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