1<html> 2<head> 3<title>Storable Rules</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%"> <font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>Storable 14 Rules</b></font></td> 15 <td width="112"><a href="http://spirit.sf.net"><img src="theme/spirit.gif" width="112" height="48" align="right" border="0"></a></td> 16 </tr> 17</table> 18<br> 19<table border="0"> 20 <tr> 21 <td width="10"></td> 22 <td width="30"><a href="../index.html"><img src="theme/u_arr.gif" border="0"></a></td> 23 <td width="30"><a href="stored_rule.html"><img src="theme/l_arr.gif" border="0"></a></td> 24 <td width="30"><a href="the_lazy_parser.html"><img src="theme/r_arr.gif" border="0"></a></td> 25 </tr> 26</table> 27<p>The rule is a weird C++ citizen, unlike any other C++ object. It does not have 28 the proper copy and assignment semantics and cannot be stored and passed around 29 by value. You cannot store rules in STL containers (vector, stack, etc) for 30 later use and you cannot pass and return rules to and from functions by value.</p> 31<p>EBNF is primarily declarative. Like in functional programming, an EBNF grammar 32 is a static recipe and there's no notion of do this then that. However, in Spirit, 33 we managed to coax imperative C++ to take in declarative EBNF. Hah! Fun!... 34 We did that by masquerading the C++ assignment operator to mimic EBNF's <tt>::=</tt>. 35 To do that, we gave the rule class' assignment operator and copy constructor 36 a different meaning and semantics. The downside is that doing so made the rule 37 unlike any other C++ object. You can't copy it. You can't assign it. </p> 38<p>We want to have the dynamic nature of C++ to our advantage. We've seen dynamic 39 Spirit in action here and there. There are indeed some interesting applications 40 of dynamic parsers using Spirit. Yet, we will not fully utilize the power of 41 dynamic parsing, unless we have a rule that behaves like any other good C++ 42 object. With such a beast, we can write full parsers that's defined at run time, 43 as opposed to compile time.</p> 44<p>We now have dynamic rules: <tt>stored_rules</tt>. Basically they are rules 45 with perfect C++ assignment/copy-constructor semantics. This means that <tt>stored_rules</tt> 46 can be stored in containers and/or dynamically created at run-time.</p> 47<pre><code><font color="#000000"><span class=identifier> </span><span class=keyword>template</span><span class=special>< 48 </span><span class=keyword>typename </span><span class=identifier>ScannerT </span><span class=special>= </span><span class=identifier>scanner</span><span class=special><>, 49 </span><span class=keyword>typename </span><span class=identifier>ContextT </span><span class=special>= </span><span class=identifier>parser_context</span><span class=special><></span><span class=identifier>, 50 </span><span class="keyword">typename</span><span class=identifier> TagT </span><span class="special">=</span><span class=identifier> parser_address_tag</span><span class=special>> 51 </span><span class=keyword>class </span><span class=identifier>stored_rule</span><span class=special>;</span></font></code></pre> 52<p>The interface is exactly the same as with the rule class (see the <a href="rule.html">section 53 on rules</a> for more information regarding the API). The only difference is 54 with the copy and assignment semantics. Now, with <tt>stored_rule</tt>s, we 55 can dynamically and algorithmically define our rules. Here are some samples... 56</p> 57<p>Say I want to dynamically create a rule for:</p> 58<pre> 59<span class=identifier> start </span><span class=special>= </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></pre> 60<p> I can write it dynamically step-by-step:</p> 61<pre> <span class=identifier> stored_rule</span><span class=special><> </span><span class=identifier>start</span><span class=special>; 62 63 </span><span class=identifier>start </span><span class=special>= </span><span class=identifier>a</span><span class=special>; 64 </span><span class=identifier>start </span><span class=special>= </span><span class=identifier>start</span><span class=special>.</span><span class=identifier>copy</span><span class=special>() </span><span class=special>| </span><span class=identifier>b</span><span class=special>; 65 </span><span class=identifier>start </span><span class=special>= </span><span class=identifier>start</span><span class=special>.</span><span class=identifier>copy</span><span class=special>() </span><span class=special>| </span><span class=identifier>c</span><span class=special>; 66 </span><span class=identifier>start </span><span class=special>= </span><span class=special>*(</span><span class=identifier>start</span><span class=special>.</span><span class=identifier>copy</span><span class=special>());</span></pre> 67<p>Later, I changed my mind and want to redefine it (again dynamically) as:</p> 68<pre><span class=identifier> start </span><span class=special>= </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=special>>> </span><span class=special>(</span><span class=identifier>start </span><span class=special>| </span><span class=identifier>b</span><span class=special>);</span> 69</pre> 70<p>I write:</p> 71<pre> <span class=special> </span><span class=identifier>start </span><span class=special>= </span><span class=identifier>b</span><span class=special>; 72 </span><span class=identifier>start </span><span class=special>= </span><span class=identifier>a </span><span class=special>| </span><span class=identifier>start</span><span class=special>.</span><span class=identifier>copy</span><span class=special>(); 73 </span><span class=identifier>start </span><span class=special>= </span><span class=identifier>start</span><span class=special>.</span><span class=identifier>copy</span><span class=special>() </span><span class=special>>> </span><span class=special>(</span><span class=identifier>start </span><span class=special>| </span><span class=identifier>b</span><span class=special>);</span></pre> 74<p>Notice the statement:</p> 75<pre> <span class=special> </span><span class=identifier>start </span><span class=special>= </span><span class=identifier>start</span><span class=special>.</span><span class=identifier>copy</span><span class=special>() </span><span class=special>| </span><span class=identifier>b</span><span class=special>;</span></pre> 76<p>Why is start.copy() required? Well, because like rules, stored rules are still 77 embedded by reference when found in the RHS (one reason is to avoid cyclic-shared-pointers). 78 If we write:</p> 79<pre> <span class=special> </span><span class=identifier>start </span><span class=special>= </span><span class=identifier>start </span><span class=special>| </span><span class=identifier>b</span><span class=special>;</span></pre> 80<p>We have <strong>left-recursion</strong>! Copying copy of start avoids self 81 referencing. What we are doing is making a copy of start, ORing it with b, then 82 destructively assigning the result back to start.</p> 83<table border="0"> 84 <tr> 85 <td width="10"></td> 86 <td width="30"><a href="../index.html"><img src="theme/u_arr.gif" border="0"></a></td> 87 <td width="30"><a href="stored_rule.html"><img src="theme/l_arr.gif" border="0"></a></td> 88 <td width="30"><a href="the_lazy_parser.html"><img src="theme/r_arr.gif" border="0"></a></td> 89 </tr> 90</table> 91<br> 92<hr size="1"> 93<p class="copyright">Copyright © 1998-2003 Joel de Guzman<br> 94 <br> 95 <font size="2">Use, modification and distribution is subject to the Boost Software 96 License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at 97 http://www.boost.org/LICENSE_1_0.txt)</font></p> 98</body> 99</html> 100
