<html>
<head>
<title>Functor Parser</title>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<link rel="stylesheet" href="theme/style.css" type="text/css">
</head>
<body>
<table width="100%" border="0" background="theme/bkd2.gif" cellspacing="2">
<tr>
<td width="10">
</td>
<td width="85%"> <font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>Functor
Parser</b></font> </td>
<td width="112"><a href="http://spirit.sf.net"><img src="theme/spirit.gif" width="112" height="48" align="right" border="0"></a></td>
</tr>
</table>
<br>
<table border="0">
<tr>
<td width="10"></td>
<td width="30"><a href="../index.html"><img src="theme/u_arr.gif" border="0"></a></td>
<td width="30"><a href="list_parsers.html"><img src="theme/l_arr.gif" border="0"></a></td>
<td width="20"><a href="refactoring.html"><img src="theme/r_arr.gif" border="0"></a></td>
</tr>
</table>
<p>The simplest way to write your hand coded parser that works well with the rest
of the Spirit library is to simply write a functor parser.</p>
<p> A functor parser is expected to have the interface:</p>
<pre>
<code><span class=keyword>struct </span><span class=identifier>functor
</span><span class=special>{
</span><span class=keyword>typedef </span><span class=identifier>T </span><span class=identifier>result_t</span><span class=special>;
</span><span class=keyword>template </span><span class=special><</span><span class=keyword>typename </span><span class=identifier>ScannerT</span><span class=special>>
</span><span class=keyword>int </span><span class=keyword>operator</span><span class=special>()(</span><span class=identifier>ScannerT </span><span class=keyword>const</span><span class=special>& </span><span class=identifier>scan</span><span class=special>, </span><span class=identifier>result_t</span><span class=special>& </span><span class=identifier>result</span><span class=special>) </span><span class=keyword>const</span><span class=special>;
</span><span class=special>};
</span></code></pre>
<p> where typedef T result_t; is the attribute type of the parser that will be
passed back to the match result (see <a href="indepth_the_parser.html">In-depth:
The Parser</a>). If the parser does not need to return an attribute, this can
simply be nil_t. The int result is the number of matching characters matched
by your parser. A negative value flags an unsucessful match.</p>
<p> A conforming functor parser can transformed into a well formed Spirit parser
by wrapping it in the functor_parser template:</p>
<pre>
<code><span class=identifier>functor_parser</span><span class=special><</span><span class=identifier>functor</span><span class=special>> </span><span class=identifier>functor_p</span><span class=special>;
</span></code></pre>
<a name="example"></a>
<h2>Example</h2>
<p> The following example puts the functor_parser into action:</p>
<pre>
<code><span class=keyword>struct </span><span class=identifier>number_parser
</span><span class=special>{
</span><span class=keyword>typedef </span><span class=keyword>int </span><span class=identifier>result_t</span><span class=special>;
</span><span class=keyword>template </span><span class=special><</span><span class=keyword>typename </span><span class=identifier>ScannerT</span><span class=special>>
</span><span class=keyword>int
</span><span class=keyword>operator</span><span class=special>()(</span><span class=identifier>ScannerT </span><span class=keyword>const</span><span class=special>& </span><span class=identifier>scan</span><span class=special>, </span><span class=identifier>result_t</span><span class=special>& </span><span class=identifier>result</span><span class=special>) </span><span class=keyword>const
</span><span class=special>{
</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>())
</span><span class=keyword>return </span><span class=special>-</span><span class=number>1</span><span class=special>;
</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>;
</span><span class=keyword>if </span><span class=special>(</span><span class=identifier>ch </span><span class=special>< </span><span class=literal>'0' </span><span class=special>|| </span><span class=identifier>ch </span><span class=special>> </span><span class=literal>'9'</span><span class=special>)
</span><span class=keyword>return </span><span class=special>-</span><span class=number>1</span><span class=special>;
</span><span class=identifier>result </span><span class=special>= </span><span class=number>0</span><span class=special>;
</span><span class=keyword>int </span><span class=identifier>len </span><span class=special>= </span><span class=number>0</span><span class=special>;
</span><span class=keyword>do
</span><span class=special>{
</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>);
</span><span class=special>++</span><span class=identifier>len</span><span class=special>;
</span><span class=special>++</span><span class=identifier>scan</span><span class=special>;
</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>&& </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>>= </span><span class=literal>'0' </span><span class=special>&& </span><span class=identifier>ch </span><span class=special><= </span><span class=literal>'9'</span><span class=special>));
</span><span class=keyword>return </span><span class=identifier>len</span><span class=special>;
</span><span class=special>}
</span><span class=special>};
</span><span class=identifier>functor_parser</span><span class=special><</span><span class=identifier>number_parser</span><span class=special>> </span><span class=identifier>number_parser_p</span><span class=special>;
</span></code></pre>
<p> To further understand the implementation, see <a href="indepth_the_scanner.html">In-depth:
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>
<pre>
<code><span class=identifier>r </span><span class=special>= </span><span class=identifier>number_parser_p </span><span class=special>>> </span><span class=special>*(</span><span class=literal>',' </span><span class=special>>> </span><span class=identifier>number_parser_p</span><span class=special>);
</span></code></pre>
<table border="0">
<tr>
<td width="10"></td>
<td width="30"><a href="../index.html"><img src="theme/u_arr.gif" border="0"></a></td>
<td width="30"><a href="list_parsers.html"><img src="theme/l_arr.gif" border="0"></a></td>
<td width="20"><a href="refactoring.html"><img src="theme/r_arr.gif" border="0"></a></td>
</tr>
</table>
<br>
<hr size="1">
<p class="copyright">Copyright © 1998-2002 Joel de Guzman<br>
<br>
<font size="2">Permission to copy, use, modify, sell and distribute this document
is granted provided this copyright notice appears in all copies. This document
is provided "as is" without express or implied warranty, and with
no claim as to its suitability for any purpose.</font></p>
<p class="copyright"> </p>
</body>
</html>
