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