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| <div class="section"> |
| <div class="titlepage"><div><div><h4 class="title"> |
| <a name="spirit.qi.reference.action"></a><a class="link" href="action.html" title="Action">Action</a> |
| </h4></div></div></div> |
| <a name="spirit.qi.reference.action.description"></a><h6> |
| <a name="id857908"></a> |
| <a class="link" href="action.html#spirit.qi.reference.action.description">Description</a> |
| </h6> |
| <p> |
| Semantic actions may be attached to any point in the grammar specification. |
| They allow to call a function or function object in order to provide the |
| value to be output by the parser attached to the semantic action. Semantic |
| actions are associated with a parser using the syntax <code class="computeroutput"><span class="identifier">p</span><span class="special">[]</span></code>, where <code class="computeroutput"><span class="identifier">p</span></code> |
| is an arbitrary parser expression. |
| </p> |
| <a name="spirit.qi.reference.action.header"></a><h6> |
| <a name="id857943"></a> |
| <a class="link" href="action.html#spirit.qi.reference.action.header">Header</a> |
| </h6> |
| <pre class="programlisting"><span class="comment">// forwards to <boost/spirit/home/qi/action.hpp> |
| </span><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">spirit</span><span class="special">/</span><span class="identifier">include</span><span class="special">/</span><span class="identifier">qi_action</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> |
| </pre> |
| <p> |
| Also, see <a class="link" href="../../structure/include.html" title="Include">Include Structure</a>. |
| </p> |
| <a name="spirit.qi.reference.action.model_of"></a><h6> |
| <a name="id858018"></a> |
| <a class="link" href="action.html#spirit.qi.reference.action.model_of">Model of</a> |
| </h6> |
| <div class="blockquote"><blockquote class="blockquote"><p> |
| <a class="link" href="parser_concepts/unaryparser.html" title="UnaryParser"><code class="computeroutput"><span class="identifier">UnaryParser</span></code></a> |
| </p></blockquote></div> |
| <div class="variablelist"> |
| <p class="title"><b>Notation</b></p> |
| <dl> |
| <dt><span class="term"><code class="computeroutput"><span class="identifier">a</span></code>, <code class="computeroutput"><span class="identifier">p</span></code></span></dt> |
| <dd><p> |
| Instances of a parser, <code class="computeroutput"><span class="identifier">P</span></code> |
| </p></dd> |
| <dt><span class="term"><code class="computeroutput"><span class="identifier">A</span></code></span></dt> |
| <dd><p> |
| Attribute type exposed by a parser, <code class="computeroutput"><span class="identifier">a</span></code> |
| </p></dd> |
| <dt><span class="term"><code class="computeroutput"><span class="identifier">fa</span></code></span></dt> |
| <dd><p> |
| A (semantic action) function with signature <code class="computeroutput"><span class="keyword">void</span><span class="special">(</span><span class="identifier">Attrib</span><span class="special">&,</span> <span class="identifier">Context</span><span class="special">,</span> <span class="keyword">bool</span><span class="special">&)</span></code>. The third parameter is a boolean |
| flag that can be set to false to force the parser to fail. Both |
| <code class="computeroutput"><span class="identifier">Context</span></code> and the boolean |
| flag are optional. For more information see below. |
| </p></dd> |
| <dt><span class="term"><code class="computeroutput"><span class="identifier">Attrib</span></code></span></dt> |
| <dd><p> |
| The attribute obtained from the parse. |
| </p></dd> |
| <dt><span class="term"><code class="computeroutput"><span class="identifier">Context</span></code></span></dt> |
| <dd><p> |
| The type of the parser execution context. For more information see |
| below. |
| </p></dd> |
| </dl> |
| </div> |
| <a name="spirit.qi.reference.action.expression_semantics"></a><h6> |
| <a name="id858216"></a> |
| <a class="link" href="action.html#spirit.qi.reference.action.expression_semantics">Expression |
| Semantics</a> |
| </h6> |
| <p> |
| Semantics of an expression is defined only where it differs from, or is |
| not defined in <a class="link" href="parser_concepts/unaryparser.html" title="UnaryParser"><code class="computeroutput"><span class="identifier">UnaryParser</span></code></a>. |
| </p> |
| <div class="informaltable"><table class="table"> |
| <colgroup> |
| <col> |
| <col> |
| </colgroup> |
| <thead><tr> |
| <th> |
| <p> |
| Expression |
| </p> |
| </th> |
| <th> |
| <p> |
| Semantics |
| </p> |
| </th> |
| </tr></thead> |
| <tbody><tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="identifier">p</span><span class="special">[</span><span class="identifier">fa</span><span class="special">]</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| If <code class="computeroutput"><span class="identifier">p</span></code> is successful, |
| call semantic action, <code class="computeroutput"><span class="identifier">fa</span></code>. |
| The function or function object <code class="computeroutput"><span class="identifier">fa</span></code> |
| is provided the attribute value parsed by the parser <code class="computeroutput"><span class="identifier">p</span></code>, plus some more context information |
| and a mutable bool flag which can be used to fail parsing. |
| </p> |
| </td> |
| </tr></tbody> |
| </table></div> |
| <p> |
| The possible signatures for functions to be used as semantic actions are: |
| </p> |
| <pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Attrib</span><span class="special">></span> |
| <span class="keyword">void</span> <span class="identifier">fa</span><span class="special">(</span><span class="identifier">Attrib</span><span class="special">&</span> <span class="identifier">attr</span><span class="special">);</span> |
| |
| <span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Attrib</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Context</span><span class="special">></span> |
| <span class="keyword">void</span> <span class="identifier">fa</span><span class="special">(</span><span class="identifier">Attrib</span><span class="special">&</span> <span class="identifier">attr</span><span class="special">,</span> <span class="identifier">Context</span><span class="special">&</span> <span class="identifier">context</span><span class="special">);</span> |
| |
| <span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Attrib</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Context</span><span class="special">></span> |
| <span class="keyword">void</span> <span class="identifier">fa</span><span class="special">(</span><span class="identifier">Attrib</span><span class="special">&</span> <span class="identifier">attr</span><span class="special">,</span> <span class="identifier">Context</span><span class="special">&</span> <span class="identifier">context</span><span class="special">,</span> <span class="keyword">bool</span><span class="special">&</span> <span class="identifier">pass</span><span class="special">);</span> |
| </pre> |
| <p> |
| The function or function object is expected to return the value to generate |
| output from by assigning it to the first parameter, <code class="computeroutput"><span class="identifier">attr</span></code>. |
| Here <code class="computeroutput"><span class="identifier">Attrib</span></code> is the attribute |
| type of the parser attached to the semantic action. |
| </p> |
| <p> |
| The type <code class="computeroutput"><span class="identifier">Context</span></code> is the |
| type of the parser execution context. This type is unspecified and depends |
| on the context the parser is invoked in. The value, <code class="computeroutput"><span class="identifier">context</span></code> |
| used by semantic actions written using <a href="../../../../../phoenix/doc/html/index.html" target="_top">Phoenix</a> |
| to access various context dependent attributes and values. For more information |
| about <a href="../../../../../phoenix/doc/html/index.html" target="_top">Phoenix</a> placeholder |
| expressions usable in semantic actions see <a class="link" href="parser_concepts/nonterminal.html" title="Nonterminal"><code class="computeroutput"><span class="identifier">Nonterminal</span></code></a>. |
| </p> |
| <p> |
| The third parameter, <code class="computeroutput"><span class="identifier">pass</span></code>, |
| can be used by the semantic action to force the associated parser to fail. |
| If pass is set to <code class="computeroutput"><span class="keyword">false</span></code> the |
| action parser will immediately return <code class="computeroutput"><span class="keyword">false</span></code> |
| as well, while not invoking <code class="computeroutput"><span class="identifier">p</span></code> |
| and not generating any output. |
| </p> |
| <a name="spirit.qi.reference.action.attributes"></a><h6> |
| <a name="id858666"></a> |
| <a class="link" href="action.html#spirit.qi.reference.action.attributes">Attributes</a> |
| </h6> |
| <div class="informaltable"><table class="table"> |
| <colgroup> |
| <col> |
| <col> |
| </colgroup> |
| <thead><tr> |
| <th> |
| <p> |
| Expression |
| </p> |
| </th> |
| <th> |
| <p> |
| Attribute |
| </p> |
| </th> |
| </tr></thead> |
| <tbody><tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="identifier">a</span><span class="special">[</span><span class="identifier">fa</span><span class="special">]</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="identifier">a</span><span class="special">:</span> |
| <span class="identifier">A</span> <span class="special">--></span> |
| <span class="identifier">a</span><span class="special">[</span><span class="identifier">fa</span><span class="special">]:</span> |
| <span class="identifier">A</span></code> |
| </p> |
| </td> |
| </tr></tbody> |
| </table></div> |
| <a name="spirit.qi.reference.action.complexity"></a><h6> |
| <a name="id858790"></a> |
| <a class="link" href="action.html#spirit.qi.reference.action.complexity">Complexity</a> |
| </h6> |
| <p> |
| The complexity of the action parser is defined by the complexity of the |
| parser the semantic action is attached to and the complexity of the function |
| or function object used as the semantic action. |
| </p> |
| <a name="spirit.qi.reference.action.example"></a><h6> |
| <a name="id858807"></a> |
| <a class="link" href="action.html#spirit.qi.reference.action.example">Example</a> |
| </h6> |
| <div class="note"><table border="0" summary="Note"> |
| <tr> |
| <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../images/note.png"></td> |
| <th align="left">Note</th> |
| </tr> |
| <tr><td align="left" valign="top"><p> |
| The test harness for the example(s) below is presented in the <a class="link" href="basics.html#spirit.qi.reference.basics.examples">Basics |
| Examples</a> section. |
| </p></td></tr> |
| </table></div> |
| <p> |
| [reference_qi_action] |
| </p> |
| <p> |
| More examples for semantic actions can be found here: <a class="link" href="../tutorials/semantic_actions.html#spirit.qi.tutorials.semantic_actions.examples_of_semantic_actions">Examples |
| of Semantic Actions</a>. |
| </p> |
| </div> |
| <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> |
| <td align="left"></td> |
| <td align="right"><div class="copyright-footer">Copyright © 2001-2010 Joel de Guzman, Hartmut Kaiser<p> |
| Distributed under the Boost Software License, Version 1.0. (See accompanying |
| file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>) |
| </p> |
| </div></td> |
| </tr></table> |
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