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25<div class="section" lang="en">
26<div class="titlepage"><div><div><h3 class="title">
27<a name="program_options.tutorial"></a>Tutorial</h3></div></div></div>
28<div class="toc"><dl>
29<dt><span class="section"><a href="tutorial.html#id2714212">Getting Started</a></span></dt>
30<dt><span class="section"><a href="tutorial.html#id2714372">Option Details</a></span></dt>
31<dt><span class="section"><a href="tutorial.html#id2714557">Multiple Sources</a></span></dt>
32</dl></div>
33<p>In this section, we'll take a look at the most common usage scenarios
34  of the program_options library, starting with the simplest one. The examples
35  show only the interesting code parts, but the complete programs can be found
36  in the "BOOST_ROOT/libs/program_options/example" directory. Through all the
37  examples, we'll assume that the following namespace alias is in effect:
38</p>
39<pre class="programlisting">namespace po = boost::program_options;</pre>
40<div class="section" lang="en">
41<div class="titlepage"><div><div><h4 class="title">
42<a name="id2714212"></a>Getting Started</h4></div></div></div>
43<p>The first example is the simplest possible: it only handles two
44    options. Here's the source code (the full program is in
45    "example/first.cpp"):
46
47</p>
48<pre class="programlisting">
49// Declare the supported options.
50po::options_description desc("Allowed options");
51desc.add_options()
52    ("help", "produce help message")
53    ("compression", po::value&lt;int&gt;(), "set compression level")
54;
55
56po::variables_map vm;
57po::store(po::parse_command_line(ac, av, desc), vm);
58po::notify(vm);   
59
60if (vm.count("help")) {
61    cout &lt;&lt; desc &lt;&lt; "\n";
62    return 1;
63}
64
65if (vm.count("compression")) {
66    cout &lt;&lt; "Compression level was set to "
67 &lt;&lt; vm["compression"].as&lt;int&gt;() &lt;&lt; ".\n";
68} else {
69    cout &lt;&lt; "Compression level was not set.\n";
70}
71</pre>
72<p>We start by declaring all allowed options using the
73    <code class="computeroutput"><a href="../options_description.html" title="Class options_description">options_description</a></code> class. The <code class="computeroutput">add_options</code> method of that
74    class returns a special proxy object that defines
75    <code class="computeroutput">operator()</code>. Calls to that operator actually declare
76    options. The parameters are option name, information about value, and option
77    description. In this example, the first option has no value, and the second
78    one has a value of type <code class="computeroutput">int</code>.
79  </p>
80<p>After that, an object of class <code class="computeroutput">variables_map</code> is
81    declared. That class is intended to store values of options, and can store
82    values of arbitrary types. Next, the calls to <code class="computeroutput">store</code>,
83    <code class="computeroutput">parse_command_line</code> and <code class="computeroutput">notify</code> functions cause
84    <code class="computeroutput">vm</code> to contain all the options found on the command
85    line.</p>
86<p>And now, finally, we can use the options as we like. The
87    <code class="computeroutput">variables_map</code> class can be used just like
88    <code class="computeroutput">std::map</code>, except that values stored there must be retrieved
89    with the <code class="computeroutput">as</code> method shown above. (If the type specified in the
90    call to the <code class="computeroutput">as</code> method is different from the actually stored
91    type, an exception is thrown.)
92  </p>
93<p>It's now a good time to try compiling the code yourself, but if
94    you're not yet ready, here's an example session:
95</p>
96<pre class="screen">
97$<strong class="userinput"><code>bin/gcc/debug/first</code></strong>
98Compression level was not set.
99$<strong class="userinput"><code>bin/gcc/debug/first --help</code></strong>
100Allowed options:
101  --help                 : produce help message
102  --compression arg      : set compression level
103$<strong class="userinput"><code>bin/gcc/debug/first --compression 10</code></strong>
104Compression level was set to 10.
105    </pre>
106</div>
107<div class="section" lang="en">
108<div class="titlepage"><div><div><h4 class="title">
109<a name="id2714372"></a>Option Details</h4></div></div></div>
110<p>An option value, surely, can have other types than <code class="computeroutput">int</code>, and
111  can have other interesting properties, which we'll discuss right now. The
112  complete version of the code snipped below can be found in
113  "example/options_description.cpp".</p>
114<p>Imagine we're writing a compiler. It should take the optimization
115    level, a number of include paths, and a number of input files, and perform some
116    interesting work. Let's describe the options:
117    </p>
118<pre class="programlisting">
119int opt;
120po::options_description desc("Allowed options");
121desc.add_options()
122    ("help", "produce help message")
123    ("optimization", po::value&lt;int&gt;(&amp;opt)-&gt;default_value(10),
124  "optimization level")
125    ("include-path,I", po::value&lt; vector&lt;string&gt; &gt;(),
126  "include path")
127    ("input-file", po::value&lt; vector&lt;string&gt; &gt;(), "input file")
128;
129</pre>
130<p>The "--help" option should be familiar from the previous example.
131    It's a good idea to have this option in all cases.</p>
132<p>The "optimization" option shows two new features. First, we specify
133    the address of the variable(<code class="computeroutput">&amp;opt</code>). After storing values, that
134    variable will have the value of the option. Second, we specify a default
135    value of 10, which will be used if no value is specified by the user.
136  </p>
137<p>The "include-path" option is an example of the only case where
138    the interface of the <code class="computeroutput">options_description</code> class serves only one
139    source -- the command line. Users typically like to use short option names
140    for common options, and the "include-path,I" name specifies that short
141    option name is "I". So, both "--include-path" and "-I" can be used.
142  </p>
143<p>The "input-file" option specifies the list of files to
144    process. That's okay for a start, but, of course, writing something like:
145    </p>
146<pre class="screen"><strong class="userinput"><code>compiler --input-file=a.cpp</code></strong></pre>
147<p>
148    is a little non-standard, compared with
149    </p>
150<pre class="screen"><strong class="userinput"><code>compiler a.cpp</code></strong></pre>
151<p>
152    We'll address this in a moment.
153  </p>
154<p>
155    The command line tokens which have no option name, as above, are
156    called "positional options" by this library. They can be handled
157    too. With a little help from the user, the library can decide that "a.cpp"
158    really means the same as "--input-file=a.cpp". Here's the additional code
159    we need:
160    </p>
161<pre class="programlisting">
162po::positional_options_description p;
163p.add("input-file", -1);
164
165po::variables_map vm;
166po::store(po::command_line_parser(ac, av).
167          options(desc).positional(p).run(), vm);
168po::notify(vm);
169    </pre>
170<p>
171    The first two lines say that all positional options should be translated
172    into "input-file" options. Also note that we use the
173    <code class="computeroutput">command_line_parser</code> class to parse the command
174    line, not the <code class="computeroutput"><a href="../parse_command_line.html" title="Function template parse_command_line">parse_command_line</a></code>
175    function. The latter is a convenient wrapper for simple cases, but now we
176    need to pass additional information.
177  </p>
178<p>By now, all options are described and parsed. We'll save ourselves the
179      trouble of implementing the rest of the compiler logic and only print the
180      options:
181    </p>
182<pre class="programlisting">
183if (vm.count("include-path"))
184{
185    cout &lt;&lt; "Include paths are: "
186         &lt;&lt; vm["include-path"].as&lt; vector&lt;string&gt; &gt;() &lt;&lt; "\n";
187}
188
189if (vm.count("input-file"))
190{
191    cout &lt;&lt; "Input files are: "
192         &lt;&lt; vm["input-file"].as&lt; vector&lt;string&gt; &gt;() &lt;&lt; "\n";
193}
194
195cout &lt;&lt; "Optimization level is " &lt;&lt; opt &lt;&lt; "\n";               
196</pre>
197<p>Here's an example session:
198    </p>
199<pre class="screen">
200$<strong class="userinput"><code>bin/gcc/debug/options_description --help</code></strong>
201Usage: options_description [options]
202Allowed options:
203  --help                 : produce help message
204  --optimization arg     : optimization level
205  -I [ --include-path ] arg : include path
206  --input-file arg       : input file
207$bin/gcc/debug/options_description
208Optimization level is 10
209$<strong class="userinput"><code>bin/gcc/debug/options_description --optimization 4 -I foo a.cpp</code></strong>
210Include paths are: foo
211Input files are: a.cpp
212Optimization level is 4
213</pre>
214<p>
215    Oops, there's a slight problem. It's still possible to specify the
216    "--input-file" option, and usage message says so, which can be confusing
217    for the user. It would be nice to hide this information, but let's wait
218    for the next example.
219  </p>
220</div>
221<div class="section" lang="en">
222<div class="titlepage"><div><div><h4 class="title">
223<a name="id2714557"></a>Multiple Sources</h4></div></div></div>
224<p>It's quite likely that specifying all options to our compiler on the
225    command line will annoy users. What if a user installs a new library and
226    wants to always pass an additional command line element? What if he has
227    made some choices which should be applied on every run? It's desirable to
228    create a config file with common settings which will be used together with
229    the command line.
230    </p>
231<p>Of course, there will be a need to combine the values from command
232    line and config file. For example, the optimization level specified on the
233    command line should override the value from the config file. On the other
234    hand, include paths should be combined.
235    </p>
236<p>Let's see the code now. The complete program is in
237      "examples/multiple_sources.cpp". The option definition has two interesting
238      details. First, we declare several instances of the
239      <code class="computeroutput">options_description</code> class. The reason is that, in general,
240      not all options are alike. Some options, like "input-file" above, should
241      not be presented in an automatic help message. Some options make sense only
242      in the config file. Finally, it's nice to have some structure in the help message,
243      not just a long list of options. Let's declare several option groups:
244      </p>
245<pre class="programlisting">
246// Declare a group of options that will be
247// allowed only on command line
248po::options_description generic("Generic options");
249generic.add_options()
250    ("version,v", "print version string")
251    ("help", "produce help message")   
252    ;
253   
254// Declare a group of options that will be
255// allowed both on command line and in
256// config file
257po::options_description config("Configuration");
258config.add_options()
259    ("optimization", po::value&lt;int&gt;(&amp;opt)-&gt;default_value(10),
260          "optimization level")
261    ("include-path,I",
262         po::value&lt; vector&lt;string&gt; &gt;()-&gt;composing(),
263         "include path")
264    ;
265
266// Hidden options, will be allowed both on command line and
267// in config file, but will not be shown to the user.
268po::options_description hidden("Hidden options");
269hidden.add_options()
270    ("input-file", po::value&lt; vector&lt;string&gt; &gt;(), "input file")
271    ;       
272</pre>
273<p>
274      Note the call to the <code class="computeroutput">composing</code> method in the declaration of the
275      "include-path" option. It tells the library that values from different sources
276      should be composed together, as we'll see shortly.
277    </p>
278<p>   
279      The <code class="computeroutput">add</code> method of the <code class="computeroutput">options_description</code>
280      class can be used to further group the options:
281      </p>
282<pre class="programlisting">
283po::options_description cmdline_options;
284cmdline_options.add(generic).add(config).add(hidden);
285
286po::options_description config_file_options;
287config_file_options.add(config).add(hidden);
288
289po::options_description visible("Allowed options");
290visible.add(generic).add(config);
291      </pre>
292<p>The parsing and storing of values follows the usual pattern, except that
293      we additionally call <code class="computeroutput">parse_config_file</code>, and
294      call the <code class="computeroutput"><a href="../id2349650.html" title="Function store">store</a></code> function twice. But what
295      happens if the same value is specified both on the command line and in
296      config file? Usually, the value stored first is preferred. This is what
297      happens for the "--optimization" option. For "composing" options, like
298      "include-file", the values are merged.
299    </p>
300<p>Here's an example session:
301</p>
302<pre class="screen">
303$<strong class="userinput"><code>bin/gcc/debug/multiple_sources</code></strong>
304Include paths are: /opt
305Optimization level is 1
306$<strong class="userinput"><code>bin/gcc/debug/multiple_sources --help</code></strong>
307Allows options:
308
309Generic options:
310  -v [ --version ]       : print version string
311  --help                 : produce help message
312
313Configuration:
314  --optimization n       : optimization level
315  -I [ --include-path ] path : include path
316
317$<strong class="userinput"><code>bin/gcc/debug/multiple_sources --optimization=4 -I foo a.cpp b.cpp</code></strong>
318Include paths are: foo /opt
319Input files are: a.cpp b.cpp
320Optimization level is 4
321</pre>
322<p>
323      The first invocation uses values from the configuration file. The second
324      invocation also uses values from command line. As we see, the include
325      paths on the command line and in the configuration file are merged,
326      while optimization is taken from the command line.
327    </p>
328</div>
329</div>
330<table width="100%"><tr>
331<td align="left"></td>
332<td align="right"><small>Copyright © 2002-2004 Vladimir Prus</small></td>
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