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| <div class="titlepage"><div><div><h5 class="title"> |
| <a name="spirit.qi.reference.directive.hold"></a><a class="link" href="hold.html" title="Parser Directive for Attribute Commit/Rollback (hold[])">Parser Directive |
| for Attribute Commit/Rollback (<code class="computeroutput"><span class="identifier">hold</span><span class="special">[]</span></code>)</a> |
| </h5></div></div></div> |
| <a name="spirit.qi.reference.directive.hold.description"></a><h6> |
| <a name="spirit.qi.reference.directive.hold.description-heading"></a> |
| <a class="link" href="hold.html#spirit.qi.reference.directive.hold.description">Description</a> |
| </h6> |
| <p> |
| The <code class="computeroutput"><span class="identifier">hold</span><span class="special">[]</span></code> |
| directive helps managing attributes, mainly for alternative parsers. |
| It instantiates a new attribute instance for the embedded parser. The |
| value of that attribute instance is copied to the outer attribute if |
| the embedded parser succeeds and it is discarded otherwise. Alternative |
| parsers normally do not rollback changes made to the outer attribute |
| by an failed alternative. Wrapping those alternatives into a <code class="computeroutput"><span class="identifier">hold</span><span class="special">[]</span></code> |
| directive ensures that only the succeeding alternative gets to modify |
| the attribute. |
| </p> |
| <a name="spirit.qi.reference.directive.hold.header"></a><h6> |
| <a name="spirit.qi.reference.directive.hold.header-heading"></a> |
| <a class="link" href="hold.html#spirit.qi.reference.directive.hold.header">Header</a> |
| </h6> |
| <pre class="programlisting"><span class="comment">// forwards to <boost/spirit/home/qi/directive/hold.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_hold</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.directive.hold.namespace"></a><h6> |
| <a name="spirit.qi.reference.directive.hold.namespace-heading"></a> |
| <a class="link" href="hold.html#spirit.qi.reference.directive.hold.namespace">Namespace</a> |
| </h6> |
| <div class="informaltable"><table class="table"> |
| <colgroup><col></colgroup> |
| <thead><tr><th> |
| <p> |
| Name |
| </p> |
| </th></tr></thead> |
| <tbody><tr><td> |
| <p> |
| <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">hold</span> <span class="comment">// alias: |
| boost::spirit::qi::hold</span></code> |
| </p> |
| </td></tr></tbody> |
| </table></div> |
| <a name="spirit.qi.reference.directive.hold.model_of"></a><h6> |
| <a name="spirit.qi.reference.directive.hold.model_of-heading"></a> |
| <a class="link" href="hold.html#spirit.qi.reference.directive.hold.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></span></dt> |
| <dd><p> |
| A <a class="link" href="../parser_concepts/parser.html" title="Parser"><code class="computeroutput"><span class="identifier">Parser</span></code></a>. |
| </p></dd> |
| </dl> |
| </div> |
| <a name="spirit.qi.reference.directive.hold.expression_semantics"></a><h6> |
| <a name="spirit.qi.reference.directive.hold.expression_semantics-heading"></a> |
| <a class="link" href="hold.html#spirit.qi.reference.directive.hold.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">hold</span><span class="special">[</span><span class="identifier">a</span><span class="special">]</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| Create a new attribute instance while parsing <code class="computeroutput"><span class="identifier">a</span></code>, copying the result to |
| the outer attribute only after <code class="computeroutput"><span class="identifier">a</span></code> |
| succeeds. |
| </p> |
| </td> |
| </tr></tbody> |
| </table></div> |
| <a name="spirit.qi.reference.directive.hold.attributes"></a><h6> |
| <a name="spirit.qi.reference.directive.hold.attributes-heading"></a> |
| <a class="link" href="hold.html#spirit.qi.reference.directive.hold.attributes">Attributes</a> |
| </h6> |
| <p> |
| See <a class="link" href="../../quick_reference/compound_attribute_rules.html#spirit.qi.quick_reference.compound_attribute_rules.notation">Compound |
| Attribute Notation</a>. |
| </p> |
| <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">hold</span><span class="special">[</span><span class="identifier">a</span><span class="special">]</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| |
| </p> |
| <pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-programlisting"><span class="identifier">a</span><span class="special">:</span> <span class="identifier">A</span> <span class="special">--></span> <span class="identifier">hold</span><span class="special">[</span><span class="identifier">a</span><span class="special">]:</span> <span class="identifier">A</span> |
| <span class="identifier">a</span><span class="special">:</span> <span class="identifier">Unused</span> <span class="special">--></span> <span class="identifier">hold</span><span class="special">[</span><span class="identifier">a</span><span class="special">]:</span> <span class="identifier">Unused</span></pre> |
| <p> |
| </p> |
| </td> |
| </tr></tbody> |
| </table></div> |
| <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 <code class="computeroutput"><span class="identifier">hold</span><span class="special">[]</span></code> |
| directive uses <code class="computeroutput"><span class="identifier">swap</span><span class="special">()</span></code> to implement the rollback/commit |
| semantics for the attribute. For this reason the attribute type needs |
| to to be usable with <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">swap</span></code> |
| (needs to either define a proper overload for <code class="computeroutput"><span class="identifier">swap</span><span class="special">(</span><span class="identifier">attribute_type</span><span class="special">&,</span> <span class="identifier">attribute_type</span><span class="special">&)</span></code> or expose a member function |
| <code class="computeroutput"><span class="identifier">attribute_type</span><span class="special">::</span><span class="identifier">swap</span><span class="special">(</span><span class="identifier">attribute_type</span><span class="special">&)</span></code>. |
| </p></td></tr> |
| </table></div> |
| <a name="spirit.qi.reference.directive.hold.complexity"></a><h6> |
| <a name="spirit.qi.reference.directive.hold.complexity-heading"></a> |
| <a class="link" href="hold.html#spirit.qi.reference.directive.hold.complexity">Complexity</a> |
| </h6> |
| <div class="blockquote"><blockquote class="blockquote"><p> |
| The complexity is defined by the complexity of the subject parser, |
| <code class="computeroutput"><span class="identifier">a</span></code> |
| </p></blockquote></div> |
| <a name="spirit.qi.reference.directive.hold.example"></a><h6> |
| <a name="spirit.qi.reference.directive.hold.example-heading"></a> |
| <a class="link" href="hold.html#spirit.qi.reference.directive.hold.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> |
| Some using declarations: |
| </p> |
| <p> |
| |
| </p> |
| <pre class="programlisting"><span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">qi</span><span class="special">::</span><span class="identifier">hold</span><span class="special">;</span> |
| <span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">qi</span><span class="special">::</span><span class="identifier">int_</span><span class="special">;</span> |
| <span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">qi</span><span class="special">::</span><span class="identifier">attr</span><span class="special">;</span> |
| </pre> |
| <p> |
| </p> |
| <p> |
| The use of <code class="computeroutput"><span class="identifier">hold</span><span class="special">[]</span></code> |
| here will make sure the changes to the attribute caused by the (failing) |
| first alternative will not be visible after the whole parsing succeeded. |
| </p> |
| <p> |
| |
| </p> |
| <pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">v</span><span class="special">;</span> |
| <span class="identifier">test_phrase_parser_attr</span><span class="special">(</span><span class="string">"123"</span><span class="special">,</span> |
| <span class="identifier">hold</span><span class="special">[</span><span class="identifier">int_</span> <span class="special">>></span> <span class="char">':'</span> <span class="special">>></span> <span class="identifier">int_</span><span class="special">]</span> <span class="special">|</span> <span class="identifier">int_</span> <span class="special">>></span> <span class="identifier">attr</span><span class="special">(</span><span class="number">0</span><span class="special">),</span> <span class="identifier">v</span><span class="special">);</span> |
| <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">v</span><span class="special">[</span><span class="number">0</span><span class="special">]</span> <span class="special"><<</span> <span class="string">","</span> <span class="special"><<</span> <span class="identifier">v</span><span class="special">[</span><span class="number">1</span><span class="special">]</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> <span class="comment">// will output: >123,0< |
| </span></pre> |
| <p> |
| </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 © 2001-2011 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> |
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