program_options/doc/tutorial.xml

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<section id="program_options.tutorial">
  <title>Tutorial</title>
  
  <para>In this section, we'll take a look at the most common usage scenarios
  of the program_options library, starting with the simplest one. The examples
  show only the interesting code parts, but the complete programs can be found
  in the "BOOST_ROOT/libs/program_options/example" directory. Through all the
  examples, we'll assume that the following namespace alias is in effect:
<programlisting>namespace po = boost::program_options;</programlisting>
  </para>

  <section>
    <title>Getting Started</title>

  <para>The first example is the simplest possible: it only handles two
    options. Here's the source code (the full program is in
    "example/first.cpp"):

<programlisting>
// Declare the supported options.
po::options_description desc(&quot;Allowed options&quot;);
desc.add_options()
    (&quot;help&quot;, &quot;produce help message&quot;)
    (&quot;compression&quot;, po::value&lt;int&gt;(), &quot;set compression level&quot;)
;

po::variables_map vm;
po::store(po::parse_command_line(ac, av, desc), vm);
po::notify(vm);    

if (vm.count(&quot;help&quot;)) {
    cout &lt;&lt; desc &lt;&lt; &quot;\n&quot;;
    return 1;
}

if (vm.count(&quot;compression&quot;)) {
    cout &lt;&lt; &quot;Compression level was set to &quot; 
 &lt;&lt; vm[&quot;compression&quot;].as&lt;int&gt;() &lt;&lt; &quot;.\n&quot;;
} else {
    cout &lt;&lt; &quot;Compression level was not set.\n&quot;;
}
</programlisting>
  </para>

  <para>We start by declaring all allowed options using the
    &options_description; class. The <code>add_options</code> method of that
    class returns a special proxy object that defines
    <code>operator()</code>. Calls to that operator actually declare
    options. The parameters are option name, information about value, and option
    description. In this example, the first option has no value, and the second
    one has a value of type <code>int</code>.
  </para>
  
  <para>After that, an object of class <code>variables_map</code> is
    declared. That class is intended to store values of options, and can store
    values of arbitrary types. Next, the calls to <code>store</code>,
    <code>parse_command_line</code> and <code>notify</code> functions cause
    <code>vm</code> to contain all the options found on the command
    line.</para>

  <para>And now, finally, we can use the options as we like. The
    <code>variables_map</code> class can be used just like
    <code>std::map</code>, except that values stored there must be retrieved
    with the <code>as</code> method shown above. (If the type specified in the
    call to the <code>as</code> method is different from the actually stored
    type, an exception is thrown.)
  </para>

  <para>It's now a good time to try compiling the code yourself, but if
    you're not yet ready, here's an example session:
<screen>
$ <userinput>bin/gcc/debug/first</userinput>
Compression level was not set.
$ <userinput>bin/gcc/debug/first --help</userinput>
Allowed options:
  --help                 : produce help message
  --compression arg      : set compression level
$ <userinput>bin/gcc/debug/first --compression 10</userinput>
Compression level was set to 10.
    </screen>
  </para>
    
  </section>

  <section>
    <title>Option Details</title>
    
  <para>An option value, surely, can have other types than <code>int</code>, and
  can have other interesting properties, which we'll discuss right now. The
  complete version of the code snipped below can be found in
  <filename>example/options_description.cpp</filename>.</para>

  <para>Imagine we're writing a compiler. It should take the optimization
    level, a number of include paths, and a number of input files, and perform some
    interesting work. Let's describe the options:
    <programlisting>
int opt;
po::options_description desc(&quot;Allowed options&quot;);
desc.add_options()
    (&quot;help&quot;, &quot;produce help message&quot;)
    (&quot;optimization&quot;, po::value&lt;int&gt;(&amp;opt)-&gt;default_value(10), 
  &quot;optimization level&quot;)
    (&quot;include-path,I&quot;, po::value&lt; vector&lt;string&gt; &gt;(), 
  &quot;include path&quot;)
    (&quot;input-file&quot;, po::value&lt; vector&lt;string&gt; &gt;(), &quot;input file&quot;)
;
</programlisting>
  </para>

  <para>The <literal>"help"</literal> option should be familiar from 
  the previous example. It's a good idea to have this option in all cases.
  </para>

  <para>The <literal>"optimization"</literal> option shows two new features. First, we specify
    the address of the variable(<code>&amp;opt</code>). After storing values, that
    variable will have the value of the option. Second, we specify a default
    value of 10, which will be used if no value is specified by the user.
  </para>

  <para>The <literal>"include-path"</literal> option is an example of the 
  only case where the interface of the <code>options_description</code> 
  class serves only one
    source -- the command line. Users typically like to use short option names
    for common options, and the "include-path,I" name specifies that short
    option name is "I". So, both "--include-path" and "-I" can be used.
  </para>

  <para>Note also that the type of the <literal>"include-path"</literal>
  option is <type>std::vector</type>. The library provides special 
  support for vectors -- it will be possible to specify the option several 
  times, and all specified values will be collected in one vector.  
  </para>
    
  <para>The "input-file" option specifies the list of files to
    process. That's okay for a start, but, of course, writing something like:
    <screen>
<userinput>compiler --input-file=a.cpp</userinput>
    </screen>
    is a little non-standard, compared with
    <screen>
<userinput>compiler a.cpp</userinput>
    </screen>
    We'll address this in a moment.
  </para>

  <para>
    The command line tokens which have no option name, as above, are
    called "positional options" by this library. They can be handled
    too. With a little help from the user, the library can decide that "a.cpp"
    really means the same as "--input-file=a.cpp". Here's the additional code
    we need:
    <programlisting>
po::positional_options_description p;
p.add(&quot;input-file&quot;, -1);

po::variables_map vm;
po::store(po::command_line_parser(ac, av).
          options(desc).positional(p).run(), vm);
po::notify(vm);
    </programlisting>    
  </para>

  <para>
    The first two lines say that all positional options should be translated
    into "input-file" options. Also note that we use the
    &command_line_parser; class to parse the command
    line, not the &parse_command_line;
    function. The latter is a convenient wrapper for simple cases, but now we
    need to pass additional information.
  </para>

  <para>By now, all options are described and parsed. We'll save ourselves the
      trouble of implementing the rest of the compiler logic and only print the
      options:
    <programlisting>
if (vm.count(&quot;include-path&quot;))
{
    cout &lt;&lt; &quot;Include paths are: &quot; 
         &lt;&lt; vm[&quot;include-path&quot;].as&lt; vector&lt;string&gt; &gt;() &lt;&lt; &quot;\n&quot;;
}

if (vm.count(&quot;input-file&quot;))
{
    cout &lt;&lt; &quot;Input files are: &quot; 
         &lt;&lt; vm[&quot;input-file&quot;].as&lt; vector&lt;string&gt; &gt;() &lt;&lt; &quot;\n&quot;;
}

cout &lt;&lt; &quot;Optimization level is &quot; &lt;&lt; opt &lt;&lt; &quot;\n&quot;;                
</programlisting>
  </para>

  <para>Here's an example session:
    <screen>
$ <userinput>bin/gcc/debug/options_description --help</userinput>
Usage: options_description [options]
Allowed options:
  --help                 : produce help message
  --optimization arg     : optimization level
  -I [ --include-path ] arg : include path
  --input-file arg       : input file
$ <userinput>bin/gcc/debug/options_description</userinput>
Optimization level is 10
$ <userinput>bin/gcc/debug/options_description --optimization 4 -I foo -I another/path --include-path third/include/path a.cpp b.cpp</userinput>
Include paths are: foo another/path third/include/path
Input files are: a.cpp b.cpp
Optimization level is 4
</screen>
  </para>

  <para>
    Oops, there's a slight problem. It's still possible to specify the
    "--input-file" option, and usage message says so, which can be confusing
    for the user. It would be nice to hide this information, but let's wait
    for the next example.
  </para>

  </section>

  <section>
    <title>Multiple Sources</title>

    <para>It's quite likely that specifying all options to our compiler on the
    command line will annoy users. What if a user installs a new library and
    wants to always pass an additional command line element? What if he has
    made some choices which should be applied on every run? It's desirable to
    create a config file with common settings which will be used together with
    the command line.
    </para>

    <para>Of course, there will be a need to combine the values from command
    line and config file. For example, the optimization level specified on the
    command line should override the value from the config file. On the other
    hand, include paths should be combined.
    </para>

    <para>Let's see the code now. The complete program is in
      "examples/multiple_sources.cpp". The option definition has two interesting
      details. First, we declare several instances of the
      <code>options_description</code> class. The reason is that, in general,
      not all options are alike. Some options, like "input-file" above, should
      not be presented in an automatic help message. Some options make sense only
      in the config file. Finally, it's nice to have some structure in the help message,
      not just a long list of options. Let's declare several option groups:
      <programlisting>
// Declare a group of options that will be 
// allowed only on command line
po::options_description generic(&quot;Generic options&quot;);
generic.add_options()
    (&quot;version,v&quot;, &quot;print version string&quot;)
    (&quot;help&quot;, &quot;produce help message&quot;)    
    ;
    
// Declare a group of options that will be 
// allowed both on command line and in
// config file
po::options_description config(&quot;Configuration&quot;);
config.add_options()
    (&quot;optimization&quot;, po::value&lt;int&gt;(&amp;opt)-&gt;default_value(10), 
          &quot;optimization level&quot;)
    (&quot;include-path,I&quot;, 
         po::value&lt; vector&lt;string&gt; &gt;()-&gt;composing(), 
         &quot;include path&quot;)
    ;

// Hidden options, will be allowed both on command line and
// in config file, but will not be shown to the user.
po::options_description hidden(&quot;Hidden options&quot;);
hidden.add_options()
    (&quot;input-file&quot;, po::value&lt; vector&lt;string&gt; &gt;(), &quot;input file&quot;)
    ;        
</programlisting>
      Note the call to the <code>composing</code> method in the declaration of the
      "include-path" option. It tells the library that values from different sources
      should be composed together, as we'll see shortly.
    </para>

    <para>    
      The <code>add</code> method of the <code>options_description</code>
      class can be used to further group the options:
      <programlisting>
po::options_description cmdline_options;
cmdline_options.add(generic).add(config).add(hidden);

po::options_description config_file_options;
config_file_options.add(config).add(hidden);

po::options_description visible(&quot;Allowed options&quot;);
visible.add(generic).add(config);
      </programlisting>
    </para>

    <para>The parsing and storing of values follows the usual pattern, except that
      we additionally call <functionname>parse_config_file</functionname>, and
      call the &store; function twice. But what
      happens if the same value is specified both on the command line and in
      config file? Usually, the value stored first is preferred. This is what
      happens for the "--optimization" option. For "composing" options, like
      "include-file", the values are merged.
    </para>

    <para>Here's an example session:
<screen>
$ <userinput>bin/gcc/debug/multiple_sources</userinput>
Include paths are: /opt
Optimization level is 1
$ <userinput>bin/gcc/debug/multiple_sources --help</userinput>
Allows options:

Generic options:
  -v [ --version ]       : print version string
  --help                 : produce help message

Configuration:
  --optimization n       : optimization level
  -I [ --include-path ] path : include path

$ <userinput>bin/gcc/debug/multiple_sources --optimization=4 -I foo a.cpp b.cpp</userinput>
Include paths are: foo /opt
Input files are: a.cpp b.cpp
Optimization level is 4
</screen>
      The first invocation uses values from the configuration file. The second
      invocation also uses values from command line. As we see, the include
      paths on the command line and in the configuration file are merged,
      while optimization is taken from the command line.
    </para>
      
  </section>

  

  

  
        
</section>

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