1<html> 2<head> 3<title>Semantic Actions</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>Semantic Actions</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="subrules.html"><img src="theme/l_arr.gif" border="0"></a></td> 25 <td width="30"><a href="indepth_the_parser.html"><img src="theme/r_arr.gif" border="0"></a></td> 26 </tr> 27</table> 28<p>Semantic actions have the form: <b>expression[action]</b></p> 29<p>Ultimately, after having defined our grammar and having generated a corresponding 30 parser, we will need to produce some output and do some work besides syntax 31 analysis; unless, of course, what we want is merely to check for the conformance 32 of an input with our grammar, which is very seldom the case. Semantic actions 33 may be attached to any expression at any level within the parser hierarchy. 34 An action is a C/C++ function or function object that will be called if a match 35 is found in the particular context where it is attached. The action function 36 serves as a hook into the parser and may be used to, for example:</p> 37<blockquote> 38 <p><img src="theme/bullet.gif" width="13" height="13"> Generate output from 39 the parser (ASTs, for example)<br> 40 <img src="theme/bullet.gif" width="13" height="13"> Report warnings or errors<br> 41 <img src="theme/bullet.gif" width="13" height="13"> Manage symbol tables</p> 42</blockquote> 43<h2>Generic Semantic Actions (Transduction Interface)</h2> 44<p>A generic semantic action can be any free function or function object that 45 is compatible with the interface:</p> 46<pre><code><font color="#000000"><span class=identifier></span><span class=keyword> void </span><span class=identifier>f</span><span class=special>(</span><span class=identifier>IteratorT </span><span class=identifier>first</span><span class=special>, </span><span class=identifier>IteratorT </span><span class=identifier>last</span><span class=special>);</span></font></code></pre> 47<p>where <tt>IteratorT</tt> is the type of iterator used, <tt>first</tt> points 48 to the current input and <tt>last</tt> points to one after the end of the input 49 (identical to STL iterator ranges). A function object (functor) should have 50 a member <tt>operator()</tt> with the same signature as above:</p> 51<pre><code><font color="#000000"><span class=special> </span><span class=keyword>struct </span><span class=identifier>my_functor 52 </span><span class=special>{ 53 </span><span class=keyword>void </span><span class=keyword>operator</span><span class=special>()(</span><span class=identifier>IteratorT </span><span class=identifier>first</span><span class=special>, </span><span class=identifier>IteratorT </span><span class=identifier>last</span><span class=special>) </span><span class=keyword>const</span><span class=special>; 54 </span><span class=special>};</span></font></code></pre> 55<p>Iterators pointing to the matching portion of the input are passed into the 56 function/functor.</p> 57<p>In general, semantic actions accept the first-last iterator pair. This is the 58 transduction interface. The action functions or functors receive the unprocessed 59 data representing the matching production directly from the input. In many cases, 60 this is sufficient. Examples are source to source translation, pre-processing, 61 etc. </p> 62<h3>Example:</h3> 63<pre><code><font color="#000000"><span class=special> </span><span class=keyword>void 64 </span><span class=identifier>my_action</span><span class=special>(</span><span class=keyword>char const</span><span class=special>* </span><span class=identifier>first</span><span class=special>, </span><span class=keyword>char const</span><span class=special>* </span><span class=identifier>last</span><span class=special>) 65 { 66 </span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special> </span><span class="identifier">str</span><span class=special>(</span><span class=identifier>first</span><span class=special>, </span><span class=identifier>last</span><span class=special>); 67 </span><span class=identifier>std</span><span class=special>::</span><span class=identifier>cout </span><span class=special><< </span><span class=identifier>str </span><span class=special><< </span><span class=identifier>std</span><span class=special>::</span><span class=identifier>endl</span><span class=special>; 68 } 69 70 </span><span class=identifier>rule</span><span class=special><> </span><span class=identifier>myrule </span><span class=special>= (</span><span class=identifier>a </span><span class=special>| </span><span class=identifier>b </span><span class=special>| *(</span><span class=identifier>c </span><span class=special>>> </span><span class=identifier>d</span><span class=special>))[&</span><span class=identifier>my_action</span><span class=special>];</span></font></code></pre> 71<p>The function <tt>my_action</tt> will be called whenever the expression <tt>(a 72 | b | *(c >> d)</tt> matches a portion of the input stream while parsing. 73 Two iterators, <tt>first</tt> and <tt>last</tt>, are passed into the function. 74 These iterators point to the start and end, respectively, of the portion of 75 input stream where the match is found.</p> 76<h3>Const-ness:</h3> 77<p>With functors, take note that the <tt>operator()</tt> should be <tt>const</tt>. 78 This implies that functors are immutable. One may wish to have some member variables 79 that are modified when the action gets called. This is not a good idea. First 80 of all, functors are preferably lightweight. Functors are passed around a lot 81 and it would incur a lot of overhead if the functors are heavily laden. Second, 82 functors are passed by value. Thus, the actual functor object that finally attaches 83 to the parser, will surely not be the original instance supplied by the client. 84 What this means is that changes to a functor's state will not affect the original 85 functor that the client passed in since they are distinct copies. If a functor 86 needs to update some state variables, which is often the case, it is better 87 to use references to external data. The following example shows how this can 88 be done:</p> 89<pre><code><font color="#000000"><span class=special> </span><span class=keyword>struct </span><span class=identifier>my_functor 90 </span><span class=special>{ 91 </span><span class=identifier>my_functor</span><span class=special>(</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special>& </span><span class=identifier>str_</span><span class=special>) 92 </span><span class=special>: </span><span class=identifier>str</span><span class=special>(</span><span class=identifier>str_</span><span class=special>) </span><span class=special>{} 93 94 </span><span class=keyword>void 95 </span><span class=keyword>operator</span><span class=special>()(</span><span class=identifier>IteratorT </span><span class=identifier>first</span><span class=special>, </span><span class=identifier>IteratorT </span><span class=identifier>last</span><span class=special>) </span><span class=keyword>const 96 </span><span class=special>{ 97 </span><span class=identifier>str</span><span class=special>.</span><span class=identifier>assign</span><span class=special>(</span><span class=identifier>first</span><span class=special>, </span><span class=identifier>last</span><span class=special>); 98 </span><span class=special>} 99 100 </span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special>& </span><span class=identifier>str</span><span class=special>; 101 </span><span class=special>};</span></font></code></pre> 102<h3>Full Example:</h3> 103<p>Here now is our calculator enhanced with semantic actions:</p> 104<pre><code><font color="#000000"><span class=special> </span><span class=keyword>namespace 105 </span><span class=special>{ 106 </span><span class=keyword>void </span><span class=identifier>do_int</span><span class=special>(</span><span class=keyword>char </span><span class=keyword>const</span><span class=special>* </span><span class=identifier>str</span><span class=special>, </span><span class=keyword>char </span><span class=keyword>const</span><span class=special>* </span><span class=identifier>end</span><span class=special>) 107 </span><span class=special>{ 108 </span><span class=identifier>string </span><span class=identifier>s</span><span class=special>(</span><span class=identifier>str</span><span class=special>, </span><span class=identifier>end</span><span class=special>); 109 </span><span class=identifier>cout </span><span class=special><< </span><span class=string>"PUSH(" </span><span class=special><< </span><span class=identifier>s </span><span class=special><< </span><span class=literal>')' </span><span class=special><< </span><span class=identifier>endl</span><span class=special>; 110 </span><span class=special>} 111 112 </span><span class=keyword>void </span><span class=identifier>do_add</span><span class=special>(</span><span class=keyword>char </span><span class=keyword>const</span><span class=special>*, </span><span class=keyword>char </span><span class=keyword>const</span><span class=special>*) </span><span class=special>{ </span><span class=identifier>cout </span><span class=special><< </span><span class=string>"ADD\n"</span><span class=special>; </span><span class=special>} 113 </span><span class=keyword>void </span><span class=identifier>do_subt</span><span class=special>(</span><span class=keyword>char </span><span class=keyword>const</span><span class=special>*, </span><span class=keyword>char </span><span class=keyword>const</span><span class=special>*) </span><span class=special>{ </span><span class=identifier>cout </span><span class=special><< </span><span class=string>"SUBTRACT\n"</span><span class=special>; </span><span class=special>} 114 </span><span class=keyword>void </span><span class=identifier>do_mult</span><span class=special>(</span><span class=keyword>char </span><span class=keyword>const</span><span class=special>*, </span><span class=keyword>char </span><span class=keyword>const</span><span class=special>*) </span><span class=special>{ </span><span class=identifier>cout </span><span class=special><< </span><span class=string>"MULTIPLY\n"</span><span class=special>; </span><span class=special>} 115 </span><span class=keyword>void </span><span class=identifier>do_div</span><span class=special>(</span><span class=keyword>char </span><span class=keyword>const</span><span class=special>*, </span><span class=keyword>char </span><span class=keyword>const</span><span class=special>*) </span><span class=special>{ </span><span class=identifier>cout </span><span class=special><< </span><span class=string>"DIVIDE\n"</span><span class=special>; </span><span class=special>} 116 </span><span class=keyword>void </span><span class=identifier>do_neg</span><span class=special>(</span><span class=keyword>char </span><span class=keyword>const</span><span class=special>*, </span><span class=keyword>char </span><span class=keyword>const</span><span class=special>*) </span><span class=special>{ </span><span class=identifier>cout </span><span class=special><< </span><span class=string>"NEGATE\n"</span><span class=special>; </span><span class=special>} 117 </span><span class=special>}</span></font></code></pre> 118<p>We augment our grammar with semantic actions:</p> 119<pre><code><font color="#000000"><span class=special> </span><span class=keyword>struct </span><span class=identifier>calculator </span><span class=special>: </span><span class=keyword>public </span><span class=identifier>grammar</span><span class=special><</span><span class=identifier>calculator</span><span class=special>> 120 </span><span class=special>{ 121 </span><span class=keyword>template </span><span class=special><</span><span class=keyword>typename </span><span class=identifier>ScannerT</span><span class=special>> 122 </span><span class=keyword>struct </span><span class=identifier>definition 123 </span><span class=special>{ 124 </span><span class=identifier>definition</span><span class=special>(</span><span class=identifier>calculator </span><span class=keyword>const</span><span class=special>& </span><span class=identifier>self</span><span class=special>) 125 </span><span class=special>{ 126 </span><span class=identifier>expression 127 </span><span class=special>= </span><span class=identifier>term 128 </span><span class=special>>> </span><span class=special>*( </span><span class=special>(</span><span class=literal>'+' </span><span class=special>>> </span><span class=identifier>term</span><span class=special>)[&</span><span class=identifier>do_add</span><span class=special>] 129 </span><span class=special>| </span><span class=special>(</span><span class=literal>'-' </span><span class=special>>> </span><span class=identifier>term</span><span class=special>)[&</span><span class=identifier>do_subt</span><span class=special>] 130 </span><span class=special>) 131 </span><span class=special>; 132 133 </span><span class=identifier>term </span><span class=special>= 134 </span><span class=identifier>factor 135 </span><span class=special>>> </span><span class=special>*( </span><span class=special>(</span><span class=literal>'*' </span><span class=special>>> </span><span class=identifier>factor</span><span class=special>)[&</span><span class=identifier>do_mult</span><span class=special>] 136 </span><span class=special>| </span><span class=special>(</span><span class=literal>'/' </span><span class=special>>> </span><span class=identifier>factor</span><span class=special>)[&</span><span class=identifier>do_div</span><span class=special>] 137 </span><span class=special>) 138 </span><span class=special>; 139 140 </span><span class=identifier>factor 141 </span><span class=special>= </span><span class=identifier>lexeme_d</span><span class=special>[(+</span><span class=identifier>digit_p</span><span class=special>)[&</span><span class=identifier>do_int</span><span class=special>]] 142 </span><span class=special>| </span><span class=literal>'(' </span><span class=special>>> </span><span class=identifier>expression </span><span class=special>>> </span><span class=literal>')' 143 </span><span class=special>| </span><span class=special>(</span><span class=literal>'-' </span><span class=special>>> </span><span class=identifier>factor</span><span class=special>)[&</span><span class=identifier>do_neg</span><span class=special>] 144 </span><span class=special>| </span><span class=special>(</span><span class=literal>'+' </span><span class=special>>> </span><span class=identifier>factor</span><span class=special>) 145 </span><span class=special>; 146 </span><span class=special>} 147 148 </span><span class=identifier>rule</span><span class=special><</span><span class=identifier>ScannerT</span><span class=special>> </span><span class=identifier>expression</span><span class=special>, </span><span class=identifier>term</span><span class=special>, </span><span class=identifier>factor</span><span class=special>; 149 150 </span><span class=identifier>rule</span><span class=special><</span><span class=identifier>ScannerT</span><span class=special>> </span><span class=keyword>const</span><span class=special>& 151 </span><span class=identifier>start</span><span class=special>() </span><span class=keyword>const </span><span class=special>{ </span><span class=keyword>return </span><span class=identifier>expression</span><span class=special>; </span><span class=special>} 152 </span><span class=special>}; 153 </span><span class=special>};</span></font></code></pre> 154<p>Feeding in the expression <tt>(-1 + 2) * (3 + -4)</tt>, for example, to the 155 rule <tt>expression</tt> will produce the expected output:</p> 156<pre><code><span class=special>-</span><span class=number>1 157</span><span class=number>2 158</span><span class=identifier>ADD 159</span><span class=number>3 160</span><span class=special>-</span><span class=number>4 161</span><span class=identifier>ADD 162</span><span class=identifier>MULT</span></code></pre> 163<p>which, by the way, is the Reverse Polish Notation (RPN) of the given expression, 164 reminiscent of some primitive calculators and the language Forth.</p> 165<p><img src="theme/lens.gif" width="15" height="16"> <a href="../example/fundamental/calc_plain.cpp">View 166 the complete source code here</a>. This is part of the Spirit distribution. 167</p> 168<h2><a name="specialized_actions"></a>Specialized Actions</h2> 169<p>In general, semantic actions accept the first-last iterator pair. There are 170 situations though where we might want to pass data in its processed form. A 171 concrete example is the numeric parser. It is unwise to pass unprocessed data 172 to a semantic action attached to a numeric parser and just throw away what has 173 been parsed by the parser. We want to pass the actual parsed number.</p> 174<p>The function and functor signature of a semantic action varies depending on 175 the parser where it is attached to. The following table lists the parsers that 176 accept unique signatures.</p> 177<table width="80%" border="0" align="center"> 178 <tr> 179 <td class="note_box"><img src="theme/note.gif" width="16" height="16"> Unless 180 explicitly stated in the documentation of a specific parser type, parsers 181 not included in the list by default expect the generic signature as explained 182 above.</td> 183 </tr> 184</table> 185<h3>Numeric Actions</h3> 186<p><b>Applies to:</b></p> 187<blockquote> 188 <p><img src="theme/bullet.gif" width="13" height="13"> uint_p<br> 189 <img src="theme/bullet.gif" width="13" height="13"> int_p<br> 190 <img src="theme/bullet.gif" width="13" height="13"> ureal_p<br> 191 <img src="theme/bullet.gif" width="13" height="13"> real_p</p> 192</blockquote> 193<p><b>Signature for functions:</b></p> 194<pre><code><font color="#000000"><span class=identifier> </span><span class=keyword>void </span><span class=identifier>func</span><span class=special>(</span><span class=identifier>NumT </span><span class=identifier>val</span><span class=special>);</span></font></code></pre> 195<p><b>Signature for functors:</b> </p> 196<pre><code><font color="#000000"><span class=special> </span><span class=keyword>struct </span><span class=identifier>ftor 197 </span><span class=special>{ 198 </span><span class=keyword>void </span><span class=keyword>operator</span><span class=special>()(</span><span class=identifier>NumT </span><span class=identifier>val</span><span class=special>) </span><span class=keyword>const</span><span class=special>; 199 </span><span class=special>};</span></font></code></pre> 200<p>Where <tt>NumT</tt> is any primitive numeric type such as <tt>int</tt>, <tt>long</tt>, 201 <tt>float</tt>, <tt>double</tt>, etc., or a user defined numeric type such as 202 big_int. <tt>NumT</tt> is the same type used as template parameter to <tt>uint_p</tt>, 203 <tt>int_p</tt>, <tt>ureal_p</tt> or <tt>real_p</tt>. The parsed number is passed 204 into the function/functor.</p> 205<h3>Character Actions</h3> 206<p><b>Applies to:</b></p> 207<blockquote> 208 <p><img src="theme/bullet.gif" width="13" height="13"> chlit, ch_p<br> 209 <img src="theme/bullet.gif" width="13" height="13"> range, range_p<br> 210 <img src="theme/bullet.gif" width="13" height="13"> anychar<br> 211 <img src="theme/bullet.gif" width="13" height="13"> alnum, alpha<br> 212 <img src="theme/bullet.gif" width="13" height="13"> cntrl, digit<br> 213 <img src="theme/bullet.gif" width="13" height="13"> graph, lower<br> 214 <img src="theme/bullet.gif" width="13" height="13"> print, punct<br> 215 <img src="theme/bullet.gif" width="13" height="13"> space, upper<br> 216 <img src="theme/bullet.gif" width="13" height="13"> xdigit</p> 217</blockquote> 218<p><b>Signature for functions:</b></p> 219<pre><code><font color="#000000"><span class=identifier> </span><span class=keyword>void </span><span class=identifier>func</span><span class=special>(</span><span class=identifier>CharT </span><span class=identifier>ch</span><span class=special>);</span></font></code></pre> 220<p><b>Signature for functors:</b></p> 221<pre><code><font color="#000000"><span class=special> </span><span class=keyword>struct </span><span class=identifier>ftor 222 </span><span class=special>{ 223 </span><span class=keyword>void </span><span class=keyword>operator</span><span class=special>()(</span><span class=identifier>CharT </span><span class=identifier>ch</span><span class=special>) </span><span class=keyword>const</span><span class=special>; 224 </span><span class=special>};</span></font></code></pre> 225<p>Where <tt>CharT</tt> is the value_type of the iterator used in parsing. A <tt>char 226 const*</tt> iterator for example has a <tt>value_type</tt> of <tt>char</tt>. 227 The matching character is passed into the function/functor.</p> 228<h2>Cascading Actions</h2> 229<p>Actions can be cascaded. Cascaded actions also inherit the function/functor 230 interface of the original. For example:</p> 231<pre><code><font color="#000000"><span class=special> </span><span class=identifier>uint_p</span><span class=special>[</span><span class=identifier>fa</span><span class=special>][</span><span class=identifier>fb</span><span class=special>][</span><span class=identifier>fc</span><span class=special>]</span></font></code></pre> 232<p>Here, the functors <tt>fa</tt>, <tt>fb</tt> and <tt>fc</tt> all expect the 233 signature <tt>void operator()(unsigned n) const</tt>.</p> 234<h2>Directives and Actions</h2> 235<p>Directives inherit the function/functor interface of the subject it is 236 enclosing. Example:</p> 237<pre><code><font color="#000000"><span class=special> </span><span class=identifier>as_lower_d</span><span class=special>[</span><span class=identifier>ch_p</span><span class=special>(</span><span class=literal>'x'</span><span class=special>)][</span><span class=identifier>f</span><span class=special>]</span></font></code></pre> 238<p>Here, the functor <tt>f</tt> expects the signature <tt>void operator()(char 239 ch) const</tt>, assuming that the iterator used is a <tt>char const*</tt>.</p> 240<table border="0"> 241 <tr> 242 <td width="10"></td> 243 <td width="30"><a href="../index.html"><img src="theme/u_arr.gif" border="0"></a></td> 244 <td width="30"><a href="subrules.html"><img src="theme/l_arr.gif" border="0"></a></td> 245 <td width="30"><a href="indepth_the_parser.html"><img src="theme/r_arr.gif" border="0"></a></td> 246 </tr> 247</table> 248<br> 249<hr size="1"> 250<p class="copyright">Copyright © 1998-2003 Joel de Guzman<br> 251 <br> 252 <font size="2">Use, modification and distribution is subject to the Boost Software 253 License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at 254 http://www.boost.org/LICENSE_1_0.txt)</font></p> 255<p> </p> 256<p> </p> 257</body> 258</html> 259
