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2<head>
3<title>In-depth: The Parser</title>
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14      <font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>In-depth: The Parser</b></font>
15    </td>
16    <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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27</table>
28<p>What makes Spirit tick? Now on to some details... The parser class is the most
29  fundamental entity in the framework. A parser accepts a scanner comprised of
30  a first-last iterator pair and returns a match object as its result. The iterators
31  delimit the data currently being parsed. The match object evaluates to true
32  if the parse succeeds, in which case the input is advanced accordingly. Each
33  parser can represent a specific pattern or algorithm, or it can be a more complex
34  parser formed as a composition of other parsers.</p>
35<p>All parsers inherit from the base template class, parser:</p>
36<pre>
37<span class=keyword>template </span><span class=special>&lt;</span><span class=keyword>typename </span><span class=identifier>DerivedT</span><span class=special>&gt;
38</span><span class=keyword>struct </span><span class=identifier>parser
39</span><span class=special>{
40    </span><span class=comment>/*...*/
41
42    </span><span class=identifier>DerivedT</span><span class=special>&amp; </span><span class=identifier>derived</span><span class=special>();
43    </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>;
44</span><span class=special>};</span></pre>
45<p>This class is a protocol base class for all parsers. The parser class does
46  not really know how to parse anything but instead relies on the template parameter
47  <tt>DerivedT</tt> to do the actual parsing. This technique is known as the <a href="references.html#curious_recurring">&quot;Curiously
48  Recurring Template Pattern&quot;</a> in template meta-programming circles. This
49  inheritance strategy gives us the power of polymorphism without the virtual
50  function overhead. In essence this is a way to implement <a href="references.html#generic_patterns">compile
51  time polymorphism</a>.</p>
52<h2> parser_category_t</h2>
53<p> Each derived parser has a typedef <tt>parser_category_t</tt> that defines
54  its category. By default, if one is not specified, it will inherit from the
55  base parser class which typedefs its parser_category_t as <tt>plain_parser_category</tt>.
56  Some template classes are provided to distinguish different types of parsers.
57  The following categories are the most generic. More specific types may inherit
58  from these.</p>
59<table width="90%" border="0" align="center">
60  <tr> 
61    <td colspan="2" class="table_title">Parser categories</td>
62  </tr>
63  <tr> 
64    <td class="table_cells" width="33%"><tt>plain_parser_category</tt></td>
65    <td class="table_cells" width="67%">Your plain vanilla parser</td>
66  </tr>
67  <tr> 
68    <td class="table_cells" width="33%"><tt>binary_parser_category</tt></td>
69    <td class="table_cells" width="67%">A parser that has subject a and b (e.g.
70      alternative)</td>
71  </tr>
72  <tr> 
73    <td class="table_cells" width="33%"><tt>unary_parser_category</tt></td>
74    <td class="table_cells" width="67%">A parser that has single subject (e.g.
75      kleene star)</td>
76  </tr>
77  <tr> 
78    <td class="table_cells" width="33%"><tt>action_parser_category</tt></td>
79    <td class="table_cells" width="67%">A parser with an attached semantic action</td>
80  </tr>
81</table>
82<pre><span class=identifier>    </span><span class=keyword>struct </span><span class=identifier>plain_parser_category </span><span class=special>{};
83    </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>{};
84    </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>{};
85    </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>
86<h2>embed_t</h2>
87<p>Each parser has a typedef <tt>embed_t</tt>. This typedef specifies how a parser
88  is embedded in a composite. By default, if one is not specified, the parser
89  will be embedded by value. That is, a copy of the parser is placed as a member
90  variable of the composite. Most parsers are embedded by value. In certain situations
91  however, this is not desirable or possible. One particular example is the <a href="rule.html">rule</a>.
92  The rule, unlike other parsers is embedded by reference.</p>
93<h2><a name="match"></a>The match</h2>
94<p>The match holds the result of a parser. A match object evaluates to true when
95  a succesful match is found, otherwise false. The length of the match is the
96  number of characters (or tokens) that is successfully matched. This can be queried
97  through its <tt>length()</tt> member function. A negative value means that the
98  match is unsucessful. </p>
99<p> Each parser may have an associated attribute. This attribute is also returned
100  back to the client on a successful parse through the match object. We can get
101  this attribute via the match's <tt>value()</tt> member function. Be warned though
102  that the match's attribute may be invalid, in which case, getting the attribute
103  will result in an exception. The member function <tt>has_valid_attribute()</tt> 
104  can be queried to know if it is safe to get the match's attribute. The attribute
105  may be set anytime through the member function <tt>value(v)</tt>where <tt>v</tt> 
106  is the new attribute value.<br>
107  <br>
108  A match attribute is valid:</p>
109<ul>
110  <li> on a successful match</li>
111  <li>when its value is set through the <tt>value(val)</tt> member function</li>
112  <li> if it is assigned or copied from a compatible match object (e.g. <tt>match&lt;double&gt;</tt> 
113    from <tt>match&lt;int&gt;</tt>) with a valid attribute. A match object <tt>A</tt> 
114    is compatible with another match object <tt>B</tt> if the attribute type of
115    <tt>A</tt> can be assigned from the attribute type of <tt></tt> <tt>B</tt> 
116    (i.e. <tt>a = b;</tt> must compile).</li>
117</ul>
118<p>The match attribute is undefined:</p>
119<ul>
120  <li>on an unsuccessful match </li>
121  <li>when an attempt to copy or assign from another match object with an incompatible
122    attribute type (e.g. <tt>match&lt;std::string&gt;</tt> from <tt>match&lt;int&gt;</tt>).</li>
123</ul>
124<h3>The match class:</h3>
125<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;
126</span><span class=keyword>    class </span><span class=identifier>match
127</span><span class=keyword>    </span><span class=special>{
128</span><span class=keyword>    public</span><span class=special>:
129
130    </span><span class=keyword>    </span><span class=comment>/*...*/
131
132</span><span class=special>    </span><span class=keyword>    typedef</span><span class="identifier"> T attr_t</span><span class="special">;<br>
133             </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>
134    <span class=keyword>    int         </span><span class=identifier>length</span><span class=special>() </span><span class=keyword>const</span>;
135        <span class="keyword">bool</span>        has_valid_attribute<span class="special">()</span> <span class="keyword">const</span><span class="special">;</span>
136   <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;
137        </span><span class=identifier>T </span><span class=keyword>const</span><span class=special>&amp;    </span><span class=identifier>value</span><span class=special>();
138</span><span class=keyword>    </span><span class=special>};</span></pre>
139<h2>match_result</h2>
140<p>It has been mentioned repeatedly that the parser returns a match object as
141  its result. This is a simplification. Actually, for the sake of genericity,
142  parsers are really not hard-coded to return a match object. More accurately,
143  a parser returns an object that adheres to a conceptual interface, of which
144  the match is an example. Nevertheless, we shall call the result type of a parser
145  a match object regardless if it is actually a match class, a derivative or a
146  totally unrelated type.</p>
147<table width="80%" border="0" align="center">
148  <tr> 
149    <td class="note_box"><img src="theme/lens.gif" width="15" height="16"> <b>Meta-functions</b><br>
150      <br>
151      What are meta-functions? We all know how functions look like. In simplest
152      terms, a function accepts some arguments and returns a result. Here is the
153      function we all love so much:<br>
154      <br>
155      <code><span class="keyword">int</span> identity_func<span class="special">(</span><span class="keyword">int</span> 
156      arg<span class="special">)</span><br>
157      <span class="special">{</span> <span class="keyword">return</span> arg<span class="special">;
158      }</span> <span class="comment">// return the argument arg</span><br>
159      </code><br>
160      Meta-functions are essentially the same. These beasts also accept arguments
161      and return a result. However, while functions work at runtime on values,
162      meta-functions work at compile time on types (or constants, but we shall
163      deal only with types). The meta-function is a template class (or struct).
164      The template parameters are the arguments to the meta-function and a typedef
165      within the class is the meta-function's return type. Here is the corresponding
166      meta-function:<code><br>
167      <br>
168      <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> 
169      ArgT<span class="special">&gt;</span><br>
170      <span class="keyword">struct</span> identity_meta_func<br>
171      <span class="special">{</span> <span class="keyword">typedef</span> ArgT
172      type<span class="special">; } </span><span class="comment">// return the
173      argument ArgT</span><br>
174      <br>
175      </code>The meta-function above is invoked as:<br>
176      <br>
177      <code><span class="keyword">typename</span> identity_meta_func<span class="special">&lt;</span>ArgT<span class="special">&gt;::</span>type</code><br>
178      <br>
179      By convention, meta-functions return the result through the typedef <tt>type</tt>.
180      Take note that <tt>typename</tt> is only required within templates.</td>
181  </tr>
182</table>
183<p>The actual match type used by the parser depends on two types: the parser's
184  attribute type and the scanner type. <tt>match_result</tt> is the meta-function
185  that returns the desired match type given an attribute type and a scanner type.
186</p>
187<p>Usage:</p>
188<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>
189<p>The meta-function basically answers the question &quot;given a scanner type
190  <tt>ScannerT</tt> and an attribute type <tt>T</tt>, what is the desired match
191  type?&quot; [<img src="theme/note.gif" width="16" height="16"> <tt>typename</tt> 
192  is only required within templates ].</p>
193<h2>The parse member function</h2>
194<p>Concrete sub-classes inheriting from parser must have a corresponding member
195  function <tt>parse(...)</tt> compatible with the conceptual Interface:<br>
196</p>
197<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;
198    </span><span class=identifier>RT
199    </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>
200<p>where <tt>RT</tt> is the desired return type of the parser. </p>
201<h2>The parser result</h2>
202<p>Concrete sub-classes inheriting from parser in most cases need to have a nested
203  meta-function <tt>result</tt> that returns the result <tt>type</tt> of the parser's
204  parse member function, given a scanner type. The meta-function has the form:</p>
205<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;
206    </span><span class=keyword>struct </span><span class=identifier>result
207    </span><span class=special>{
208        </span><span class=keyword>typedef </span>RT <span class=identifier></span><span class=identifier>type</span><span class=special>;
209    </span><span class=special>};</span></pre>
210<p>where <tt>RT</tt> is the desired return type of the parser. This is usually,
211  but not always, dependent on the template parameter <tt>ScannerT</tt>. For example,
212  given an attribute type <tt>int</tt>, we can use the match_result metafunction:</p>
213<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;
214    </span><span class=keyword>struct </span><span class=identifier>result
215    </span><span class=special>{
216        </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>;
217    };</span></pre>
218<p>If a parser does not supply a result metafunction, a default is provided by
219  the base parser class.<span class=special> </span>The default is declared as:</p>
220<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;
221    </span><span class=keyword>struct </span><span class=identifier>result
222    </span><span class=special>{
223        </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>;
224    };</span></pre>
225<p>Without a result metafunction, notice that the parser's default attribute is
226  <tt>nil_t</tt> (i.e. the parser has no attribute).</p>
227<h2><span class=special></span>parser_result</h2>
228<p>Given a a scanner type <tt>ScannerT</tt> and a parser type <tt>ParserT</tt>,
229  what will be the actual result of the parser? The answer to this question is
230  provided to by the <tt>parser_result</tt> meta-function.</p>
231<p>Usage:</p>
232<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>
233<p>In general, the meta-function just forwards the invocation to the parser's
234  result meta-function:</p>
235<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;
236    </span><span class=keyword>struct </span><span class=identifier>parser_result
237    </span><span class=special>{
238        </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>;
239    </span><span class=special>};</span></pre>
240<p>This is similar to a global function calling a member function. Most of the
241  time, the usage above is equivalent to:</p>
242<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>
243<p>Yet, this should not be relied upon to be true all the time because the parser_result
244  metafunction might be specialized for specific parser and/or scanner types.</p>
245<p>The parser_result metafunction makes the signature of the required parse member
246  function almost canonical:</p>
247<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;
248    </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>
249<p>where<span class=special></span> <tt>self_t</tt> is a typedef to the parser.</p>
250<h2>parser class declaration</h2>
251<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;
252    </span><span class=keyword>struct </span><span class=identifier>parser
253    </span><span class=special>{
254        </span><span class=keyword>typedef </span><span class=identifier>DerivedT                embed_t</span><span class=special>;
255        </span><span class=keyword>typedef </span><span class=identifier>DerivedT                derived_t</span><span class=special>;
256        </span><span class=keyword>typedef </span><span class=identifier>plain_parser_category   parser_category_t</span><span class=special>;
257
258        </span><span class=keyword>template </span><span class=special>&lt;</span><span class="keyword">typename</span> ScannerT<span class=special>&gt;
259        </span><span class=keyword>struct </span><span class=identifier>result
260        </span><span class=special>{
261            </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>;
262        };
263
264        </span><span class=identifier>DerivedT</span><span class=special>&amp; </span><span class=identifier>derived</span><span class=special>();
265        </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>;
266
267        </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;
268        </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;
269        </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>;
270    };</span></pre>
271<table border="0">
272  <tr> 
273    <td width="10"></td>
274    <td width="30"><a href="../index.html"><img src="theme/u_arr.gif" border="0"></a></td>
275    <td width="30"><a href="semantic_actions.html"><img src="theme/l_arr.gif" border="0"></a></td>
276    <td width="30"><a href="indepth_the_scanner.html"><img src="theme/r_arr.gif" border="0"></a></td>
277  </tr>
278</table>
279<br>
280<hr size="1">
281<p class="copyright">Copyright &copy; 1998-2003 Joel de Guzman<br>
282  <br>
283<font size="2">Use, modification and distribution is subject to the Boost Software
284    License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
285    http://www.boost.org/LICENSE_1_0.txt) </font> </p>
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