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| <td width="85%"> <font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>Storable |
| Rules</b></font></td> |
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| <p>The rule is a weird C++ citizen, unlike any other C++ object. It does not have |
| the proper copy and assignment semantics and cannot be stored and passed around |
| by value. You cannot store rules in STL containers (vector, stack, etc) for |
| later use and you cannot pass and return rules to and from functions by value.</p> |
| <p>EBNF is primarily declarative. Like in functional programming, an EBNF grammar |
| is a static recipe and there's no notion of do this then that. However, in Spirit, |
| we managed to coax imperative C++ to take in declarative EBNF. Hah! Fun!... |
| We did that by masquerading the C++ assignment operator to mimic EBNF's <tt>::=</tt>. |
| To do that, we gave the rule class' assignment operator and copy constructor |
| a different meaning and semantics. The downside is that doing so made the rule |
| unlike any other C++ object. You can't copy it. You can't assign it. </p> |
| <p>We want to have the dynamic nature of C++ to our advantage. We've seen dynamic |
| Spirit in action here and there. There are indeed some interesting applications |
| of dynamic parsers using Spirit. Yet, we will not fully utilize the power of |
| dynamic parsing, unless we have a rule that behaves like any other good C++ |
| object. With such a beast, we can write full parsers that's defined at run time, |
| as opposed to compile time.</p> |
| <p>We now have dynamic rules: <tt>stored_rules</tt>. Basically they are rules |
| with perfect C++ assignment/copy-constructor semantics. This means that <tt>stored_rules</tt> |
| can be stored in containers and/or dynamically created at run-time.</p> |
| <pre><code><font color="#000000"><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>= </span><span class=identifier>scanner</span><span class=special><>, |
| </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>, |
| </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>> |
| </span><span class=keyword>class </span><span class=identifier>stored_rule</span><span class=special>;</span></font></code></pre> |
| <p>The interface is exactly the same as with the rule class (see the <a href="rule.html">section |
| on rules</a> for more information regarding the API). The only difference is |
| with the copy and assignment semantics. Now, with <tt>stored_rule</tt>s, we |
| can dynamically and algorithmically define our rules. Here are some samples... |
| </p> |
| <p>Say I want to dynamically create a rule for:</p> |
| <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=identifier>c</span><span class=special>);</span></pre> |
| <p> I can write it dynamically step-by-step:</p> |
| <pre> <span class=identifier> stored_rule</span><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>a</span><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><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>; |
| </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> |
| <p>Later, I changed my mind and want to redefine it (again dynamically) as:</p> |
| <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> |
| </pre> |
| <p>I write:</p> |
| <pre> <span class=special> </span><span class=identifier>start </span><span class=special>= </span><span class=identifier>b</span><span class=special>; |
| </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>(); |
| </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> |
| <p>Notice the statement:</p> |
| <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> |
| <p>Why is start.copy() required? Well, because like rules, stored rules are still |
| embedded by reference when found in the RHS (one reason is to avoid cyclic-shared-pointers). |
| If we write:</p> |
| <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> |
| <p>We have <strong>left-recursion</strong>! Copying copy of start avoids self |
| referencing. What we are doing is making a copy of start, ORing it with b, then |
| destructively assigning the result back to start.</p> |
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| <p class="copyright">Copyright © 1998-2003 Joel de Guzman<br> |
| <br> |
| <font size="2">Use, modification and distribution is subject to the Boost Software |
| License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at |
| http://www.boost.org/LICENSE_1_0.txt)</font></p> |
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