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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>