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   9
  10 <div class="doc_title">
  11   LLVM Coding Standards
  12 </div>
  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_class_struct">Use of class/struct Keywords</a></li>
  33         </ol></li>
  34     </ol></li>
  35   <li><a href="#styleissues">Style Issues</a>
  36     <ol>
  37       <li><a href="#macro">The High Level Issues</a>
  38         <ol>
  39           <li><a href="#hl_module">A Public Header File <b>is</b> a
  40               Module</a></li>
  41           <li><a href="#hl_dontinclude">#include as Little as Possible</a></li>
  42           <li><a href="#hl_privateheaders">Keep "internal" Headers
  43               Private</a></li>
  44           <li><a href="#ll_iostream"><tt>#include &lt;iostream&gt;</tt> is
  45               <em>forbidden</em></a></li>
  46         </ol></li>
  47       <li><a href="#micro">The Low Level Issues</a>
  48         <ol>
  49           <li><a href="#ll_assert">Assert Liberally</a></li>
  50           <li><a href="#ll_ns_std">Do not use 'using namespace std'</a></li>
  51           <li><a href="#ll_virtual_anch">Provide a virtual method anchor for
  52               classes in headers</a></li>
  53           <li><a href="#ll_preincrement">Prefer Preincrement</a></li>
  54           <li><a href="#ll_avoidendl">Avoid <tt>std::endl</tt></a></li>
  55         </ol></li>
  56     </ol></li>
  57   <li><a href="#seealso">See Also</a></li>
  58 </ol>
  59
  60 <div class="doc_author">
  61   <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a> and
  62      <a href="mailto:void@nondot.org">Bill Wendling</a></p>
  63 </div>
  64
  65
  66 <!-- *********************************************************************** -->
  67 <div class="doc_section">
  68   <a name="introduction">Introduction</a>
  69 </div>
  70 <!-- *********************************************************************** -->
  71
  72 <div class="doc_text">
  73
  74 <p>This document attempts to describe a few coding standards that are being used
  75 in the LLVM source tree.  Although no coding standards should be regarded as
  76 absolute requirements to be followed in all instances, coding standards can be
  77 useful.</p>
  78
  79 <p>This document intentionally does not prescribe fixed standards for religious
  80 issues such as brace placement and space usage.  For issues like this, follow
  81 the golden rule:</p>
  82
  83 <blockquote>
  84
  85 <p><b><a name="goldenrule">If you are adding a significant body of source to a
  86 project, feel free to use whatever style you are most comfortable with.  If you
  87 are extending, enhancing, or bug fixing already implemented code, use the style
  88 that is already being used so that the source is uniform and easy to
  89 follow.</a></b></p>
  90
  91 </blockquote>
  92
  93 <p>The ultimate goal of these guidelines is the increase readability and
  94 maintainability of our common source base. If you have suggestions for topics to
  95 be included, please mail them to <a
  96 href="mailto:sabre@nondot.org">Chris</a>.</p>
  97
  98 </div>
  99
 100 <!-- *********************************************************************** -->
 101 <div class="doc_section">
 102   <a name="mechanicalissues">Mechanical Source Issues</a>
 103 </div>
 104 <!-- *********************************************************************** -->
 105
 106 <!-- ======================================================================= -->
 107 <div class="doc_subsection">
 108   <a name="sourceformating">Source Code Formatting</a>
 109 </div>
 110
 111 <!-- _______________________________________________________________________ -->
 112 <div class="doc_subsubsection">
 113   <a name="scf_commenting">Commenting</a>
 114 </div>
 115
 116 <div class="doc_text">
 117
 118 <p>Comments are one critical part of readability and maintainability.  Everyone
 119 knows they should comment, so should you.  Although we all should probably
 120 comment our code more than we do, there are a few very critical places that
 121 documentation is very useful:</p>
 122
 123 <b>File Headers</b>
 124
 125 <p>Every source file should have a header on it that describes the basic
 126 purpose of the file.  If a file does not have a header, it should not be
 127 checked into Subversion.  Most source trees will probably have a standard
 128 file header format.  The standard format for the LLVM source tree looks like
 129 this:</p>
 130
 131 <div class="doc_code">
 132 <pre>
 133 //===-- llvm/Instruction.h - Instruction class definition -------*- C++ -*-===//
 134 //
 135 //                     The LLVM Compiler Infrastructure
 136 //
 137 // This file is distributed under the University of Illinois Open Source
 138 // License. See LICENSE.TXT for details.
 139 //
 140 //===----------------------------------------------------------------------===//
 141 //
 142 // This file contains the declaration of the Instruction class, which is the
 143 // base class for all of the VM instructions.
 144 //
 145 //===----------------------------------------------------------------------===//
 146 </pre>
 147 </div>
 148
 149 <p>A few things to note about this particular format:  The "<tt>-*- C++
 150 -*-</tt>" string on the first line is there to tell Emacs that the source file
 151 is a C++ file, not a C file (Emacs assumes .h files are C files by default).
 152 Note that this tag is not necessary in .cpp files.  The name of the file is also
 153 on the first line, along with a very short description of the purpose of the
 154 file.  This is important when printing out code and flipping though lots of
 155 pages.</p>
 156
 157 <p>The next section in the file is a concise note that defines the license
 158 that the file is released under.  This makes it perfectly clear what terms the
 159 source code can be distributed under and should not be modified in any way.</p>
 160
 161 <p>The main body of the description does not have to be very long in most cases.
 162 Here it's only two lines.  If an algorithm is being implemented or something
 163 tricky is going on, a reference to the paper where it is published should be
 164 included, as well as any notes or "gotchas" in the code to watch out for.</p>
 165
 166 <b>Class overviews</b>
 167
 168 <p>Classes are one fundamental part of a good object oriented design.  As such,
 169 a class definition should have a comment block that explains what the class is
 170 used for... if it's not obvious.  If it's so completely obvious your grandma
 171 could figure it out, it's probably safe to leave it out.  Naming classes
 172 something sane goes a long ways towards avoiding writing documentation.</p>
 173
 174
 175 <b>Method information</b>
 176
 177 <p>Methods defined in a class (as well as any global functions) should also be
 178 documented properly.  A quick note about what it does any a description of the
 179 borderline behaviour is all that is necessary here (unless something
 180 particularly tricky or insideous is going on).  The hope is that people can
 181 figure out how to use your interfaces without reading the code itself... that is
 182 the goal metric.</p>
 183
 184 <p>Good things to talk about here are what happens when something unexpected
 185 happens: does the method return null?  Abort?  Format your hard disk?</p>
 186
 187 </div>
 188
 189 <!-- _______________________________________________________________________ -->
 190 <div class="doc_subsubsection">
 191   <a name="scf_commentformat">Comment Formatting</a>
 192 </div>
 193
 194 <div class="doc_text">
 195
 196 <p>In general, prefer C++ style (<tt>//</tt>) comments.  They take less space,
 197 require less typing, don't have nesting problems, etc.  There are a few cases
 198 when it is useful to use C style (<tt>/* */</tt>) comments however:</p>
 199
 200 <ol>
 201   <li>When writing a C code: Obviously if you are writing C code, use C style
 202       comments.</li>
 203   <li>When writing a header file that may be <tt>#include</tt>d by a C source
 204       file.</li>
 205   <li>When writing a source file that is used by a tool that only accepts C
 206       style comments.</li>
 207 </ol>
 208
 209 <p>To comment out a large block of code, use <tt>#if 0</tt> and <tt>#endif</tt>.
 210 These nest properly and are better behaved in general than C style comments.</p>
 211
 212 </div>
 213
 214 <!-- _______________________________________________________________________ -->
 215 <div class="doc_subsubsection">
 216   <a name="scf_includes"><tt>#include</tt> Style</a>
 217 </div>
 218
 219 <div class="doc_text">
 220
 221 <p>Immediately after the <a href="#scf_commenting">header file comment</a> (and
 222 include guards if working on a header file), the <a
 223 href="#hl_dontinclude">minimal</a> list of <tt>#include</tt>s required by the
 224 file should be listed.  We prefer these <tt>#include</tt>s to be listed in this
 225 order:</p>
 226
 227 <ol>
 228   <li><a href="#mmheader">Main Module header</a></li>
 229   <li><a href="#hl_privateheaders">Local/Private Headers</a></li>
 230   <li><tt>llvm/*</tt></li>
 231   <li><tt>llvm/Analysis/*</tt></li>
 232   <li><tt>llvm/Assembly/*</tt></li>
 233   <li><tt>llvm/Bytecode/*</tt></li>
 234   <li><tt>llvm/CodeGen/*</tt></li>
 235   <li>...</li>
 236   <li><tt>Support/*</tt></li>
 237   <li><tt>Config/*</tt></li>
 238   <li>System <tt>#includes</tt></li>
 239 </ol>
 240
 241 <p>... and each category should be sorted by name.</p>
 242
 243 <p><a name="mmheader">The "Main Module Header"</a> file applies to .cpp file
 244 which implement an interface defined by a .h file.  This <tt>#include</tt>
 245 should always be included <b>first</b> regardless of where it lives on the file
 246 system.  By including a header file first in the .cpp files that implement the
 247 interfaces, we ensure that the header does not have any hidden dependencies
 248 which are not explicitly #included in the header, but should be.  It is also a
 249 form of documentation in the .cpp file to indicate where the interfaces it
 250 implements are defined.</p>
 251
 252 </div>
 253
 254 <!-- _______________________________________________________________________ -->
 255 <div class="doc_subsubsection">
 256   <a name="scf_codewidth">Source Code Width</a>
 257 </div>
 258
 259 <div class="doc_text">
 260
 261 <p>Write your code to fit within 80 columns of text.  This helps those of us who
 262 like to print out code and look at your code in an xterm without resizing
 263 it.</p>
 264
 265 <p>The longer answer is that there must be some limit to the width of the code
 266 in order to reasonably allow developers to have multiple files side-by-side in
 267 windows on a modest display.  If you are going to pick a width limit, it is
 268 somewhat arbitrary but you might as well pick something standard.  Going with
 269 90 columns (for example) instead of 80 columns wouldn't add any significant
 270 value and would be detrimental to printing out code.  Also many other projects
 271 have standardized on 80 columns, so some people have already configured their
 272 editors for it (vs something else, like 90 columns).</p>
 273
 274 <p>This is one of many contentious issues in coding standards, but is not up
 275 for debate.</p>
 276
 277 </div>
 278
 279 <!-- _______________________________________________________________________ -->
 280 <div class="doc_subsubsection">
 281   <a name="scf_spacestabs">Use Spaces Instead of Tabs</a>
 282 </div>
 283
 284 <div class="doc_text">
 285
 286 <p>In all cases, prefer spaces to tabs in source files.  People have different
 287 prefered indentation levels, and different styles of indentation that they
 288 like... this is fine.  What isn't is that different editors/viewers expand tabs
 289 out to different tab stops.  This can cause your code to look completely
 290 unreadable, and it is not worth dealing with.</p>
 291
 292 <p>As always, follow the <a href="#goldenrule">Golden Rule</a> above: follow the
 293 style of existing code if your are modifying and extending it.  If you like four
 294 spaces of indentation, <b>DO NOT</b> do that in the middle of a chunk of code
 295 with two spaces of indentation.  Also, do not reindent a whole source file: it
 296 makes for incredible diffs that are absolutely worthless.</p>
 297
 298 </div>
 299
 300 <!-- _______________________________________________________________________ -->
 301 <div class="doc_subsubsection">
 302   <a name="scf_indentation">Indent Code Consistently</a>
 303 </div>
 304
 305 <div class="doc_text">
 306
 307 <p>Okay, your first year of programming you were told that indentation is
 308 important.  If you didn't believe and internalize this then, now is the time.
 309 Just do it.</p>
 310
 311 </div>
 312
 313
 314 <!-- ======================================================================= -->
 315 <div class="doc_subsection">
 316   <a name="compilerissues">Compiler Issues</a>
 317 </div>
 318
 319
 320 <!-- _______________________________________________________________________ -->
 321 <div class="doc_subsubsection">
 322   <a name="ci_warningerrors">Treat Compiler Warnings Like Errors</a>
 323 </div>
 324
 325 <div class="doc_text">
 326
 327 <p>If your code has compiler warnings in it, something is wrong: you aren't
 328 casting values correctly, your have "questionable" constructs in your code, or
 329 you are doing something legitimately wrong.  Compiler warnings can cover up
 330 legitimate errors in output and make dealing with a translation unit
 331 difficult.</p>
 332
 333 <p>It is not possible to prevent all warnings from all compilers, nor is it
 334 desirable.  Instead, pick a standard compiler (like <tt>gcc</tt>) that provides
 335 a good thorough set of warnings, and stick to them.  At least in the case of
 336 <tt>gcc</tt>, it is possible to work around any spurious errors by changing the
 337 syntax of the code slightly.  For example, an warning that annoys me occurs when
 338 I write code like this:</p>
 339
 340 <div class="doc_code">
 341 <pre>
 342 if (V = getValue()) {
 343   ...
 344 }
 345 </pre>
 346 </div>
 347
 348 <p><tt>gcc</tt> will warn me that I probably want to use the <tt>==</tt>
 349 operator, and that I probably mistyped it.  In most cases, I haven't, and I
 350 really don't want the spurious errors.  To fix this particular problem, I
 351 rewrite the code like this:</p>
 352
 353 <div class="doc_code">
 354 <pre>
 355 if ((V = getValue())) {
 356   ...
 357 }
 358 </pre>
 359 </div>
 360
 361 <p>...which shuts <tt>gcc</tt> up.  Any <tt>gcc</tt> warning that annoys you can
 362 be fixed by massaging the code appropriately.</p>
 363
 364 <p>These are the <tt>gcc</tt> warnings that I prefer to enable: <tt>-Wall
 365 -Winline -W -Wwrite-strings -Wno-unused</tt></p>
 366
 367 </div>
 368
 369 <!-- _______________________________________________________________________ -->
 370 <div class="doc_subsubsection">
 371   <a name="ci_portable_code">Write Portable Code</a>
 372 </div>
 373
 374 <div class="doc_text">
 375
 376 <p>In almost all cases, it is possible and within reason to write completely
 377 portable code.  If there are cases where it isn't possible to write portable
 378 code, isolate it behind a well defined (and well documented) interface.</p>
 379
 380 <p>In practice, this means that you shouldn't assume much about the host
 381 compiler, including its support for "high tech" features like partial
 382 specialization of templates.  If these features are used, they should only be
 383 an implementation detail of a library which has a simple exposed API.</p>
 384
 385 </div>
 386
 387 <!-- _______________________________________________________________________ -->
 388 <div class="doc_subsubsection">
 389 <a name="ci_class_struct">Use of <tt>class</tt> and <tt>struct</tt> Keywords</a>
 390 </div>
 391 <div class="doc_text">
 392
 393 <p>In C++, the <tt>class</tt> and <tt>struct</tt> keywords can be used almost
 394 interchangeably. The only difference is when they are used to declare a class:
 395 <tt>class</tt> makes all members private by default while <tt>struct</tt> makes
 396 all members public by default.</p>
 397
 398 <p>Unfortunately, not all compilers follow the rules and some will generate
 399 different symbols based on whether <tt>class</tt> or <tt>struct</tt> was used to
 400 declare the symbol.  This can lead to problems at link time.</p>
 401
 402 <p>So, the rule for LLVM is to always use the <tt>class</tt> keyword, unless
 403 <b>all</b> members are public, in which case <tt>struct</tt> is allowed.</p>
 404
 405 </div>
 406
 407 <!-- *********************************************************************** -->
 408 <div class="doc_section">
 409   <a name="styleissues">Style Issues</a>
 410 </div>
 411 <!-- *********************************************************************** -->
 412
 413
 414 <!-- ======================================================================= -->
 415 <div class="doc_subsection">
 416   <a name="macro">The High Level Issues</a>
 417 </div>
 418
 419
 420 <!-- _______________________________________________________________________ -->
 421 <div class="doc_subsubsection">
 422   <a name="hl_module">A Public Header File <b>is</b> a Module</a>
 423 </div>
 424
 425 <div class="doc_text">
 426
 427 <p>C++ doesn't do too well in the modularity department.  There is no real
 428 encapsulation or data hiding (unless you use expensive protocol classes), but it
 429 is what we have to work with.  When you write a public header file (in the LLVM
 430 source tree, they live in the top level "include" directory), you are defining a
 431 module of functionality.</p>
 432
 433 <p>Ideally, modules should be completely independent of each other, and their
 434 header files should only include the absolute minimum number of headers
 435 possible. A module is not just a class, a function, or a namespace: <a
 436 href="http://www.cuj.com/articles/2000/0002/0002c/0002c.htm">it's a collection
 437 of these</a> that defines an interface.  This interface may be several
 438 functions, classes or data structures, but the important issue is how they work
 439 together.</p>
 440
 441 <p>In general, a module should be implemented with one or more <tt>.cpp</tt>
 442 files.  Each of these <tt>.cpp</tt> files should include the header that defines
 443 their interface first.  This ensure that all of the dependences of the module
 444 header have been properly added to the module header itself, and are not
 445 implicit.  System headers should be included after user headers for a
 446 translation unit.</p>
 447
 448 </div>
 449
 450 <!-- _______________________________________________________________________ -->
 451 <div class="doc_subsubsection">
 452   <a name="hl_dontinclude"><tt>#include</tt> as Little as Possible</a>
 453 </div>
 454
 455 <div class="doc_text">
 456
 457 <p><tt>#include</tt> hurts compile time performance.  Don't do it unless you
 458 have to, especially in header files.</p>
 459
 460 <p>But wait, sometimes you need to have the definition of a class to use it, or
 461 to inherit from it.  In these cases go ahead and <tt>#include</tt> that header
 462 file.  Be aware however that there are many cases where you don't need to have
 463 the full definition of a class.  If you are using a pointer or reference to a
 464 class, you don't need the header file.  If you are simply returning a class
 465 instance from a prototyped function or method, you don't need it.  In fact, for
 466 most cases, you simply don't need the definition of a class... and not
 467 <tt>#include</tt>'ing speeds up compilation.</p>
 468
 469 <p>It is easy to try to go too overboard on this recommendation, however.  You
 470 <b>must</b> include all of the header files that you are using -- you can
 471 include them either directly
 472 or indirectly (through another header file).  To make sure that you don't
 473 accidently forget to include a header file in your module header, make sure to
 474 include your module header <b>first</b> in the implementation file (as mentioned
 475 above).  This way there won't be any hidden dependencies that you'll find out
 476 about later...</p>
 477
 478 </div>
 479
 480 <!-- _______________________________________________________________________ -->
 481 <div class="doc_subsubsection">
 482   <a name="hl_privateheaders">Keep "internal" Headers Private</a>
 483 </div>
 484
 485 <div class="doc_text">
 486
 487 <p>Many modules have a complex implementation that causes them to use more than
 488 one implementation (<tt>.cpp</tt>) file.  It is often tempting to put the
 489 internal communication interface (helper classes, extra functions, etc) in the
 490 public module header file.  Don't do this.</p>
 491
 492 <p>If you really need to do something like this, put a private header file in
 493 the same directory as the source files, and include it locally.  This ensures
 494 that your private interface remains private and undisturbed by outsiders.</p>
 495
 496 <p>Note however, that it's okay to put extra implementation methods a public
 497 class itself... just make them private (or protected), and all is well.</p>
 498
 499 </div>
 500
 501 <!-- _______________________________________________________________________ -->
 502 <div class="doc_subsubsection">
 503   <a name="ll_iostream"><tt>#include &lt;iostream&gt;</tt> is forbidden</a>
 504 </div>
 505
 506 <div class="doc_text">
 507
 508 <p>The use of <tt>#include &lt;iostream&gt;</tt> in library files is
 509 hereby <b><em>forbidden</em></b>. The primary reason for doing this is to
 510 support clients using LLVM libraries as part of larger systems. In particular,
 511 we statically link LLVM into some dynamic libraries. Even if LLVM isn't used,
 512 the static c'tors are run whenever an application start up that uses the dynamic
 513 library. There are two problems with this:</p>
 514
 515 <ol>
 516   <li>The time to run the static c'tors impacts startup time of
 517       applications&mdash;a critical time for GUI apps.</li>
 518   <li>The static c'tors cause the app to pull many extra pages of memory off the
 519       disk: both the code for the static c'tors in each <tt>.o</tt> file and the
 520       small amount of data that gets touched. In addition, touched/dirty pages
 521       put more pressure on the VM system on low-memory machines.</li>
 522 </ol>
 523
 524 <p>Note that using the other stream headers (<tt>&lt;sstream&gt;</tt> for
 525 example) is allowed normally, it is just <tt>&lt;iostream&gt;</tt> that is
 526 causing problems.</p>
 527
 528 <p>The preferred replacement for stream functionality is the
 529 <tt>llvm::raw_ostream</tt> class (for writing to output streams of various
 530 sorts) and the <tt>llvm::MemoryBuffer</tt> API (for reading in files).</p>
 531
 532 </div>
 533
 534
 535 <!-- ======================================================================= -->
 536 <div class="doc_subsection">
 537   <a name="micro">The Low Level Issues</a>
 538 </div>
 539
 540
 541 <!-- _______________________________________________________________________ -->
 542 <div class="doc_subsubsection">
 543   <a name="ll_assert">Assert Liberally</a>
 544 </div>
 545
 546 <div class="doc_text">
 547
 548 <p>Use the "<tt>assert</tt>" function to its fullest.  Check all of your
 549 preconditions and assumptions, you never know when a bug (not neccesarily even
 550 yours) might be caught early by an assertion, which reduces debugging time
 551 dramatically.  The "<tt>&lt;cassert&gt;</tt>" header file is probably already
 552 included by the header files you are using, so it doesn't cost anything to use
 553 it.</p>
 554
 555 <p>To further assist with debugging, make sure to put some kind of error message
 556 in the assertion statement (which is printed if the assertion is tripped). This
 557 helps the poor debugging make sense of why an assertion is being made and
 558 enforced, and hopefully what to do about it.  Here is one complete example:</p>
 559
 560 <div class="doc_code">
 561 <pre>
 562 inline Value *getOperand(unsigned i) {
 563   assert(i &lt; Operands.size() &amp;&amp; "getOperand() out of range!");
 564   return Operands[i];
 565 }
 566 </pre>
 567 </div>
 568
 569 <p>Here are some examples:</p>
 570
 571 <div class="doc_code">
 572 <pre>
 573 assert(Ty-&gt;isPointerType() &amp;&amp; "Can't allocate a non pointer type!");
 574
 575 assert((Opcode == Shl || Opcode == Shr) &amp;&amp; "ShiftInst Opcode invalid!");
 576
 577 assert(idx &lt; getNumSuccessors() &amp;&amp; "Successor # out of range!");
 578
 579 assert(V1.getType() == V2.getType() &amp;&amp; "Constant types must be identical!");
 580
 581 assert(isa&lt;PHINode&gt;(Succ-&gt;front()) &amp;&amp; "Only works on PHId BBs!");
 582 </pre>
 583 </div>
 584
 585 <p>You get the idea...</p>
 586
 587 <p>Please be aware when adding assert statements that not all compilers are aware of
 588 the semantics of the assert.  In some places, asserts are used to indicate a piece of
 589 code that should not be reached.  These are typically of the form:</p>
 590
 591 <div class="doc_code">
 592 <pre>
 593 assert(0 &amp;&amp; "Some helpful error message");
 594 </pre>
 595 </div>
 596
 597 <p>When used in a function that returns a value, they should be followed with a return
 598 statement and a comment indicating that this line is never reached.  This will prevent
 599 a compiler which is unable to deduce that the assert statement never returns from
 600 generating a warning.</p>
 601
 602 <div class="doc_code">
 603 <pre>
 604 assert(0 &amp;&amp; "Some helpful error message");
 605 // Not reached
 606 return 0;
 607 </pre>
 608 </div>
 609
 610 </div>
 611
 612 <!-- _______________________________________________________________________ -->
 613 <div class="doc_subsubsection">
 614   <a name="ll_ns_std">Do not use '<tt>using namespace std</tt>'</a>
 615 </div>
 616
 617 <div class="doc_text">
 618 <p>In LLVM, we prefer to explicitly prefix all identifiers from the standard
 619 namespace with an "<tt>std::</tt>" prefix, rather than rely on
 620 "<tt>using namespace std;</tt>".</p>
 621
 622 <p> In header files, adding a '<tt>using namespace XXX</tt>' directive pollutes
 623 the namespace of any source file that <tt>#include</tt>s the header.  This is
 624 clearly a bad thing.</p>
 625
 626 <p>In implementation files (e.g. .cpp files), the rule is more of a stylistic
 627 rule, but is still important.  Basically, using explicit namespace prefixes
 628 makes the code <b>clearer</b>, because it is immediately obvious what facilities
 629 are being used and where they are coming from, and <b>more portable</b>, because
 630 namespace clashes cannot occur between LLVM code and other namespaces.  The
 631 portability rule is important because different standard library implementations
 632 expose different symbols (potentially ones they shouldn't), and future revisions
 633 to the C++ standard will add more symbols to the <tt>std</tt> namespace.  As
 634 such, we never use '<tt>using namespace std;</tt>' in LLVM.</p>
 635
 636 <p>The exception to the general rule (i.e. it's not an exception for
 637 the <tt>std</tt> namespace) is for implementation files.  For example, all of
 638 the code in the LLVM project implements code that lives in the 'llvm' namespace.
 639 As such, it is ok, and actually clearer, for the .cpp files to have a '<tt>using
 640 namespace llvm</tt>' directive at their top, after the <tt>#include</tt>s.  The
 641 general form of this rule is that any .cpp file that implements code in any
 642 namespace may use that namespace (and its parents'), but should not use any
 643 others.</p>
 644
 645 </div>
 646
 647 <!-- _______________________________________________________________________ -->
 648 <div class="doc_subsubsection">
 649   <a name="ll_virtual_anch">Provide a virtual method anchor for classes
 650   in headers</a>
 651 </div>
 652
 653 <div class="doc_text">
 654
 655 <p>If a class is defined in a header file and has a v-table (either it has
 656 virtual methods or it derives from classes with virtual methods), it must
 657 always have at least one out-of-line virtual method in the class.  Without
 658 this, the compiler will copy the vtable and RTTI into every <tt>.o</tt> file
 659 that <tt>#include</tt>s the header, bloating <tt>.o</tt> file sizes and
 660 increasing link times.</p>
 661
 662 </div>
 663
 664
 665 <!-- _______________________________________________________________________ -->
 666 <div class="doc_subsubsection">
 667   <a name="ll_preincrement">Prefer Preincrement</a>
 668 </div>
 669
 670 <div class="doc_text">
 671
 672 <p>Hard fast rule: Preincrement (<tt>++X</tt>) may be no slower than
 673 postincrement (<tt>X++</tt>) and could very well be a lot faster than it.  Use
 674 preincrementation whenever possible.</p>
 675
 676 <p>The semantics of postincrement include making a copy of the value being
 677 incremented, returning it, and then preincrementing the "work value".  For
 678 primitive types, this isn't a big deal... but for iterators, it can be a huge
 679 issue (for example, some iterators contains stack and set objects in them...
 680 copying an iterator could invoke the copy ctor's of these as well).  In general,
 681 get in the habit of always using preincrement, and you won't have a problem.</p>
 682
 683 </div>
 684
 685 <!-- _______________________________________________________________________ -->
 686 <div class="doc_subsubsection">
 687   <a name="ll_avoidendl">Avoid <tt>std::endl</tt></a>
 688 </div>
 689
 690 <div class="doc_text">
 691
 692 <p>The <tt>std::endl</tt> modifier, when used with iostreams outputs a newline
 693 to the output stream specified.  In addition to doing this, however, it also
 694 flushes the output stream.  In other words, these are equivalent:</p>
 695
 696 <div class="doc_code">
 697 <pre>
 698 std::cout &lt;&lt; std::endl;
 699 std::cout &lt;&lt; '\n' &lt;&lt; std::flush;
 700 </pre>
 701 </div>
 702
 703 <p>Most of the time, you probably have no reason to flush the output stream, so
 704 it's better to use a literal <tt>'\n'</tt>.</p>
 705
 706 </div>
 707
 708
 709 <!-- *********************************************************************** -->
 710 <div class="doc_section">
 711   <a name="seealso">See Also</a>
 712 </div>
 713 <!-- *********************************************************************** -->
 714
 715 <div class="doc_text">
 716
 717 <p>A lot of these comments and recommendations have been culled for other
 718 sources.  Two particularly important books for our work are:</p>
 719
 720 <ol>
 721
 722 <li><a href="http://www.amazon.com/Effective-Specific-Addison-Wesley-Professional-Computing/dp/0321334876">Effective
 723 C++</a> by Scott Meyers.  Also
 724 interesting and useful are "More Effective C++" and "Effective STL" by the same
 725 author.</li>
 726
 727 <li>Large-Scale C++ Software Design by John Lakos</li>
 728
 729 </ol>
 730
 731 <p>If you get some free time, and you haven't read them: do so, you might learn
 732 something.</p>
 733
 734 </div>
 735
 736 <!-- *********************************************************************** -->
 737
 738 <hr>
 739 <address>
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 744
 745   <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
 746   <a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
 747   Last modified: $Date$
 748 </address>
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