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27 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
28 <a name="program_options.overview"></a>Library Overview</h2></div></div></div>
29 <div class="toc"><dl class="toc">
30 <dt><span class="section"><a href="overview.html#idp435792752">Options Description Component</a></span></dt>
31 <dt><span class="section"><a href="overview.html#idp435855552">Parsers Component</a></span></dt>
32 <dt><span class="section"><a href="overview.html#idp435865968">Storage Component</a></span></dt>
33 <dt><span class="section"><a href="overview.html#idp435877344">Specific parsers</a></span></dt>
34 <dt><span class="section"><a href="overview.html#idp435909200">Annotated List of Symbols</a></span></dt>
36 <p>In the tutorial section, we saw several examples of library usage.
37 Here we will describe the overall library design including the primary
38 components and their function.
40 <p>The library has three main components:
42 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
43 <li class="listitem"><p>The options description component, which describes the allowed options
44 and what to do with the values of the options.
46 <li class="listitem"><p>The parsers component, which uses this information to find option names
47 and values in the input sources and return them.
49 <li class="listitem"><p>The storage component, which provides the
50 interface to access the value of an option. It also converts the string
51 representation of values that parsers return into desired C++ types.
56 <p>To be a little more concrete, the <code class="computeroutput">options_description</code>
57 class is from the options description component, the
58 <code class="computeroutput">parse_command_line</code> function is from the parsers component, and the
59 <code class="computeroutput">variables_map</code> class is from the storage component. </p>
60 <p>In the tutorial we've learned how those components can be used by the
61 <code class="computeroutput">main</code> function to parse the command line and config
62 file. Before going into the details of each component, a few notes about
63 the world outside of <code class="computeroutput">main</code>.
66 For that outside world, the storage component is the most important. It
67 provides a class which stores all option values and that class can be
68 freely passed around your program to modules which need access to the
69 options. All the other components can be used only in the place where
70 the actual parsing is the done. However, it might also make sense for the
71 individual program modules to describe their options and pass them to the
72 main module, which will merge all options. Of course, this is only
73 important when the number of options is large and declaring them in one
74 place becomes troublesome.
77 <div class="titlepage"><div><div><h3 class="title">
78 <a name="idp435792752"></a>Options Description Component</h3></div></div></div>
79 <div class="toc"><dl class="toc">
80 <dt><span class="section"><a href="overview.html#idp435820112">Syntactic Information</a></span></dt>
81 <dt><span class="section"><a href="overview.html#idp435838768">Semantic Information</a></span></dt>
82 <dt><span class="section"><a href="overview.html#idp435843360">Positional Options</a></span></dt>
84 <p>The options description component has three main classes:
85 <code class="computeroutput"><a class="link" href="../boost/program_options/option_description.html" title="Class option_description">option_description</a></code>, <code class="computeroutput"><a class="link" href="../boost/program_options/value_semantic.html" title="Class value_semantic">value_semantic</a></code> and <code class="computeroutput"><a class="link" href="../boost/program_options/options_description.html" title="Class options_description">options_description</a></code>. The
86 first two together describe a single option. The <code class="computeroutput"><a class="link" href="../boost/program_options/option_description.html" title="Class option_description">option_description</a></code>
87 class contains the option's name, description and a pointer to <code class="computeroutput"><a class="link" href="../boost/program_options/value_semantic.html" title="Class value_semantic">value_semantic</a></code>,
88 which, in turn, knows the type of the option's value and can parse the value,
89 apply the default value, and so on. The <code class="computeroutput"><a class="link" href="../boost/program_options/options_description.html" title="Class options_description">options_description</a></code> class is a
90 container for instances of <code class="computeroutput"><a class="link" href="../boost/program_options/option_description.html" title="Class option_description">option_description</a></code>.
92 <p>For almost every library, those classes could be created in a
93 conventional way: that is, you'd create new options using constructors and
94 then call the <code class="computeroutput">add</code> method of <code class="computeroutput"><a class="link" href="../boost/program_options/options_description.html" title="Class options_description">options_description</a></code>. However,
95 that's overly verbose for declaring 20 or 30 options. This concern led
96 to creation of the syntax that you've already seen:
98 <pre class="programlisting">
99 options_description desc;
101 ("help", "produce help")
102 ("optimization", value<int>()->default_value(10), "optimization level")
107 <p>The call to the <code class="computeroutput">value</code> function creates an instance of
108 a class derived from the <code class="computeroutput">value_semantic</code> class: <code class="computeroutput">typed_value</code>.
109 That class contains the code to parse
110 values of a specific type, and contains a number of methods which can be
111 called by the user to specify additional information. (This
112 essentially emulates named parameters of the constructor.) Calls to
113 <code class="computeroutput">operator()</code> on the object returned by <code class="computeroutput">add_options</code>
114 forward arguments to the constructor of the <code class="computeroutput">option_description</code>
115 class and add the new instance.
118 Note that in addition to the
119 <code class="computeroutput">value</code>, library provides the <code class="computeroutput">bool_switch</code>
120 function, and user can write his own function which will return
121 other subclasses of <code class="computeroutput">value_semantic</code> with
122 different behaviour. For the remainder of this section, we'll talk only
123 about the <code class="computeroutput">value</code> function.
125 <p>The information about an option is divided into syntactic and
126 semantic. Syntactic information includes the name of the option and the
127 number of tokens which can be used to specify the value. This
128 information is used by parsers to group tokens into (name, value) pairs,
129 where value is just a vector of strings
130 (<code class="computeroutput">std::vector<std::string></code>). The semantic layer
131 is responsible for converting the value of the option into more usable C++
134 <p>This separation is an important part of library design. The parsers
135 use only the syntactic layer, which takes away some of the freedom to
136 use overly complex structures. For example, it's not easy to parse
138 <pre class="screen">calc --expression=1 + 2/3</pre>
140 possible to parse </p>
141 <pre class="screen">1 + 2/3</pre>
142 <p> without knowing that it's a C
143 expression. With a little help from the user the task becomes trivial,
144 and the syntax clear: </p>
145 <pre class="screen">calc --expression="1 + 2/3"</pre>
148 <div class="section">
149 <div class="titlepage"><div><div><h4 class="title">
150 <a name="idp435820112"></a>Syntactic Information</h4></div></div></div>
151 <div class="toc"><dl class="toc"><dt><span class="section"><a href="overview.html#idp435829872">Description formatting</a></span></dt></dl></div>
152 <p>The syntactic information is provided by the
153 <code class="computeroutput"><a class="link" href="../boost/program_options/options_description.html" title="Class options_description">boost::program_options::options_description</a></code> class
154 and some methods of the
155 <code class="computeroutput"><a class="link" href="../boost/program_options/value_semantic.html" title="Class value_semantic">boost::program_options::value_semantic</a></code> class
158 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
159 <li class="listitem"><p>
160 name of the option, used to identify the option inside the
163 <li class="listitem"><p>
164 description of the option, which can be presented to the user,
166 <li class="listitem"><p>
167 the allowed number of source tokens that comprise options's
168 value, which is used during parsing.
173 <p>Consider the following example:
175 <pre class="programlisting">
176 options_description desc;
178 ("help", "produce help message")
179 ("compression", value<string>(), "compression level")
180 ("verbose", value<string>()->implicit_value("0"), "verbosity level")
181 ("email", value<string>()->multitoken(), "email to send to")
185 For the first parameter, we specify only the name and the
186 description. No value can be specified in the parsed source.
187 For the first option, the user must specify a value, using a single
188 token. For the third option, the user may either provide a single token
189 for the value, or no token at all. For the last option, the value can
190 span several tokens. For example, the following command line is OK:
193 test --help --compression 10 --verbose --email beadle@mars beadle2@mars
197 <div class="section">
198 <div class="titlepage"><div><div><h5 class="title">
199 <a name="idp435829872"></a>Description formatting</h5></div></div></div>
201 Sometimes the description can get rather long, for example, when
202 several option's values need separate documentation. Below we
203 describe some simple formatting mechanisms you can use.
205 <p>The description string has one or more paragraphs, separated by
206 the newline character ('\n'). When an option is output, the library
207 will compute the indentation for options's description. Each of the
208 paragraph is output as a separate line with that intentation. If
209 a paragraph does not fit on one line it is spanned over multiple
210 lines (which will have the same indentation).
212 <p>You may specify additional indent for the first specified by
213 inserting spaces at the beginning of a paragraph. For example:
215 <pre class="programlisting">
216 options.add_options()
217 ("help", " A long help msg a long help msg a long help msg a long help
218 msg a long help msg a long help msg a long help msg a long help msg ")
222 will specify a four-space indent for the first line. The output will
226 --help A long help msg a long
227 help msg a long help msg
228 a long help msg a long
229 help msg a long help msg
230 a long help msg a long
236 <p>For the case where line is wrapped, you can want an additional
237 indent for wrapped text. This can be done by
238 inserting a tabulator character ('\t') at the desired position. For
241 <pre class="programlisting">
242 options.add_options()
243 ("well_formated", "As you can see this is a very well formatted
244 option description.\n"
245 "You can do this for example:\n\n"
247 " Value1: \tdoes this and that, bla bla bla bla
248 bla bla bla bla bla bla bla bla bla bla bla\n"
249 " Value2: \tdoes something else, bla bla bla bla
250 bla bla bla bla bla bla bla bla bla bla bla\n\n"
251 " This paragraph has a first line indent only,
252 bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla");
258 --well_formated As you can see this is a
265 Value1: does this and
271 Value2: does something
279 first line indent only,
280 bla bla bla bla bla bla
281 bla bla bla bla bla bla
285 The tab character is removed before output. Only one tabulator per
286 paragraph is allowed, otherwise an exception of type
287 program_options::error is thrown. Finally, the tabulator is ignored if
288 it is not on the first line of the paragraph or is on the last
289 possible position of the first line.
293 <div class="section">
294 <div class="titlepage"><div><div><h4 class="title">
295 <a name="idp435838768"></a>Semantic Information</h4></div></div></div>
296 <p>The semantic information is completely provided by the
297 <code class="computeroutput"><a class="link" href="../boost/program_options/value_semantic.html" title="Class value_semantic">boost::program_options::value_semantic</a></code> class. For
300 <pre class="programlisting">
301 options_description desc;
303 ("compression", value<int>()->default_value(10), "compression level")
304 ("email", value< vector<string> >()
305 ->composing()->notifier(&your_function), "email")
309 These declarations specify that default value of the first option is 10,
310 that the second option can appear several times and all instances should
311 be merged, and that after parsing is done, the library will call
312 function <code class="computeroutput">&your_function</code>, passing the value of the
313 "email" option as argument.
316 <div class="section">
317 <div class="titlepage"><div><div><h4 class="title">
318 <a name="idp435843360"></a>Positional Options</h4></div></div></div>
319 <p>Our definition of option as (name, value) pairs is simple and
320 useful, but in one special case of the command line, there's a
321 problem. A command line can include a <em class="firstterm">positional option</em>,
322 which does not specify any name at all, for example:
325 archiver --compression=9 /etc/passwd
328 Here, the "/etc/passwd" element does not have any option name.
330 <p>One solution is to ask the user to extract positional options
331 himself and process them as he likes. However, there's a nicer approach
332 -- provide a method to automatically assign the names for positional
333 options, so that the above command line can be interpreted the same way
337 archiver --compression=9 --input-file=/etc/passwd
341 <p>The <code class="computeroutput"><a class="link" href="../boost/program_options/positional_op_idp682411312.html" title="Class positional_options_description">positional_options_description</a></code> class allows the command line
342 parser to assign the names. The class specifies how many positional options
343 are allowed, and for each allowed option, specifies the name. For example:
345 <pre class="programlisting">
346 positional_options_description pd; pd.add("input-file", 1);
348 <p> specifies that for exactly one, first, positional
349 option the name will be "input-file".
351 <p>It's possible to specify that a number, or even all positional options, be
354 <pre class="programlisting">
355 positional_options_description pd;
356 pd.add("output-file", 2).add("input-file", -1);
359 In the above example, the first two positional options will be associated
360 with name "output-file", and any others with the name "input-file".
362 <div class="warning"><table border="0" summary="Warning">
364 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Warning]" src="../../../doc/src/images/warning.png"></td>
365 <th align="left">Warning</th>
367 <tr><td align="left" valign="top"><p>The <code class="computeroutput"><a class="link" href="../boost/program_options/positional_op_idp682411312.html" title="Class positional_options_description">positional_options_description</a></code> class only specifies translation from
368 position to name, and the option name should still be registered with
369 an instance of the <code class="computeroutput"><a class="link" href="../boost/program_options/options_description.html" title="Class options_description">options_description</a></code> class.</p></td></tr>
373 <div class="section">
374 <div class="titlepage"><div><div><h3 class="title">
375 <a name="idp435855552"></a>Parsers Component</h3></div></div></div>
376 <p>The parsers component splits input sources into (name, value) pairs.
377 Each parser looks for possible options and consults the options
378 description component to determine if the option is known and how its value
379 is specified. In the simplest case, the name is explicitly specified,
380 which allows the library to decide if such option is known. If it is known, the
381 <code class="computeroutput"><a class="link" href="../boost/program_options/value_semantic.html" title="Class value_semantic">value_semantic</a></code> instance determines how the value is specified. (If
382 it is not known, an exception is thrown.) Common
383 cases are when the value is explicitly specified by the user, and when
384 the value cannot be specified by the user, but the presence of the
385 option implies some value (for example, <code class="computeroutput">true</code>). So, the
386 parser checks that the value is specified when needed and not specified
387 when not needed, and returns new (name, value) pair.
390 To invoke a parser you typically call a function, passing the options
391 description and command line or config file or something else.
392 The results of parsing are returned as an instance of the <code class="computeroutput"><a class="link" href="reference.html#boost.program_options.parsed_options">parsed_options</a></code>
393 class. Typically, that object is passed directly to the storage
394 component. However, it also can be used directly, or undergo some additional
398 There are three exceptions to the above model -- all related to
399 traditional usage of the command line. While they require some support
400 from the options description component, the additional complexity is
403 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
404 <li class="listitem"><p>The name specified on the command line may be
405 different from the option name -- it's common to provide a "short option
406 name" alias to a longer name. It's also common to allow an abbreviated name
407 to be specified on the command line.
409 <li class="listitem"><p>Sometimes it's desirable to specify value as several
410 tokens. For example, an option "--email-recipient" may be followed
411 by several emails, each as a separate command line token. This
412 behaviour is supported, though it can lead to parsing ambiguities
413 and is not enabled by default.
415 <li class="listitem"><p>The command line may contain positional options -- elements
416 which don't have any name. The command line parser provides a
417 mechanism to guess names for such options, as we've seen in the
424 <div class="section">
425 <div class="titlepage"><div><div><h3 class="title">
426 <a name="idp435865968"></a>Storage Component</h3></div></div></div>
427 <p>The storage component is responsible for:
429 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
430 <li class="listitem"><p>Storing the final values of an option into a special class and in
431 regular variables</p></li>
432 <li class="listitem"><p>Handling priorities among different sources.</p></li>
433 <li class="listitem"><p>Calling user-specified <code class="computeroutput">notify</code> functions with the final
434 values of options.</p></li>
438 <p>Let's consider an example:
440 <pre class="programlisting">
442 store(parse_command_line(argc, argv, desc), vm);
443 store(parse_config_file("example.cfg", desc), vm);
447 The <code class="computeroutput">variables_map</code> class is used to store the option
448 values. The two calls to the <code class="computeroutput">store</code> function add values
449 found on the command line and in the config file. Finally the call to
450 the <code class="computeroutput">notify</code> function runs the user-specified notify
451 functions and stores the values into regular variables, if needed.
453 <p>The priority is handled in a simple way: the <code class="computeroutput">store</code>
454 function will not change the value of an option if it's already
455 assigned. In this case, if the command line specifies the value for an
456 option, any value in the config file is ignored.
458 <div class="warning"><table border="0" summary="Warning">
460 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Warning]" src="../../../doc/src/images/warning.png"></td>
461 <th align="left">Warning</th>
463 <tr><td align="left" valign="top"><p>Don't forget to call the <code class="computeroutput">notify</code> function after you've
464 stored all parsed values.</p></td></tr>
467 <div class="section">
468 <div class="titlepage"><div><div><h3 class="title">
469 <a name="idp435877344"></a>Specific parsers</h3></div></div></div>
470 <div class="toc"><dl class="toc">
471 <dt><span class="section"><a href="overview.html#idp435878016">Configuration file parser</a></span></dt>
472 <dt><span class="section"><a href="overview.html#idp435889856">Environment variables parser</a></span></dt>
474 <div class="section">
475 <div class="titlepage"><div><div><h4 class="title">
476 <a name="idp435878016"></a>Configuration file parser</h4></div></div></div>
477 <p>The <code class="computeroutput"><a class="link" href="../boost/program_options/parse_config__idp682372336.html" title="Function template parse_config_file">parse_config_file</a></code> function implements parsing
478 of simple INI-like configuration files. Configuration file
479 syntax is line based:
481 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
482 <li class="listitem">
483 <p>A line in the form:</p>
485 <em class="replaceable"><code>name</code></em>=<em class="replaceable"><code>value</code></em>
487 <p>gives a value to an option.</p>
489 <li class="listitem">
490 <p>A line in the form:</p>
492 [<em class="replaceable"><code>section name</code></em>]
494 <p>introduces a new section in the configuration file.</p>
496 <li class="listitem"><p>The <code class="literal">#</code> character introduces a
497 comment that spans until the end of the line.</p></li>
499 <p>The option names are relative to the section names, so
500 the following configuration file part:</p>
505 <p>is equivalent to</p>
507 gui.accessibility.visual_bell=yes
510 <div class="section">
511 <div class="titlepage"><div><div><h4 class="title">
512 <a name="idp435889856"></a>Environment variables parser</h4></div></div></div>
513 <p><em class="firstterm">Environment variables</em> are string variables
514 which are available to all programs via the <code class="computeroutput">getenv</code> function
515 of C runtime library. The operating system allows to set initial values
516 for a given user, and the values can be further changed on the command
517 line. For example, on Windows one can use the
518 <code class="filename">autoexec.bat</code> file or (on recent versions) the
519 <code class="filename">Control Panel/System/Advanced/Environment Variables</code>
520 dialog, and on Unix —, the <code class="filename">/etc/profile</code>,
521 <code class="filename">~/.profile</code> and <code class="filename">~/.bash_profile</code>
522 files. Because environment variables can be set for the entire system,
523 they are particularly suitable for options which apply to all programs.
525 <p>The environment variables can be parsed with the
526 <code class="computeroutput"><a class="link" href="../boost/program_options/parse_environ_idp682387952.html" title="Function parse_environment">parse_environment</a></code> function. The function have several overloaded
527 versions. The first parameter is always an <code class="computeroutput"><a class="link" href="../boost/program_options/options_description.html" title="Class options_description">options_description</a></code>
528 instance, and the second specifies what variables must be processed, and
529 what option names must correspond to it. To describe the second
530 parameter we need to consider naming conventions for environment
532 <p>If you have an option that should be specified via environment
533 variable, you need make up the variable's name. To avoid name clashes,
534 we suggest that you use a sufficiently unique prefix for environment
535 variables. Also, while option names are most likely in lower case,
536 environment variables conventionally use upper case. So, for an option
537 name <code class="literal">proxy</code> the environment variable might be called
538 <code class="envar">BOOST_PROXY</code>. During parsing, we need to perform reverse
539 conversion of the names. This is accomplished by passing the choosen
540 prefix as the second parameter of the <code class="computeroutput"><a class="link" href="../boost/program_options/parse_environ_idp682387952.html" title="Function parse_environment">parse_environment</a></code> function.
541 Say, if you pass <code class="literal">BOOST_</code> as the prefix, and there are
542 two variables, <code class="envar">CVSROOT</code> and <code class="envar">BOOST_PROXY</code>, the
543 first variable will be ignored, and the second one will be converted to
544 option <code class="literal">proxy</code>.
546 <p>The above logic is sufficient in many cases, but it is also
547 possible to pass, as the second parameter of the <code class="computeroutput"><a class="link" href="../boost/program_options/parse_environ_idp682387952.html" title="Function parse_environment">parse_environment</a></code>
548 function, any function taking a <code class="computeroutput">std::string</code> and returning
549 <code class="computeroutput">std::string</code>. That function will be called for each
550 environment variable and should return either the name of the option, or
551 empty string if the variable should be ignored.
555 <div class="section">
556 <div class="titlepage"><div><div><h3 class="title">
557 <a name="idp435909200"></a>Annotated List of Symbols</h3></div></div></div>
558 <p>The following table describes all the important symbols in the
559 library, for quick access.</p>
560 <div class="informaltable"><table class="table" width="100%">
570 <tr><td colspan="2">Options description component</td></tr>
572 <td><code class="computeroutput"><a class="link" href="../boost/program_options/options_description.html" title="Class options_description">options_description</a></code></td>
573 <td>describes a number of options</td>
576 <td><code class="computeroutput"><a class="link" href="../boost/program_options/value_idp682542144.html" title="Function value">value</a></code></td>
577 <td>defines the option's value</td>
579 <tr><td colspan="2">Parsers component</td></tr>
581 <td><code class="computeroutput"><a class="link" href="../boost/program_options/parse_command_line.html" title="Function template parse_command_line">parse_command_line</a></code></td>
582 <td>parses command line (simpified interface)</td>
585 <td><code class="computeroutput"><a class="link" href="../boost/program_options/basic_command_line_parser.html" title="Class template basic_command_line_parser">basic_command_line_parser</a></code></td>
586 <td>parses command line (extended interface)</td>
589 <td><code class="computeroutput"><a class="link" href="../boost/program_options/parse_config__idp682372336.html" title="Function template parse_config_file">parse_config_file</a></code></td>
590 <td>parses config file</td>
593 <td><code class="computeroutput"><a class="link" href="../boost/program_options/parse_environ_idp682387952.html" title="Function parse_environment">parse_environment</a></code></td>
594 <td>parses environment</td>
596 <tr><td colspan="2">Storage component</td></tr>
598 <td><code class="computeroutput"><a class="link" href="../boost/program_options/variables_map.html" title="Class variables_map">variables_map</a></code></td>
599 <td>storage for option values</td>
605 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
606 <td align="left"></td>
607 <td align="right"><div class="copyright-footer">Copyright © 2002-2004 Vladimir Prus<p>Distributed under the Boost Software License, Version 1.0.
608 (See accompanying file <code class="filename">LICENSE_1_0.txt</code> or copy at
609 <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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