort of code is that it gets very 652 nested very quickly. Meaning that the reader of the code has to keep a lot of 653 context in their brain to remember what is going immediately on in the loop, 654 because they don't know if/when the <tt>if</tt> conditions will have elses etc. 655 It is strongly preferred to structure the loop like this:</p> 656 657 <div class="doc_code"> 658 <pre> 659 for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ++II) { 660 BinaryOperator *BO = dyn_cast<BinaryOperator>(II); 661 if (!BO) continue; 662 663 Value *LHS = BO->getOperand(0); 664 Value *RHS = BO->getOperand(1); 665 if (LHS == RHS) continue; 666 667 ... 668 } 669 </pre> 670 </div> 671 672 <p>This has all the benefits of using early exits for functions: it reduces 673 nesting of the loop, it makes it easier to describe why the conditions are true, 674 and it makes it obvious to the reader that there is no <tt>else</tt> coming up 675 that they have to push context into their brain for. If a loop is large, this 676 can be a big understandability win.</p> 677 678 </div> 679 680 <!-- _______________________________________________________________________ --> 681 <h4> 682 <a name="hl_else_after_return">Don't use <tt>else</tt> after a <tt>return</tt></a> 683 </h4> 684 685 <div> 686 687 <p>For similar reasons above (reduction of indentation and easier reading), 688 please do not use '<tt>else</tt>' or '<tt>else if</tt>' after something that 689 interrupts control flow — like <tt>return</tt>, <tt>break</tt>, 690 <tt>continue</tt>, <tt>goto</tt>, etc. For example, this is <em>bad</em>:</p> 691 692 <div class="doc_code"> 693 <pre> 694 case 'J': { 695 if (Signed) { 696 Type = Context.getsigjmp_bufType(); 697 if (Type.isNull()) { 698 Error = ASTContext::GE_Missing_sigjmp_buf; 699 return QualType(); 700 <b>} else { 701 break; 702 }</b> 703 } else { 704 Type = Context.getjmp_bufType(); 705 if (Type.isNull()) { 706 Error = ASTContext::GE_Missing_jmp_buf; 707 return QualType(); 708 <b>} else { 709 break; 710 }</b> 711 } 712 } 713 } 714 </pre> 715 </div> 716 717 <p>It is better to write it like this:</p> 718 719 <div class="doc_code"> 720 <pre> 721 case 'J': 722 if (Signed) { 723 Type = Context.getsigjmp_bufType(); 724 if (Type.isNull()) { 725 Error = ASTContext::GE_Missing_sigjmp_buf; 726 return QualType(); 727 } 728 } else { 729 Type = Context.getjmp_bufType(); 730 if (Type.isNull()) { 731 Error = ASTContext::GE_Missing_jmp_buf; 732 return QualType(); 733 } 734 } 735 <b>break;</b> 736 </pre> 737 </div> 738 739 <p>Or better yet (in this case) as:</p> 740 741 <div class="doc_code"> 742 <pre> 743 case 'J': 744 if (Signed) 745 Type = Context.getsigjmp_bufType(); 746 else 747 Type = Context.getjmp_bufType(); 748 749 if (Type.isNull()) { 750 Error = Signed ? ASTContext::GE_Missing_sigjmp_buf : 751 ASTContext::GE_Missing_jmp_buf; 752 return QualType(); 753 } 754 <b>break;</b> 755 </pre> 756 </div> 757 758 <p>The idea is to reduce indentation and the amount of code you have to keep 759 track of when reading the code.</p> 760 761 </div> 762 763 <!-- _______________________________________________________________________ --> 764 <h4> 765 <a name="hl_predicateloops">Turn Predicate Loops into Predicate Functions</a> 766 </h4> 767 768 <div> 769 770 <p>It is very common to write small loops that just compute a boolean value. 771 There are a number of ways that people commonly write these, but an example of 772 this sort of thing is:</p> 773 774 <div class="doc_code"> 775 <pre> 776 <b>bool FoundFoo = false;</b> 777 for (unsigned i = 0, e = BarList.size(); i != e; ++i) 778 if (BarList[i]->isFoo()) { 779 <b>FoundFoo = true;</b> 780 break; 781 } 782 783 <b>if (FoundFoo) {</b> 784 ... 785 } 786 </pre> 787 </div> 788 789 <p>This sort of code is awkward to write, and is almost always a bad sign. 790 Instead of this sort of loop, we strongly prefer to use a predicate function 791 (which may be <a href="#micro_anonns">static</a>) that uses 792 <a href="#hl_earlyexit">early exits</a> to compute the predicate. We prefer 793 the code to be structured like this:</p> 794 795 <div class="doc_code"> 796 <pre> 797 /// ListContainsFoo - Return true if the specified list has an element that is 798 /// a foo. 799 static bool ListContainsFoo(const std::vector<Bar*> &List) { 800 for (unsigned i = 0, e = List.size(); i != e; ++i) 801 if (List[i]->isFoo()) 802 return true; 803 return false; 804 } 805 ... 806 807 <b>if (ListContainsFoo(BarList)) {</b> 808 ... 809 } 810 </pre> 811 </div> 812 813 <p>There are many reasons for doing this: it reduces indentation and factors out 814 code which can often be shared by other code that checks for the same predicate. 815 More importantly, it <em>forces you to pick a name</em> for the function, and 816 forces you to write a comment for it. In this silly example, this doesn't add 817 much value. However, if the condition is complex, this can make it a lot easier 818 for the reader to understand the code that queries for this predicate. Instead 819 of being faced with the in-line details of how we check to see if the BarList 820 contains a foo, we can trust the function name and continue reading with better 821 locality.</p> 822 823 </div> 824 825 </div> 826 827 <!-- ======================================================================= --> 828 <h3> 829 <a name="micro">The Low-Level Issues</a> 830 </h3> 831 <!-- ======================================================================= --> 832 833 <div> 834 835 <!-- _______________________________________________________________________ --> 836 <h4> 837 <a name="ll_naming"> 838 Name Types, Functions, Variables, and Enumerators Properly 839 </a> 840 </h4> 841 842 <div> 843 844 <p>Poorly-chosen names can mislead the reader and cause bugs. We cannot stress 845 enough how important it is to use <em>descriptive</em> names. Pick names that 846 match the semantics and role of the underlying entities, within reason. Avoid 847 abbreviations unless they are well known. After picking a good name, make sure 848 to use consistent capitalization for the name, as inconsistency requires clients 849 to either memorize the APIs or to look it up to find the exact spelling.</p> 850 851 <p>In general, names should be in camel case (e.g. <tt>TextFileReader</tt> 852 and <tt>isLValue()</tt>). Different kinds of declarations have different 853 rules:</p> 854 855 <ul> 856 <li><p><b>Type names</b> (including classes, structs, enums, typedefs, etc) 857 should be nouns and start with an upper-case letter (e.g. 858 <tt>TextFileReader</tt>).</p></li> 859 860 <li><p><b>Function names</b> should be verb phrases (as they represent 861 actions), and command-like function should be imperative. The name should 862 be camel case, and start with a lower case letter (e.g. <tt>openFile()</tt> 863 or <tt>isFoo()</tt>).</p></li> 864 865 <li><p><b>Enum declarations</b> (e.g. <tt>enum Foo {...}</tt>) are types, so 866 they should follow the naming conventions for types. A common use for enums 867 is as a discriminator for a union, or an indicator of a subclass. When an 868 enum is used for something like this, it should have a <tt>Kind</tt> suffix 869 (e.g. <tt>ValueKind</tt>).</p></li> 870 871 <li><p><b>Enumerators</b> (e.g. <tt>enum { Foo, Bar }</tt>) and <b>public member 872 variables</b> should start with an upper-case letter, just like types. 873 Unless the enumerators are defined in their own small namespace or inside a 874 class, enumerators should have a prefix corresponding to the enum 875 declaration name. For example, <tt>enum ValueKind { ... };</tt> may contain 876 enumerators like <tt>VK_Argument</tt>, <tt>VK_BasicBlock</tt>, etc. 877 Enumerators that are just convenience constants are exempt from the 878 requirement for a prefix. For instance:</p> 879 880 <div class="doc_code"> 881 <pre> 882 enum { 883 MaxSize = 42, 884 Density = 12 885 }; 886 </pre> 887 </div> 888 </li> 889 890 </ul> 891 892 <p>As an exception, classes that mimic STL classes can have member names in 893 STL's style of lower-case words separated by underscores (e.g. <tt>begin()</tt>, 894 <tt>push_back()</tt>, and <tt>empty()</tt>).</p> 895 896 <p>Here are some examples of good and bad names:</p> 897 898 <div class="doc_code"> 899 <pre> 900 class VehicleMaker { 901 ... 902 Factory<Tire> F; // Bad -- abbreviation and non-descriptive. 903 Factory<Tire> Factory; // Better. 904 Factory<Tire> TireFactory; // Even better -- if VehicleMaker has more than one 905 // kind of factories. 906 }; 907 908 Vehicle MakeVehicle(VehicleType Type) { 909 VehicleMaker M; // Might be OK if having a short life-span. 910 Tire tmp1 = M.makeTire(); // Bad -- 'tmp1' provides no information. 911 Light headlight = M.makeLight("head"); // Good -- descriptive. 912 ... 913 } 914 </pre> 915 </div> 916 917 </div> 918 919 920 <!-- _______________________________________________________________________ --> 921 <h4> 922 <a name="ll_assert">Assert Liberally</a> 923 </h4> 924 925 <div> 926 927 <p>Use the "<tt>assert</tt>" macro to its fullest. Check all of your 928 preconditions and assumptions, you never know when a bug (not necessarily even 929 yours) might be caught early by an assertion, which reduces debugging time 930 dramatically. The "<tt><cassert></tt>" header file is probably already 931 included by the header files you are using, so it doesn't cost anything to use 932 it.</p> 933 934 <p>To further assist with debugging, make sure to put some kind of error message 935 in the assertion statement, which is printed if the assertion is tripped. This 936 helps the poor debugger make sense of why an assertion is being made and 937 enforced, and hopefully what to do about it. Here is one complete example:</p> 938 939 <div class="doc_code"> 940 <pre> 941 inline Value *getOperand(unsigned i) { 942 assert(i < Operands.size() && "getOperand() out of range!"); 943 return Operands[i]; 944 } 945 </pre> 946 </div> 947 948 <p>Here are more examples:</p> 949 950 <div class="doc_code"> 951 <pre> 952 assert(Ty->isPointerType() && "Can't allocate a non pointer type!"); 953 954 assert((Opcode == Shl || Opcode == Shr) && "ShiftInst Opcode invalid!"); 955 956 assert(idx < getNumSuccessors() && "Successor # out of range!"); 957 958 assert(V1.getType() == V2.getType() && "Constant types must be identical!"); 959 960 assert(isa<PHINode>(Succ->front()) && "Only works on PHId BBs!"); 961 </pre> 962 </div> 963 964 <p>You get the idea.</p> 965 966 <p>Please be aware that, when adding assert statements, not all compilers are aware of 967 the semantics of the assert. In some places, asserts are used to indicate a piece of 968 code that should not be reached. These are typically of the form:</p> 969 970 <div class="doc_code"> 971 <pre> 972 assert(0 && "Some helpful error message"); 973 </pre> 974 </div> 975 976 <p>When used in a function that returns a value, they should be followed with a return 977 statement and a comment indicating that this line is never reached. This will prevent 978 a compiler which is unable to deduce that the assert statement never returns from 979 generating a warning.</p> 980 981 <div class="doc_code"> 982 <pre> 983 assert(0 && "Some helpful error message"); 984 // Not reached 985 return 0; 986 </pre> 987 </div> 988 989 <p>Another issue is that values used only by assertions will produce an "unused 990 value" warning when assertions are disabled. For example, this code will 991 warn:</p> 992 993 <div class="doc_code"> 994 <pre> 995 unsigned Size = V.size(); 996 assert(Size > 42 && "Vector smaller than it should be"); 997 998 bool NewToSet = Myset.insert(Value); 999 assert(NewToSet && "The value shouldn't be in the set yet"); 1000 </pre> 1001 </div> 1002 1003 <p>These are two interesting different cases. In the first case, the call to 1004 V.size() is only useful for the assert, and we don't want it executed when 1005 assertions are disabled. Code like this should move the call into the assert 1006 itself. In the second case, the side effects of the call must happen whether 1007 the assert is enabled or not. In this case, the value should be cast to void to 1008 disable the warning. To be specific, it is preferred to write the code like 1009 this:</p> 1010 1011 <div class="doc_code"> 1012 <pre> 1013 assert(V.size() > 42 && "Vector smaller than it should be"); 1014 1015 bool NewToSet = Myset.insert(Value); (void)NewToSet; 1016 assert(NewToSet && "The value shouldn't be in the set yet"); 1017 </pre> 1018 </div> 1019 1020 1021 </div> 1022 1023 <!-- _______________________________________________________________________ --> 1024 <h4> 1025 <a name="ll_ns_std">Do Not Use '<tt>using namespace std</tt>'</a> 1026 </h4> 1027 1028 <div> 1029 1030 <p>In LLVM, we prefer to explicitly prefix all identifiers from the standard 1031 namespace with an "<tt>std::</tt>" prefix, rather than rely on 1032 "<tt>using namespace std;</tt>".</p> 1033 1034 <p> In header files, adding a '<tt>using namespace XXX</tt>' directive pollutes 1035 the namespace of any source file that <tt>#include</tt>s the header. This is 1036 clearly a bad thing.</p> 1037 1038 <p>In implementation files (e.g. <tt>.cpp</tt> files), the rule is more of a stylistic 1039 rule, but is still important. Basically, using explicit namespace prefixes 1040 makes the code <b>clearer</b>, because it is immediately obvious what facilities 1041 are being used and where they are coming from. And <b>more portable</b>, because 1042 namespace clashes cannot occur between LLVM code and other namespaces. The 1043 portability rule is important because different standard library implementations 1044 expose different symbols (potentially ones they shouldn't), and future revisions 1045 to the C++ standard will add more symbols to the <tt>std</tt> namespace. As 1046 such, we never use '<tt>using namespace std;</tt>' in LLVM.</p> 1047 1048 <p>The exception to the general rule (i.e. it's not an exception for 1049 the <tt>std</tt> namespace) is for implementation files. For example, all of 1050 the code in the LLVM project implements code that lives in the 'llvm' namespace. 1051 As such, it is ok, and actually clearer, for the <tt>.cpp</tt> files to have a 1052 '<tt>using namespace llvm;</tt>' directive at the top, after the 1053 <tt>#include</tt>s. This reduces indentation in the body of the file for source 1054 editors that indent based on braces, and keeps the conceptual context cleaner. 1055 The general form of this rule is that any <tt>.cpp</tt> file that implements 1056 code in any namespace may use that namespace (and its parents'), but should not 1057 use any others.</p> 1058 1059 </div> 1060 1061 <!-- _______________________________________________________________________ --> 1062 <h4> 1063 <a name="ll_virtual_anch"> 1064 Provide a Virtual Method Anchor for Classes in Headers 1065 </a> 1066 </h4> 1067 1068 <div> 1069 1070 <p>If a class is defined in a header file and has a v-table (either it has 1071 virtual methods or it derives from classes with virtual methods), it must 1072 always have at least one out-of-line virtual method in the class. Without 1073 this, the compiler will copy the vtable and RTTI into every <tt>.o</tt> file 1074 that <tt>#include</tt>s the header, bloating <tt>.o</tt> file sizes and 1075 increasing link times.</p> 1076 1077 </div> 1078 1079 <!-- _______________________________________________________________________ --> 1080 <h4> 1081 <a name="ll_end">Don't evaluate <tt>end()</tt> every time through a loop</a> 1082 </h4> 1083 1084 <div> 1085 1086 <p>Because C++ doesn't have a standard "<tt>foreach</tt>" loop (though it can be 1087 emulated with macros and may be coming in C++'0x) we end up writing a lot of 1088 loops that manually iterate from begin to end on a variety of containers or 1089 through other data structures. One common mistake is to write a loop in this 1090 style:</p> 1091 1092 <div class="doc_code"> 1093 <pre> 1094 BasicBlock *BB = ... 1095 for (BasicBlock::iterator I = BB->begin(); I != <b>BB->end()</b>; ++I) 1096 ... use I ... 1097 </pre> 1098 </div> 1099 1100 <p>The problem with this construct is that it evaluates "<tt>BB->end()</tt>" 1101 every time through the loop. Instead of writing the loop like this, we strongly 1102 prefer loops to be written so that they evaluate it once before the loop starts. 1103 A convenient way to do this is like so:</p> 1104 1105 <div class="doc_code"> 1106 <pre> 1107 BasicBlock *BB = ... 1108 for (BasicBlock::iterator I = BB->begin(), E = <b>BB->end()</b>; I != E; ++I) 1109 ... use I ... 1110 </pre> 1111 </div> 1112 1113 <p>The observant may quickly point out that these two loops may have different 1114 semantics: if the container (a basic block in this case) is being mutated, then 1115 "<tt>BB->end()</tt>" may change its value every time through the loop and the 1116 second loop may not in fact be correct. If you actually do depend on this 1117 behavior, please write the loop in the first form and add a comment indicating 1118 that you did it intentionally.</p> 1119 1120 <p>Why do we prefer the second form (when correct)? Writing the loop in the 1121 first form has two problems. First it may be less efficient than evaluating it 1122 at the start of the loop. In this case, the cost is probably minor — a 1123 few extra loads every time through the loop. However, if the base expression is 1124 more complex, then the cost can rise quickly. I've seen loops where the end 1125 expression was actually something like: "<tt>SomeMap[x]->end()</tt>" and map 1126 lookups really aren't cheap. By writing it in the second form consistently, you 1127 eliminate the issue entirely and don't even have to think about it.</p> 1128 1129 <p>The second (even bigger) issue is that writing the loop in the first form 1130 hints to the reader that the loop is mutating the container (a fact that a 1131 comment would handily confirm!). If you write the loop in the second form, it 1132 is immediately obvious without even looking at the body of the loop that the 1133 container isn't being modified, which makes it easier to read the code and 1134 understand what it does.</p> 1135 1136 <p>While the second form of the loop is a few extra keystrokes, we do strongly 1137 prefer it.</p> 1138 1139 </div> 1140 1141 <!-- _______________________________________________________________________ --> 1142 <h4> 1143 <a name="ll_iostream"><tt>#include <iostream></tt> is Forbidden</a> 1144 </h4> 1145 1146 <div> 1147 1148 <p>The use of <tt>#include <iostream></tt> in library files is 1149 hereby <b><em>forbidden</em></b>. The primary reason for doing this is to 1150 support clients using LLVM libraries as part of larger systems. In particular, 1151 we statically link LLVM into some dynamic libraries. Even if LLVM isn't used, 1152 the static constructors are run whenever an application starts up that uses the 1153 dynamic library. There are two problems with this:</p> 1154 1155 <ol> 1156 <li>The time to run the static c'tors impacts startup time of applications 1157 — a critical time for GUI apps.</li> 1158 1159 <li>The static c'tors cause the app to pull many extra pages of memory off the 1160 disk: both the code for the static c'tors in each <tt>.o</tt> file and the 1161 small amount of data that gets touched. In addition, touched/dirty pages 1162 put more pressure on the VM system on low-memory machines.</li> 1163 </ol> 1164 1165 <p>Note that using the other stream headers (<tt><sstream></tt> for 1166 example) is not problematic in this regard — 1167 just <tt><iostream></tt>. However, <tt>raw_ostream</tt> provides various 1168 APIs that are better performing for almost every use than <tt>std::ostream</tt> 1169 style APIs. <b>Therefore new code should always 1170 use <a href="#ll_raw_ostream"><tt>raw_ostream</tt></a> for writing, or 1171 the <tt>llvm::MemoryBuffer</tt> API for reading files.</b></p> 1172 1173 </div> 1174 1175 1176 <!-- _______________________________________________________________________ --> 1177 <h4> 1178 <a name="ll_raw_ostream">Use <tt>raw_ostream</tt></a> 1179 </h4> 1180 1181 <div> 1182 1183 <p>LLVM includes a lightweight, simple, and efficient stream implementation 1184 in <tt>llvm/Support/raw_ostream.h</tt>, which provides all of the common 1185 features of <tt>std::ostream</tt>. All new code should use <tt>raw_ostream</tt> 1186 instead of <tt>ostream</tt>.</p> 1187 1188 <p>Unlike <tt>std::ostream</tt>, <tt>raw_ostream</tt> is not a template and can 1189 be forward declared as <tt>class raw_ostream</tt>. Public headers should 1190 generally not include the <tt>raw_ostream</tt> header, but use forward 1191 declarations and constant references to <tt>raw_ostream</tt> instances.</p> 1192 1193 </div> 1194 1195 1196 <!-- _______________________________________________________________________ --> 1197 <h4> 1198 <a name="ll_avoidendl">Avoid <tt>std::endl</tt></a> 1199 </h4> 1200 1201 <div> 1202 1203 <p>The <tt>std::endl</tt> modifier, when used with <tt>iostreams</tt> outputs a 1204 newline to the output stream specified. In addition to doing this, however, it 1205 also flushes the output stream. In other words, these are equivalent:</p> 1206 1207 <div class="doc_code"> 1208 <pre> 1209 std::cout << std::endl; 1210 std::cout << '\n' << std::flush; 1211 </pre> 1212 </div> 1213 1214 <p>Most of the time, you probably have no reason to flush the output stream, so 1215 it's better to use a literal <tt>'\n'</tt>.</p> 1216 1217 </div> 1218 1219 </div> 1220 1221 <!-- ======================================================================= --> 1222 <h3> 1223 <a name="nano">Microscopic Details</a> 1224 </h3> 1225 <!-- ======================================================================= --> 1226 1227 <div> 1228 1229 <p>This section describes preferred low-level formatting guidelines along with 1230 reasoning on why we prefer them.</p> 1231 1232 <!-- _______________________________________________________________________ --> 1233 <h4> 1234 <a name="micro_spaceparen">Spaces Before Parentheses</a> 1235 </h4> 1236 1237 <div> 1238 1239 <p>We prefer to put a space before an open parenthesis only in control flow 1240 statements, but not in normal function call expressions and function-like 1241 macros. For example, this is good:</p> 1242 1243 <div class="doc_code"> 1244 <pre> 1245 <b>if (</b>x) ... 1246 <b>for (</b>i = 0; i != 100; ++i) ... 1247 <b>while (</b>llvm_rocks) ... 1248 1249 <b>somefunc(</b>42); 1250 <b><a href="#ll_assert">assert</a>(</b>3 != 4 && "laws of math are failing me"); 1251 1252 a = <b>foo(</b>42, 92) + <b>bar(</b>x); 1253 </pre> 1254 </div> 1255 1256 <p>and this is bad:</p> 1257 1258 <div class="doc_code"> 1259 <pre> 1260 <b>if(</b>x) ... 1261 <b>for(</b>i = 0; i != 100; ++i) ... 1262 <b>while(</b>llvm_rocks) ... 1263 1264 <b>somefunc (</b>42); 1265 <b><a href="#ll_assert">assert</a> (</b>3 != 4 && "laws of math are failing me"); 1266 1267 a = <b>foo (</b>42, 92) + <b>bar (</b>x); 1268 </pre> 1269 </div> 1270 1271 <p>The reason for doing this is not completely arbitrary. This style makes 1272 control flow operators stand out more, and makes expressions flow better. The 1273 function call operator binds very tightly as a postfix operator. Putting a 1274 space after a function name (as in the last example) makes it appear that the 1275 code might bind the arguments of the left-hand-side of a binary operator with 1276 the argument list of a function and the name of the right side. More 1277 specifically, it is easy to misread the "a" example as:</p> 1278 1279 <div class="doc_code"> 1280 <pre> 1281 a = foo <b>(</b>(42, 92) + bar<b>)</b> (x); 1282 </pre> 1283 </div> 1284 1285 <p>when skimming through the code. By avoiding a space in a function, we avoid 1286 this misinterpretation.</p> 1287 1288 </div> 1289 1290 <!-- _______________________________________________________________________ --> 1291 <h4> 1292 <a name="micro_preincrement">Prefer Preincrement</a> 1293 </h4> 1294 1295 <div> 1296 1297 <p>Hard fast rule: Preincrement (<tt>++X</tt>) may be no slower than 1298 postincrement (<tt>X++</tt>) and could very well be a lot faster than it. Use 1299 preincrementation whenever possible.</p> 1300 1301 <p>The semantics of postincrement include making a copy of the value being 1302 incremented, returning it, and then preincrementing the "work value". For 1303 primitive types, this isn't a big deal... but for iterators, it can be a huge 1304 issue (for example, some iterators contains stack and set objects in them... 1305 copying an iterator could invoke the copy ctor's of these as well). In general, 1306 get in the habit of always using preincrement, and you won't have a problem.</p> 1307 1308 </div> 1309 1310 <!-- _______________________________________________________________________ --> 1311 <h4> 1312 <a name="micro_namespaceindent">Namespace Indentation</a> 1313 </h4> 1314 1315 <div> 1316 1317 <p> 1318 In general, we strive to reduce indentation wherever possible. This is useful 1319 because we want code to <a href="#scf_codewidth">fit into 80 columns</a> without 1320 wrapping horribly, but also because it makes it easier to understand the code. 1321 Namespaces are a funny thing: they are often large, and we often desire to put 1322 lots of stuff into them (so they can be large). Other times they are tiny, 1323 because they just hold an enum or something similar. In order to balance this, 1324 we use different approaches for small versus large namespaces. 1325 </p> 1326 1327 <p> 1328 If a namespace definition is small and <em>easily</em> fits on a screen (say, 1329 less than 35 lines of code), then you should indent its body. Here's an 1330 example: 1331 </p> 1332 1333 <div class="doc_code"> 1334 <pre> 1335 namespace llvm { 1336 namespace X86 { 1337 /// RelocationType - An enum for the x86 relocation codes. Note that 1338 /// the terminology here doesn't follow x86 convention - word means 1339 /// 32-bit and dword means 64-bit. 1340 enum RelocationType { 1341 /// reloc_pcrel_word - PC relative relocation, add the relocated value to 1342 /// the value already in memory, after we adjust it for where the PC is. 1343 reloc_pcrel_word = 0, 1344 1345 /// reloc_picrel_word - PIC base relative relocation, add the relocated 1346 /// value to the value already in memory, after we adjust it for where the 1347 /// PIC base is. 1348 reloc_picrel_word = 1, 1349 1350 /// reloc_absolute_word, reloc_absolute_dword - Absolute relocation, just 1351 /// add the relocated value to the value already in memory. 1352 reloc_absolute_word = 2, 1353 reloc_absolute_dword = 3 1354 }; 1355 } 1356 } 1357 </pre> 1358 </div> 1359 1360 <p>Since the body is small, indenting adds value because it makes it very clear 1361 where the namespace starts and ends, and it is easy to take the whole thing in 1362 in one "gulp" when reading the code. If the blob of code in the namespace is 1363 larger (as it typically is in a header in the <tt>llvm</tt> or <tt>clang</tt> namespaces), do not 1364 indent the code, and add a comment indicating what namespace is being closed. 1365 For example:</p> 1366 1367 <div class="doc_code"> 1368 <pre> 1369 namespace llvm { 1370 namespace knowledge { 1371 1372 /// Grokable - This class represents things that Smith can have an intimate 1373 /// understanding of and contains the data associated with it. 1374 class Grokable { 1375 ... 1376 public: 1377 explicit Grokable() { ... } 1378 virtual ~Grokable() = 0; 1379 1380 ... 1381 1382 }; 1383 1384 } // end namespace knowledge 1385 } // end namespace llvm 1386 </pre> 1387 </div> 1388 1389 <p>Because the class is large, we don't expect that the reader can easily 1390 understand the entire concept in a glance, and the end of the file (where the 1391 namespaces end) may be a long ways away from the place they open. As such, 1392 indenting the contents of the namespace doesn't add any value, and detracts from 1393 the readability of the class. In these cases it is best to <em>not</em> indent 1394 the contents of the namespace.</p> 1395 1396 </div> 1397 1398 <!-- _______________________________________________________________________ --> 1399 <h4> 1400 <a name="micro_anonns">Anonymous Namespaces</a> 1401 </h4> 1402 1403 <div> 1404 1405 <p>After talking about namespaces in general, you may be wondering about 1406 anonymous namespaces in particular. 1407 Anonymous namespaces are a great language feature that tells the C++ compiler 1408 that the contents of the namespace are only visible within the current 1409 translation unit, allowing more aggressive optimization and eliminating the 1410 possibility of symbol name collisions. Anonymous namespaces are to C++ as 1411 "static" is to C functions and global variables. While "static" is available 1412 in C++, anonymous namespaces are more general: they can make entire classes 1413 private to a file.</p> 1414 1415 <p>The problem with anonymous namespaces is that they naturally want to 1416 encourage indentation of their body, and they reduce locality of reference: if 1417 you see a random function definition in a C++ file, it is easy to see if it is 1418 marked static, but seeing if it is in an anonymous namespace requires scanning 1419 a big chunk of the file.</p> 1420 1421 <p>Because of this, we have a simple guideline: make anonymous namespaces as 1422 small as possible, and only use them for class declarations. For example, this 1423 is good:</p> 1424 1425 <div class="doc_code"> 1426 <pre> 1427 <b>namespace {</b> 1428 class StringSort { 1429 ... 1430 public: 1431 StringSort(...) 1432 bool operator<(const char *RHS) const; 1433 }; 1434 <b>} // end anonymous namespace</b> 1435 1436 static void Helper() { 1437 ... 1438 } 1439 1440 bool StringSort::operator<(const char *RHS) const { 1441 ... 1442 } 1443 1444 </pre> 1445 </div> 1446 1447 <p>This is bad:</p> 1448 1449 1450 <div class="doc_code"> 1451 <pre> 1452 <b>namespace {</b> 1453 class StringSort { 1454 ... 1455 public: 1456 StringSort(...) 1457 bool operator<(const char *RHS) const; 1458 }; 1459 1460 void Helper() { 1461 ... 1462 } 1463 1464 bool StringSort::operator<(const char *RHS) const { 1465 ... 1466 } 1467 1468 <b>} // end anonymous namespace</b> 1469 1470 </pre> 1471 </div> 1472 1473 1474 <p>This is bad specifically because if you're looking at "Helper" in the middle 1475 of a large C++ file, that you have no immediate way to tell if it is local to 1476 the file. When it is marked static explicitly, this is immediately obvious. 1477 Also, there is no reason to enclose the definition of "operator<" in the 1478 namespace just because it was declared there. 1479 </p> 1480 1481 </div> 1482 1483 </div> 1484 1485 </div> 1486 1487 <!-- *********************************************************************** --> 1488 <h2> 1489 <a name="seealso">See Also</a> 1490 </h2> 1491 <!-- *********************************************************************** --> 1492 1493 <div> 1494 1495 <p>A lot of these comments and recommendations have been culled for other 1496 sources. Two particularly important books for our work are:</p> 1497 1498 <ol> 1499 1500 <li><a href="http://www.amazon.com/Effective-Specific-Addison-Wesley-Professional-Computing/dp/0321334876">Effective 1501 C++</a> by Scott Meyers. Also 1502 interesting and useful are "More Effective C++" and "Effective STL" by the same 1503 author.</li> 1504 1505 <li>Large-Scale C++ Software Design by John Lakos</li> 1506 1507 </ol> 1508 1509 <p>If you get some free time, and you haven't read them: do so, you might learn 1510 something.</p> 1511 1512 </div> 1513 1514 <!-- *********************************************************************** --> 1515 1516 <hr> 1517 <address> 1518 <a href="http://jigsaw.w3.org/css-validator/check/referer"><img 1519 src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a> 1520 <a href="http://validator.w3.org/check/referer"><img 1521 src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a> 1522 1523 <a href="mailto:sabre (a] nondot.org">Chris Lattner</a><br> 1524 <a href="http://llvm.org/">LLVM Compiler Infrastructure</a><br> 1525 Last modified: $Date$ 1526 </address> 1527 1528 </body> 1529 </html> 1530
