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       <font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>In-depth: The Parser</b></font>
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 <p>What makes Spirit tick? Now on to some details... The parser class is the most
   fundamental entity in the framework. A parser accepts a scanner comprised of
   a first-last iterator pair and returns a match object as its result. The iterators
   delimit the data currently being parsed. The match object evaluates to true
   if the parse succeeds, in which case the input is advanced accordingly. Each
   parser can represent a specific pattern or algorithm, or it can be a more complex
   parser formed as a composition of other parsers.</p>
 <p>All parsers inherit from the base template class, parser:</p>
 <pre>
 <span class=keyword>template </span><span class=special>&lt;</span><span class=keyword>typename </span><span class=identifier>DerivedT</span><span class=special>&gt;
 </span><span class=keyword>struct </span><span class=identifier>parser
 </span><span class=special>{
     </span><span class=comment>/*...*/

     </span><span class=identifier>DerivedT</span><span class=special>&amp; </span><span class=identifier>derived</span><span class=special>();
     </span><span class=identifier>DerivedT </span><span class=keyword>const</span><span class=special>&amp; </span><span class=identifier>derived</span><span class=special>() </span><span class=keyword>const</span><span class=special>;
 </span><span class=special>};</span></pre>
 <p>This class is a protocol base class for all parsers. The parser class does
   not really know how to parse anything but instead relies on the template parameter
   <tt>DerivedT</tt> to do the actual parsing. This technique is known as the <a href="references.html#curious_recurring">&quot;Curiously
   Recurring Template Pattern&quot;</a> in template meta-programming circles. This
   inheritance strategy gives us the power of polymorphism without the virtual
   function overhead. In essence this is a way to implement <a href="references.html#generic_patterns">compile
   time polymorphism</a>.</p>
 <h2> parser_category_t</h2>
 <p> Each derived parser has a typedef <tt>parser_category_t</tt> that defines
   its category. By default, if one is not specified, it will inherit from the
   base parser class which typedefs its parser_category_t as <tt>plain_parser_category</tt>.
   Some template classes are provided to distinguish different types of parsers.
   The following categories are the most generic. More specific types may inherit
   from these.</p>
 <table width="90%" border="0" align="center">
   <tr>
     <td colspan="2" class="table_title">Parser categories</td>
   </tr>
   <tr>
     <td class="table_cells" width="33%"><tt>plain_parser_category</tt></td>
     <td class="table_cells" width="67%">Your plain vanilla parser</td>
   </tr>
   <tr>
     <td class="table_cells" width="33%"><tt>binary_parser_category</tt></td>
     <td class="table_cells" width="67%">A parser that has subject a and b (e.g.
       alternative)</td>
   </tr>
   <tr>
     <td class="table_cells" width="33%"><tt>unary_parser_category</tt></td>
     <td class="table_cells" width="67%">A parser that has single subject (e.g.
       kleene star)</td>
   </tr>
   <tr>
     <td class="table_cells" width="33%"><tt>action_parser_category</tt></td>
     <td class="table_cells" width="67%">A parser with an attached semantic action</td>
   </tr>
 </table>
 <pre><span class=identifier>    </span><span class=keyword>struct </span><span class=identifier>plain_parser_category </span><span class=special>{};
     </span><span class=keyword>struct </span><span class=identifier>binary_parser_category       </span><span class=special>: </span><span class=identifier>plain_parser_category </span><span class=special>{};
     </span><span class=keyword>struct </span><span class=identifier>unary_parser_category        </span><span class=special>: </span><span class=identifier>plain_parser_category </span><span class=special>{};
     </span><span class=keyword>struct </span><span class=identifier>action_parser_category       </span><span class=special>: </span><span class=identifier>unary_parser_category </span><span class=special>{};</span></pre>
 <h2>embed_t</h2>
 <p>Each parser has a typedef <tt>embed_t</tt>. This typedef specifies how a parser
   is embedded in a composite. By default, if one is not specified, the parser
   will be embedded by value. That is, a copy of the parser is placed as a member
   variable of the composite. Most parsers are embedded by value. In certain situations
   however, this is not desirable or possible. One particular example is the <a href="rule.html">rule</a>.
   The rule, unlike other parsers is embedded by reference.</p>
 <h2><a name="match"></a>The match</h2>
 <p>The match holds the result of a parser. A match object evaluates to true when
   a succesful match is found, otherwise false. The length of the match is the
   number of characters (or tokens) that is successfully matched. This can be queried
   through its <tt>length()</tt> member function. A negative value means that the
   match is unsucessful. </p>
 <p> Each parser may have an associated attribute. This attribute is also returned
   back to the client on a successful parse through the match object. We can get
   this attribute via the match's <tt>value()</tt> member function. Be warned though
   that the match's attribute may be invalid, in which case, getting the attribute
   will result in an exception. The member function <tt>has_valid_attribute()</tt>
   can be queried to know if it is safe to get the match's attribute. The attribute
   may be set anytime through the member function <tt>value(v)</tt>where <tt>v</tt>
   is the new attribute value.<br>
   <br>
   A match attribute is valid:</p>
 <ul>
   <li> on a successful match</li>
   <li>when its value is set through the <tt>value(val)</tt> member function</li>
   <li> if it is assigned or copied from a compatible match object (e.g. <tt>match&lt;double&gt;</tt>
     from <tt>match&lt;int&gt;</tt>) with a valid attribute. A match object <tt>A</tt>
     is compatible with another match object <tt>B</tt> if the attribute type of
     <tt>A</tt> can be assigned from the attribute type of <tt></tt> <tt>B</tt>
     (i.e. <tt>a = b;</tt> must compile).</li>
 </ul>
 <p>The match attribute is undefined:</p>
 <ul>
   <li>on an unsuccessful match </li>
   <li>when an attempt to copy or assign from another match object with an incompatible
     attribute type (e.g. <tt>match&lt;std::string&gt;</tt> from <tt>match&lt;int&gt;</tt>).</li>
 </ul>
 <h3>The match class:</h3>
 <pre><span class=keyword>    template </span><span class=special>&lt;</span><span class=keyword>typename </span><span class=identifier>T</span><span class=special>&gt;
 </span><span class=keyword>    class </span><span class=identifier>match
 </span><span class=keyword>    </span><span class=special>{
 </span><span class=keyword>    public</span><span class=special>:

     </span><span class=keyword>    </span><span class=comment>/*...*/

 </span><span class=special>    </span><span class=keyword>    typedef</span><span class="identifier"> T attr_t</span><span class="special">;<br>
              </span><span class=keyword>    </span><span class="special">   </span><span class=keyword>operator safe_bool</span><span class=special>() </span><span class=keyword>const</span>; <span class="comment">// convertible to a bool</span>
     <span class=keyword>    int         </span><span class=identifier>length</span><span class=special>() </span><span class=keyword>const</span>;
         <span class="keyword">bool</span>        has_valid_attribute<span class="special">()</span> <span class="keyword">const</span><span class="special">;</span>
    <span class=keyword>    </span> <span class=identifier>void</span><span class=special>        </span><span class=identifier>value</span><span class=special>(</span><span class="identifier">T </span><span class="keyword">const</span><span class=special>&amp;) </span><span class=keyword>const;
         </span><span class=identifier>T </span><span class=keyword>const</span><span class=special>&amp;    </span><span class=identifier>value</span><span class=special>();
 </span><span class=keyword>    </span><span class=special>};</span></pre>
 <h2>match_result</h2>
 <p>It has been mentioned repeatedly that the parser returns a match object as
   its result. This is a simplification. Actually, for the sake of genericity,
   parsers are really not hard-coded to return a match object. More accurately,
   a parser returns an object that adheres to a conceptual interface, of which
   the match is an example. Nevertheless, we shall call the result type of a parser
   a match object regardless if it is actually a match class, a derivative or a
   totally unrelated type.</p>
 <table width="80%" border="0" align="center">
   <tr>
     <td class="note_box"><img src="theme/lens.gif" width="15" height="16"> <b>Meta-functions</b><br>
       <br>
       What are meta-functions? We all know how functions look like. In simplest
       terms, a function accepts some arguments and returns a result. Here is the
       function we all love so much:<br>
       <br>
       <code><span class="keyword">int</span> identity_func<span class="special">(</span><span class="keyword">int</span>
       arg<span class="special">)</span><br>
       <span class="special">{</span> <span class="keyword">return</span> arg<span class="special">;
       }</span> <span class="comment">// return the argument arg</span><br>
       </code><br>
       Meta-functions are essentially the same. These beasts also accept arguments
       and return a result. However, while functions work at runtime on values,
       meta-functions work at compile time on types (or constants, but we shall
       deal only with types). The meta-function is a template class (or struct).
       The template parameters are the arguments to the meta-function and a typedef
       within the class is the meta-function's return type. Here is the corresponding
       meta-function:<code><br>
       <br>
       <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span>
       ArgT<span class="special">&gt;</span><br>
       <span class="keyword">struct</span> identity_meta_func<br>
       <span class="special">{</span> <span class="keyword">typedef</span> ArgT
       type<span class="special">; } </span><span class="comment">// return the
       argument ArgT</span><br>
       <br>
       </code>The meta-function above is invoked as:<br>
       <br>
       <code><span class="keyword">typename</span> identity_meta_func<span class="special">&lt;</span>ArgT<span class="special">&gt;::</span>type</code><br>
       <br>
       By convention, meta-functions return the result through the typedef <tt>type</tt>.
       Take note that <tt>typename</tt> is only required within templates.</td>
   </tr>
 </table>
 <p>The actual match type used by the parser depends on two types: the parser's
   attribute type and the scanner type. <tt>match_result</tt> is the meta-function
   that returns the desired match type given an attribute type and a scanner type.
 </p>
 <p>Usage:</p>
 <pre>    <span class=keyword>typename </span><span class=identifier>match_result</span><span class=special>&lt;</span><span class=identifier>ScannerT</span><span class=special>, </span><span class=identifier>T</span><span class=special>&gt;::</span><span class=identifier>type</span></pre>
 <p>The meta-function basically answers the question &quot;given a scanner type
   <tt>ScannerT</tt> and an attribute type <tt>T</tt>, what is the desired match
   type?&quot; [<img src="theme/note.gif" width="16" height="16"> <tt>typename</tt>
   is only required within templates ].</p>
 <h2>The parse member function</h2>
 <p>Concrete sub-classes inheriting from parser must have a corresponding member
   function <tt>parse(...)</tt> compatible with the conceptual Interface:<br>
 </p>
 <pre><span class=identifier>    </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;
     </span><span class=identifier>RT
     </span><span class=identifier>parse</span><span class=special>(</span><span class=identifier>ScannerT</span><span class=special></span> const<span class=special>&amp; </span>scan<span class=identifier></span><span class=special>) </span><span class=keyword>const</span><span class=special>;</span></pre>
 <p>where <tt>RT</tt> is the desired return type of the parser. </p>
 <h2>The parser result</h2>
 <p>Concrete sub-classes inheriting from parser in most cases need to have a nested
   meta-function <tt>result</tt> that returns the result <tt>type</tt> of the parser's
   parse member function, given a scanner type. The meta-function has the form:</p>
 <pre><span class=keyword>    template </span><span class=special>&lt;</span><span class=keyword>typename </span><span class=identifier>ScannerT</span><span class=special>&gt;
     </span><span class=keyword>struct </span><span class=identifier>result
     </span><span class=special>{
         </span><span class=keyword>typedef </span>RT <span class=identifier></span><span class=identifier>type</span><span class=special>;
     </span><span class=special>};</span></pre>
 <p>where <tt>RT</tt> is the desired return type of the parser. This is usually,
   but not always, dependent on the template parameter <tt>ScannerT</tt>. For example,
   given an attribute type <tt>int</tt>, we can use the match_result metafunction:</p>
 <pre><span class=keyword>    template </span><span class=special>&lt;</span><span class=keyword>typename </span><span class=identifier>ScannerT</span><span class=special>&gt;
     </span><span class=keyword>struct </span><span class=identifier>result
     </span><span class=special>{
         </span><span class=keyword>typedef typename </span><span class=identifier>match_result</span><span class=special>&lt;</span><span class=identifier>ScannerT</span><span class=special>, </span><span class="keyword">int</span><span class=special>&gt;::</span><span class=identifier>type type</span><span class=special>;
     };</span></pre>
 <p>If a parser does not supply a result metafunction, a default is provided by
   the base parser class.<span class=special> </span>The default is declared as:</p>
 <pre><span class=keyword>    template </span><span class=special>&lt;</span><span class=keyword>typename </span><span class=identifier>ScannerT</span><span class=special>&gt;
     </span><span class=keyword>struct </span><span class=identifier>result
     </span><span class=special>{
         </span><span class=keyword>typedef typename </span><span class=identifier>match_result</span><span class=special>&lt;</span><span class=identifier>ScannerT</span><span class=special>, </span><span class="identifier">nil_t</span><span class=special>&gt;::</span><span class=identifier>type type</span><span class=special>;
     };</span></pre>
 <p>Without a result metafunction, notice that the parser's default attribute is
   <tt>nil_t</tt> (i.e. the parser has no attribute).</p>
 <h2><span class=special></span>parser_result</h2>
 <p>Given a a scanner type <tt>ScannerT</tt> and a parser type <tt>ParserT</tt>,
   what will be the actual result of the parser? The answer to this question is
   provided to by the <tt>parser_result</tt> meta-function.</p>
 <p>Usage:</p>
 <pre>    <span class=keyword>typename </span><span class=identifier>parser_result</span><span class=special>&lt;</span><span class=identifier>ParserT, ScannerT</span><span class=special>&gt;::</span><span class=identifier>type</span></pre>
 <p>In general, the meta-function just forwards the invocation to the parser's
   result meta-function:</p>
 <pre><span class=identifier>    </span><span class=keyword>template </span><span class=special>&lt;</span><span class=keyword>typename </span><span class=identifier>ParserT</span><span class=special>, </span><span class=keyword>typename </span><span class=identifier>ScannerT</span><span class=special>&gt;
     </span><span class=keyword>struct </span><span class=identifier>parser_result
     </span><span class=special>{
         </span><span class=keyword>typedef </span><span class=keyword>typename </span><span class=identifier>ParserT</span><span class=special>::</span><span class=keyword>template </span><span class=identifier>result</span><span class=special>&lt;</span><span class=identifier>ScannerT</span><span class=special>&gt;::</span><span class=identifier>type </span><span class=identifier>type</span><span class=special>;
     </span><span class=special>};</span></pre>
 <p>This is similar to a global function calling a member function. Most of the
   time, the usage above is equivalent to:</p>
 <pre><span class=keyword>    typename </span><span class=identifier>ParserT</span><span class=special>::</span><span class=keyword>template </span><span class=identifier>result</span><span class=special>&lt;</span><span class=identifier>ScannerT</span><span class=special>&gt;::</span><span class=identifier>type</span></pre>
 <p>Yet, this should not be relied upon to be true all the time because the parser_result
   metafunction might be specialized for specific parser and/or scanner types.</p>
 <p>The parser_result metafunction makes the signature of the required parse member
   function almost canonical:</p>
 <pre><span class=identifier>    </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;
     </span><span class=keyword>typename </span><span class=identifier>parser_result</span><span class=special>&lt;</span><span class=identifier>self_t, ScannerT</span><span class=special>&gt;::</span><span class=identifier>type</span><br>    <span class=identifier>parse</span><span class=special>(</span><span class=identifier>ScannerT</span><span class=special></span> const<span class=special>&amp; </span>scan<span class=identifier></span><span class=special>) </span><span class=keyword>const</span><span class=special>;</span></pre>
 <p>where<span class=special></span> <tt>self_t</tt> is a typedef to the parser.</p>
 <h2>parser class declaration</h2>
 <pre><span class=identifier>    </span><span class=keyword>template </span><span class=special>&lt;</span><span class=keyword>typename </span><span class=identifier>DerivedT</span><span class=special>&gt;
     </span><span class=keyword>struct </span><span class=identifier>parser
     </span><span class=special>{
         </span><span class=keyword>typedef </span><span class=identifier>DerivedT                embed_t</span><span class=special>;
         </span><span class=keyword>typedef </span><span class=identifier>DerivedT                derived_t</span><span class=special>;
         </span><span class=keyword>typedef </span><span class=identifier>plain_parser_category   parser_category_t</span><span class=special>;

         </span><span class=keyword>template </span><span class=special>&lt;</span><span class="keyword">typename</span> ScannerT<span class=special>&gt;
         </span><span class=keyword>struct </span><span class=identifier>result
         </span><span class=special>{
             </span><span class=keyword>typedef typename </span><span class=identifier>match_result</span><span class=special>&lt;</span><span class=identifier>ScannerT</span><span class=special>, </span><span class=identifier>nil_t</span><span class=special>&gt;::</span><span class=identifier>type type</span><span class=special>;
         };

         </span><span class=identifier>DerivedT</span><span class=special>&amp; </span><span class=identifier>derived</span><span class=special>();
         </span><span class=identifier>DerivedT </span><span class=keyword>const</span><span class=special>&amp; </span><span class=identifier>derived</span><span class=special>() </span><span class=keyword>const</span><span class=special>;

         </span><span class=keyword>template </span><span class=special>&lt;</span><span class=keyword>typename </span><span class=identifier>ActionT</span><span class=special>&gt;
         </span><span class=identifier>action</span><span class=special>&lt;</span><span class=identifier>DerivedT</span><span class=special>, </span><span class=identifier>ActionT</span><span class=special>&gt;
         </span><span class=keyword>operator</span><span class=special>[](</span><span class=identifier>ActionT </span><span class=keyword>const</span><span class=special>&amp; </span><span class=identifier>actor</span><span class=special>) </span><span class=keyword>const</span><span class=special>;
     };</span></pre>
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 <p class="copyright">Copyright &copy; 1998-2003 Joel de Guzman<br>
   <br>
 <font size="2">Use, modification and distribution is subject to the Boost Software
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