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   6   <title>A Few Coding Standards</title>
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   9
  10 <div class="doc_title">
  11   A Few 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
 126 describes the basic purpose of the file.  If a file does not have a header, it
 127 should not be checked into CVS.  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 was developed by &lt;whoever started the file&gt; and is distributed under
 138 // the University of Illinois Open Source 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 'developed by' line
 150 should be the name of the person or organization who initially contributed the
 151 file.  The "<tt>-*- C++
 152 -*-</tt>" string on the first line is there to tell Emacs that the source file
 153 is a C++ file, not a C file (Emacs assumes .h files are C files by default).
 154 Note that this tag is not necessary in .cpp files.  The name of the file is also
 155 on the first line, along with a very short description of the purpose of the
 156 file.  This is important when printing out code and flipping though lots of
 157 pages.</p>
 158
 159 <p>The next section in the file is a concise note that defines the license that
 160 the file is released under.  This makes it perfectly clear what terms the source
 161 code can be distributed under.</p>
 162
 163 <p>The main body of the description does not have to be very long in most cases.
 164 Here it's only two lines.  If an algorithm is being implemented or something
 165 tricky is going on, a reference to the paper where it is published should be
 166 included, as well as any notes or "gotchas" in the code to watch out for.</p>
 167
 168 <b>Class overviews</b>
 169
 170 <p>Classes are one fundamental part of a good object oriented design.  As such,
 171 a class definition should have a comment block that explains what the class is
 172 used for... if it's not obvious.  If it's so completely obvious your grandma
 173 could figure it out, it's probably safe to leave it out.  Naming classes
 174 something sane goes a long ways towards avoiding writing documentation.</p>
 175
 176
 177 <b>Method information</b>
 178
 179 <p>Methods defined in a class (as well as any global functions) should also be
 180 documented properly.  A quick note about what it does any a description of the
 181 borderline behaviour is all that is necessary here (unless something
 182 particularly tricky or insideous is going on).  The hope is that people can
 183 figure out how to use your interfaces without reading the code itself... that is
 184 the goal metric.</p>
 185
 186 <p>Good things to talk about here are what happens when something unexpected
 187 happens: does the method return null?  Abort?  Format your hard disk?</p>
 188
 189 </div>
 190
 191 <!-- _______________________________________________________________________ -->
 192 <div class="doc_subsubsection">
 193   <a name="scf_commentformat">Comment Formatting</a>
 194 </div>
 195
 196 <div class="doc_text">
 197
 198 <p>In general, prefer C++ style (<tt>//</tt>) comments.  They take less space,
 199 require less typing, don't have nesting problems, etc.  There are a few cases
 200 when it is useful to use C style (<tt>/* */</tt>) comments however:</p>
 201
 202 <ol>
 203   <li>When writing a C code: Obviously if you are writing C code, use C style
 204       comments.</li>
 205   <li>When writing a header file that may be <tt>#include</tt>d by a C source
 206       file.</li>
 207   <li>When writing a source file that is used by a tool that only accepts C
 208       style comments.</li>
 209 </ol>
 210
 211 <p>To comment out a large block of code, use <tt>#if 0</tt> and <tt>#endif</tt>.
 212 These nest properly and are better behaved in general than C style comments.</p>
 213
 214 </div>
 215
 216 <!-- _______________________________________________________________________ -->
 217 <div class="doc_subsubsection">
 218   <a name="scf_includes"><tt>#include</tt> Style</a>
 219 </div>
 220
 221 <div class="doc_text">
 222
 223 <p>Immediately after the <a href="#scf_commenting">header file comment</a> (and
 224 include guards if working on a header file), the <a
 225 href="#hl_dontinclude">minimal</a> list of <tt>#include</tt>s required by the
 226 file should be listed.  We prefer these <tt>#include</tt>s to be listed in this
 227 order:</p>
 228
 229 <ol>
 230   <li><a href="#mmheader">Main Module header</a></li>
 231   <li><a href="#hl_privateheaders">Local/Private Headers</a></li>
 232   <li><tt>llvm/*</tt></li>
 233   <li><tt>llvm/Analysis/*</tt></li>
 234   <li><tt>llvm/Assembly/*</tt></li>
 235   <li><tt>llvm/Bytecode/*</tt></li>
 236   <li><tt>llvm/CodeGen/*</tt></li>
 237   <li>...</li>
 238   <li><tt>Support/*</tt></li>
 239   <li><tt>Config/*</tt></li>
 240   <li>System <tt>#includes</tt></li>
 241 </ol>
 242
 243 <p>... and each catagory should be sorted by name.</p>
 244
 245 <p><a name="mmheader">The "Main Module Header"</a> file applies to .cpp file
 246 which implement an interface defined by a .h file.  This <tt>#include</tt>
 247 should always be included <b>first</b> regardless of where it lives on the file
 248 system.  By including a header file first in the .cpp files that implement the
 249 interfaces, we ensure that the header does not have any hidden dependencies
 250 which are not explicitly #included in the header, but should be.  It is also a
 251 form of documentation in the .cpp file to indicate where the interfaces it
 252 implements are defined.</p>
 253
 254 </div>
 255
 256 <!-- _______________________________________________________________________ -->
 257 <div class="doc_subsubsection">
 258   <a name="scf_codewidth">Source Code Width</a>
 259 </div>
 260
 261 <div class="doc_text">
 262
 263 <p>Write your code to fit within 80 columns of text.  This helps those of us who
 264 like to print out code and look at your code in an xterm without resizing
 265 it.</p>
 266
 267 </div>
 268
 269 <!-- _______________________________________________________________________ -->
 270 <div class="doc_subsubsection">
 271   <a name="scf_spacestabs">Use Spaces Instead of Tabs</a>
 272 </div>
 273
 274 <div class="doc_text">
 275
 276 <p>In all cases, prefer spaces to tabs in source files.  People have different
 277 prefered indentation levels, and different styles of indentation that they
 278 like... this is fine.  What isn't is that different editors/viewers expand tabs
 279 out to different tab stops.  This can cause your code to look completely
 280 unreadable, and it is not worth dealing with.</p>
 281
 282 <p>As always, follow the <a href="#goldenrule">Golden Rule</a> above: follow the
 283 style of existing code if your are modifying and extending it.  If you like four
 284 spaces of indentation, <b>DO NOT</b> do that in the middle of a chunk of code
 285 with two spaces of indentation.  Also, do not reindent a whole source file: it
 286 makes for incredible diffs that are absolutely worthless.</p>
 287
 288 </div>
 289
 290 <!-- _______________________________________________________________________ -->
 291 <div class="doc_subsubsection">
 292   <a name="scf_indentation">Indent Code Consistently</a>
 293 </div>
 294
 295 <div class="doc_text">
 296
 297 <p>Okay, your first year of programming you were told that indentation is
 298 important.  If you didn't believe and internalize this then, now is the time.
 299 Just do it.</p>
 300
 301 </div>
 302
 303
 304 <!-- ======================================================================= -->
 305 <div class="doc_subsection">
 306   <a name="compilerissues">Compiler Issues</a>
 307 </div>
 308
 309
 310 <!-- _______________________________________________________________________ -->
 311 <div class="doc_subsubsection">
 312   <a name="ci_warningerrors">Treat Compiler Warnings Like Errors</a>
 313 </div>
 314
 315 <div class="doc_text">
 316
 317 <p>If your code has compiler warnings in it, something is wrong: you aren't
 318 casting values correctly, your have "questionable" constructs in your code, or
 319 you are doing something legitimately wrong.  Compiler warnings can cover up
 320 legitimate errors in output and make dealing with a translation unit
 321 difficult.</p>
 322
 323 <p>It is not possible to prevent all warnings from all compilers, nor is it
 324 desirable.  Instead, pick a standard compiler (like <tt>gcc</tt>) that provides
 325 a good thorough set of warnings, and stick to them.  At least in the case of
 326 <tt>gcc</tt>, it is possible to work around any spurious errors by changing the
 327 syntax of the code slightly.  For example, an warning that annoys me occurs when
 328 I write code like this:</p>
 329
 330 <div class="doc_code">
 331 <pre>
 332 if (V = getValue()) {
 333   ...
 334 }
 335 </pre>
 336 </div>
 337
 338 <p><tt>gcc</tt> will warn me that I probably want to use the <tt>==</tt>
 339 operator, and that I probably mistyped it.  In most cases, I haven't, and I
 340 really don't want the spurious errors.  To fix this particular problem, I
 341 rewrite the code like this:</p>
 342
 343 <div class="doc_code">
 344 <pre>
 345 if ((V = getValue())) {
 346   ...
 347 }
 348 </pre>
 349 </div>
 350
 351 <p>...which shuts <tt>gcc</tt> up.  Any <tt>gcc</tt> warning that annoys you can
 352 be fixed by massaging the code appropriately.</p>
 353
 354 <p>These are the <tt>gcc</tt> warnings that I prefer to enable: <tt>-Wall
 355 -Winline -W -Wwrite-strings -Wno-unused</tt></p>
 356
 357 </div>
 358
 359 <!-- _______________________________________________________________________ -->
 360 <div class="doc_subsubsection">
 361   <a name="ci_portable_code">Write Portable Code</a>
 362 </div>
 363
 364 <div class="doc_text">
 365
 366 <p>In almost all cases, it is possible and within reason to write completely
 367 portable code.  If there are cases where it isn't possible to write portable
 368 code, isolate it behind a well defined (and well documented) interface.</p>
 369
 370 <p>In practice, this means that you shouldn't assume much about the host
 371 compiler, including its support for "high tech" features like partial
 372 specialization of templates.  In fact, Visual C++ 6 could be an important target
 373 for our work in the future, and we don't want to have to rewrite all of our code
 374 to support it.</p>
 375
 376 </div>
 377
 378 <!-- _______________________________________________________________________ -->
 379 <div class="doc_subsubsection">
 380 <a name="ci_class_struct">Use of <tt>class</tt> and <tt>struct</tt> Keywords</a>
 381 </div>
 382 <div class="doc_text">
 383
 384 <p>In C++, the <tt>class</tt> and <tt>struct</tt> keywords can be used almost
 385 interchangeably. The only difference is when they are used to declare a class:
 386 <tt>class</tt> makes all members private by default while <tt>struct</tt> makes
 387 all members public by default.</p>
 388
 389 <p>Unfortunately, not all compilers follow the rules and some will generate
 390 different symbols based on whether <tt>class</tt> or <tt>struct</tt> was used to
 391 declare the symbol.  This can lead to problems at link time.</p>
 392
 393 <p>So, the rule for LLVM is to always use the <tt>class</tt> keyword, unless
 394 <b>all</b> members are public, in which case <tt>struct</tt> is allowed.</p>
 395
 396 </div>
 397
 398 <!-- *********************************************************************** -->
 399 <div class="doc_section">
 400   <a name="styleissues">Style Issues</a>
 401 </div>
 402 <!-- *********************************************************************** -->
 403
 404
 405 <!-- ======================================================================= -->
 406 <div class="doc_subsection">
 407   <a name="macro">The High Level Issues</a>
 408 </div>
 409
 410
 411 <!-- _______________________________________________________________________ -->
 412 <div class="doc_subsubsection">
 413   <a name="hl_module">A Public Header File <b>is</b> a Module</a>
 414 </div>
 415
 416 <div class="doc_text">
 417
 418 <p>C++ doesn't do too well in the modularity department.  There is no real
 419 encapsulation or data hiding (unless you use expensive protocol classes), but it
 420 is what we have to work with.  When you write a public header file (in the LLVM
 421 source tree, they live in the top level "include" directory), you are defining a
 422 module of functionality.</p>
 423
 424 <p>Ideally, modules should be completely independent of each other, and their
 425 header files should only include the absolute minimum number of headers
 426 possible. A module is not just a class, a function, or a namespace: <a
 427 href="http://www.cuj.com/articles/2000/0002/0002c/0002c.htm">it's a collection
 428 of these</a> that defines an interface.  This interface may be several
 429 functions, classes or data structures, but the important issue is how they work
 430 together.</p>
 431
 432 <p>In general, a module should be implemented with one or more <tt>.cpp</tt>
 433 files.  Each of these <tt>.cpp</tt> files should include the header that defines
 434 their interface first.  This ensure that all of the dependences of the module
 435 header have been properly added to the module header itself, and are not
 436 implicit.  System headers should be included after user headers for a
 437 translation unit.</p>
 438
 439 </div>
 440
 441 <!-- _______________________________________________________________________ -->
 442 <div class="doc_subsubsection">
 443   <a name="hl_dontinclude"><tt>#include</tt> as Little as Possible</a>
 444 </div>
 445
 446 <div class="doc_text">
 447
 448 <p><tt>#include</tt> hurts compile time performance.  Don't do it unless you
 449 have to, especially in header files.</p>
 450
 451 <p>But wait, sometimes you need to have the definition of a class to use it, or
 452 to inherit from it.  In these cases go ahead and <tt>#include</tt> that header
 453 file.  Be aware however that there are many cases where you don't need to have
 454 the full definition of a class.  If you are using a pointer or reference to a
 455 class, you don't need the header file.  If you are simply returning a class
 456 instance from a prototyped function or method, you don't need it.  In fact, for
 457 most cases, you simply don't need the definition of a class... and not
 458 <tt>#include</tt>'ing speeds up compilation.</p>
 459
 460 <p>It is easy to try to go too overboard on this recommendation, however.  You
 461 <b>must</b> include all of the header files that you are using -- you can
 462 include them either directly
 463 or indirectly (through another header file).  To make sure that you don't
 464 accidently forget to include a header file in your module header, make sure to
 465 include your module header <b>first</b> in the implementation file (as mentioned
 466 above).  This way there won't be any hidden dependencies that you'll find out
 467 about later...</p>
 468
 469 </div>
 470
 471 <!-- _______________________________________________________________________ -->
 472 <div class="doc_subsubsection">
 473   <a name="hl_privateheaders">Keep "internal" Headers Private</a>
 474 </div>
 475
 476 <div class="doc_text">
 477
 478 <p>Many modules have a complex implementation that causes them to use more than
 479 one implementation (<tt>.cpp</tt>) file.  It is often tempting to put the
 480 internal communication interface (helper classes, extra functions, etc) in the
 481 public module header file.  Don't do this.</p>
 482
 483 <p>If you really need to do something like this, put a private header file in
 484 the same directory as the source files, and include it locally.  This ensures
 485 that your private interface remains private and undisturbed by outsiders.</p>
 486
 487 <p>Note however, that it's okay to put extra implementation methods a public
 488 class itself... just make them private (or protected), and all is well.</p>
 489
 490 </div>
 491
 492 <!-- _______________________________________________________________________ -->
 493 <div class="doc_subsubsection">
 494   <a name="ll_iostream"><tt>#include &lt;iostream&gt;</tt> is forbidden</a>
 495 </div>
 496
 497 <div class="doc_text">
 498
 499 <p>The use of <tt>#include &lt;iostream&gt;</tt> in library files is
 500 hereby <b><em>forbidden</em></b>. The primary reason for doing this is to
 501 support clients using LLVM libraries as part of larger systems. In particular,
 502 we statically link LLVM into some dynamic libraries. Even if LLVM isn't used,
 503 the static c'tors are run whenever an application start up that uses the dynamic
 504 library. There are two problems with this:</p>
 505
 506 <ol>
 507   <li>The time to run the static c'tors impacts startup time of
 508       applications&mdash;a critical time for gui apps.</li>
 509   <li>The static c'tors cause the app to pull many extra pages of memory off the
 510       disk: both the code for the static c'tors in each .o file and the small
 511       amount of data that gets touched.  In addition, touched/dirty pages put
 512       more pressure on the VM system on low-memory machines.</li>
 513 </ol>
 514
 515 <table align="center">
 516   <tbody>
 517     <tr>
 518       <th>Old Way</th>
 519       <th>New Way</th>
 520     </tr>
 521     <tr>
 522       <td align="left"><pre>#include &lt;iostream&gt;</pre></td>
 523       <td align="left"><pre>#include "llvm/Support/Streams.h"</pre></td>
 524     </tr>
 525     <tr>
 526       <td align="left"><pre>DEBUG(std::cerr &lt;&lt; ...);
 527 DEBUG(dump(std::cerr));</pre></td>
 528       <td align="left"><pre>DOUT &lt;&lt; ...;
 529 dump(DOUT);</pre></td>
 530     </tr>
 531     <tr>
 532       <td align="left"><pre>std::cerr &lt;&lt; "Hello world\n";</pre></td>
 533       <td align="left"><pre>llvm::cerr &lt;&lt; "Hello world\n";</pre></td>
 534     </tr>
 535     <tr>
 536       <td align="left"><pre>std::cout &lt;&lt; "Hello world\n";</pre></td>
 537       <td align="left"><pre>llvm::cout &lt;&lt; "Hello world\n";</pre></td>
 538     </tr>
 539     <tr>
 540       <td align="left"><pre>std::cin &gt;&gt; Var;</pre></td>
 541       <td align="left"><pre>llvm::cin &gt;&gt; Var;</pre></td>
 542     </tr>
 543     <tr>
 544       <td align="left"><pre>std::ostream</pre></td>
 545       <td align="left"><pre>llvm::OStream</pre></td>
 546     </tr>
 547     <tr>
 548       <td align="left"><pre>std::istream</pre></td>
 549       <td align="left"><pre>llvm::IStream</pre></td>
 550     </tr>
 551     <tr>
 552       <td align="left"><pre>std::stringstream</pre></td>
 553       <td align="left"><pre>llvm::StringStream</pre></td>
 554     </tr>
 555     <tr>
 556       <td align="left"><pre>void print(std::ostream &Out);
 557 // ...
 558 print(std::cerr);</pre></td>
 559       <td align="left"><pre>void print(std::ostream &Out);
 560 void print(std::ostream *Out) { if (Out) print(*Out) }
 561 // ...
 562 print(llvm::cerr);</pre>
 563
 564 <ul><i>N.B.</i> The second <tt>print</tt> method is called by the <tt>print</tt>
 565 expression. It prevents the execution of the first <tt>print</tt> method if the
 566 stream is <tt>cnull</tt>.</ul></td>
 567    </tbody>
 568 </table>
 569
 570 </div>
 571
 572
 573 <!-- ======================================================================= -->
 574 <div class="doc_subsection">
 575   <a name="micro">The Low Level Issues</a>
 576 </div>
 577
 578
 579 <!-- _______________________________________________________________________ -->
 580 <div class="doc_subsubsection">
 581   <a name="ll_assert">Assert Liberally</a>
 582 </div>
 583
 584 <div class="doc_text">
 585
 586 <p>Use the "<tt>assert</tt>" function to its fullest.  Check all of your
 587 preconditions and assumptions, you never know when a bug (not neccesarily even
 588 yours) might be caught early by an assertion, which reduces debugging time
 589 dramatically.  The "<tt>&lt;cassert&gt;</tt>" header file is probably already
 590 included by the header files you are using, so it doesn't cost anything to use
 591 it.</p>
 592
 593 <p>To further assist with debugging, make sure to put some kind of error message
 594 in the assertion statement (which is printed if the assertion is tripped). This
 595 helps the poor debugging make sense of why an assertion is being made and
 596 enforced, and hopefully what to do about it.  Here is one complete example:</p>
 597
 598 <div class="doc_code">
 599 <pre>
 600 inline Value *getOperand(unsigned i) {
 601   assert(i &lt; Operands.size() &amp;&amp; "getOperand() out of range!");
 602   return Operands[i];
 603 }
 604 </pre>
 605 </div>
 606
 607 <p>Here are some examples:</p>
 608
 609 <div class="doc_code">
 610 <pre>
 611 assert(Ty-&gt;isPointerType() &amp;&amp; "Can't allocate a non pointer type!");
 612
 613 assert((Opcode == Shl || Opcode == Shr) &amp;&amp; "ShiftInst Opcode invalid!");
 614
 615 assert(idx &lt; getNumSuccessors() &amp;&amp; "Successor # out of range!");
 616
 617 assert(V1.getType() == V2.getType() &amp;&amp; "Constant types must be identical!");
 618
 619 assert(isa&lt;PHINode&gt;(Succ-&gt;front()) &amp;&amp; "Only works on PHId BBs!");
 620 </pre>
 621 </div>
 622
 623 <p>You get the idea...</p>
 624
 625 </div>
 626
 627 <!-- _______________________________________________________________________ -->
 628 <div class="doc_subsubsection">
 629   <a name="ll_ns_std">Do not use '<tt>using namespace std</tt>'</a>
 630 </div>
 631
 632 <div class="doc_text">
 633 <p>In LLVM, we prefer to explicitly prefix all identifiers from the standard
 634 namespace with an "<tt>std::</tt>" prefix, rather than rely on
 635 "<tt>using namespace std;</tt>".</p>
 636
 637 <p> In header files, adding a '<tt>using namespace XXX</tt>' directive pollutes
 638 the namespace of any source file that includes the header.  This is clearly a
 639 bad thing.</p>
 640
 641 <p>In implementation files (e.g. .cpp files), the rule is more of a stylistic
 642 rule, but is still important.  Basically, using explicit namespace prefixes
 643 makes the code <b>clearer</b>, because it is immediately obvious what facilities
 644 are being used and where they are coming from, and <b>more portable</b>, because
 645 namespace clashes cannot occur between LLVM code and other namespaces.  The
 646 portability rule is important because different standard library implementations
 647 expose different symbols (potentially ones they shouldn't), and future revisions
 648 to the C++ standard will add more symbols to the <tt>std</tt> namespace.  As
 649 such, we never use '<tt>using namespace std;</tt>' in LLVM.</p>
 650
 651 <p>The exception to the general rule (i.e. it's not an exception for
 652 the <tt>std</tt> namespace) is for implementation files.  For example, all of
 653 the code in the LLVM project implements code that lives in the 'llvm' namespace.
 654 As such, it is ok, and actually clearer, for the .cpp files to have a '<tt>using
 655 namespace llvm</tt>' directive at their top, after the <tt>#include</tt>s.  The
 656 general form of this rule is that any .cpp file that implements code in any
 657 namespace may use that namespace (and its parents'), but should not use any
 658 others.</p>
 659
 660 </div>
 661
 662 <!-- _______________________________________________________________________ -->
 663 <div class="doc_subsubsection">
 664   <a name="ll_virtual_anch">Provide a virtual method anchor for classes
 665   in headers</a>
 666 </div>
 667
 668 <div class="doc_text">
 669
 670 <p>If a class is defined in a header file and has a v-table (either it has
 671 virtual methods or it derives from classes with virtual methods), it must
 672 always have at least one out-of-line virtual method in the class.  Without
 673 this, the compiler will copy the vtable and RTTI into every .o file that
 674 #includes the header, bloating .o file sizes and increasing link times.
 675 </p>
 676
 677 </div>
 678
 679
 680 <!-- _______________________________________________________________________ -->
 681 <div class="doc_subsubsection">
 682   <a name="ll_preincrement">Prefer Preincrement</a>
 683 </div>
 684
 685 <div class="doc_text">
 686
 687 <p>Hard fast rule: Preincrement (<tt>++X</tt>) may be no slower than
 688 postincrement (<tt>X++</tt>) and could very well be a lot faster than it.  Use
 689 preincrementation whenever possible.</p>
 690
 691 <p>The semantics of postincrement include making a copy of the value being
 692 incremented, returning it, and then preincrementing the "work value".  For
 693 primitive types, this isn't a big deal... but for iterators, it can be a huge
 694 issue (for example, some iterators contains stack and set objects in them...
 695 copying an iterator could invoke the copy ctor's of these as well).  In general,
 696 get in the habit of always using preincrement, and you won't have a problem.</p>
 697
 698 </div>
 699
 700 <!-- _______________________________________________________________________ -->
 701 <div class="doc_subsubsection">
 702   <a name="ll_avoidendl">Avoid <tt>std::endl</tt></a>
 703 </div>
 704
 705 <div class="doc_text">
 706
 707 <p>The <tt>std::endl</tt> modifier, when used with iostreams outputs a newline
 708 to the output stream specified.  In addition to doing this, however, it also
 709 flushes the output stream.  In other words, these are equivalent:</p>
 710
 711 <div class="doc_code">
 712 <pre>
 713 std::cout &lt;&lt; std::endl;
 714 std::cout &lt;&lt; '\n' &lt;&lt; std::flush;
 715 </pre>
 716 </div>
 717
 718 <p>Most of the time, you probably have no reason to flush the output stream, so
 719 it's better to use a literal <tt>'\n'</tt>.</p>
 720
 721 </div>
 722
 723
 724 <!-- *********************************************************************** -->
 725 <div class="doc_section">
 726   <a name="seealso">See Also</a>
 727 </div>
 728 <!-- *********************************************************************** -->
 729
 730 <div class="doc_text">
 731
 732 <p>A lot of these comments and recommendations have been culled for other
 733 sources.  Two particularly important books for our work are:</p>
 734
 735 <ol>
 736
 737 <li><a href="http://www.aw-bc.com/catalog/academic/product/0,1144,0201310155,00.html">Effective
 738 C++</a> by Scott Meyers.  There is an online version of the book (only some
 739 chapters though) <a
 740 href="http://www.awlonline.com/cseng/meyerscddemo/">available as well</a>.  Also
 741 interesting and useful are "More Effective C++" and "Effective STL" by the same
 742 author.</li>
 743
 744 <li><a href="http://cseng.aw.com/book/0,3828,0201633620,00.html">Large-Scale C++
 745 Software Design</a> by John Lakos</li>
 746
 747 </ol>
 748
 749 <p>If you get some free time, and you haven't read them: do so, you might learn
 750 something.</p>
 751
 752 </div>
 753
 754 <!-- *********************************************************************** -->
 755
 756 <hr>
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 762
 763   <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
 764   <a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
 765   Last modified: $Date$
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