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1<html>
2<head>
3<title>Functor Parser</title>
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10  <tr> 
11    <td width="10"> 
12    </td>
13    <td width="85%"> <font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>Functor
14      Parser</b></font> </td>
15    <td width="112"><a href="http://spirit.sf.net"><img src="theme/spirit.gif" width="112" height="48" align="right" border="0"></a></td>
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20  <tr>
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25  </tr>
26</table>
27<p>The simplest way to write your hand coded parser that works well with the rest
28  of the Spirit library is to simply write a functor parser.</p>
29<p> A functor parser is expected to have the interface:</p>
30<pre>
31    <code><span class=keyword>struct </span><span class=identifier>functor
32    </span><span class=special>{
33        </span><span class=keyword>typedef </span><span class=identifier>T </span><span class=identifier>result_t</span><span class=special>;
34
35        </span><span class=keyword>template </span><span class=special>&lt;</span><span class=keyword>typename </span><span class=identifier>ScannerT</span><span class=special>&gt;
36        </span><span class=keyword>std::ptrdiff_t
37        </span><span class=keyword>operator</span><span class=special>()(</span><span class=identifier>ScannerT </span><span class=keyword>const</span><span class=special>&amp; </span><span class=identifier>scan</span><span class=special>, </span><span class=identifier>result_t</span><span class=special>&amp; </span><span class=identifier>result</span><span class=special>) </span><span class=keyword>const</span><span class=special>;
38    </span><span class=special>};
39</span></code></pre>
40<p> where typedef T result_t; is the attribute type of the parser that will be
41  passed back to the match result (see <a href="indepth_the_parser.html">In-depth:
42  The Parser</a>). If the parser does not need to return an attribute, this can
43  simply be nil_t. The <span class=keyword><tt>std::ptrdiff_t</tt></span> result
44  is the number of matching characters matched by your parser. A negative value
45  flags an unsucessful match.</p>
46<p> A conforming functor parser can transformed into a well formed Spirit parser
47  by wrapping it in the functor_parser template:</p>
48<pre>
49    <code><span class=identifier>functor_parser</span><span class=special>&lt;</span><span class=identifier>functor</span><span class=special>&gt; </span><span class=identifier>functor_p</span><span class=special>;
50</span></code></pre>
51
52<h2>Example</h2>
53<p> The following example puts the functor_parser into action:</p>
54<pre>
55    <code><span class=keyword>struct </span><span class=identifier>number_parser
56    </span><span class=special>{
57        </span><span class=keyword>typedef </span><span class=keyword>int </span><span class=identifier>result_t</span><span class=special>;
58        </span><span class=keyword>template </span><span class=special>&lt;</span><span class=keyword>typename </span><span class=identifier>ScannerT</span><span class=special>&gt;
59        </span><span class=keyword>std::ptrdiff_t</span>
60        <span class=keyword>operator</span><span class=special>()(</span><span class=identifier>ScannerT </span><span class=keyword>const</span><span class=special>&amp; </span><span class=identifier>scan</span><span class=special>, </span><span class=identifier>result_t</span><span class=special>&amp; </span><span class=identifier>result</span><span class=special>) </span><span class=keyword>const
61        </span><span class=special>{
62            </span><span class=keyword>if </span><span class=special>(</span><span class=identifier>scan</span><span class=special>.</span><span class=identifier>at_end</span><span class=special>())
63                </span><span class=keyword>return </span><span class=special>-</span><span class=number>1</span><span class=special>;
64
65            </span><span class=keyword>char </span><span class=identifier>ch </span><span class=special>= </span><span class=special>*</span><span class=identifier>scan</span><span class=special>;
66            </span><span class=keyword>if </span><span class=special>(</span><span class=identifier>ch </span><span class=special>&lt; </span><span class=literal>'0' </span><span class=special>|| </span><span class=identifier>ch </span><span class=special>&gt; </span><span class=literal>'9'</span><span class=special>)
67                </span><span class=keyword>return </span><span class=special>-</span><span class=number>1</span><span class=special>;
68
69            </span><span class=identifier>result </span><span class=special>= </span><span class=number>0</span><span class=special>;
70            </span><span class=keyword>std::ptrdiff_t</span> <span class=identifier>len </span><span class=special>= </span><span class=number>0</span><span class=special>;
71
72            </span><span class=keyword>do
73            </span><span class=special>{
74                </span><span class=identifier>result </span><span class=special>= </span><span class=identifier>result</span><span class=special>*</span><span class=number>10 </span><span class=special>+ </span><span class=keyword>int</span><span class=special>(</span><span class=identifier>ch </span><span class=special>- </span><span class=literal>'0'</span><span class=special>);
75                </span><span class=special>++</span><span class=identifier>len</span><span class=special>;
76                </span><span class=special>++</span><span class=identifier>scan</span><span class=special>;
77            </span><span class=special>} </span><span class=keyword>while </span><span class=special>(!</span><span class=identifier>scan</span><span class=special>.</span><span class=identifier>at_end</span><span class=special>() </span><span class=special>&amp;&amp; </span><span class=special>(</span><span class=identifier>ch </span><span class=special>= </span><span class=special>*</span><span class=identifier>scan</span><span class=special>, </span><span class=identifier>ch </span><span class=special>&gt;= </span><span class=literal>'0' </span><span class=special>&amp;&amp; </span><span class=identifier>ch </span><span class=special>&lt;= </span><span class=literal>'9'</span><span class=special>));
78
79            </span><span class=keyword>return </span><span class=identifier>len</span><span class=special>;
80        </span><span class=special>}
81    </span><span class=special>};
82
83    </span><span class=identifier>functor_parser</span><span class=special>&lt;</span><span class=identifier>number_parser</span><span class=special>&gt; </span><span class=identifier>number_parser_p</span><span class=special>;
84</span></code></pre>
85<p>  <img src="theme/lens.gif" width="15" height="16"> The full source code can be <a href="../example/fundamental/functor_parser.cpp">viewed here</a>. This is part of the Spirit distribution. </p>
86<p>To further understand the implementation, see <a href="indepth_the_scanner.html">In-depth:
87    The Scanner</a> for the scanner API details. We now have a parser <tt>number_parser_p</tt> that we can use just like any other Spirit parser. Example:</p>
88<pre>
89    <code><span class=identifier>r </span><span class=special>= </span><span class=identifier>number_parser_p </span><span class=special>&gt;&gt; </span><span class=special>*(</span><span class=literal>',' </span><span class=special>&gt;&gt; </span><span class=identifier>number_parser_p</span><span class=special>);
90</span></code></pre>
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92  <tr> 
93    <td width="10"></td>
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97  </tr>
98</table>
99<br>
100<hr size="1">
101<p class="copyright">Copyright &copy; 1998-2003 Joel de Guzman<br>
102  <br>
103<font size="2">Use, modification and distribution is subject to the Boost Software
104    License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
105    http://www.boost.org/LICENSE_1_0.txt)</font></p>
106<p class="copyright">&nbsp;</p>
107</body>
108</html>
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