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>Variable names</b> should be nouns (as they represent state). The 861 name should be camel case, and start with an upper case letter (e.g. 862 <tt>Leader</tt> or <tt>Boats</tt>).</p></li> 863 864 <li><p><b>Function names</b> should be verb phrases (as they represent 865 actions), and command-like function should be imperative. The name should 866 be camel case, and start with a lower case letter (e.g. <tt>openFile()</tt> 867 or <tt>isFoo()</tt>).</p></li> 868 869 <li><p><b>Enum declarations</b> (e.g. <tt>enum Foo {...}</tt>) are types, so 870 they should follow the naming conventions for types. A common use for enums 871 is as a discriminator for a union, or an indicator of a subclass. When an 872 enum is used for something like this, it should have a <tt>Kind</tt> suffix 873 (e.g. <tt>ValueKind</tt>).</p></li> 874 875 <li><p><b>Enumerators</b> (e.g. <tt>enum { Foo, Bar }</tt>) and <b>public member 876 variables</b> should start with an upper-case letter, just like types. 877 Unless the enumerators are defined in their own small namespace or inside a 878 class, enumerators should have a prefix corresponding to the enum 879 declaration name. For example, <tt>enum ValueKind { ... };</tt> may contain 880 enumerators like <tt>VK_Argument</tt>, <tt>VK_BasicBlock</tt>, etc. 881 Enumerators that are just convenience constants are exempt from the 882 requirement for a prefix. For instance:</p> 883 884 <div class="doc_code"> 885 <pre> 886 enum { 887 MaxSize = 42, 888 Density = 12 889 }; 890 </pre> 891 </div> 892 </li> 893 894 </ul> 895 896 <p>As an exception, classes that mimic STL classes can have member names in 897 STL's style of lower-case words separated by underscores (e.g. <tt>begin()</tt>, 898 <tt>push_back()</tt>, and <tt>empty()</tt>).</p> 899 900 <p>Here are some examples of good and bad names:</p> 901 902 <div class="doc_code"> 903 <pre> 904 class VehicleMaker { 905 ... 906 Factory<Tire> F; // Bad -- abbreviation and non-descriptive. 907 Factory<Tire> Factory; // Better. 908 Factory<Tire> TireFactory; // Even better -- if VehicleMaker has more than one 909 // kind of factories. 910 }; 911 912 Vehicle MakeVehicle(VehicleType Type) { 913 VehicleMaker M; // Might be OK if having a short life-span. 914 Tire tmp1 = M.makeTire(); // Bad -- 'tmp1' provides no information. 915 Light headlight = M.makeLight("head"); // Good -- descriptive. 916 ... 917 } 918 </pre> 919 </div> 920 921 </div> 922 923 924 <!-- _______________________________________________________________________ --> 925 <h4> 926 <a name="ll_assert">Assert Liberally</a> 927 </h4> 928 929 <div> 930 931 <p>Use the "<tt>assert</tt>" macro to its fullest. Check all of your 932 preconditions and assumptions, you never know when a bug (not necessarily even 933 yours) might be caught early by an assertion, which reduces debugging time 934 dramatically. The "<tt><cassert></tt>" header file is probably already 935 included by the header files you are using, so it doesn't cost anything to use 936 it.</p> 937 938 <p>To further assist with debugging, make sure to put some kind of error message 939 in the assertion statement, which is printed if the assertion is tripped. This 940 helps the poor debugger make sense of why an assertion is being made and 941 enforced, and hopefully what to do about it. Here is one complete example:</p> 942 943 <div class="doc_code"> 944 <pre> 945 inline Value *getOperand(unsigned i) { 946 assert(i < Operands.size() && "getOperand() out of range!"); 947 return Operands[i]; 948 } 949 </pre> 950 </div> 951 952 <p>Here are more examples:</p> 953 954 <div class="doc_code"> 955 <pre> 956 assert(Ty->isPointerType() && "Can't allocate a non pointer type!"); 957 958 assert((Opcode == Shl || Opcode == Shr) && "ShiftInst Opcode invalid!"); 959 960 assert(idx < getNumSuccessors() && "Successor # out of range!"); 961 962 assert(V1.getType() == V2.getType() && "Constant types must be identical!"); 963 964 assert(isa<PHINode>(Succ->front()) && "Only works on PHId BBs!"); 965 </pre> 966 </div> 967 968 <p>You get the idea.</p> 969 970 <p>Please be aware that, when adding assert statements, not all compilers are aware of 971 the semantics of the assert. In some places, asserts are used to indicate a piece of 972 code that should not be reached. These are typically of the form:</p> 973 974 <div class="doc_code"> 975 <pre> 976 assert(0 && "Some helpful error message"); 977 </pre> 978 </div> 979 980 <p>When used in a function that returns a value, they should be followed with a return 981 statement and a comment indicating that this line is never reached. This will prevent 982 a compiler which is unable to deduce that the assert statement never returns from 983 generating a warning.</p> 984 985 <div class="doc_code"> 986 <pre> 987 assert(0 && "Some helpful error message"); 988 // Not reached 989 return 0; 990 </pre> 991 </div> 992 993 <p>Another issue is that values used only by assertions will produce an "unused 994 value" warning when assertions are disabled. For example, this code will 995 warn:</p> 996 997 <div class="doc_code"> 998 <pre> 999 unsigned Size = V.size(); 1000 assert(Size > 42 && "Vector smaller than it should be"); 1001 1002 bool NewToSet = Myset.insert(Value); 1003 assert(NewToSet && "The value shouldn't be in the set yet"); 1004 </pre> 1005 </div> 1006 1007 <p>These are two interesting different cases. In the first case, the call to 1008 V.size() is only useful for the assert, and we don't want it executed when 1009 assertions are disabled. Code like this should move the call into the assert 1010 itself. In the second case, the side effects of the call must happen whether 1011 the assert is enabled or not. In this case, the value should be cast to void to 1012 disable the warning. To be specific, it is preferred to write the code like 1013 this:</p> 1014 1015 <div class="doc_code"> 1016 <pre> 1017 assert(V.size() > 42 && "Vector smaller than it should be"); 1018 1019 bool NewToSet = Myset.insert(Value); (void)NewToSet; 1020 assert(NewToSet && "The value shouldn't be in the set yet"); 1021 </pre> 1022 </div> 1023 1024 1025 </div> 1026 1027 <!-- _______________________________________________________________________ --> 1028 <h4> 1029 <a name="ll_ns_std">Do Not Use '<tt>using namespace std</tt>'</a> 1030 </h4> 1031 1032 <div> 1033 1034 <p>In LLVM, we prefer to explicitly prefix all identifiers from the standard 1035 namespace with an "<tt>std::</tt>" prefix, rather than rely on 1036 "<tt>using namespace std;</tt>".</p> 1037 1038 <p> In header files, adding a '<tt>using namespace XXX</tt>' directive pollutes 1039 the namespace of any source file that <tt>#include</tt>s the header. This is 1040 clearly a bad thing.</p> 1041 1042 <p>In implementation files (e.g. <tt>.cpp</tt> files), the rule is more of a stylistic 1043 rule, but is still important. Basically, using explicit namespace prefixes 1044 makes the code <b>clearer</b>, because it is immediately obvious what facilities 1045 are being used and where they are coming from. And <b>more portable</b>, because 1046 namespace clashes cannot occur between LLVM code and other namespaces. The 1047 portability rule is important because different standard library implementations 1048 expose different symbols (potentially ones they shouldn't), and future revisions 1049 to the C++ standard will add more symbols to the <tt>std</tt> namespace. As 1050 such, we never use '<tt>using namespace std;</tt>' in LLVM.</p> 1051 1052 <p>The exception to the general rule (i.e. it's not an exception for 1053 the <tt>std</tt> namespace) is for implementation files. For example, all of 1054 the code in the LLVM project implements code that lives in the 'llvm' namespace. 1055 As such, it is ok, and actually clearer, for the <tt>.cpp</tt> files to have a 1056 '<tt>using namespace llvm;</tt>' directive at the top, after the 1057 <tt>#include</tt>s. This reduces indentation in the body of the file for source 1058 editors that indent based on braces, and keeps the conceptual context cleaner. 1059 The general form of this rule is that any <tt>.cpp</tt> file that implements 1060 code in any namespace may use that namespace (and its parents'), but should not 1061 use any others.</p> 1062 1063 </div> 1064 1065 <!-- _______________________________________________________________________ --> 1066 <h4> 1067 <a name="ll_virtual_anch"> 1068 Provide a Virtual Method Anchor for Classes in Headers 1069 </a> 1070 </h4> 1071 1072 <div> 1073 1074 <p>If a class is defined in a header file and has a v-table (either it has 1075 virtual methods or it derives from classes with virtual methods), it must 1076 always have at least one out-of-line virtual method in the class. Without 1077 this, the compiler will copy the vtable and RTTI into every <tt>.o</tt> file 1078 that <tt>#include</tt>s the header, bloating <tt>.o</tt> file sizes and 1079 increasing link times.</p> 1080 1081 </div> 1082 1083 <!-- _______________________________________________________________________ --> 1084 <h4> 1085 <a name="ll_end">Don't evaluate <tt>end()</tt> every time through a loop</a> 1086 </h4> 1087 1088 <div> 1089 1090 <p>Because C++ doesn't have a standard "<tt>foreach</tt>" loop (though it can be 1091 emulated with macros and may be coming in C++'0x) we end up writing a lot of 1092 loops that manually iterate from begin to end on a variety of containers or 1093 through other data structures. One common mistake is to write a loop in this 1094 style:</p> 1095 1096 <div class="doc_code"> 1097 <pre> 1098 BasicBlock *BB = ... 1099 for (BasicBlock::iterator I = BB->begin(); I != <b>BB->end()</b>; ++I) 1100 ... use I ... 1101 </pre> 1102 </div> 1103 1104 <p>The problem with this construct is that it evaluates "<tt>BB->end()</tt>" 1105 every time through the loop. Instead of writing the loop like this, we strongly 1106 prefer loops to be written so that they evaluate it once before the loop starts. 1107 A convenient way to do this is like so:</p> 1108 1109 <div class="doc_code"> 1110 <pre> 1111 BasicBlock *BB = ... 1112 for (BasicBlock::iterator I = BB->begin(), E = <b>BB->end()</b>; I != E; ++I) 1113 ... use I ... 1114 </pre> 1115 </div> 1116 1117 <p>The observant may quickly point out that these two loops may have different 1118 semantics: if the container (a basic block in this case) is being mutated, then 1119 "<tt>BB->end()</tt>" may change its value every time through the loop and the 1120 second loop may not in fact be correct. If you actually do depend on this 1121 behavior, please write the loop in the first form and add a comment indicating 1122 that you did it intentionally.</p> 1123 1124 <p>Why do we prefer the second form (when correct)? Writing the loop in the 1125 first form has two problems. First it may be less efficient than evaluating it 1126 at the start of the loop. In this case, the cost is probably minor — a 1127 few extra loads every time through the loop. However, if the base expression is 1128 more complex, then the cost can rise quickly. I've seen loops where the end 1129 expression was actually something like: "<tt>SomeMap[x]->end()</tt>" and map 1130 lookups really aren't cheap. By writing it in the second form consistently, you 1131 eliminate the issue entirely and don't even have to think about it.</p> 1132 1133 <p>The second (even bigger) issue is that writing the loop in the first form 1134 hints to the reader that the loop is mutating the container (a fact that a 1135 comment would handily confirm!). If you write the loop in the second form, it 1136 is immediately obvious without even looking at the body of the loop that the 1137 container isn't being modified, which makes it easier to read the code and 1138 understand what it does.</p> 1139 1140 <p>While the second form of the loop is a few extra keystrokes, we do strongly 1141 prefer it.</p> 1142 1143 </div> 1144 1145 <!-- _______________________________________________________________________ --> 1146 <h4> 1147 <a name="ll_iostream"><tt>#include <iostream></tt> is Forbidden</a> 1148 </h4> 1149 1150 <div> 1151 1152 <p>The use of <tt>#include <iostream></tt> in library files is 1153 hereby <b><em>forbidden</em></b>. The primary reason for doing this is to 1154 support clients using LLVM libraries as part of larger systems. In particular, 1155 we statically link LLVM into some dynamic libraries. Even if LLVM isn't used, 1156 the static constructors are run whenever an application starts up that uses the 1157 dynamic library. There are two problems with this:</p> 1158 1159 <ol> 1160 <li>The time to run the static c'tors impacts startup time of applications 1161 — a critical time for GUI apps.</li> 1162 1163 <li>The static c'tors cause the app to pull many extra pages of memory off the 1164 disk: both the code for the static c'tors in each <tt>.o</tt> file and the 1165 small amount of data that gets touched. In addition, touched/dirty pages 1166 put more pressure on the VM system on low-memory machines.</li> 1167 </ol> 1168 1169 <p>Note that using the other stream headers (<tt><sstream></tt> for 1170 example) is not problematic in this regard — 1171 just <tt><iostream></tt>. However, <tt>raw_ostream</tt> provides various 1172 APIs that are better performing for almost every use than <tt>std::ostream</tt> 1173 style APIs. <b>Therefore new code should always 1174 use <a href="#ll_raw_ostream"><tt>raw_ostream</tt></a> for writing, or 1175 the <tt>llvm::MemoryBuffer</tt> API for reading files.</b></p> 1176 1177 </div> 1178 1179 1180 <!-- _______________________________________________________________________ --> 1181 <h4> 1182 <a name="ll_raw_ostream">Use <tt>raw_ostream</tt></a> 1183 </h4> 1184 1185 <div> 1186 1187 <p>LLVM includes a lightweight, simple, and efficient stream implementation 1188 in <tt>llvm/Support/raw_ostream.h</tt>, which provides all of the common 1189 features of <tt>std::ostream</tt>. All new code should use <tt>raw_ostream</tt> 1190 instead of <tt>ostream</tt>.</p> 1191 1192 <p>Unlike <tt>std::ostream</tt>, <tt>raw_ostream</tt> is not a template and can 1193 be forward declared as <tt>class raw_ostream</tt>. Public headers should 1194 generally not include the <tt>raw_ostream</tt> header, but use forward 1195 declarations and constant references to <tt>raw_ostream</tt> instances.</p> 1196 1197 </div> 1198 1199 1200 <!-- _______________________________________________________________________ --> 1201 <h4> 1202 <a name="ll_avoidendl">Avoid <tt>std::endl</tt></a> 1203 </h4> 1204 1205 <div> 1206 1207 <p>The <tt>std::endl</tt> modifier, when used with <tt>iostreams</tt> outputs a 1208 newline to the output stream specified. In addition to doing this, however, it 1209 also flushes the output stream. In other words, these are equivalent:</p> 1210 1211 <div class="doc_code"> 1212 <pre> 1213 std::cout << std::endl; 1214 std::cout << '\n' << std::flush; 1215 </pre> 1216 </div> 1217 1218 <p>Most of the time, you probably have no reason to flush the output stream, so 1219 it's better to use a literal <tt>'\n'</tt>.</p> 1220 1221 </div> 1222 1223 </div> 1224 1225 <!-- ======================================================================= --> 1226 <h3> 1227 <a name="nano">Microscopic Details</a> 1228 </h3> 1229 <!-- ======================================================================= --> 1230 1231 <div> 1232 1233 <p>This section describes preferred low-level formatting guidelines along with 1234 reasoning on why we prefer them.</p> 1235 1236 <!-- _______________________________________________________________________ --> 1237 <h4> 1238 <a name="micro_spaceparen">Spaces Before Parentheses</a> 1239 </h4> 1240 1241 <div> 1242 1243 <p>We prefer to put a space before an open parenthesis only in control flow 1244 statements, but not in normal function call expressions and function-like 1245 macros. For example, this is good:</p> 1246 1247 <div class="doc_code"> 1248 <pre> 1249 <b>if (</b>x) ... 1250 <b>for (</b>i = 0; i != 100; ++i) ... 1251 <b>while (</b>llvm_rocks) ... 1252 1253 <b>somefunc(</b>42); 1254 <b><a href="#ll_assert">assert</a>(</b>3 != 4 && "laws of math are failing me"); 1255 1256 a = <b>foo(</b>42, 92) + <b>bar(</b>x); 1257 </pre> 1258 </div> 1259 1260 <p>and this is bad:</p> 1261 1262 <div class="doc_code"> 1263 <pre> 1264 <b>if(</b>x) ... 1265 <b>for(</b>i = 0; i != 100; ++i) ... 1266 <b>while(</b>llvm_rocks) ... 1267 1268 <b>somefunc (</b>42); 1269 <b><a href="#ll_assert">assert</a> (</b>3 != 4 && "laws of math are failing me"); 1270 1271 a = <b>foo (</b>42, 92) + <b>bar (</b>x); 1272 </pre> 1273 </div> 1274 1275 <p>The reason for doing this is not completely arbitrary. This style makes 1276 control flow operators stand out more, and makes expressions flow better. The 1277 function call operator binds very tightly as a postfix operator. Putting a 1278 space after a function name (as in the last example) makes it appear that the 1279 code might bind the arguments of the left-hand-side of a binary operator with 1280 the argument list of a function and the name of the right side. More 1281 specifically, it is easy to misread the "a" example as:</p> 1282 1283 <div class="doc_code"> 1284 <pre> 1285 a = foo <b>(</b>(42, 92) + bar<b>)</b> (x); 1286 </pre> 1287 </div> 1288 1289 <p>when skimming through the code. By avoiding a space in a function, we avoid 1290 this misinterpretation.</p> 1291 1292 </div> 1293 1294 <!-- _______________________________________________________________________ --> 1295 <h4> 1296 <a name="micro_preincrement">Prefer Preincrement</a> 1297 </h4> 1298 1299 <div> 1300 1301 <p>Hard fast rule: Preincrement (<tt>++X</tt>) may be no slower than 1302 postincrement (<tt>X++</tt>) and could very well be a lot faster than it. Use 1303 preincrementation whenever possible.</p> 1304 1305 <p>The semantics of postincrement include making a copy of the value being 1306 incremented, returning it, and then preincrementing the "work value". For 1307 primitive types, this isn't a big deal... but for iterators, it can be a huge 1308 issue (for example, some iterators contains stack and set objects in them... 1309 copying an iterator could invoke the copy ctor's of these as well). In general, 1310 get in the habit of always using preincrement, and you won't have a problem.</p> 1311 1312 </div> 1313 1314 <!-- _______________________________________________________________________ --> 1315 <h4> 1316 <a name="micro_namespaceindent">Namespace Indentation</a> 1317 </h4> 1318 1319 <div> 1320 1321 <p> 1322 In general, we strive to reduce indentation wherever possible. This is useful 1323 because we want code to <a href="#scf_codewidth">fit into 80 columns</a> without 1324 wrapping horribly, but also because it makes it easier to understand the code. 1325 Namespaces are a funny thing: they are often large, and we often desire to put 1326 lots of stuff into them (so they can be large). Other times they are tiny, 1327 because they just hold an enum or something similar. In order to balance this, 1328 we use different approaches for small versus large namespaces. 1329 </p> 1330 1331 <p> 1332 If a namespace definition is small and <em>easily</em> fits on a screen (say, 1333 less than 35 lines of code), then you should indent its body. Here's an 1334 example: 1335 </p> 1336 1337 <div class="doc_code"> 1338 <pre> 1339 namespace llvm { 1340 namespace X86 { 1341 /// RelocationType - An enum for the x86 relocation codes. Note that 1342 /// the terminology here doesn't follow x86 convention - word means 1343 /// 32-bit and dword means 64-bit. 1344 enum RelocationType { 1345 /// reloc_pcrel_word - PC relative relocation, add the relocated value to 1346 /// the value already in memory, after we adjust it for where the PC is. 1347 reloc_pcrel_word = 0, 1348 1349 /// reloc_picrel_word - PIC base relative relocation, add the relocated 1350 /// value to the value already in memory, after we adjust it for where the 1351 /// PIC base is. 1352 reloc_picrel_word = 1, 1353 1354 /// reloc_absolute_word, reloc_absolute_dword - Absolute relocation, just 1355 /// add the relocated value to the value already in memory. 1356 reloc_absolute_word = 2, 1357 reloc_absolute_dword = 3 1358 }; 1359 } 1360 } 1361 </pre> 1362 </div> 1363 1364 <p>Since the body is small, indenting adds value because it makes it very clear 1365 where the namespace starts and ends, and it is easy to take the whole thing in 1366 in one "gulp" when reading the code. If the blob of code in the namespace is 1367 larger (as it typically is in a header in the <tt>llvm</tt> or <tt>clang</tt> namespaces), do not 1368 indent the code, and add a comment indicating what namespace is being closed. 1369 For example:</p> 1370 1371 <div class="doc_code"> 1372 <pre> 1373 namespace llvm { 1374 namespace knowledge { 1375 1376 /// Grokable - This class represents things that Smith can have an intimate 1377 /// understanding of and contains the data associated with it. 1378 class Grokable { 1379 ... 1380 public: 1381 explicit Grokable() { ... } 1382 virtual ~Grokable() = 0; 1383 1384 ... 1385 1386 }; 1387 1388 } // end namespace knowledge 1389 } // end namespace llvm 1390 </pre> 1391 </div> 1392 1393 <p>Because the class is large, we don't expect that the reader can easily 1394 understand the entire concept in a glance, and the end of the file (where the 1395 namespaces end) may be a long ways away from the place they open. As such, 1396 indenting the contents of the namespace doesn't add any value, and detracts from 1397 the readability of the class. In these cases it is best to <em>not</em> indent 1398 the contents of the namespace.</p> 1399 1400 </div> 1401 1402 <!-- _______________________________________________________________________ --> 1403 <h4> 1404 <a name="micro_anonns">Anonymous Namespaces</a> 1405 </h4> 1406 1407 <div> 1408 1409 <p>After talking about namespaces in general, you may be wondering about 1410 anonymous namespaces in particular. 1411 Anonymous namespaces are a great language feature that tells the C++ compiler 1412 that the contents of the namespace are only visible within the current 1413 translation unit, allowing more aggressive optimization and eliminating the 1414 possibility of symbol name collisions. Anonymous namespaces are to C++ as 1415 "static" is to C functions and global variables. While "static" is available 1416 in C++, anonymous namespaces are more general: they can make entire classes 1417 private to a file.</p> 1418 1419 <p>The problem with anonymous namespaces is that they naturally want to 1420 encourage indentation of their body, and they reduce locality of reference: if 1421 you see a random function definition in a C++ file, it is easy to see if it is 1422 marked static, but seeing if it is in an anonymous namespace requires scanning 1423 a big chunk of the file.</p> 1424 1425 <p>Because of this, we have a simple guideline: make anonymous namespaces as 1426 small as possible, and only use them for class declarations. For example, this 1427 is good:</p> 1428 1429 <div class="doc_code"> 1430 <pre> 1431 <b>namespace {</b> 1432 class StringSort { 1433 ... 1434 public: 1435 StringSort(...) 1436 bool operator<(const char *RHS) const; 1437 }; 1438 <b>} // end anonymous namespace</b> 1439 1440 static void Helper() { 1441 ... 1442 } 1443 1444 bool StringSort::operator<(const char *RHS) const { 1445 ... 1446 } 1447 1448 </pre> 1449 </div> 1450 1451 <p>This is bad:</p> 1452 1453 1454 <div class="doc_code"> 1455 <pre> 1456 <b>namespace {</b> 1457 class StringSort { 1458 ... 1459 public: 1460 StringSort(...) 1461 bool operator<(const char *RHS) const; 1462 }; 1463 1464 void Helper() { 1465 ... 1466 } 1467 1468 bool StringSort::operator<(const char *RHS) const { 1469 ... 1470 } 1471 1472 <b>} // end anonymous namespace</b> 1473 1474 </pre> 1475 </div> 1476 1477 1478 <p>This is bad specifically because if you're looking at "Helper" in the middle 1479 of a large C++ file, that you have no immediate way to tell if it is local to 1480 the file. When it is marked static explicitly, this is immediately obvious. 1481 Also, there is no reason to enclose the definition of "operator<" in the 1482 namespace just because it was declared there. 1483 </p> 1484 1485 </div> 1486 1487 </div> 1488 1489 </div> 1490 1491 <!-- *********************************************************************** --> 1492 <h2> 1493 <a name="seealso">See Also</a> 1494 </h2> 1495 <!-- *********************************************************************** --> 1496 1497 <div> 1498 1499 <p>A lot of these comments and recommendations have been culled for other 1500 sources. Two particularly important books for our work are:</p> 1501 1502 <ol> 1503 1504 <li><a href="http://www.amazon.com/Effective-Specific-Addison-Wesley-Professional-Computing/dp/0321334876">Effective 1505 C++</a> by Scott Meyers. Also 1506 interesting and useful are "More Effective C++" and "Effective STL" by the same 1507 author.</li> 1508 1509 <li>Large-Scale C++ Software Design by John Lakos</li> 1510 1511 </ol> 1512 1513 <p>If you get some free time, and you haven't read them: do so, you might learn 1514 something.</p> 1515 1516 </div> 1517 1518 <!-- *********************************************************************** --> 1519 1520 <hr> 1521 <address> 1522 <a href="http://jigsaw.w3.org/css-validator/check/referer"><img 1523 src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a> 1524 <a href="http://validator.w3.org/check/referer"><img 1525 src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a> 1526 1527 <a href="mailto:sabre (a] nondot.org">Chris Lattner</a><br> 1528 <a href="http://llvm.org/">LLVM Compiler Infrastructure</a><br> 1529 Last modified: $Date$ 1530 </address> 1531 1532 </body> 1533 </html> 1534
