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<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="program_options.overview"></a>Library Overview</h2></div></div></div>
<div class="toc"><dl>
<dt><span class="section"><a href="overview.html#id2073547">Options Description Component</a></span></dt>
<dt><span class="section"><a href="overview.html#id2074129">Parsers Component</a></span></dt>
<dt><span class="section"><a href="overview.html#id2074238">Storage Component</a></span></dt>
<dt><span class="section"><a href="overview.html#id2074341">Specific parsers</a></span></dt>
<dt><span class="section"><a href="overview.html#id2074635">Annotated List of Symbols</a></span></dt>
</dl></div>
<p>In the tutorial section, we saw several examples of library usage.
Here we will describe the overall library design including the primary
components and their function.
</p>
<p>The library has three main components:
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem"><p>The options description component, which describes the allowed options
and what to do with the values of the options.
</p></li>
<li class="listitem"><p>The parsers component, which uses this information to find option names
and values in the input sources and return them.
</p></li>
<li class="listitem"><p>The storage component, which provides the
interface to access the value of an option. It also converts the string
representation of values that parsers return into desired C++ types.
</p></li>
</ul></div>
<p>
</p>
<p>To be a little more concrete, the <code class="computeroutput">options_description</code>
class is from the options description component, the
<code class="computeroutput">parse_command_line</code> function is from the parsers component, and the
<code class="computeroutput">variables_map</code> class is from the storage component. </p>
<p>In the tutorial we've learned how those components can be used by the
<code class="computeroutput">main</code> function to parse the command line and config
file. Before going into the details of each component, a few notes about
the world outside of <code class="computeroutput">main</code>.
</p>
<p>
For that outside world, the storage component is the most important. It
provides a class which stores all option values and that class can be
freely passed around your program to modules which need access to the
options. All the other components can be used only in the place where
the actual parsing is the done. However, it might also make sense for the
individual program modules to describe their options and pass them to the
main module, which will merge all options. Of course, this is only
important when the number of options is large and declaring them in one
place becomes troublesome.
</p>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="id2073547"></a>Options Description Component</h3></div></div></div>
<div class="toc"><dl>
<dt><span class="section"><a href="overview.html#id2073801">Syntactic Information</a></span></dt>
<dt><span class="section"><a href="overview.html#id2073964">Semantic Information</a></span></dt>
<dt><span class="section"><a href="overview.html#id2074010">Positional Options</a></span></dt>
</dl></div>
<p>The options description component has three main classes:
<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
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>
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>,
which, in turn, knows the type of the option's value and can parse the value,
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
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>.
</p>
<p>For almost every library, those classes could be created in a
conventional way: that is, you'd create new options using constructors and
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,
that's overly verbose for declaring 20 or 30 options. This concern led
to creation of the syntax that you've already seen:
</p>
<pre class="programlisting">
options_description desc;
desc.add_options()
("help", "produce help")
("optimization", value&lt;int&gt;()-&gt;default_value(10), "optimization level")
;
</pre>
<p>
</p>
<p>The call to the <code class="computeroutput">value</code> function creates an instance of
a class derived from the <code class="computeroutput">value_semantic</code> class: <code class="computeroutput">typed_value</code>.
That class contains the code to parse
values of a specific type, and contains a number of methods which can be
called by the user to specify additional information. (This
essentially emulates named parameters of the constructor.) Calls to
<code class="computeroutput">operator()</code> on the object returned by <code class="computeroutput">add_options</code>
forward arguments to the constructor of the <code class="computeroutput">option_description</code>
class and add the new instance.
</p>
<p>
Note that in addition to the
<code class="computeroutput">value</code>, library provides the <code class="computeroutput">bool_switch</code>
function, and user can write his own function which will return
other subclasses of <code class="computeroutput">value_semantic</code> with
different behaviour. For the remainder of this section, we'll talk only
about the <code class="computeroutput">value</code> function.
</p>
<p>The information about an option is divided into syntactic and
semantic. Syntactic information includes the name of the option and the
number of tokens which can be used to specify the value. This
information is used by parsers to group tokens into (name, value) pairs,
where value is just a vector of strings
(<code class="computeroutput">std::vector&lt;std::string&gt;</code>). The semantic layer
is responsible for converting the value of the option into more usable C++
types.
</p>
<p>This separation is an important part of library design. The parsers
use only the syntactic layer, which takes away some of the freedom to
use overly complex structures. For example, it's not easy to parse
syntax like: </p>
<pre class="screen">calc --expression=1 + 2/3</pre>
<p> because it's not
possible to parse </p>
<pre class="screen">1 + 2/3</pre>
<p> without knowing that it's a C
expression. With a little help from the user the task becomes trivial,
and the syntax clear: </p>
<pre class="screen">calc --expression="1 + 2/3"</pre>
<p>
</p>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="id2073801"></a>Syntactic Information</h4></div></div></div>
<div class="toc"><dl><dt><span class="section"><a href="overview.html#id2073892">Description formatting</a></span></dt></dl></div>
<p>The syntactic information is provided by the
<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
and some methods of the
<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
and includes:
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem"><p>
name of the option, used to identify the option inside the
program,
</p></li>
<li class="listitem"><p>
description of the option, which can be presented to the user,
</p></li>
<li class="listitem"><p>
the allowed number of source tokens that comprise options's
value, which is used during parsing.
</p></li>
</ul></div>
<p>
</p>
<p>Consider the following example:
</p>
<pre class="programlisting">
options_description desc;
desc.add_options()
("help", "produce help message")
("compression", value&lt;string&gt;(), "compression level")
("verbose", value&lt;string&gt;()-&gt;zero_tokens(), "verbosity level")
("email", value&lt;string&gt;()-&gt;multitoken(), "email to send to")
;
</pre>
<p>
For the first parameter, we specify only the name and the
description. No value can be specified in the parsed source.
For the first option, the user must specify a value, using a single
token. For the third option, the user may either provide a single token
for the value, or no token at all. For the last option, the value can
span several tokens. For example, the following command line is OK:
</p>
<pre class="screen">
test --help --compression 10 --verbose --email beadle@mars beadle2@mars
</pre>
<p>
</p>
<div class="section">
<div class="titlepage"><div><div><h5 class="title">
<a name="id2073892"></a>Description formatting</h5></div></div></div>
<p>
Sometimes the description can get rather long, for example, when
several option's values need separate documentation. Below we
describe some simple formatting mechanisms you can use.
</p>
<p>The description string has one or more paragraphs, separated by
the newline character ('\n'). When an option is output, the library
will compute the indentation for options's description. Each of the
paragraph is output as a separate line with that intentation. If
a paragraph does not fit on one line it is spanned over multiple
lines (which will have the same indentation).
</p>
<p>You may specify additional indent for the first specified by
inserting spaces at the beginning of a paragraph. For example:
</p>
<pre class="programlisting">
options.add_options()
("help", " A long help msg a long help msg a long help msg a long help
msg a long help msg a long help msg a long help msg a long help msg ")
;
</pre>
<p>
will specify a four-space indent for the first line. The output will
look like:
</p>
<pre class="screen">
--help A long help msg a long
help msg a long help msg
a long help msg a long
help msg a long help msg
a long help msg a long
help msg
</pre>
<p>
</p>
<p>For the case where line is wrapped, you can want an additional
indent for wrapped text. This can be done by
inserting a tabulator character ('\t') at the desired position. For
example:
</p>
<pre class="programlisting">
options.add_options()
("well_formated", "As you can see this is a very well formatted
option description.\n"
"You can do this for example:\n\n"
"Values:\n"
" Value1: \tdoes this and that, bla bla bla bla
bla bla bla bla bla bla bla bla bla bla bla\n"
" Value2: \tdoes something else, bla bla bla bla
bla bla bla bla bla bla bla bla bla bla bla\n\n"
" This paragraph has a first line indent only,
bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla");
</pre>
<p>
will produce:
</p>
<pre class="screen">
--well_formated As you can see this is a
very well formatted
option description.
You can do this for
example:
Values:
Value1: does this and
that, bla bla
bla bla bla bla
bla bla bla bla
bla bla bla bla
bla
Value2: does something
else, bla bla
bla bla bla bla
bla bla bla bla
bla bla bla bla
bla
This paragraph has a
first line indent only,
bla bla bla bla bla bla
bla bla bla bla bla bla
bla bla bla
</pre>
<p>
The tab character is removed before output. Only one tabulator per
paragraph is allowed, otherwisee an exception of type
program_options::error is thrown. Finally, the tabulator is ignored if
it's is not on the first line of the paragraph or is on the last
possible position of the first line.
</p>
</div>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="id2073964"></a>Semantic Information</h4></div></div></div>
<p>The semantic information is completely provided by the
<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
example:
</p>
<pre class="programlisting">
options_description desc;
desc.add_options()
("compression", value&lt;int&gt;()-&gt;default_value(10), "compression level")
("email", value&lt; vector&lt;string&gt; &gt;()
-&gt;composing()-&gt;notifier(&amp;your_function), "email")
;
</pre>
<p>
These declarations specify that default value of the first option is 10,
that the second option can appear several times and all instances should
be merged, and that after parsing is done, the library will call
function <code class="computeroutput">&amp;your_function</code>, passing the value of the
"email" option as argument.
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="id2074010"></a>Positional Options</h4></div></div></div>
<p>Our definition of option as (name, value) pairs is simple and
useful, but in one special case of the command line, there's a
problem. A command line can include a <em class="firstterm">positional option</em>,
which does not specify any name at all, for example:
</p>
<pre class="screen">
archiver --compression=9 /etc/passwd
</pre>
<p>
Here, the "/etc/passwd" element does not have any option name.
</p>
<p>One solution is to ask the user to extract positional options
himself and process them as he likes. However, there's a nicer approach
-- provide a method to automatically assign the names for positional
options, so that the above command line can be interpreted the same way
as:
</p>
<pre class="screen">
archiver --compression=9 --input-file=/etc/passwd
</pre>
<p>
</p>
<p>The <code class="computeroutput"><a class="link" href="../boost/program_options/positional_options_desc_id993016.html" title="Class positional_options_description">positional_options_description</a></code> class allows the command line
parser to assign the names. The class specifies how many positional options
are allowed, and for each allowed option, specifies the name. For example:
</p>
<pre class="programlisting">
positional_options_description pd; pd.add("input-file", 1);
</pre>
<p> specifies that for exactly one, first, positional
option the name will be "input-file".
</p>
<p>It's possible to specify that a number, or even all positional options, be
given the same name.
</p>
<pre class="programlisting">
positional_options_description pd;
pd.add("output-file", 2).add("input-file", -1);
</pre>
<p>
In the above example, the first two positional options will be associated
with name "output-file", and any others with the name "input-file".
</p>
<div class="warning"><table border="0" summary="Warning">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Warning]" src="../../../doc/src/images/warning.png"></td>
<th align="left">Warning</th>
</tr>
<tr><td align="left" valign="top"><p>The <code class="computeroutput"><a class="link" href="../boost/program_options/positional_options_desc_id993016.html" title="Class positional_options_description">positional_options_description</a></code> class only specifies translation from
position to name, and the option name should still be registered with
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>
</table></div>
</div>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="id2074129"></a>Parsers Component</h3></div></div></div>
<p>The parsers component splits input sources into (name, value) pairs.
Each parser looks for possible options and consults the options
description component to determine if the option is known and how its value
is specified. In the simplest case, the name is explicitly specified,
which allows the library to decide if such option is known. If it is known, the
<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
it is not known, an exception is thrown.) Common
cases are when the value is explicitly specified by the user, and when
the value cannot be specified by the user, but the presence of the
option implies some value (for example, <code class="computeroutput">true</code>). So, the
parser checks that the value is specified when needed and not specified
when not needed, and returns new (name, value) pair.
</p>
<p>
To invoke a parser you typically call a function, passing the options
description and command line or config file or something else.
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>
class. Typically, that object is passed directly to the storage
component. However, it also can be used directly, or undergo some additional
processing.
</p>
<p>
There are three exceptions to the above model -- all related to
traditional usage of the command line. While they require some support
from the options description component, the additional complexity is
tolerable.
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem"><p>The name specified on the command line may be
different from the option name -- it's common to provide a "short option
name" alias to a longer name. It's also common to allow an abbreviated name
to be specified on the command line.
</p></li>
<li class="listitem"><p>Sometimes it's desirable to specify value as several
tokens. For example, an option "--email-recipient" may be followed
by several emails, each as a separate command line token. This
behaviour is supported, though it can lead to parsing ambiguities
and is not enabled by default.
</p></li>
<li class="listitem"><p>The command line may contain positional options -- elements
which don't have any name. The command line parser provides a
mechanism to guess names for such options, as we've seen in the
tutorial.
</p></li>
</ul></div>
<p>
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="id2074238"></a>Storage Component</h3></div></div></div>
<p>The storage component is responsible for:
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem"><p>Storing the final values of an option into a special class and in
regular variables</p></li>
<li class="listitem"><p>Handling priorities among different sources.</p></li>
<li class="listitem"><p>Calling user-specified <code class="computeroutput">notify</code> functions with the final
values of options.</p></li>
</ul></div>
<p>
</p>
<p>Let's consider an example:
</p>
<pre class="programlisting">
variables_map vm;
store(parse_command_line(argc, argv, desc), vm);
store(parse_config_file("example.cfg", desc), vm);
notify(vm);
</pre>
<p>
The <code class="computeroutput">variables_map</code> class is used to store the option
values. The two calls to the <code class="computeroutput">store</code> function add values
found on the command line and in the config file. Finally the call to
the <code class="computeroutput">notify</code> function runs the user-specified notify
functions and stores the values into regular variables, if needed.
</p>
<p>The priority is handled in a simple way: the <code class="computeroutput">store</code>
function will not change the value of an option if it's already
assigned. In this case, if the command line specifies the value for an
option, any value in the config file is ignored.
</p>
<div class="warning"><table border="0" summary="Warning">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Warning]" src="../../../doc/src/images/warning.png"></td>
<th align="left">Warning</th>
</tr>
<tr><td align="left" valign="top"><p>Don't forget to call the <code class="computeroutput">notify</code> function after you've
stored all parsed values.</p></td></tr>
</table></div>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="id2074341"></a>Specific parsers</h3></div></div></div>
<div class="toc"><dl>
<dt><span class="section"><a href="overview.html#id2074347">Configuration file parser</a></span></dt>
<dt><span class="section"><a href="overview.html#id2074451">Environment variables parser</a></span></dt>
</dl></div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="id2074347"></a>Configuration file parser</h4></div></div></div>
<p>The <code class="computeroutput"><a class="link" href="../boost/program_options/parse_config_file_id991860.html" title="Function template parse_config_file">parse_config_file</a></code> function implements parsing
of simple INI-like configuration files. Configuration file
syntax is line based:
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
<p>A line in the form:</p>
<pre class="screen">
<em class="replaceable"><code>name</code></em>=<em class="replaceable"><code>value</code></em>
</pre>
<p>gives a value to an option.</p>
</li>
<li class="listitem">
<p>A line in the form:</p>
<pre class="screen">
[<em class="replaceable"><code>section name</code></em>]
</pre>
<p>introduces a new section in the configuration file.</p>
</li>
<li class="listitem"><p>The <code class="literal">#</code> character introduces a
comment that spans until the end of the line.</p></li>
</ul></div>
<p>The option names are relative to the section names, so
the following configuration file part:</p>
<pre class="screen">
[gui.accessibility]
visual_bell=yes
</pre>
<p>is equivalent to</p>
<pre class="screen">
gui.accessibility.visual_bell=yes
</pre>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="id2074451"></a>Environment variables parser</h4></div></div></div>
<p><em class="firstterm">Environment variables</em> are string variables
which are available to all programs via the <code class="computeroutput">getenv</code> function
of C runtime library. The operating system allows to set initial values
for a given user, and the values can be further changed on the command
line. For example, on Windows one can use the
<code class="filename">autoexec.bat</code> file or (on recent versions) the
<code class="filename">Control Panel/System/Advanced/Environment Variables</code>
dialog, and on Unix &#8212;, the <code class="filename">/etc/profile</code>,
<code class="filename">~/.profile</code> and <code class="filename">~/.bash_profile</code>
files. Because environment variables can be set for the entire system,
they are particularly suitable for options which apply to all programs.
</p>
<p>The environment variables can be parsed with the
<code class="computeroutput"><a class="link" href="../boost/program_options/parse_environment_id898481.html" title="Function parse_environment">parse_environment</a></code> function. The function have several overloaded
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>
instance, and the second specifies what variables must be processed, and
what option names must correspond to it. To describe the second
parameter we need to consider naming conventions for environment
variables.</p>
<p>If you have an option that should be specified via environment
variable, you need make up the variable's name. To avoid name clashes,
we suggest that you use a sufficiently unique prefix for environment
variables. Also, while option names are most likely in lower case,
environment variables conventionally use upper case. So, for an option
name <code class="literal">proxy</code> the environment variable might be called
<code class="envar">BOOST_PROXY</code>. During parsing, we need to perform reverse
conversion of the names. This is accomplished by passing the choosen
prefix as the second parameter of the <code class="computeroutput"><a class="link" href="../boost/program_options/parse_environment_id898481.html" title="Function parse_environment">parse_environment</a></code> function.
Say, if you pass <code class="literal">BOOST_</code> as the prefix, and there are
two variables, <code class="envar">CVSROOT</code> and <code class="envar">BOOST_PROXY</code>, the
first variable will be ignored, and the second one will be converted to
option <code class="literal">proxy</code>.
</p>
<p>The above logic is sufficient in many cases, but it is also
possible to pass, as the second parameter of the <code class="computeroutput"><a class="link" href="../boost/program_options/parse_environment_id898481.html" title="Function parse_environment">parse_environment</a></code>
function, any function taking a <code class="computeroutput">std::string</code> and returning
<code class="computeroutput">std::string</code>. That function will be called for each
environment variable and should return either the name of the option, or
empty string if the variable should be ignored.
</p>
</div>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="id2074635"></a>Annotated List of Symbols</h3></div></div></div>
<p>The following table describes all the important symbols in the
library, for quick access.</p>
<div class="informaltable"><table class="table" width="100%">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>Symbol</th>
<th>Description</th>
</tr></thead>
<tbody>
<tr><td colspan="2">Options description component</td></tr>
<tr>
<td><code class="computeroutput"><a class="link" href="../boost/program_options/options_description.html" title="Class options_description">options_description</a></code></td>
<td>describes a number of options</td>
</tr>
<tr>
<td><code class="computeroutput"><a class="link" href="../boost/program_options/value_id929613.html" title="Function value">value</a></code></td>
<td>defines the option's value</td>
</tr>
<tr><td colspan="2">Parsers component</td></tr>
<tr>
<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>
<td>parses command line (simpified interface)</td>
</tr>
<tr>
<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>
<td>parses command line (extended interface)</td>
</tr>
<tr>
<td><code class="computeroutput"><a class="link" href="../boost/program_options/parse_config_file_id991860.html" title="Function template parse_config_file">parse_config_file</a></code></td>
<td>parses config file</td>
</tr>
<tr>
<td><code class="computeroutput"><a class="link" href="../boost/program_options/parse_environment_id898481.html" title="Function parse_environment">parse_environment</a></code></td>
<td>parses environment</td>
</tr>
<tr><td colspan="2">Storage component</td></tr>
<tr>
<td><code class="computeroutput"><a class="link" href="../boost/program_options/variables_map.html" title="Class variables_map">variables_map</a></code></td>
<td>storage for option values</td>
</tr>
</tbody>
</table></div>
</div>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright &#169; 2002-2004 Vladimir Prus<p>Distributed under the Boost Software License, Version 1.0.
(See accompanying file <code class="filename">LICENSE_1_0.txt</code> or copy at
<a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
</p>
</div></td>
</tr></table>
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