1<html> 2<head> 3<title>In-depth: The Parser</title> 4<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"> 5<link rel="stylesheet" href="theme/style.css" type="text/css"> 6</head> 7 8<body> 9<table width="100%" border="0" background="theme/bkd2.gif" cellspacing="2"> 10 <tr> 11 <td width="10"> 12 </td> 13 <td width="85%"> 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> 17 </tr> 18</table> 19<br> 20<table border="0"> 21 <tr> 22 <td width="10"></td> 23 <td width="30"><a href="../index.html"><img src="theme/u_arr.gif" border="0"></a></td> 24 <td width="30"><a href="semantic_actions.html"><img src="theme/l_arr.gif" border="0"></a></td> 25 <td width="30"><a href="indepth_the_scanner.html"><img src="theme/r_arr.gif" border="0"></a></td> 26 </tr> 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><</span><span class=keyword>typename </span><span class=identifier>DerivedT</span><span class=special>> 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>& </span><span class=identifier>derived</span><span class=special>(); 43 </span><span class=identifier>DerivedT </span><span class=keyword>const</span><span class=special>& </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">"Curiously 48 Recurring Template Pattern"</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 successful 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 unsuccessful. </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<double></tt> 113 from <tt>match<int></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<std::string></tt> from <tt>match<int></tt>).</li> 123</ul> 124<h3>The match class:</h3> 125<pre><span class=keyword> template </span><span class=special><</span><span class=keyword>typename </span><span class=identifier>T</span><span class=special>> 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>&) </span><span class=keyword>const; 137 </span><span class=identifier>T </span><span class=keyword>const</span><span class=special>& </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"><</span><span class="keyword">typename</span> 169 ArgT<span class="special">></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"><</span>ArgT<span class="special">>::</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><</span><span class=identifier>ScannerT</span><span class=special>, </span><span class=identifier>T</span><span class=special>>::</span><span class=identifier>type</span></pre> 189<p>The meta-function basically answers the question "given a scanner type 190 <tt>ScannerT</tt> and an attribute type <tt>T</tt>, what is the desired match 191 type?" [<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><</span><span class=keyword>typename </span><span class=identifier>ScannerT</span><span class=special>> 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>& </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><</span><span class=keyword>typename </span><span class=identifier>ScannerT</span><span class=special>> 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><</span><span class=keyword>typename </span><span class=identifier>ScannerT</span><span class=special>> 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><</span><span class=identifier>ScannerT</span><span class=special>, </span><span class="keyword">int</span><span class=special>>::</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><</span><span class=keyword>typename </span><span class=identifier>ScannerT</span><span class=special>> 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><</span><span class=identifier>ScannerT</span><span class=special>, </span><span class="identifier">nil_t</span><span class=special>>::</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><</span><span class=identifier>ParserT, ScannerT</span><span class=special>>::</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><</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>> 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><</span><span class=identifier>ScannerT</span><span class=special>>::</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><</span><span class=identifier>ScannerT</span><span class=special>>::</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><</span><span class=keyword>typename </span><span class=identifier>ScannerT</span><span class=special>> 248 </span><span class=keyword>typename </span><span class=identifier>parser_result</span><span class=special><</span><span class=identifier>self_t, ScannerT</span><span class=special>>::</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>& </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><</span><span class=keyword>typename </span><span class=identifier>DerivedT</span><span class=special>> 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><</span><span class="keyword">typename</span> ScannerT<span class=special>> 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><</span><span class=identifier>ScannerT</span><span class=special>, </span><span class=identifier>nil_t</span><span class=special>>::</span><span class=identifier>type type</span><span class=special>; 262 }; 263 264 </span><span class=identifier>DerivedT</span><span class=special>& </span><span class=identifier>derived</span><span class=special>(); 265 </span><span class=identifier>DerivedT </span><span class=keyword>const</span><span class=special>& </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><</span><span class=keyword>typename </span><span class=identifier>ActionT</span><span class=special>> 268 </span><span class=identifier>action</span><span class=special><</span><span class=identifier>DerivedT</span><span class=special>, </span><span class=identifier>ActionT</span><span class=special>> 269 </span><span class=keyword>operator</span><span class=special>[](</span><span class=identifier>ActionT </span><span class=keyword>const</span><span class=special>& </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 © 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> 286</body> 287</html> 288
