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  10
  11 <h1>
  12   CommandLine 2.0 Library Manual
  13 </h1>
  14
  15 <ol>
  16   <li><a href="#introduction">Introduction</a></li>
  17
  18   <li><a href="#quickstart">Quick Start Guide</a>
  19     <ol>
  20       <li><a href="#bool">Boolean Arguments</a></li>
  21       <li><a href="#alias">Argument Aliases</a></li>
  22       <li><a href="#onealternative">Selecting an alternative from a
  23                                     set of possibilities</a></li>
  24       <li><a href="#namedalternatives">Named alternatives</a></li>
  25       <li><a href="#list">Parsing a list of options</a></li>
  26       <li><a href="#bits">Collecting options as a set of flags</a></li>
  27       <li><a href="#description">Adding freeform text to help output</a></li>
  28     </ol></li>
  29
  30   <li><a href="#referenceguide">Reference Guide</a>
  31     <ol>
  32       <li><a href="#positional">Positional Arguments</a>
  33         <ul>
  34         <li><a href="#--">Specifying positional options with hyphens</a></li>
  35         <li><a href="#getPosition">Determining absolute position with
  36           getPosition</a></li>
  37         <li><a href="#cl::ConsumeAfter">The <tt>cl::ConsumeAfter</tt>
  38              modifier</a></li>
  39         </ul></li>
  40
  41       <li><a href="#storage">Internal vs External Storage</a></li>
  42
  43       <li><a href="#attributes">Option Attributes</a></li>
  44
  45       <li><a href="#modifiers">Option Modifiers</a>
  46         <ul>
  47         <li><a href="#hiding">Hiding an option from <tt>-help</tt>
  48             output</a></li>
  49         <li><a href="#numoccurrences">Controlling the number of occurrences
  50                                      required and allowed</a></li>
  51         <li><a href="#valrequired">Controlling whether or not a value must be
  52                                    specified</a></li>
  53         <li><a href="#formatting">Controlling other formatting options</a></li>
  54         <li><a href="#misc">Miscellaneous option modifiers</a></li>
  55         <li><a href="#response">Response files</a></li>
  56         </ul></li>
  57
  58       <li><a href="#toplevel">Top-Level Classes and Functions</a>
  59         <ul>
  60         <li><a href="#cl::ParseCommandLineOptions">The
  61             <tt>cl::ParseCommandLineOptions</tt> function</a></li>
  62         <li><a href="#cl::ParseEnvironmentOptions">The
  63             <tt>cl::ParseEnvironmentOptions</tt> function</a></li>
  64         <li><a href="#cl::SetVersionPrinter">The <tt>cl::SetVersionPrinter</tt>
  65           function</a></li>
  66         <li><a href="#cl::opt">The <tt>cl::opt</tt> class</a></li>
  67         <li><a href="#cl::list">The <tt>cl::list</tt> class</a></li>
  68         <li><a href="#cl::bits">The <tt>cl::bits</tt> class</a></li>
  69         <li><a href="#cl::alias">The <tt>cl::alias</tt> class</a></li>
  70         <li><a href="#cl::extrahelp">The <tt>cl::extrahelp</tt> class</a></li>
  71         </ul></li>
  72
  73       <li><a href="#builtinparsers">Builtin parsers</a>
  74         <ul>
  75         <li><a href="#genericparser">The Generic <tt>parser&lt;t&gt;</tt>
  76             parser</a></li>
  77         <li><a href="#boolparser">The <tt>parser&lt;bool&gt;</tt>
  78             specialization</a></li>
  79         <li><a href="#boolOrDefaultparser">The <tt>parser&lt;boolOrDefault&gt;</tt>
  80             specialization</a></li>
  81         <li><a href="#stringparser">The <tt>parser&lt;string&gt;</tt>
  82             specialization</a></li>
  83         <li><a href="#intparser">The <tt>parser&lt;int&gt;</tt>
  84             specialization</a></li>
  85         <li><a href="#doubleparser">The <tt>parser&lt;double&gt;</tt> and
  86             <tt>parser&lt;float&gt;</tt> specializations</a></li>
  87         </ul></li>
  88     </ol></li>
  89   <li><a href="#extensionguide">Extension Guide</a>
  90     <ol>
  91       <li><a href="#customparser">Writing a custom parser</a></li>
  92       <li><a href="#explotingexternal">Exploiting external storage</a></li>
  93       <li><a href="#dynamicopts">Dynamically adding command line
  94           options</a></li>
  95     </ol></li>
  96 </ol>
  97
  98 <div class="doc_author">
  99   <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p>
 100 </div>
 101
 102 <!-- *********************************************************************** -->
 103 <h2>
 104   <a name="introduction">Introduction</a>
 105 </h2>
 106 <!-- *********************************************************************** -->
 107
 108 <div>
 109
 110 <p>This document describes the CommandLine argument processing library.  It will
 111 show you how to use it, and what it can do.  The CommandLine library uses a
 112 declarative approach to specifying the command line options that your program
 113 takes.  By default, these options declarations implicitly hold the value parsed
 114 for the option declared (of course this <a href="#storage">can be
 115 changed</a>).</p>
 116
 117 <p>Although there are a <b>lot</b> of command line argument parsing libraries
 118 out there in many different languages, none of them fit well with what I needed.
 119 By looking at the features and problems of other libraries, I designed the
 120 CommandLine library to have the following features:</p>
 121
 122 <ol>
 123 <li>Speed: The CommandLine library is very quick and uses little resources.  The
 124 parsing time of the library is directly proportional to the number of arguments
 125 parsed, not the the number of options recognized.  Additionally, command line
 126 argument values are captured transparently into user defined global variables,
 127 which can be accessed like any other variable (and with the same
 128 performance).</li>
 129
 130 <li>Type Safe: As a user of CommandLine, you don't have to worry about
 131 remembering the type of arguments that you want (is it an int?  a string? a
 132 bool? an enum?) and keep casting it around.  Not only does this help prevent
 133 error prone constructs, it also leads to dramatically cleaner source code.</li>
 134
 135 <li>No subclasses required: To use CommandLine, you instantiate variables that
 136 correspond to the arguments that you would like to capture, you don't subclass a
 137 parser.  This means that you don't have to write <b>any</b> boilerplate
 138 code.</li>
 139
 140 <li>Globally accessible: Libraries can specify command line arguments that are
 141 automatically enabled in any tool that links to the library.  This is possible
 142 because the application doesn't have to keep a list of arguments to pass to
 143 the parser.  This also makes supporting <a href="#dynamicopts">dynamically
 144 loaded options</a> trivial.</li>
 145
 146 <li>Cleaner: CommandLine supports enum and other types directly, meaning that
 147 there is less error and more security built into the library.  You don't have to
 148 worry about whether your integral command line argument accidentally got
 149 assigned a value that is not valid for your enum type.</li>
 150
 151 <li>Powerful: The CommandLine library supports many different types of
 152 arguments, from simple <a href="#boolparser">boolean flags</a> to <a
 153 href="#cl::opt">scalars arguments</a> (<a href="#stringparser">strings</a>, <a
 154 href="#intparser">integers</a>, <a href="#genericparser">enums</a>, <a
 155 href="#doubleparser">doubles</a>), to <a href="#cl::list">lists of
 156 arguments</a>.  This is possible because CommandLine is...</li>
 157
 158 <li>Extensible: It is very simple to add a new argument type to CommandLine.
 159 Simply specify the parser that you want to use with the command line option when
 160 you declare it.  <a href="#customparser">Custom parsers</a> are no problem.</li>
 161
 162 <li>Labor Saving: The CommandLine library cuts down on the amount of grunt work
 163 that you, the user, have to do.  For example, it automatically provides a
 164 <tt>-help</tt> option that shows the available command line options for your
 165 tool.  Additionally, it does most of the basic correctness checking for
 166 you.</li>
 167
 168 <li>Capable: The CommandLine library can handle lots of different forms of
 169 options often found in real programs.  For example, <a
 170 href="#positional">positional</a> arguments, <tt>ls</tt> style <a
 171 href="#cl::Grouping">grouping</a> options (to allow processing '<tt>ls
 172 -lad</tt>' naturally), <tt>ld</tt> style <a href="#cl::Prefix">prefix</a>
 173 options (to parse '<tt>-lmalloc -L/usr/lib</tt>'), and <a
 174 href="#cl::ConsumeAfter">interpreter style options</a>.</li>
 175
 176 </ol>
 177
 178 <p>This document will hopefully let you jump in and start using CommandLine in
 179 your utility quickly and painlessly.  Additionally it should be a simple
 180 reference manual to figure out how stuff works.  If it is failing in some area
 181 (or you want an extension to the library), nag the author, <a
 182 href="mailto:sabre@nondot.org">Chris Lattner</a>.</p>
 183
 184 </div>
 185
 186 <!-- *********************************************************************** -->
 187 <h2>
 188   <a name="quickstart">Quick Start Guide</a>
 189 </h2>
 190 <!-- *********************************************************************** -->
 191
 192 <div>
 193
 194 <p>This section of the manual runs through a simple CommandLine'ification of a
 195 basic compiler tool.  This is intended to show you how to jump into using the
 196 CommandLine library in your own program, and show you some of the cool things it
 197 can do.</p>
 198
 199 <p>To start out, you need to include the CommandLine header file into your
 200 program:</p>
 201
 202 <div class="doc_code"><pre>
 203   #include "llvm/Support/CommandLine.h"
 204 </pre></div>
 205
 206 <p>Additionally, you need to add this as the first line of your main
 207 program:</p>
 208
 209 <div class="doc_code"><pre>
 210 int main(int argc, char **argv) {
 211   <a href="#cl::ParseCommandLineOptions">cl::ParseCommandLineOptions</a>(argc, argv);
 212   ...
 213 }
 214 </pre></div>
 215
 216 <p>... which actually parses the arguments and fills in the variable
 217 declarations.</p>
 218
 219 <p>Now that you are ready to support command line arguments, we need to tell the
 220 system which ones we want, and what type of arguments they are.  The CommandLine
 221 library uses a declarative syntax to model command line arguments with the
 222 global variable declarations that capture the parsed values.  This means that
 223 for every command line option that you would like to support, there should be a
 224 global variable declaration to capture the result.  For example, in a compiler,
 225 we would like to support the Unix-standard '<tt>-o &lt;filename&gt;</tt>' option
 226 to specify where to put the output.  With the CommandLine library, this is
 227 represented like this:</p>
 228
 229 <a name="value_desc_example"></a>
 230 <div class="doc_code"><pre>
 231 <a href="#cl::opt">cl::opt</a>&lt;string&gt; OutputFilename("<i>o</i>", <a href="#cl::desc">cl::desc</a>("<i>Specify output filename</i>"), <a href="#cl::value_desc">cl::value_desc</a>("<i>filename</i>"));
 232 </pre></div>
 233
 234 <p>This declares a global variable &quot;<tt>OutputFilename</tt>&quot; that is used to
 235 capture the result of the &quot;<tt>o</tt>&quot; argument (first parameter).  We specify
 236 that this is a simple scalar option by using the &quot;<tt><a
 237 href="#cl::opt">cl::opt</a></tt>&quot; template (as opposed to the <a
 238 href="#list">&quot;<tt>cl::list</tt> template</a>), and tell the CommandLine library
 239 that the data type that we are parsing is a string.</p>
 240
 241 <p>The second and third parameters (which are optional) are used to specify what
 242 to output for the "<tt>-help</tt>" option.  In this case, we get a line that
 243 looks like this:</p>
 244
 245 <div class="doc_code"><pre>
 246 USAGE: compiler [options]
 247
 248 OPTIONS:
 249   -help             - display available options (-help-hidden for more)
 250   <b>-o &lt;filename&gt;     - Specify output filename</b>
 251 </pre></div>
 252
 253 <p>Because we specified that the command line option should parse using the
 254 <tt>string</tt> data type, the variable declared is automatically usable as a
 255 real string in all contexts that a normal C++ string object may be used.  For
 256 example:</p>
 257
 258 <div class="doc_code"><pre>
 259   ...
 260   std::ofstream Output(OutputFilename.c_str());
 261   if (Output.good()) ...
 262   ...
 263 </pre></div>
 264
 265 <p>There are many different options that you can use to customize the command
 266 line option handling library, but the above example shows the general interface
 267 to these options.  The options can be specified in any order, and are specified
 268 with helper functions like <a href="#cl::desc"><tt>cl::desc(...)</tt></a>, so
 269 there are no positional dependencies to remember.  The available options are
 270 discussed in detail in the <a href="#referenceguide">Reference Guide</a>.</p>
 271
 272 <p>Continuing the example, we would like to have our compiler take an input
 273 filename as well as an output filename, but we do not want the input filename to
 274 be specified with a hyphen (ie, not <tt>-filename.c</tt>).  To support this
 275 style of argument, the CommandLine library allows for <a
 276 href="#positional">positional</a> arguments to be specified for the program.
 277 These positional arguments are filled with command line parameters that are not
 278 in option form.  We use this feature like this:</p>
 279
 280 <div class="doc_code"><pre>
 281 <a href="#cl::opt">cl::opt</a>&lt;string&gt; InputFilename(<a href="#cl::Positional">cl::Positional</a>, <a href="#cl::desc">cl::desc</a>("<i>&lt;input file&gt;</i>"), <a href="#cl::init">cl::init</a>("<i>-</i>"));
 282 </pre></div>
 283
 284 <p>This declaration indicates that the first positional argument should be
 285 treated as the input filename.  Here we use the <tt><a
 286 href="#cl::init">cl::init</a></tt> option to specify an initial value for the
 287 command line option, which is used if the option is not specified (if you do not
 288 specify a <tt><a href="#cl::init">cl::init</a></tt> modifier for an option, then
 289 the default constructor for the data type is used to initialize the value).
 290 Command line options default to being optional, so if we would like to require
 291 that the user always specify an input filename, we would add the <tt><a
 292 href="#cl::Required">cl::Required</a></tt> flag, and we could eliminate the
 293 <tt><a href="#cl::init">cl::init</a></tt> modifier, like this:</p>
 294
 295 <div class="doc_code"><pre>
 296 <a href="#cl::opt">cl::opt</a>&lt;string&gt; InputFilename(<a href="#cl::Positional">cl::Positional</a>, <a href="#cl::desc">cl::desc</a>("<i>&lt;input file&gt;</i>"), <b><a href="#cl::Required">cl::Required</a></b>);
 297 </pre></div>
 298
 299 <p>Again, the CommandLine library does not require the options to be specified
 300 in any particular order, so the above declaration is equivalent to:</p>
 301
 302 <div class="doc_code"><pre>
 303 <a href="#cl::opt">cl::opt</a>&lt;string&gt; InputFilename(<a href="#cl::Positional">cl::Positional</a>, <a href="#cl::Required">cl::Required</a>, <a href="#cl::desc">cl::desc</a>("<i>&lt;input file&gt;</i>"));
 304 </pre></div>
 305
 306 <p>By simply adding the <tt><a href="#cl::Required">cl::Required</a></tt> flag,
 307 the CommandLine library will automatically issue an error if the argument is not
 308 specified, which shifts all of the command line option verification code out of
 309 your application into the library.  This is just one example of how using flags
 310 can alter the default behaviour of the library, on a per-option basis.  By
 311 adding one of the declarations above, the <tt>-help</tt> option synopsis is now
 312 extended to:</p>
 313
 314 <div class="doc_code"><pre>
 315 USAGE: compiler [options] <b>&lt;input file&gt;</b>
 316
 317 OPTIONS:
 318   -help             - display available options (-help-hidden for more)
 319   -o &lt;filename&gt;     - Specify output filename
 320 </pre></div>
 321
 322 <p>... indicating that an input filename is expected.</p>
 323
 324 <!-- ======================================================================= -->
 325 <h3>
 326   <a name="bool">Boolean Arguments</a>
 327 </h3>
 328
 329 <div>
 330
 331 <p>In addition to input and output filenames, we would like the compiler example
 332 to support three boolean flags: "<tt>-f</tt>" to force writing binary output to
 333 a terminal, "<tt>--quiet</tt>" to enable quiet mode, and "<tt>-q</tt>" for
 334 backwards compatibility with some of our users.  We can support these by
 335 declaring options of boolean type like this:</p>
 336
 337 <div class="doc_code"><pre>
 338 <a href="#cl::opt">cl::opt</a>&lt;bool&gt; Force ("<i>f</i>", <a href="#cl::desc">cl::desc</a>("<i>Enable binary output on terminals</i>"));
 339 <a href="#cl::opt">cl::opt</a>&lt;bool&gt; Quiet ("<i>quiet</i>", <a href="#cl::desc">cl::desc</a>("<i>Don't print informational messages</i>"));
 340 <a href="#cl::opt">cl::opt</a>&lt;bool&gt; Quiet2("<i>q</i>", <a href="#cl::desc">cl::desc</a>("<i>Don't print informational messages</i>"), <a href="#cl::Hidden">cl::Hidden</a>);
 341 </pre></div>
 342
 343 <p>This does what you would expect: it declares three boolean variables
 344 ("<tt>Force</tt>", "<tt>Quiet</tt>", and "<tt>Quiet2</tt>") to recognize these
 345 options.  Note that the "<tt>-q</tt>" option is specified with the "<a
 346 href="#cl::Hidden"><tt>cl::Hidden</tt></a>" flag.  This modifier prevents it
 347 from being shown by the standard "<tt>-help</tt>" output (note that it is still
 348 shown in the "<tt>-help-hidden</tt>" output).</p>
 349
 350 <p>The CommandLine library uses a <a href="#builtinparsers">different parser</a>
 351 for different data types.  For example, in the string case, the argument passed
 352 to the option is copied literally into the content of the string variable... we
 353 obviously cannot do that in the boolean case, however, so we must use a smarter
 354 parser.  In the case of the boolean parser, it allows no options (in which case
 355 it assigns the value of true to the variable), or it allows the values
 356 "<tt>true</tt>" or "<tt>false</tt>" to be specified, allowing any of the
 357 following inputs:</p>
 358
 359 <div class="doc_code"><pre>
 360  compiler -f          # No value, 'Force' == true
 361  compiler -f=true     # Value specified, 'Force' == true
 362  compiler -f=TRUE     # Value specified, 'Force' == true
 363  compiler -f=FALSE    # Value specified, 'Force' == false
 364 </pre></div>
 365
 366 <p>... you get the idea.  The <a href="#boolparser">bool parser</a> just turns
 367 the string values into boolean values, and rejects things like '<tt>compiler
 368 -f=foo</tt>'.  Similarly, the <a href="#doubleparser">float</a>, <a
 369 href="#doubleparser">double</a>, and <a href="#intparser">int</a> parsers work
 370 like you would expect, using the '<tt>strtol</tt>' and '<tt>strtod</tt>' C
 371 library calls to parse the string value into the specified data type.</p>
 372
 373 <p>With the declarations above, "<tt>compiler -help</tt>" emits this:</p>
 374
 375 <div class="doc_code"><pre>
 376 USAGE: compiler [options] &lt;input file&gt;
 377
 378 OPTIONS:
 379   <b>-f     - Enable binary output on terminals</b>
 380   -o     - Override output filename
 381   <b>-quiet - Don't print informational messages</b>
 382   -help  - display available options (-help-hidden for more)
 383 </pre></div>
 384
 385 <p>and "<tt>compiler -help-hidden</tt>" prints this:</p>
 386
 387 <div class="doc_code"><pre>
 388 USAGE: compiler [options] &lt;input file&gt;
 389
 390 OPTIONS:
 391   -f     - Enable binary output on terminals
 392   -o     - Override output filename
 393   <b>-q     - Don't print informational messages</b>
 394   -quiet - Don't print informational messages
 395   -help  - display available options (-help-hidden for more)
 396 </pre></div>
 397
 398 <p>This brief example has shown you how to use the '<tt><a
 399 href="#cl::opt">cl::opt</a></tt>' class to parse simple scalar command line
 400 arguments.  In addition to simple scalar arguments, the CommandLine library also
 401 provides primitives to support CommandLine option <a href="#alias">aliases</a>,
 402 and <a href="#list">lists</a> of options.</p>
 403
 404 </div>
 405
 406 <!-- ======================================================================= -->
 407 <h3>
 408   <a name="alias">Argument Aliases</a>
 409 </h3>
 410
 411 <div>
 412
 413 <p>So far, the example works well, except for the fact that we need to check the
 414 quiet condition like this now:</p>
 415
 416 <div class="doc_code"><pre>
 417 ...
 418   if (!Quiet &amp;&amp; !Quiet2) printInformationalMessage(...);
 419 ...
 420 </pre></div>
 421
 422 <p>... which is a real pain!  Instead of defining two values for the same
 423 condition, we can use the "<tt><a href="#cl::alias">cl::alias</a></tt>" class to make the "<tt>-q</tt>"
 424 option an <b>alias</b> for the "<tt>-quiet</tt>" option, instead of providing
 425 a value itself:</p>
 426
 427 <div class="doc_code"><pre>
 428 <a href="#cl::opt">cl::opt</a>&lt;bool&gt; Force ("<i>f</i>", <a href="#cl::desc">cl::desc</a>("<i>Overwrite output files</i>"));
 429 <a href="#cl::opt">cl::opt</a>&lt;bool&gt; Quiet ("<i>quiet</i>", <a href="#cl::desc">cl::desc</a>("<i>Don't print informational messages</i>"));
 430 <a href="#cl::alias">cl::alias</a>     QuietA("<i>q</i>", <a href="#cl::desc">cl::desc</a>("<i>Alias for -quiet</i>"), <a href="#cl::aliasopt">cl::aliasopt</a>(Quiet));
 431 </pre></div>
 432
 433 <p>The third line (which is the only one we modified from above) defines a
 434 "<tt>-q</tt>" alias that updates the "<tt>Quiet</tt>" variable (as specified by
 435 the <tt><a href="#cl::aliasopt">cl::aliasopt</a></tt> modifier) whenever it is
 436 specified.  Because aliases do not hold state, the only thing the program has to
 437 query is the <tt>Quiet</tt> variable now.  Another nice feature of aliases is
 438 that they automatically hide themselves from the <tt>-help</tt> output
 439 (although, again, they are still visible in the <tt>-help-hidden
 440 output</tt>).</p>
 441
 442 <p>Now the application code can simply use:</p>
 443
 444 <div class="doc_code"><pre>
 445 ...
 446   if (!Quiet) printInformationalMessage(...);
 447 ...
 448 </pre></div>
 449
 450 <p>... which is much nicer!  The "<tt><a href="#cl::alias">cl::alias</a></tt>"
 451 can be used to specify an alternative name for any variable type, and has many
 452 uses.</p>
 453
 454 </div>
 455
 456 <!-- ======================================================================= -->
 457 <h3>
 458   <a name="onealternative">Selecting an alternative from a set of
 459   possibilities</a>
 460 </h3>
 461
 462 <div>
 463
 464 <p>So far we have seen how the CommandLine library handles builtin types like
 465 <tt>std::string</tt>, <tt>bool</tt> and <tt>int</tt>, but how does it handle
 466 things it doesn't know about, like enums or '<tt>int*</tt>'s?</p>
 467
 468 <p>The answer is that it uses a table-driven generic parser (unless you specify
 469 your own parser, as described in the <a href="#extensionguide">Extension
 470 Guide</a>).  This parser maps literal strings to whatever type is required, and
 471 requires you to tell it what this mapping should be.</p>
 472
 473 <p>Let's say that we would like to add four optimization levels to our
 474 optimizer, using the standard flags "<tt>-g</tt>", "<tt>-O0</tt>",
 475 "<tt>-O1</tt>", and "<tt>-O2</tt>".  We could easily implement this with boolean
 476 options like above, but there are several problems with this strategy:</p>
 477
 478 <ol>
 479 <li>A user could specify more than one of the options at a time, for example,
 480 "<tt>compiler -O3 -O2</tt>".  The CommandLine library would not be able to
 481 catch this erroneous input for us.</li>
 482
 483 <li>We would have to test 4 different variables to see which ones are set.</li>
 484
 485 <li>This doesn't map to the numeric levels that we want... so we cannot easily
 486 see if some level &gt;= "<tt>-O1</tt>" is enabled.</li>
 487
 488 </ol>
 489
 490 <p>To cope with these problems, we can use an enum value, and have the
 491 CommandLine library fill it in with the appropriate level directly, which is
 492 used like this:</p>
 493
 494 <div class="doc_code"><pre>
 495 enum OptLevel {
 496   g, O1, O2, O3
 497 };
 498
 499 <a href="#cl::opt">cl::opt</a>&lt;OptLevel&gt; OptimizationLevel(<a href="#cl::desc">cl::desc</a>("<i>Choose optimization level:</i>"),
 500   <a href="#cl::values">cl::values</a>(
 501     clEnumVal(g , "<i>No optimizations, enable debugging</i>"),
 502     clEnumVal(O1, "<i>Enable trivial optimizations</i>"),
 503     clEnumVal(O2, "<i>Enable default optimizations</i>"),
 504     clEnumVal(O3, "<i>Enable expensive optimizations</i>"),
 505    clEnumValEnd));
 506
 507 ...
 508   if (OptimizationLevel &gt;= O2) doPartialRedundancyElimination(...);
 509 ...
 510 </pre></div>
 511
 512 <p>This declaration defines a variable "<tt>OptimizationLevel</tt>" of the
 513 "<tt>OptLevel</tt>" enum type.  This variable can be assigned any of the values
 514 that are listed in the declaration (Note that the declaration list must be
 515 terminated with the "<tt>clEnumValEnd</tt>" argument!).  The CommandLine
 516 library enforces
 517 that the user can only specify one of the options, and it ensure that only valid
 518 enum values can be specified.  The "<tt>clEnumVal</tt>" macros ensure that the
 519 command line arguments matched the enum values.  With this option added, our
 520 help output now is:</p>
 521
 522 <div class="doc_code"><pre>
 523 USAGE: compiler [options] &lt;input file&gt;
 524
 525 OPTIONS:
 526   <b>Choose optimization level:
 527     -g          - No optimizations, enable debugging
 528     -O1         - Enable trivial optimizations
 529     -O2         - Enable default optimizations
 530     -O3         - Enable expensive optimizations</b>
 531   -f            - Enable binary output on terminals
 532   -help         - display available options (-help-hidden for more)
 533   -o &lt;filename&gt; - Specify output filename
 534   -quiet        - Don't print informational messages
 535 </pre></div>
 536
 537 <p>In this case, it is sort of awkward that flag names correspond directly to
 538 enum names, because we probably don't want a enum definition named "<tt>g</tt>"
 539 in our program.  Because of this, we can alternatively write this example like
 540 this:</p>
 541
 542 <div class="doc_code"><pre>
 543 enum OptLevel {
 544   Debug, O1, O2, O3
 545 };
 546
 547 <a href="#cl::opt">cl::opt</a>&lt;OptLevel&gt; OptimizationLevel(<a href="#cl::desc">cl::desc</a>("<i>Choose optimization level:</i>"),
 548   <a href="#cl::values">cl::values</a>(
 549    clEnumValN(Debug, "g", "<i>No optimizations, enable debugging</i>"),
 550     clEnumVal(O1        , "<i>Enable trivial optimizations</i>"),
 551     clEnumVal(O2        , "<i>Enable default optimizations</i>"),
 552     clEnumVal(O3        , "<i>Enable expensive optimizations</i>"),
 553    clEnumValEnd));
 554
 555 ...
 556   if (OptimizationLevel == Debug) outputDebugInfo(...);
 557 ...
 558 </pre></div>
 559
 560 <p>By using the "<tt>clEnumValN</tt>" macro instead of "<tt>clEnumVal</tt>", we
 561 can directly specify the name that the flag should get.  In general a direct
 562 mapping is nice, but sometimes you can't or don't want to preserve the mapping,
 563 which is when you would use it.</p>
 564
 565 </div>
 566
 567 <!-- ======================================================================= -->
 568 <h3>
 569   <a name="namedalternatives">Named Alternatives</a>
 570 </h3>
 571
 572 <div>
 573
 574 <p>Another useful argument form is a named alternative style.  We shall use this
 575 style in our compiler to specify different debug levels that can be used.
 576 Instead of each debug level being its own switch, we want to support the
 577 following options, of which only one can be specified at a time:
 578 "<tt>--debug-level=none</tt>", "<tt>--debug-level=quick</tt>",
 579 "<tt>--debug-level=detailed</tt>".  To do this, we use the exact same format as
 580 our optimization level flags, but we also specify an option name.  For this
 581 case, the code looks like this:</p>
 582
 583 <div class="doc_code"><pre>
 584 enum DebugLev {
 585   nodebuginfo, quick, detailed
 586 };
 587
 588 // Enable Debug Options to be specified on the command line
 589 <a href="#cl::opt">cl::opt</a>&lt;DebugLev&gt; DebugLevel("<i>debug_level</i>", <a href="#cl::desc">cl::desc</a>("<i>Set the debugging level:</i>"),
 590   <a href="#cl::values">cl::values</a>(
 591     clEnumValN(nodebuginfo, "none", "<i>disable debug information</i>"),
 592      clEnumVal(quick,               "<i>enable quick debug information</i>"),
 593      clEnumVal(detailed,            "<i>enable detailed debug information</i>"),
 594     clEnumValEnd));
 595 </pre></div>
 596
 597 <p>This definition defines an enumerated command line variable of type "<tt>enum
 598 DebugLev</tt>", which works exactly the same way as before.  The difference here
 599 is just the interface exposed to the user of your program and the help output by
 600 the "<tt>-help</tt>" option:</p>
 601
 602 <div class="doc_code"><pre>
 603 USAGE: compiler [options] &lt;input file&gt;
 604
 605 OPTIONS:
 606   Choose optimization level:
 607     -g          - No optimizations, enable debugging
 608     -O1         - Enable trivial optimizations
 609     -O2         - Enable default optimizations
 610     -O3         - Enable expensive optimizations
 611   <b>-debug_level  - Set the debugging level:
 612     =none       - disable debug information
 613     =quick      - enable quick debug information
 614     =detailed   - enable detailed debug information</b>
 615   -f            - Enable binary output on terminals
 616   -help         - display available options (-help-hidden for more)
 617   -o &lt;filename&gt; - Specify output filename
 618   -quiet        - Don't print informational messages
 619 </pre></div>
 620
 621 <p>Again, the only structural difference between the debug level declaration and
 622 the optimization level declaration is that the debug level declaration includes
 623 an option name (<tt>"debug_level"</tt>), which automatically changes how the
 624 library processes the argument.  The CommandLine library supports both forms so
 625 that you can choose the form most appropriate for your application.</p>
 626
 627 </div>
 628
 629 <!-- ======================================================================= -->
 630 <h3>
 631   <a name="list">Parsing a list of options</a>
 632 </h3>
 633
 634 <div>
 635
 636 <p>Now that we have the standard run-of-the-mill argument types out of the way,
 637 lets get a little wild and crazy.  Lets say that we want our optimizer to accept
 638 a <b>list</b> of optimizations to perform, allowing duplicates.  For example, we
 639 might want to run: "<tt>compiler -dce -constprop -inline -dce -strip</tt>".  In
 640 this case, the order of the arguments and the number of appearances is very
 641 important.  This is what the "<tt><a href="#cl::list">cl::list</a></tt>"
 642 template is for.  First, start by defining an enum of the optimizations that you
 643 would like to perform:</p>
 644
 645 <div class="doc_code"><pre>
 646 enum Opts {
 647   // 'inline' is a C++ keyword, so name it 'inlining'
 648   dce, constprop, inlining, strip
 649 };
 650 </pre></div>
 651
 652 <p>Then define your "<tt><a href="#cl::list">cl::list</a></tt>" variable:</p>
 653
 654 <div class="doc_code"><pre>
 655 <a href="#cl::list">cl::list</a>&lt;Opts&gt; OptimizationList(<a href="#cl::desc">cl::desc</a>("<i>Available Optimizations:</i>"),
 656   <a href="#cl::values">cl::values</a>(
 657     clEnumVal(dce               , "<i>Dead Code Elimination</i>"),
 658     clEnumVal(constprop         , "<i>Constant Propagation</i>"),
 659    clEnumValN(inlining, "<i>inline</i>", "<i>Procedure Integration</i>"),
 660     clEnumVal(strip             , "<i>Strip Symbols</i>"),
 661   clEnumValEnd));
 662 </pre></div>
 663
 664 <p>This defines a variable that is conceptually of the type
 665 "<tt>std::vector&lt;enum Opts&gt;</tt>".  Thus, you can access it with standard
 666 vector methods:</p>
 667
 668 <div class="doc_code"><pre>
 669   for (unsigned i = 0; i != OptimizationList.size(); ++i)
 670     switch (OptimizationList[i])
 671        ...
 672 </pre></div>
 673
 674 <p>... to iterate through the list of options specified.</p>
 675
 676 <p>Note that the "<tt><a href="#cl::list">cl::list</a></tt>" template is
 677 completely general and may be used with any data types or other arguments that
 678 you can use with the "<tt><a href="#cl::opt">cl::opt</a></tt>" template.  One
 679 especially useful way to use a list is to capture all of the positional
 680 arguments together if there may be more than one specified.  In the case of a
 681 linker, for example, the linker takes several '<tt>.o</tt>' files, and needs to
 682 capture them into a list.  This is naturally specified as:</p>
 683
 684 <div class="doc_code"><pre>
 685 ...
 686 <a href="#cl::list">cl::list</a>&lt;std::string&gt; InputFilenames(<a href="#cl::Positional">cl::Positional</a>, <a href="#cl::desc">cl::desc</a>("&lt;Input files&gt;"), <a href="#cl::OneOrMore">cl::OneOrMore</a>);
 687 ...
 688 </pre></div>
 689
 690 <p>This variable works just like a "<tt>vector&lt;string&gt;</tt>" object.  As
 691 such, accessing the list is simple, just like above.  In this example, we used
 692 the <tt><a href="#cl::OneOrMore">cl::OneOrMore</a></tt> modifier to inform the
 693 CommandLine library that it is an error if the user does not specify any
 694 <tt>.o</tt> files on our command line.  Again, this just reduces the amount of
 695 checking we have to do.</p>
 696
 697 </div>
 698
 699 <!-- ======================================================================= -->
 700 <h3>
 701   <a name="bits">Collecting options as a set of flags</a>
 702 </h3>
 703
 704 <div>
 705
 706 <p>Instead of collecting sets of options in a list, it is also possible to
 707 gather information for enum values in a <b>bit vector</b>.  The representation used by
 708 the <a href="#bits"><tt>cl::bits</tt></a> class is an <tt>unsigned</tt>
 709 integer.  An enum value is represented by a 0/1 in the enum's ordinal value bit
 710 position. 1 indicating that the enum was specified, 0 otherwise.  As each
 711 specified value is parsed, the resulting enum's bit is set in the option's bit
 712 vector:</p>
 713
 714 <div class="doc_code"><pre>
 715   <i>bits</i> |= 1 << (unsigned)<i>enum</i>;
 716 </pre></div>
 717
 718 <p>Options that are specified multiple times are redundant.  Any instances after
 719 the first are discarded.</p>
 720
 721 <p>Reworking the above list example, we could replace <a href="#list">
 722 <tt>cl::list</tt></a> with <a href="#bits"><tt>cl::bits</tt></a>:</p>
 723
 724 <div class="doc_code"><pre>
 725 <a href="#cl::bits">cl::bits</a>&lt;Opts&gt; OptimizationBits(<a href="#cl::desc">cl::desc</a>("<i>Available Optimizations:</i>"),
 726   <a href="#cl::values">cl::values</a>(
 727     clEnumVal(dce               , "<i>Dead Code Elimination</i>"),
 728     clEnumVal(constprop         , "<i>Constant Propagation</i>"),
 729    clEnumValN(inlining, "<i>inline</i>", "<i>Procedure Integration</i>"),
 730     clEnumVal(strip             , "<i>Strip Symbols</i>"),
 731   clEnumValEnd));
 732 </pre></div>
 733
 734 <p>To test to see if <tt>constprop</tt> was specified, we can use the
 735 <tt>cl:bits::isSet</tt> function:</p>
 736
 737 <div class="doc_code"><pre>
 738   if (OptimizationBits.isSet(constprop)) {
 739     ...
 740   }
 741 </pre></div>
 742
 743 <p>It's also possible to get the raw bit vector using the
 744 <tt>cl::bits::getBits</tt> function:</p>
 745
 746 <div class="doc_code"><pre>
 747   unsigned bits = OptimizationBits.getBits();
 748 </pre></div>
 749
 750 <p>Finally, if external storage is used, then the location specified must be of
 751 <b>type</b> <tt>unsigned</tt>. In all other ways a <a
 752 href="#bits"><tt>cl::bits</tt></a> option is equivalent to a <a
 753 href="#list"> <tt>cl::list</tt></a> option.</p>
 754
 755 </div>
 756
 757
 758 <!-- ======================================================================= -->
 759 <h3>
 760   <a name="description">Adding freeform text to help output</a>
 761 </h3>
 762
 763 <div>
 764
 765 <p>As our program grows and becomes more mature, we may decide to put summary
 766 information about what it does into the help output.  The help output is styled
 767 to look similar to a Unix <tt>man</tt> page, providing concise information about
 768 a program.  Unix <tt>man</tt> pages, however often have a description about what
 769 the program does.  To add this to your CommandLine program, simply pass a third
 770 argument to the <a
 771 href="#cl::ParseCommandLineOptions"><tt>cl::ParseCommandLineOptions</tt></a>
 772 call in main.  This additional argument is then printed as the overview
 773 information for your program, allowing you to include any additional information
 774 that you want.  For example:</p>
 775
 776 <div class="doc_code"><pre>
 777 int main(int argc, char **argv) {
 778   <a href="#cl::ParseCommandLineOptions">cl::ParseCommandLineOptions</a>(argc, argv, " CommandLine compiler example\n\n"
 779                               "  This program blah blah blah...\n");
 780   ...
 781 }
 782 </pre></div>
 783
 784 <p>would yield the help output:</p>
 785
 786 <div class="doc_code"><pre>
 787 <b>OVERVIEW: CommandLine compiler example
 788
 789   This program blah blah blah...</b>
 790
 791 USAGE: compiler [options] &lt;input file&gt;
 792
 793 OPTIONS:
 794   ...
 795   -help             - display available options (-help-hidden for more)
 796   -o &lt;filename&gt;     - Specify output filename
 797 </pre></div>
 798
 799 </div>
 800
 801 </div>
 802
 803 <!-- *********************************************************************** -->
 804 <h2>
 805   <a name="referenceguide">Reference Guide</a>
 806 </h2>
 807 <!-- *********************************************************************** -->
 808
 809 <div>
 810
 811 <p>Now that you know the basics of how to use the CommandLine library, this
 812 section will give you the detailed information you need to tune how command line
 813 options work, as well as information on more "advanced" command line option
 814 processing capabilities.</p>
 815
 816 <!-- ======================================================================= -->
 817 <h3>
 818   <a name="positional">Positional Arguments</a>
 819 </h3>
 820
 821 <div>
 822
 823 <p>Positional arguments are those arguments that are not named, and are not
 824 specified with a hyphen.  Positional arguments should be used when an option is
 825 specified by its position alone.  For example, the standard Unix <tt>grep</tt>
 826 tool takes a regular expression argument, and an optional filename to search
 827 through (which defaults to standard input if a filename is not specified).
 828 Using the CommandLine library, this would be specified as:</p>
 829
 830 <div class="doc_code"><pre>
 831 <a href="#cl::opt">cl::opt</a>&lt;string&gt; Regex   (<a href="#cl::Positional">cl::Positional</a>, <a href="#cl::desc">cl::desc</a>("<i>&lt;regular expression&gt;</i>"), <a href="#cl::Required">cl::Required</a>);
 832 <a href="#cl::opt">cl::opt</a>&lt;string&gt; Filename(<a href="#cl::Positional">cl::Positional</a>, <a href="#cl::desc">cl::desc</a>("<i>&lt;input file&gt;</i>"), <a href="#cl::init">cl::init</a>("<i>-</i>"));
 833 </pre></div>
 834
 835 <p>Given these two option declarations, the <tt>-help</tt> output for our grep
 836 replacement would look like this:</p>
 837
 838 <div class="doc_code"><pre>
 839 USAGE: spiffygrep [options] <b>&lt;regular expression&gt; &lt;input file&gt;</b>
 840
 841 OPTIONS:
 842   -help - display available options (-help-hidden for more)
 843 </pre></div>
 844
 845 <p>... and the resultant program could be used just like the standard
 846 <tt>grep</tt> tool.</p>
 847
 848 <p>Positional arguments are sorted by their order of construction.  This means
 849 that command line options will be ordered according to how they are listed in a
 850 .cpp file, but will not have an ordering defined if the positional arguments
 851 are defined in multiple .cpp files.  The fix for this problem is simply to
 852 define all of your positional arguments in one .cpp file.</p>
 853
 854 <!-- _______________________________________________________________________ -->
 855 <h4>
 856   <a name="--">Specifying positional options with hyphens</a>
 857 </h4>
 858
 859 <div>
 860
 861 <p>Sometimes you may want to specify a value to your positional argument that
 862 starts with a hyphen (for example, searching for '<tt>-foo</tt>' in a file).  At
 863 first, you will have trouble doing this, because it will try to find an argument
 864 named '<tt>-foo</tt>', and will fail (and single quotes will not save you).
 865 Note that the system <tt>grep</tt> has the same problem:</p>
 866
 867 <div class="doc_code"><pre>
 868   $ spiffygrep '-foo' test.txt
 869   Unknown command line argument '-foo'.  Try: spiffygrep -help'
 870
 871   $ grep '-foo' test.txt
 872   grep: illegal option -- f
 873   grep: illegal option -- o
 874   grep: illegal option -- o
 875   Usage: grep -hblcnsviw pattern file . . .
 876 </pre></div>
 877
 878 <p>The solution for this problem is the same for both your tool and the system
 879 version: use the '<tt>--</tt>' marker.  When the user specifies '<tt>--</tt>' on
 880 the command line, it is telling the program that all options after the
 881 '<tt>--</tt>' should be treated as positional arguments, not options.  Thus, we
 882 can use it like this:</p>
 883
 884 <div class="doc_code"><pre>
 885   $ spiffygrep -- -foo test.txt
 886     ...output...
 887 </pre></div>
 888
 889 </div>
 890
 891 <!-- _______________________________________________________________________ -->
 892 <h4>
 893   <a name="getPosition">Determining absolute position with getPosition()</a>
 894 </h4>
 895 <div>
 896   <p>Sometimes an option can affect or modify the meaning of another option. For
 897   example, consider <tt>gcc</tt>'s <tt>-x LANG</tt> option. This tells
 898   <tt>gcc</tt> to ignore the suffix of subsequent positional arguments and force
 899   the file to be interpreted as if it contained source code in language
 900   <tt>LANG</tt>. In order to handle this properly, you need to know the
 901   absolute position of each argument, especially those in lists, so their
 902   interaction(s) can be applied correctly. This is also useful for options like
 903   <tt>-llibname</tt> which is actually a positional argument that starts with
 904   a dash.</p>
 905   <p>So, generally, the problem is that you have two <tt>cl::list</tt> variables
 906   that interact in some way. To ensure the correct interaction, you can use the
 907   <tt>cl::list::getPosition(optnum)</tt> method. This method returns the
 908   absolute position (as found on the command line) of the <tt>optnum</tt>
 909   item in the <tt>cl::list</tt>.</p>
 910   <p>The idiom for usage is like this:</p>
 911
 912   <div class="doc_code"><pre>
 913   static cl::list&lt;std::string&gt; Files(cl::Positional, cl::OneOrMore);
 914   static cl::list&lt;std::string&gt; Libraries("l", cl::ZeroOrMore);
 915
 916   int main(int argc, char**argv) {
 917     // ...
 918     std::vector&lt;std::string&gt;::iterator fileIt = Files.begin();
 919     std::vector&lt;std::string&gt;::iterator libIt  = Libraries.begin();
 920     unsigned libPos = 0, filePos = 0;
 921     while ( 1 ) {
 922       if ( libIt != Libraries.end() )
 923         libPos = Libraries.getPosition( libIt - Libraries.begin() );
 924       else
 925         libPos = 0;
 926       if ( fileIt != Files.end() )
 927         filePos = Files.getPosition( fileIt - Files.begin() );
 928       else
 929         filePos = 0;
 930
 931       if ( filePos != 0 &amp;&amp; (libPos == 0 || filePos &lt; libPos) ) {
 932         // Source File Is next
 933         ++fileIt;
 934       }
 935       else if ( libPos != 0 &amp;&amp; (filePos == 0 || libPos &lt; filePos) ) {
 936         // Library is next
 937         ++libIt;
 938       }
 939       else
 940         break; // we're done with the list
 941     }
 942   }</pre></div>
 943
 944   <p>Note that, for compatibility reasons, the <tt>cl::opt</tt> also supports an
 945   <tt>unsigned getPosition()</tt> option that will provide the absolute position
 946   of that option. You can apply the same approach as above with a
 947   <tt>cl::opt</tt> and a <tt>cl::list</tt> option as you can with two lists.</p>
 948 </div>
 949
 950 <!-- _______________________________________________________________________ -->
 951 <h4>
 952   <a name="cl::ConsumeAfter">The <tt>cl::ConsumeAfter</tt> modifier</a>
 953 </h4>
 954
 955 <div>
 956
 957 <p>The <tt>cl::ConsumeAfter</tt> <a href="#formatting">formatting option</a> is
 958 used to construct programs that use "interpreter style" option processing.  With
 959 this style of option processing, all arguments specified after the last
 960 positional argument are treated as special interpreter arguments that are not
 961 interpreted by the command line argument.</p>
 962
 963 <p>As a concrete example, lets say we are developing a replacement for the
 964 standard Unix Bourne shell (<tt>/bin/sh</tt>).  To run <tt>/bin/sh</tt>, first
 965 you specify options to the shell itself (like <tt>-x</tt> which turns on trace
 966 output), then you specify the name of the script to run, then you specify
 967 arguments to the script.  These arguments to the script are parsed by the Bourne
 968 shell command line option processor, but are not interpreted as options to the
 969 shell itself.  Using the CommandLine library, we would specify this as:</p>
 970
 971 <div class="doc_code"><pre>
 972 <a href="#cl::opt">cl::opt</a>&lt;string&gt; Script(<a href="#cl::Positional">cl::Positional</a>, <a href="#cl::desc">cl::desc</a>("<i>&lt;input script&gt;</i>"), <a href="#cl::init">cl::init</a>("-"));
 973 <a href="#cl::list">cl::list</a>&lt;string&gt;  Argv(<a href="#cl::ConsumeAfter">cl::ConsumeAfter</a>, <a href="#cl::desc">cl::desc</a>("<i>&lt;program arguments&gt;...</i>"));
 974 <a href="#cl::opt">cl::opt</a>&lt;bool&gt;    Trace("<i>x</i>", <a href="#cl::desc">cl::desc</a>("<i>Enable trace output</i>"));
 975 </pre></div>
 976
 977 <p>which automatically provides the help output:</p>
 978
 979 <div class="doc_code"><pre>
 980 USAGE: spiffysh [options] <b>&lt;input script&gt; &lt;program arguments&gt;...</b>
 981
 982 OPTIONS:
 983   -help - display available options (-help-hidden for more)
 984   <b>-x    - Enable trace output</b>
 985 </pre></div>
 986
 987 <p>At runtime, if we run our new shell replacement as `<tt>spiffysh -x test.sh
 988 -a -x -y bar</tt>', the <tt>Trace</tt> variable will be set to true, the
 989 <tt>Script</tt> variable will be set to "<tt>test.sh</tt>", and the
 990 <tt>Argv</tt> list will contain <tt>["-a", "-x", "-y", "bar"]</tt>, because they
 991 were specified after the last positional argument (which is the script
 992 name).</p>
 993
 994 <p>There are several limitations to when <tt>cl::ConsumeAfter</tt> options can
 995 be specified.  For example, only one <tt>cl::ConsumeAfter</tt> can be specified
 996 per program, there must be at least one <a href="#positional">positional
 997 argument</a> specified, there must not be any <a href="#cl::list">cl::list</a>
 998 positional arguments, and the <tt>cl::ConsumeAfter</tt> option should be a <a
 999 href="#cl::list">cl::list</a> option.</p>
1000
1001 </div>
1002
1003 </div>
1004
1005 <!-- ======================================================================= -->
1006 <h3>
1007   <a name="storage">Internal vs External Storage</a>
1008 </h3>
1009
1010 <div>
1011
1012 <p>By default, all command line options automatically hold the value that they
1013 parse from the command line.  This is very convenient in the common case,
1014 especially when combined with the ability to define command line options in the
1015 files that use them.  This is called the internal storage model.</p>
1016
1017 <p>Sometimes, however, it is nice to separate the command line option processing
1018 code from the storage of the value parsed.  For example, lets say that we have a
1019 '<tt>-debug</tt>' option that we would like to use to enable debug information
1020 across the entire body of our program.  In this case, the boolean value
1021 controlling the debug code should be globally accessible (in a header file, for
1022 example) yet the command line option processing code should not be exposed to
1023 all of these clients (requiring lots of .cpp files to #include
1024 <tt>CommandLine.h</tt>).</p>
1025
1026 <p>To do this, set up your .h file with your option, like this for example:</p>
1027
1028 <div class="doc_code">
1029 <pre>
1030 <i>// DebugFlag.h - Get access to the '-debug' command line option
1031 //
1032
1033 // DebugFlag - This boolean is set to true if the '-debug' command line option
1034 // is specified.  This should probably not be referenced directly, instead, use
1035 // the DEBUG macro below.
1036 //</i>
1037 extern bool DebugFlag;
1038
1039 <i>// DEBUG macro - This macro should be used by code to emit debug information.
1040 // In the '-debug' option is specified on the command line, and if this is a
1041 // debug build, then the code specified as the option to the macro will be
1042 // executed.  Otherwise it will not be.</i>
1043 <span class="doc_hilite">#ifdef NDEBUG
1044 #define DEBUG(X)
1045 #else
1046 #define DEBUG(X)</span> do { if (DebugFlag) { X; } } while (0)
1047 <span class="doc_hilite">#endif</span>
1048 </pre>
1049 </div>
1050
1051 <p>This allows clients to blissfully use the <tt>DEBUG()</tt> macro, or the
1052 <tt>DebugFlag</tt> explicitly if they want to.  Now we just need to be able to
1053 set the <tt>DebugFlag</tt> boolean when the option is set.  To do this, we pass
1054 an additional argument to our command line argument processor, and we specify
1055 where to fill in with the <a href="#cl::location">cl::location</a>
1056 attribute:</p>
1057
1058 <div class="doc_code">
1059 <pre>
1060 bool DebugFlag;                  <i>// the actual value</i>
1061 static <a href="#cl::opt">cl::opt</a>&lt;bool, true&gt;       <i>// The parser</i>
1062 Debug("<i>debug</i>", <a href="#cl::desc">cl::desc</a>("<i>Enable debug output</i>"), <a href="#cl::Hidden">cl::Hidden</a>, <a href="#cl::location">cl::location</a>(DebugFlag));
1063 </pre>
1064 </div>
1065
1066 <p>In the above example, we specify "<tt>true</tt>" as the second argument to
1067 the <tt><a href="#cl::opt">cl::opt</a></tt> template, indicating that the
1068 template should not maintain a copy of the value itself.  In addition to this,
1069 we specify the <tt><a href="#cl::location">cl::location</a></tt> attribute, so
1070 that <tt>DebugFlag</tt> is automatically set.</p>
1071
1072 </div>
1073
1074 <!-- ======================================================================= -->
1075 <h3>
1076   <a name="attributes">Option Attributes</a>
1077 </h3>
1078
1079 <div>
1080
1081 <p>This section describes the basic attributes that you can specify on
1082 options.</p>
1083
1084 <ul>
1085
1086 <li>The option name attribute (which is required for all options, except <a
1087 href="#positional">positional options</a>) specifies what the option name is.
1088 This option is specified in simple double quotes:
1089
1090 <pre>
1091 <a href="#cl::opt">cl::opt</a>&lt;<b>bool</b>&gt; Quiet("<i>quiet</i>");
1092 </pre>
1093
1094 </li>
1095
1096 <li><a name="cl::desc">The <b><tt>cl::desc</tt></b></a> attribute specifies a
1097 description for the option to be shown in the <tt>-help</tt> output for the
1098 program.</li>
1099
1100 <li><a name="cl::value_desc">The <b><tt>cl::value_desc</tt></b></a> attribute
1101 specifies a string that can be used to fine tune the <tt>-help</tt> output for
1102 a command line option.  Look <a href="#value_desc_example">here</a> for an
1103 example.</li>
1104
1105 <li><a name="cl::init">The <b><tt>cl::init</tt></b></a> attribute specifies an
1106 initial value for a <a href="#cl::opt">scalar</a> option.  If this attribute is
1107 not specified then the command line option value defaults to the value created
1108 by the default constructor for the type. <b>Warning</b>: If you specify both
1109 <b><tt>cl::init</tt></b> and <b><tt>cl::location</tt></b> for an option,
1110 you must specify <b><tt>cl::location</tt></b> first, so that when the
1111 command-line parser sees <b><tt>cl::init</tt></b>, it knows where to put the
1112 initial value. (You will get an error at runtime if you don't put them in
1113 the right order.)</li>
1114
1115 <li><a name="cl::location">The <b><tt>cl::location</tt></b></a> attribute where
1116 to store the value for a parsed command line option if using external storage.
1117 See the section on <a href="#storage">Internal vs External Storage</a> for more
1118 information.</li>
1119
1120 <li><a name="cl::aliasopt">The <b><tt>cl::aliasopt</tt></b></a> attribute
1121 specifies which option a <tt><a href="#cl::alias">cl::alias</a></tt> option is
1122 an alias for.</li>
1123
1124 <li><a name="cl::values">The <b><tt>cl::values</tt></b></a> attribute specifies
1125 the string-to-value mapping to be used by the generic parser.  It takes a
1126 <b>clEnumValEnd terminated</b> list of (option, value, description) triplets
1127 that
1128 specify the option name, the value mapped to, and the description shown in the
1129 <tt>-help</tt> for the tool.  Because the generic parser is used most
1130 frequently with enum values, two macros are often useful:
1131
1132 <ol>
1133
1134 <li><a name="clEnumVal">The <b><tt>clEnumVal</tt></b></a> macro is used as a
1135 nice simple way to specify a triplet for an enum.  This macro automatically
1136 makes the option name be the same as the enum name.  The first option to the
1137 macro is the enum, the second is the description for the command line
1138 option.</li>
1139
1140 <li><a name="clEnumValN">The <b><tt>clEnumValN</tt></b></a> macro is used to
1141 specify macro options where the option name doesn't equal the enum name.  For
1142 this macro, the first argument is the enum value, the second is the flag name,
1143 and the second is the description.</li>
1144
1145 </ol>
1146
1147 You will get a compile time error if you try to use cl::values with a parser
1148 that does not support it.</li>
1149
1150 <li><a name="cl::multi_val">The <b><tt>cl::multi_val</tt></b></a>
1151 attribute specifies that this option takes has multiple values
1152 (example: <tt>-sectalign segname sectname sectvalue</tt>). This
1153 attribute takes one unsigned argument - the number of values for the
1154 option. This attribute is valid only on <tt>cl::list</tt> options (and
1155 will fail with compile error if you try to use it with other option
1156 types). It is allowed to use all of the usual modifiers on
1157 multi-valued options (besides <tt>cl::ValueDisallowed</tt>,
1158 obviously).</li>
1159
1160 </ul>
1161
1162 </div>
1163
1164 <!-- ======================================================================= -->
1165 <h3>
1166   <a name="modifiers">Option Modifiers</a>
1167 </h3>
1168
1169 <div>
1170
1171 <p>Option modifiers are the flags and expressions that you pass into the
1172 constructors for <tt><a href="#cl::opt">cl::opt</a></tt> and <tt><a
1173 href="#cl::list">cl::list</a></tt>.  These modifiers give you the ability to
1174 tweak how options are parsed and how <tt>-help</tt> output is generated to fit
1175 your application well.</p>
1176
1177 <p>These options fall into five main categories:</p>
1178
1179 <ol>
1180 <li><a href="#hiding">Hiding an option from <tt>-help</tt> output</a></li>
1181 <li><a href="#numoccurrences">Controlling the number of occurrences
1182                              required and allowed</a></li>
1183 <li><a href="#valrequired">Controlling whether or not a value must be
1184                            specified</a></li>
1185 <li><a href="#formatting">Controlling other formatting options</a></li>
1186 <li><a href="#misc">Miscellaneous option modifiers</a></li>
1187 </ol>
1188
1189 <p>It is not possible to specify two options from the same category (you'll get
1190 a runtime error) to a single option, except for options in the miscellaneous
1191 category.  The CommandLine library specifies defaults for all of these settings
1192 that are the most useful in practice and the most common, which mean that you
1193 usually shouldn't have to worry about these.</p>
1194
1195 <!-- _______________________________________________________________________ -->
1196 <h4>
1197   <a name="hiding">Hiding an option from <tt>-help</tt> output</a>
1198 </h4>
1199
1200 <div>
1201
1202 <p>The <tt>cl::NotHidden</tt>, <tt>cl::Hidden</tt>, and
1203 <tt>cl::ReallyHidden</tt> modifiers are used to control whether or not an option
1204 appears in the <tt>-help</tt> and <tt>-help-hidden</tt> output for the
1205 compiled program:</p>
1206
1207 <ul>
1208
1209 <li><a name="cl::NotHidden">The <b><tt>cl::NotHidden</tt></b></a> modifier
1210 (which is the default for <tt><a href="#cl::opt">cl::opt</a></tt> and <tt><a
1211 href="#cl::list">cl::list</a></tt> options) indicates the option is to appear
1212 in both help listings.</li>
1213
1214 <li><a name="cl::Hidden">The <b><tt>cl::Hidden</tt></b></a> modifier (which is the
1215 default for <tt><a href="#cl::alias">cl::alias</a></tt> options) indicates that
1216 the option should not appear in the <tt>-help</tt> output, but should appear in
1217 the <tt>-help-hidden</tt> output.</li>
1218
1219 <li><a name="cl::ReallyHidden">The <b><tt>cl::ReallyHidden</tt></b></a> modifier
1220 indicates that the option should not appear in any help output.</li>
1221
1222 </ul>
1223
1224 </div>
1225
1226 <!-- _______________________________________________________________________ -->
1227 <h4>
1228   <a name="numoccurrences">Controlling the number of occurrences required and
1229   allowed</a>
1230 </h4>
1231
1232 <div>
1233
1234 <p>This group of options is used to control how many time an option is allowed
1235 (or required) to be specified on the command line of your program.  Specifying a
1236 value for this setting allows the CommandLine library to do error checking for
1237 you.</p>
1238
1239 <p>The allowed values for this option group are:</p>
1240
1241 <ul>
1242
1243 <li><a name="cl::Optional">The <b><tt>cl::Optional</tt></b></a> modifier (which
1244 is the default for the <tt><a href="#cl::opt">cl::opt</a></tt> and <tt><a
1245 href="#cl::alias">cl::alias</a></tt> classes) indicates that your program will
1246 allow either zero or one occurrence of the option to be specified.</li>
1247
1248 <li><a name="cl::ZeroOrMore">The <b><tt>cl::ZeroOrMore</tt></b></a> modifier
1249 (which is the default for the <tt><a href="#cl::list">cl::list</a></tt> class)
1250 indicates that your program will allow the option to be specified zero or more
1251 times.</li>
1252
1253 <li><a name="cl::Required">The <b><tt>cl::Required</tt></b></a> modifier
1254 indicates that the specified option must be specified exactly one time.</li>
1255
1256 <li><a name="cl::OneOrMore">The <b><tt>cl::OneOrMore</tt></b></a> modifier
1257 indicates that the option must be specified at least one time.</li>
1258
1259 <li>The <b><tt>cl::ConsumeAfter</tt></b> modifier is described in the <a
1260 href="#positional">Positional arguments section</a>.</li>
1261
1262 </ul>
1263
1264 <p>If an option is not specified, then the value of the option is equal to the
1265 value specified by the <tt><a href="#cl::init">cl::init</a></tt> attribute.  If
1266 the <tt><a href="#cl::init">cl::init</a></tt> attribute is not specified, the
1267 option value is initialized with the default constructor for the data type.</p>
1268
1269 <p>If an option is specified multiple times for an option of the <tt><a
1270 href="#cl::opt">cl::opt</a></tt> class, only the last value will be
1271 retained.</p>
1272
1273 </div>
1274
1275 <!-- _______________________________________________________________________ -->
1276 <h4>
1277   <a name="valrequired">Controlling whether or not a value must be specified</a>
1278 </h4>
1279
1280 <div>
1281
1282 <p>This group of options is used to control whether or not the option allows a
1283 value to be present.  In the case of the CommandLine library, a value is either
1284 specified with an equal sign (e.g. '<tt>-index-depth=17</tt>') or as a trailing
1285 string (e.g. '<tt>-o a.out</tt>').</p>
1286
1287 <p>The allowed values for this option group are:</p>
1288
1289 <ul>
1290
1291 <li><a name="cl::ValueOptional">The <b><tt>cl::ValueOptional</tt></b></a> modifier
1292 (which is the default for <tt>bool</tt> typed options) specifies that it is
1293 acceptable to have a value, or not.  A boolean argument can be enabled just by
1294 appearing on the command line, or it can have an explicit '<tt>-foo=true</tt>'.
1295 If an option is specified with this mode, it is illegal for the value to be
1296 provided without the equal sign.  Therefore '<tt>-foo true</tt>' is illegal.  To
1297 get this behavior, you must use the <a
1298 href="#cl::ValueRequired">cl::ValueRequired</a> modifier.</li>
1299
1300 <li><a name="cl::ValueRequired">The <b><tt>cl::ValueRequired</tt></b></a> modifier
1301 (which is the default for all other types except for <a
1302 href="#onealternative">unnamed alternatives using the generic parser</a>)
1303 specifies that a value must be provided.  This mode informs the command line
1304 library that if an option is not provides with an equal sign, that the next
1305 argument provided must be the value.  This allows things like '<tt>-o
1306 a.out</tt>' to work.</li>
1307
1308 <li><a name="cl::ValueDisallowed">The <b><tt>cl::ValueDisallowed</tt></b></a>
1309 modifier (which is the default for <a href="#onealternative">unnamed
1310 alternatives using the generic parser</a>) indicates that it is a runtime error
1311 for the user to specify a value.  This can be provided to disallow users from
1312 providing options to boolean options (like '<tt>-foo=true</tt>').</li>
1313
1314 </ul>
1315
1316 <p>In general, the default values for this option group work just like you would
1317 want them to.  As mentioned above, you can specify the <a
1318 href="#cl::ValueDisallowed">cl::ValueDisallowed</a> modifier to a boolean
1319 argument to restrict your command line parser.  These options are mostly useful
1320 when <a href="#extensionguide">extending the library</a>.</p>
1321
1322 </div>
1323
1324 <!-- _______________________________________________________________________ -->
1325 <h4>
1326   <a name="formatting">Controlling other formatting options</a>
1327 </h4>
1328
1329 <div>
1330
1331 <p>The formatting option group is used to specify that the command line option
1332 has special abilities and is otherwise different from other command line
1333 arguments.  As usual, you can only specify one of these arguments at most.</p>
1334
1335 <ul>
1336
1337 <li><a name="cl::NormalFormatting">The <b><tt>cl::NormalFormatting</tt></b></a>
1338 modifier (which is the default all options) specifies that this option is
1339 "normal".</li>
1340
1341 <li><a name="cl::Positional">The <b><tt>cl::Positional</tt></b></a> modifier
1342 specifies that this is a positional argument that does not have a command line
1343 option associated with it.  See the <a href="#positional">Positional
1344 Arguments</a> section for more information.</li>
1345
1346 <li>The <b><a href="#cl::ConsumeAfter"><tt>cl::ConsumeAfter</tt></a></b> modifier
1347 specifies that this option is used to capture "interpreter style" arguments.  See <a href="#cl::ConsumeAfter">this section for more information</a>.</li>
1348
1349 <li><a name="cl::Prefix">The <b><tt>cl::Prefix</tt></b></a> modifier specifies
1350 that this option prefixes its value.  With 'Prefix' options, the equal sign does
1351 not separate the value from the option name specified. Instead, the value is
1352 everything after the prefix, including any equal sign if present. This is useful
1353 for processing odd arguments like <tt>-lmalloc</tt> and <tt>-L/usr/lib</tt> in a
1354 linker tool or <tt>-DNAME=value</tt> in a compiler tool.   Here, the
1355 '<tt>l</tt>', '<tt>D</tt>' and '<tt>L</tt>' options are normal string (or list)
1356 options, that have the <b><tt><a href="#cl::Prefix">cl::Prefix</a></tt></b>
1357 modifier added to allow the CommandLine library to recognize them.  Note that
1358 <b><tt><a href="#cl::Prefix">cl::Prefix</a></tt></b> options must not have the
1359 <b><tt><a href="#cl::ValueDisallowed">cl::ValueDisallowed</a></tt></b> modifier
1360 specified.</li>
1361
1362 <li><a name="cl::Grouping">The <b><tt>cl::Grouping</tt></b></a> modifier is used
1363 to implement Unix-style tools (like <tt>ls</tt>) that have lots of single letter
1364 arguments, but only require a single dash.  For example, the '<tt>ls -labF</tt>'
1365 command actually enables four different options, all of which are single
1366 letters.  Note that <b><tt><a href="#cl::Grouping">cl::Grouping</a></tt></b>
1367 options cannot have values.</li>
1368
1369 </ul>
1370
1371 <p>The CommandLine library does not restrict how you use the <b><tt><a
1372 href="#cl::Prefix">cl::Prefix</a></tt></b> or <b><tt><a
1373 href="#cl::Grouping">cl::Grouping</a></tt></b> modifiers, but it is possible to
1374 specify ambiguous argument settings.  Thus, it is possible to have multiple
1375 letter options that are prefix or grouping options, and they will still work as
1376 designed.</p>
1377
1378 <p>To do this, the CommandLine library uses a greedy algorithm to parse the
1379 input option into (potentially multiple) prefix and grouping options.  The
1380 strategy basically looks like this:</p>
1381
1382 <div class="doc_code"><tt>parse(string OrigInput) {</tt>
1383
1384 <ol>
1385 <li><tt>string input = OrigInput;</tt>
1386 <li><tt>if (isOption(input)) return getOption(input).parse();</tt>&nbsp;&nbsp;&nbsp;&nbsp;<i>// Normal option</i>
1387 <li><tt>while (!isOption(input) &amp;&amp; !input.empty()) input.pop_back();</tt>&nbsp;&nbsp;&nbsp;&nbsp;<i>// Remove the last letter</i>
1388 <li><tt>if (input.empty()) return error();</tt>&nbsp;&nbsp;&nbsp;&nbsp;<i>// No matching option</i>
1389 <li><tt>if (getOption(input).isPrefix())<br>
1390 &nbsp;&nbsp;return getOption(input).parse(input);</tt>
1391 <li><tt>while (!input.empty()) {&nbsp;&nbsp;&nbsp;&nbsp;<i>// Must be grouping options</i><br>
1392 &nbsp;&nbsp;getOption(input).parse();<br>
1393 &nbsp;&nbsp;OrigInput.erase(OrigInput.begin(), OrigInput.begin()+input.length());<br>
1394 &nbsp;&nbsp;input = OrigInput;<br>
1395 &nbsp;&nbsp;while (!isOption(input) &amp;&amp; !input.empty()) input.pop_back();<br>
1396 }</tt>
1397 <li><tt>if (!OrigInput.empty()) error();</tt></li>
1398 </ol>
1399
1400 <p><tt>}</tt></p>
1401 </div>
1402
1403 </div>
1404
1405 <!-- _______________________________________________________________________ -->
1406 <h4>
1407   <a name="misc">Miscellaneous option modifiers</a>
1408 </h4>
1409
1410 <div>
1411
1412 <p>The miscellaneous option modifiers are the only flags where you can specify
1413 more than one flag from the set: they are not mutually exclusive.  These flags
1414 specify boolean properties that modify the option.</p>
1415
1416 <ul>
1417
1418 <li><a name="cl::CommaSeparated">The <b><tt>cl::CommaSeparated</tt></b></a> modifier
1419 indicates that any commas specified for an option's value should be used to
1420 split the value up into multiple values for the option.  For example, these two
1421 options are equivalent when <tt>cl::CommaSeparated</tt> is specified:
1422 "<tt>-foo=a -foo=b -foo=c</tt>" and "<tt>-foo=a,b,c</tt>".  This option only
1423 makes sense to be used in a case where the option is allowed to accept one or
1424 more values (i.e. it is a <a href="#cl::list">cl::list</a> option).</li>
1425
1426 <li><a name="cl::PositionalEatsArgs">The
1427 <b><tt>cl::PositionalEatsArgs</tt></b></a> modifier (which only applies to
1428 positional arguments, and only makes sense for lists) indicates that positional
1429 argument should consume any strings after it (including strings that start with
1430 a "-") up until another recognized positional argument.  For example, if you
1431 have two "eating" positional arguments, "<tt>pos1</tt>" and "<tt>pos2</tt>", the
1432 string "<tt>-pos1 -foo -bar baz -pos2 -bork</tt>" would cause the "<tt>-foo -bar
1433 -baz</tt>" strings to be applied to the "<tt>-pos1</tt>" option and the
1434 "<tt>-bork</tt>" string to be applied to the "<tt>-pos2</tt>" option.</li>
1435
1436 <li><a name="cl::Sink">The <b><tt>cl::Sink</tt></b></a> modifier is
1437 used to handle unknown options. If there is at least one option with
1438 <tt>cl::Sink</tt> modifier specified, the parser passes
1439 unrecognized option strings to it as values instead of signaling an
1440 error. As with <tt>cl::CommaSeparated</tt>, this modifier
1441 only makes sense with a <a href="#cl::list">cl::list</a> option.</li>
1442
1443 </ul>
1444
1445 <p>So far, these are the only three miscellaneous option modifiers.</p>
1446
1447 </div>
1448
1449 <!-- _______________________________________________________________________ -->
1450 <h4>
1451   <a name="response">Response files</a>
1452 </h4>
1453
1454 <div>
1455
1456 <p>Some systems, such as certain variants of Microsoft Windows and
1457 some older Unices have a relatively low limit on command-line
1458 length. It is therefore customary to use the so-called 'response
1459 files' to circumvent this restriction. These files are mentioned on
1460 the command-line (using the "@file") syntax. The program reads these
1461 files and inserts the contents into argv, thereby working around the
1462 command-line length limits. Response files are enabled by an optional
1463 fourth argument to
1464 <a href="#cl::ParseEnvironmentOptions"><tt>cl::ParseEnvironmentOptions</tt></a>
1465 and
1466 <a href="#cl::ParseCommandLineOptions"><tt>cl::ParseCommandLineOptions</tt></a>.
1467 </p>
1468
1469 </div>
1470
1471 </div>
1472
1473 <!-- ======================================================================= -->
1474 <h3>
1475   <a name="toplevel">Top-Level Classes and Functions</a>
1476 </h3>
1477
1478 <div>
1479
1480 <p>Despite all of the built-in flexibility, the CommandLine option library
1481 really only consists of one function (<a
1482 href="#cl::ParseCommandLineOptions"><tt>cl::ParseCommandLineOptions</tt></a>)
1483 and three main classes: <a href="#cl::opt"><tt>cl::opt</tt></a>, <a
1484 href="#cl::list"><tt>cl::list</tt></a>, and <a
1485 href="#cl::alias"><tt>cl::alias</tt></a>.  This section describes these three
1486 classes in detail.</p>
1487
1488 <!-- _______________________________________________________________________ -->
1489 <h4>
1490   <a name="cl::ParseCommandLineOptions">The <tt>cl::ParseCommandLineOptions</tt>
1491   function</a>
1492 </h4>
1493
1494 <div>
1495
1496 <p>The <tt>cl::ParseCommandLineOptions</tt> function is designed to be called
1497 directly from <tt>main</tt>, and is used to fill in the values of all of the
1498 command line option variables once <tt>argc</tt> and <tt>argv</tt> are
1499 available.</p>
1500
1501 <p>The <tt>cl::ParseCommandLineOptions</tt> function requires two parameters
1502 (<tt>argc</tt> and <tt>argv</tt>), but may also take an optional third parameter
1503 which holds <a href="#description">additional extra text</a> to emit when the
1504 <tt>-help</tt> option is invoked, and a fourth boolean parameter that enables
1505 <a href="#response">response files</a>.</p>
1506
1507 </div>
1508
1509 <!-- _______________________________________________________________________ -->
1510 <h4>
1511   <a name="cl::ParseEnvironmentOptions">The <tt>cl::ParseEnvironmentOptions</tt>
1512   function</a>
1513 </h4>
1514
1515 <div>
1516
1517 <p>The <tt>cl::ParseEnvironmentOptions</tt> function has mostly the same effects
1518 as <a
1519 href="#cl::ParseCommandLineOptions"><tt>cl::ParseCommandLineOptions</tt></a>,
1520 except that it is designed to take values for options from an environment
1521 variable, for those cases in which reading the command line is not convenient or
1522 desired. It fills in the values of all the command line option variables just
1523 like <a
1524 href="#cl::ParseCommandLineOptions"><tt>cl::ParseCommandLineOptions</tt></a>
1525 does.</p>
1526
1527 <p>It takes four parameters: the name of the program (since <tt>argv</tt> may
1528 not be available, it can't just look in <tt>argv[0]</tt>), the name of the
1529 environment variable to examine, the optional
1530 <a href="#description">additional extra text</a> to emit when the
1531 <tt>-help</tt> option is invoked, and the boolean
1532 switch that controls whether <a href="#response">response files</a>
1533 should be read.</p>
1534
1535 <p><tt>cl::ParseEnvironmentOptions</tt> will break the environment
1536 variable's value up into words and then process them using
1537 <a href="#cl::ParseCommandLineOptions"><tt>cl::ParseCommandLineOptions</tt></a>.
1538 <b>Note:</b> Currently <tt>cl::ParseEnvironmentOptions</tt> does not support
1539 quoting, so an environment variable containing <tt>-option "foo bar"</tt> will
1540 be parsed as three words, <tt>-option</tt>, <tt>"foo</tt>, and <tt>bar"</tt>,
1541 which is different from what you would get from the shell with the same
1542 input.</p>
1543
1544 </div>
1545
1546 <!-- _______________________________________________________________________ -->
1547 <h4>
1548   <a name="cl::SetVersionPrinter">The <tt>cl::SetVersionPrinter</tt>
1549   function</a>
1550 </h4>
1551
1552 <div>
1553
1554 <p>The <tt>cl::SetVersionPrinter</tt> function is designed to be called
1555 directly from <tt>main</tt> and <i>before</i>
1556 <tt>cl::ParseCommandLineOptions</tt>. Its use is optional. It simply arranges
1557 for a function to be called in response to the <tt>--version</tt> option instead
1558 of having the <tt>CommandLine</tt> library print out the usual version string
1559 for LLVM. This is useful for programs that are not part of LLVM but wish to use
1560 the <tt>CommandLine</tt> facilities. Such programs should just define a small
1561 function that takes no arguments and returns <tt>void</tt> and that prints out
1562 whatever version information is appropriate for the program. Pass the address
1563 of that function to <tt>cl::SetVersionPrinter</tt> to arrange for it to be
1564 called when the <tt>--version</tt> option is given by the user.</p>
1565
1566 </div>
1567 <!-- _______________________________________________________________________ -->
1568 <h4>
1569   <a name="cl::opt">The <tt>cl::opt</tt> class</a>
1570 </h4>
1571
1572 <div>
1573
1574 <p>The <tt>cl::opt</tt> class is the class used to represent scalar command line
1575 options, and is the one used most of the time.  It is a templated class which
1576 can take up to three arguments (all except for the first have default values
1577 though):</p>
1578
1579 <div class="doc_code"><pre>
1580 <b>namespace</b> cl {
1581   <b>template</b> &lt;<b>class</b> DataType, <b>bool</b> ExternalStorage = <b>false</b>,
1582             <b>class</b> ParserClass = parser&lt;DataType&gt; &gt;
1583   <b>class</b> opt;
1584 }
1585 </pre></div>
1586
1587 <p>The first template argument specifies what underlying data type the command
1588 line argument is, and is used to select a default parser implementation.  The
1589 second template argument is used to specify whether the option should contain
1590 the storage for the option (the default) or whether external storage should be
1591 used to contain the value parsed for the option (see <a href="#storage">Internal
1592 vs External Storage</a> for more information).</p>
1593
1594 <p>The third template argument specifies which parser to use.  The default value
1595 selects an instantiation of the <tt>parser</tt> class based on the underlying
1596 data type of the option.  In general, this default works well for most
1597 applications, so this option is only used when using a <a
1598 href="#customparser">custom parser</a>.</p>
1599
1600 </div>
1601
1602 <!-- _______________________________________________________________________ -->
1603 <h4>
1604   <a name="cl::list">The <tt>cl::list</tt> class</a>
1605 </h4>
1606
1607 <div>
1608
1609 <p>The <tt>cl::list</tt> class is the class used to represent a list of command
1610 line options.  It too is a templated class which can take up to three
1611 arguments:</p>
1612
1613 <div class="doc_code"><pre>
1614 <b>namespace</b> cl {
1615   <b>template</b> &lt;<b>class</b> DataType, <b>class</b> Storage = <b>bool</b>,
1616             <b>class</b> ParserClass = parser&lt;DataType&gt; &gt;
1617   <b>class</b> list;
1618 }
1619 </pre></div>
1620
1621 <p>This class works the exact same as the <a
1622 href="#cl::opt"><tt>cl::opt</tt></a> class, except that the second argument is
1623 the <b>type</b> of the external storage, not a boolean value.  For this class,
1624 the marker type '<tt>bool</tt>' is used to indicate that internal storage should
1625 be used.</p>
1626
1627 </div>
1628
1629 <!-- _______________________________________________________________________ -->
1630 <h4>
1631   <a name="cl::bits">The <tt>cl::bits</tt> class</a>
1632 </h4>
1633
1634 <div>
1635
1636 <p>The <tt>cl::bits</tt> class is the class used to represent a list of command
1637 line options in the form of a bit vector.  It is also a templated class which
1638 can take up to three arguments:</p>
1639
1640 <div class="doc_code"><pre>
1641 <b>namespace</b> cl {
1642   <b>template</b> &lt;<b>class</b> DataType, <b>class</b> Storage = <b>bool</b>,
1643             <b>class</b> ParserClass = parser&lt;DataType&gt; &gt;
1644   <b>class</b> bits;
1645 }
1646 </pre></div>
1647
1648 <p>This class works the exact same as the <a
1649 href="#cl::opt"><tt>cl::lists</tt></a> class, except that the second argument
1650 must be of <b>type</b> <tt>unsigned</tt> if external storage is used.</p>
1651
1652 </div>
1653
1654 <!-- _______________________________________________________________________ -->
1655 <h4>
1656   <a name="cl::alias">The <tt>cl::alias</tt> class</a>
1657 </h4>
1658
1659 <div>
1660
1661 <p>The <tt>cl::alias</tt> class is a nontemplated class that is used to form
1662 aliases for other arguments.</p>
1663
1664 <div class="doc_code"><pre>
1665 <b>namespace</b> cl {
1666   <b>class</b> alias;
1667 }
1668 </pre></div>
1669
1670 <p>The <a href="#cl::aliasopt"><tt>cl::aliasopt</tt></a> attribute should be
1671 used to specify which option this is an alias for.  Alias arguments default to
1672 being <a href="#cl::Hidden">Hidden</a>, and use the aliased options parser to do
1673 the conversion from string to data.</p>
1674
1675 </div>
1676
1677 <!-- _______________________________________________________________________ -->
1678 <h4>
1679   <a name="cl::extrahelp">The <tt>cl::extrahelp</tt> class</a>
1680 </h4>
1681
1682 <div>
1683
1684 <p>The <tt>cl::extrahelp</tt> class is a nontemplated class that allows extra
1685 help text to be printed out for the <tt>-help</tt> option.</p>
1686
1687 <div class="doc_code"><pre>
1688 <b>namespace</b> cl {
1689   <b>struct</b> extrahelp;
1690 }
1691 </pre></div>
1692
1693 <p>To use the extrahelp, simply construct one with a <tt>const char*</tt>
1694 parameter to the constructor. The text passed to the constructor will be printed
1695 at the bottom of the help message, verbatim. Note that multiple
1696 <tt>cl::extrahelp</tt> <b>can</b> be used, but this practice is discouraged. If
1697 your tool needs to print additional help information, put all that help into a
1698 single <tt>cl::extrahelp</tt> instance.</p>
1699 <p>For example:</p>
1700 <div class="doc_code"><pre>
1701   cl::extrahelp("\nADDITIONAL HELP:\n\n  This is the extra help\n");
1702 </pre></div>
1703 </div>
1704
1705 </div>
1706
1707 <!-- ======================================================================= -->
1708 <h3>
1709   <a name="builtinparsers">Builtin parsers</a>
1710 </h3>
1711
1712 <div>
1713
1714 <p>Parsers control how the string value taken from the command line is
1715 translated into a typed value, suitable for use in a C++ program.  By default,
1716 the CommandLine library uses an instance of <tt>parser&lt;type&gt;</tt> if the
1717 command line option specifies that it uses values of type '<tt>type</tt>'.
1718 Because of this, custom option processing is specified with specializations of
1719 the '<tt>parser</tt>' class.</p>
1720
1721 <p>The CommandLine library provides the following builtin parser
1722 specializations, which are sufficient for most applications. It can, however,
1723 also be extended to work with new data types and new ways of interpreting the
1724 same data.  See the <a href="#customparser">Writing a Custom Parser</a> for more
1725 details on this type of library extension.</p>
1726
1727 <ul>
1728
1729 <li><a name="genericparser">The <b>generic <tt>parser&lt;t&gt;</tt> parser</b></a>
1730 can be used to map strings values to any data type, through the use of the <a
1731 href="#cl::values">cl::values</a> property, which specifies the mapping
1732 information.  The most common use of this parser is for parsing enum values,
1733 which allows you to use the CommandLine library for all of the error checking to
1734 make sure that only valid enum values are specified (as opposed to accepting
1735 arbitrary strings).  Despite this, however, the generic parser class can be used
1736 for any data type.</li>
1737
1738 <li><a name="boolparser">The <b><tt>parser&lt;bool&gt;</tt> specialization</b></a>
1739 is used to convert boolean strings to a boolean value.  Currently accepted
1740 strings are "<tt>true</tt>", "<tt>TRUE</tt>", "<tt>True</tt>", "<tt>1</tt>",
1741 "<tt>false</tt>", "<tt>FALSE</tt>", "<tt>False</tt>", and "<tt>0</tt>".</li>
1742
1743 <li><a name="boolOrDefaultparser">The <b><tt>parser&lt;boolOrDefault&gt;</tt>
1744  specialization</b></a> is used for cases where the value is boolean,
1745 but we also need to know whether the option was specified at all.  boolOrDefault
1746 is an enum with 3 values, BOU_UNSET, BOU_TRUE and BOU_FALSE.  This parser accepts
1747 the same strings as <b><tt>parser&lt;bool&gt;</tt></b>.</li>
1748
1749 <li><a name="stringparser">The <b><tt>parser&lt;string&gt;</tt>
1750 specialization</b></a> simply stores the parsed string into the string value
1751 specified.  No conversion or modification of the data is performed.</li>
1752
1753 <li><a name="intparser">The <b><tt>parser&lt;int&gt;</tt> specialization</b></a>
1754 uses the C <tt>strtol</tt> function to parse the string input.  As such, it will
1755 accept a decimal number (with an optional '+' or '-' prefix) which must start
1756 with a non-zero digit.  It accepts octal numbers, which are identified with a
1757 '<tt>0</tt>' prefix digit, and hexadecimal numbers with a prefix of
1758 '<tt>0x</tt>' or '<tt>0X</tt>'.</li>
1759
1760 <li><a name="doubleparser">The <b><tt>parser&lt;double&gt;</tt></b></a> and
1761 <b><tt>parser&lt;float&gt;</tt> specializations</b> use the standard C
1762 <tt>strtod</tt> function to convert floating point strings into floating point
1763 values.  As such, a broad range of string formats is supported, including
1764 exponential notation (ex: <tt>1.7e15</tt>) and properly supports locales.
1765 </li>
1766
1767 </ul>
1768
1769 </div>
1770
1771 </div>
1772
1773 <!-- *********************************************************************** -->
1774 <h2>
1775   <a name="extensionguide">Extension Guide</a>
1776 </h2>
1777 <!-- *********************************************************************** -->
1778
1779 <div>
1780
1781 <p>Although the CommandLine library has a lot of functionality built into it
1782 already (as discussed previously), one of its true strengths lie in its
1783 extensibility.  This section discusses how the CommandLine library works under
1784 the covers and illustrates how to do some simple, common, extensions.</p>
1785
1786 <!-- ======================================================================= -->
1787 <h3>
1788   <a name="customparser">Writing a custom parser</a>
1789 </h3>
1790
1791 <div>
1792
1793 <p>One of the simplest and most common extensions is the use of a custom parser.
1794 As <a href="#builtinparsers">discussed previously</a>, parsers are the portion
1795 of the CommandLine library that turns string input from the user into a
1796 particular parsed data type, validating the input in the process.</p>
1797
1798 <p>There are two ways to use a new parser:</p>
1799
1800 <ol>
1801
1802 <li>
1803
1804 <p>Specialize the <a href="#genericparser"><tt>cl::parser</tt></a> template for
1805 your custom data type.<p>
1806
1807 <p>This approach has the advantage that users of your custom data type will
1808 automatically use your custom parser whenever they define an option with a value
1809 type of your data type.  The disadvantage of this approach is that it doesn't
1810 work if your fundamental data type is something that is already supported.</p>
1811
1812 </li>
1813
1814 <li>
1815
1816 <p>Write an independent class, using it explicitly from options that need
1817 it.</p>
1818
1819 <p>This approach works well in situations where you would line to parse an
1820 option using special syntax for a not-very-special data-type.  The drawback of
1821 this approach is that users of your parser have to be aware that they are using
1822 your parser instead of the builtin ones.</p>
1823
1824 </li>
1825
1826 </ol>
1827
1828 <p>To guide the discussion, we will discuss a custom parser that accepts file
1829 sizes, specified with an optional unit after the numeric size.  For example, we
1830 would like to parse "102kb", "41M", "1G" into the appropriate integer value.  In
1831 this case, the underlying data type we want to parse into is
1832 '<tt>unsigned</tt>'.  We choose approach #2 above because we don't want to make
1833 this the default for all <tt>unsigned</tt> options.</p>
1834
1835 <p>To start out, we declare our new <tt>FileSizeParser</tt> class:</p>
1836
1837 <div class="doc_code"><pre>
1838 <b>struct</b> FileSizeParser : <b>public</b> cl::basic_parser&lt;<b>unsigned</b>&gt; {
1839   <i>// parse - Return true on error.</i>
1840   <b>bool</b> parse(cl::Option &amp;O, <b>const char</b> *ArgName, <b>const</b> std::string &amp;ArgValue,
1841              <b>unsigned</b> &amp;Val);
1842 };
1843 </pre></div>
1844
1845 <p>Our new class inherits from the <tt>cl::basic_parser</tt> template class to
1846 fill in the default, boiler plate code for us.  We give it the data type that
1847 we parse into, the last argument to the <tt>parse</tt> method, so that clients of
1848 our custom parser know what object type to pass in to the parse method.  (Here we
1849 declare that we parse into '<tt>unsigned</tt>' variables.)</p>
1850
1851 <p>For most purposes, the only method that must be implemented in a custom
1852 parser is the <tt>parse</tt> method.  The <tt>parse</tt> method is called
1853 whenever the option is invoked, passing in the option itself, the option name,
1854 the string to parse, and a reference to a return value.  If the string to parse
1855 is not well-formed, the parser should output an error message and return true.
1856 Otherwise it should return false and set '<tt>Val</tt>' to the parsed value.  In
1857 our example, we implement <tt>parse</tt> as:</p>
1858
1859 <div class="doc_code"><pre>
1860 <b>bool</b> FileSizeParser::parse(cl::Option &amp;O, <b>const char</b> *ArgName,
1861                            <b>const</b> std::string &amp;Arg, <b>unsigned</b> &amp;Val) {
1862   <b>const char</b> *ArgStart = Arg.c_str();
1863   <b>char</b> *End;
1864
1865   <i>// Parse integer part, leaving 'End' pointing to the first non-integer char</i>
1866   Val = (unsigned)strtol(ArgStart, &amp;End, 0);
1867
1868   <b>while</b> (1) {
1869     <b>switch</b> (*End++) {
1870     <b>case</b> 0: <b>return</b> false;   <i>// No error</i>
1871     <b>case</b> 'i':               <i>// Ignore the 'i' in KiB if people use that</i>
1872     <b>case</b> 'b': <b>case</b> 'B':     <i>// Ignore B suffix</i>
1873       <b>break</b>;
1874
1875     <b>case</b> 'g': <b>case</b> 'G': Val *= 1024*1024*1024; <b>break</b>;
1876     <b>case</b> 'm': <b>case</b> 'M': Val *= 1024*1024;      <b>break</b>;
1877     <b>case</b> 'k': <b>case</b> 'K': Val *= 1024;           <b>break</b>;
1878
1879     default:
1880       <i>// Print an error message if unrecognized character!</i>
1881       <b>return</b> O.error("'" + Arg + "' value invalid for file size argument!");
1882     }
1883   }
1884 }
1885 </pre></div>
1886
1887 <p>This function implements a very simple parser for the kinds of strings we are
1888 interested in.  Although it has some holes (it allows "<tt>123KKK</tt>" for
1889 example), it is good enough for this example.  Note that we use the option
1890 itself to print out the error message (the <tt>error</tt> method always returns
1891 true) in order to get a nice error message (shown below).  Now that we have our
1892 parser class, we can use it like this:</p>
1893
1894 <div class="doc_code"><pre>
1895 <b>static</b> <a href="#cl::opt">cl::opt</a>&lt;<b>unsigned</b>, <b>false</b>, FileSizeParser&gt;
1896 MFS(<i>"max-file-size"</i>, <a href="#cl::desc">cl::desc</a>(<i>"Maximum file size to accept"</i>),
1897     <a href="#cl::value_desc">cl::value_desc</a>("<i>size</i>"));
1898 </pre></div>
1899
1900 <p>Which adds this to the output of our program:</p>
1901
1902 <div class="doc_code"><pre>
1903 OPTIONS:
1904   -help                 - display available options (-help-hidden for more)
1905   ...
1906   <b>-max-file-size=&lt;size&gt; - Maximum file size to accept</b>
1907 </pre></div>
1908
1909 <p>And we can test that our parse works correctly now (the test program just
1910 prints out the max-file-size argument value):</p>
1911
1912 <div class="doc_code"><pre>
1913 $ ./test
1914 MFS: 0
1915 $ ./test -max-file-size=123MB
1916 MFS: 128974848
1917 $ ./test -max-file-size=3G
1918 MFS: 3221225472
1919 $ ./test -max-file-size=dog
1920 -max-file-size option: 'dog' value invalid for file size argument!
1921 </pre></div>
1922
1923 <p>It looks like it works.  The error message that we get is nice and helpful,
1924 and we seem to accept reasonable file sizes.  This wraps up the "custom parser"
1925 tutorial.</p>
1926
1927 </div>
1928
1929 <!-- ======================================================================= -->
1930 <h3>
1931   <a name="explotingexternal">Exploiting external storage</a>
1932 </h3>
1933
1934 <div>
1935   <p>Several of the LLVM libraries define static <tt>cl::opt</tt> instances that
1936   will automatically be included in any program that links with that library.
1937   This is a feature. However, sometimes it is necessary to know the value of the
1938   command line option outside of the library. In these cases the library does or
1939   should provide an external storage location that is accessible to users of the
1940   library. Examples of this include the <tt>llvm::DebugFlag</tt> exported by the
1941   <tt>lib/Support/Debug.cpp</tt> file and the <tt>llvm::TimePassesIsEnabled</tt>
1942   flag exported by the <tt>lib/VMCore/Pass.cpp</tt> file.</p>
1943
1944 <p>TODO: complete this section</p>
1945
1946 </div>
1947
1948 <!-- ======================================================================= -->
1949 <h3>
1950   <a name="dynamicopts">Dynamically adding command line options</a>
1951 </h3>
1952
1953 <div>
1954
1955 <p>TODO: fill in this section</p>
1956
1957 </div>
1958
1959 </div>
1960
1961 <!-- *********************************************************************** -->
1962
1963 <hr>
1964 <address>
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1970   <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
1971   <a href="http://llvm.org/">LLVM Compiler Infrastructure</a><br>
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