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<td width="85%"> <font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>Parametric
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Parsers</b></font> </td>
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<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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<p>We already have a hint of the dynamic nature of the Spirit framework. This
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capability is fundamental to Spirit. Dynamic parsing is a very powerful concept.
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We shall take this concept further through run-time parametric parsers. We are
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able to handle parsing tasks that are impossible to do with any EBNF syntax
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alone.</p>
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<h2>A Little Secret</h2>
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<p> A little critter called <tt>boost::ref</tt> lurking in the boost distribution
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is quite powerful beast when used with Spirit's primitive parsers. We are used
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to seeing the Spirit primitive parsers created with string or character literals
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such as:</p>
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<pre>
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<code><span class=identifier>ch_p</span><span class=special>(</span><span class=literal>'A'</span><span class=special>)
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</span><span class=identifier>range_p</span><span class=special>(</span><span class=literal>'A'</span><span class=special>, </span><span class=literal>'Z'</span><span class=special>)
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</span><span class=identifier>str_p</span><span class=special>(</span><span class=string>"Hello World"</span><span class=special>)</span></code></pre>
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<p> str_p has a second form that accepts two iterators over the string:</p>
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<pre>
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<code><span class=keyword>char </span><span class=keyword>const</span><span class=special>* </span><span class=identifier>first </span><span class=special>= </span><span class=string>"My oh my"</span><span class=special>;
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</span><span class=keyword>char </span><span class=keyword>const</span><span class=special>* </span><span class=identifier>last </span><span class=special>= </span><span class=identifier>first </span><span class=special>+ </span><span class=identifier>std</span><span class=special>::</span><span class=identifier>strlen</span><span class=special>(</span><span class=identifier>first</span><span class=special>);
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</span><span class=identifier>str_p</span><span class=special>(</span><span class=identifier>first</span><span class=special>, </span><span class=identifier>last</span><span class=special>)</span></code></pre>
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<p> What is not obvious is that we can use <tt>boost::ref</tt> as well:</p>
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<pre>
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<code><span class=keyword>char </span><span class=identifier>ch </span><span class=special>= </span><span class=literal>'A'</span><span class=special>;
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</span><span class=keyword>char </span><span class=identifier>from </span><span class=special>= </span><span class=literal>'A'</span><span class=special>;
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</span><span class=keyword>char </span><span class=identifier>to </span><span class=special>= </span><span class=literal>'Z'</span><span class=special>;
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</span><span class=identifier>ch_p</span><span class=special>(</span><span class=identifier>boost</span><span class=special>::</span><span class=identifier>ref</span><span class=special>(</span><span class=identifier>ch</span><span class=special>))
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</span><span class=identifier>range_p</span><span class=special>(</span><span class=identifier>boost</span><span class=special>::</span><span class=identifier>ref</span><span class=special>(</span><span class=identifier>from</span><span class=special>), </span><span class=identifier>boost</span><span class=special>::</span><span class=identifier>ref</span><span class=special>(</span><span class=identifier>to</span><span class=special>))</span></code></pre>
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<p> When <tt>boost::ref</tt> is used, the actual parameters to <tt>ch_p</tt> and
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<tt>range_p</tt> are held by reference. This means that we can change the values
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of <tt>ch</tt>, <tt>from</tt> and <tt>to</tt> anytime and the corresponding
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<tt>ch_p</tt> and <tt>range_p</tt> parser will follow their dynamic values.
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Of course, since they are held by reference, you must make sure that the referenced
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object is not destructed while parsing.</p>
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<p> What about <tt>str_p</tt>?</p>
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<p> While the first form of <tt>str_p</tt> (the single argument form) is reserved
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for null terminated string constants, the second form (the two argument first/last
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iterator form) may be used:</p>
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<pre>
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<code><span class=keyword>char </span><span class=keyword>const</span><span class=special>* </span><span class=identifier>first </span><span class=special>= </span><span class=string>"My oh my"</span><span class=special>;
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</span><span class=keyword>char </span><span class=keyword>const</span><span class=special>* </span><span class=identifier>last </span><span class=special>= </span><span class=identifier>first </span><span class=special>+ </span><span class=identifier>std</span><span class=special>::</span><span class=identifier>strlen</span><span class=special>(</span><span class=identifier>first</span><span class=special>);
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</span><span class=identifier>str_p</span><span class=special>(</span><span class=identifier>boost</span><span class=special>::</span><span class=identifier>ref</span><span class=special>(</span><span class=identifier>first</span><span class=special>), </span><span class=identifier>boost</span><span class=special>::</span><span class=identifier>ref</span><span class=special>(</span><span class=identifier>last</span><span class=special>))</span></code></pre>
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<table width="80%" border="0" align="center">
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<tr>
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<td class="note_box"> <img src="theme/note.gif" width="16" height="16"> Hey,
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don't forget <tt>chseq_p</tt>. All these apply to this seldom used primitive
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as well. </td>
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</tr>
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</table>
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<h2>Functional Parametric Primitives</h2>
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<pre> <span class="preprocessor">#include</span> <span class="special"><</span>boost<span class="special">/</span>spirit<span class="special">/</span>attribute<span class="special">/</span><span class="identifier">parametric</span><span class="special">.</span>hpp<span class="special">></span></pre>
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<p> Taking this further, Spirit includes functional versions of the primitives.
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Rather than taking in characters, strings or references to characters and strings
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(using boost::ref), the functional versions take in functions or functors.</p>
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<h3>f_chlit and f_ch_p</h3>
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<p> The functional version of <tt>chlit</tt>. This parser takes in a function
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or functor (function object). The function is expected to have an interface
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compatible with:</p>
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<pre>
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<code><span class=identifier>CharT </span><span class=identifier>func</span><span class=special>()</span></code></pre>
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<p> where CharT is the character type (e.g. <tt>char</tt>, <tt>int</tt>, <tt>wchar_t</tt>).</p>
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<p> The functor is expected to have an interface compatible with:</p>
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<pre>
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<code><span class=keyword>struct </span><span class=identifier>functor
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</span><span class=special>{
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</span><span class=identifier>CharT </span><span class=keyword>operator</span><span class=special>()() </span><span class=keyword>const</span><span class=special>;
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</span><span class=special>};</span></code></pre>
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<p> where CharT is the character type (e.g. <tt>char</tt>, <tt>int</tt>, <tt>wchar_t</tt>).</p>
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<p> Here's a contrived example:</p>
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<pre>
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<code><span class=keyword>struct </span><span class=identifier>X
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</span><span class=special>{
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</span><span class=keyword>char </span><span class=keyword>operator</span><span class=special>()() </span><span class=keyword>const
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</span><span class=special>{
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</span><span class=keyword>return </span><span class=literal>'X'</span><span class=special>; </span><span class=special>
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}
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</span><span class=special>};</span></code></pre>
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<p> Now we can use X to create our f_chlit parser:</p>
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<pre>
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<code><span class=identifier>f_ch_p</span><span class=special>(</span><span class=identifier>X</span><span class=special>())</span></code></pre>
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<h3>f_range and f_range_p</h3>
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<p> The functional version of <tt>range</tt>. This parser takes in a function
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or functor compatible with the interfaces above. The difference is that <tt>f_range</tt>
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(and <tt>f_range_p</tt>) expects two functors. One for the start and one for
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the end of the range.</p>
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<h3>f_chseq and f_chseq_p</h3>
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<p> The functional version of <tt>chseq</tt>. This parser takes in two functions
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or functors. One for the begin iterator and one for the end iterator. The function
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is expected to have an interface compatible with:</p>
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<pre>
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<code><span class=identifier>IteratorT </span><span class=identifier>func</span><span class=special>()</span></code></pre>
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<p> where <tt>IteratorT</tt> is the iterator type (e.g. <tt>char const*</tt>,
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<tt>wchar_t const*</tt>).</p>
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<p> The functor is expected to have an interface compatible with:</p>
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<pre>
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<code><span class=keyword>struct </span><span class=identifier>functor
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</span><span class=special>{
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</span><span class=identifier>IteratorT </span><span class=keyword>operator</span><span class=special>()() </span><span class=keyword>const</span><span class=special>;
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</span><span class=special>};</span></code></pre>
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<p> where <tt>IteratorT</tt> is the iterator type (e.g. <tt>char const*</tt>,
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<tt>wchar_t const*</tt>).</p>
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<h3>f_strlit and f_str_p</h3>
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<p> The functional version of <tt>strlit</tt>. This parser takes in two functions
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or functors compatible with the interfaces that <tt>f_chseq</tt> expects.</p>
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<table border="0">
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<tr>
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<td width="10"></td>
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<td width="30"><a href="../index.html"><img src="theme/u_arr.gif" border="0"></a></td>
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<td width="30"><a href="predefined_actors.html"><img src="theme/l_arr.gif" border="0"></a></td>
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<td width="30"><a href="functional.html"><img src="theme/r_arr.gif" border="0"></a></td>
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</tr>
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</table>
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<br>
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<hr size="1">
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<p class="copyright">Copyright © 1998-2003 Joel de Guzman<br>
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<br>
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<font size="2">Use, modification and distribution is subject to the Boost Software
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License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
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http://www.boost.org/LICENSE_1_0.txt)</font></p>
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<p class="copyright"> </p>
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