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| <div class="titlepage"><div><div><h5 class="title"> |
| <a name="spirit.karma.reference.directive.omit"></a><a class="link" href="omit.html" title="Consume Attribute (omit[] and skip[])">Consume Attribute |
| (<code class="computeroutput"><span class="identifier">omit</span><span class="special">[]</span></code> |
| and <code class="computeroutput"><span class="identifier">skip</span><span class="special">[]</span></code>)</a> |
| </h5></div></div></div> |
| <a name="spirit.karma.reference.directive.omit.description"></a><h6> |
| <a name="id1051273"></a> |
| <a class="link" href="omit.html#spirit.karma.reference.directive.omit.description">Description</a> |
| </h6> |
| <p> |
| The directives <code class="computeroutput"><span class="identifier">omit</span><span class="special">[]</span></code> |
| and <code class="computeroutput"><span class="identifier">skip</span><span class="special">[]</span></code> |
| consumes the attribute type of the embedded generator without generating |
| any output. The <code class="computeroutput"><span class="identifier">omit</span><span class="special">[]</span></code> directive will still execute the embedded |
| generator while discarding the generated output afterwards. The <code class="computeroutput"><span class="identifier">skip</span><span class="special">[]</span></code> |
| directive will not execute the embedded generator, but will use it only |
| to extract the exposed attribute type. |
| </p> |
| <a name="spirit.karma.reference.directive.omit.header"></a><h6> |
| <a name="id1051333"></a> |
| <a class="link" href="omit.html#spirit.karma.reference.directive.omit.header">Header</a> |
| </h6> |
| <pre class="programlisting"><span class="comment">// forwards to <boost/spirit/home/karma/directive/omit.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">karma_omit</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> |
| <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">omit</span> <span class="comment">// alias: |
| boost::spirit::karma::omit</span></code> |
| </p> |
| </td></tr> |
| <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">skip</span> <span class="comment">// alias: |
| boost::spirit::karma::skip</span></code> |
| </p> |
| </td></tr> |
| </tbody> |
| </table></div> |
| <a name="spirit.karma.reference.directive.omit.model_of"></a><h6> |
| <a name="id1051512"></a> |
| <a class="link" href="omit.html#spirit.karma.reference.directive.omit.model_of">Model |
| of</a> |
| </h6> |
| <div class="blockquote"><blockquote class="blockquote"><p> |
| <a class="link" href="../generator_concepts/unarygenerator.html" title="UnaryGenerator"><code class="computeroutput"><span class="identifier">UnaryGenerator</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 generator object |
| </p></dd> |
| <dt><span class="term"><code class="computeroutput"><span class="identifier">A</span></code></span></dt> |
| <dd><p> |
| Attribute type of generator <code class="computeroutput"><span class="identifier">a</span></code> |
| </p></dd> |
| </dl> |
| </div> |
| <a name="spirit.karma.reference.directive.omit.expression_semantics"></a><h6> |
| <a name="id1051599"></a> |
| <a class="link" href="omit.html#spirit.karma.reference.directive.omit.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="../generator_concepts/unarygenerator.html" title="UnaryGenerator"><code class="computeroutput"><span class="identifier">UnaryGenerator</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">omit</span><span class="special">[</span><span class="identifier">a</span><span class="special">]</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| The <code class="computeroutput"><span class="identifier">omit</span></code> directive |
| consumes the attribute type of the embedded generator <code class="computeroutput"><span class="identifier">A</span></code> without generating any |
| output. It succeeds always. The embedded generator is executed |
| and any generated output is discarded. |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="identifier">skip</span><span class="special">[</span><span class="identifier">a</span><span class="special">]</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| The <code class="computeroutput"><span class="identifier">skip</span></code> directive |
| consumes the attribute type of the embedded generator <code class="computeroutput"><span class="identifier">A</span></code> without generating any |
| output. It succeeds always. The embedded generator is not executed. |
| </p> |
| </td> |
| </tr> |
| </tbody> |
| </table></div> |
| <a name="spirit.karma.reference.directive.omit.attributes"></a><h6> |
| <a name="id1051766"></a> |
| <a class="link" href="omit.html#spirit.karma.reference.directive.omit.attributes">Attributes</a> |
| </h6> |
| <p> |
| See <a class="link" href="../../quick_reference/compound_attribute_rules.html#spirit.karma.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">omit</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">omit</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">omit</span><span class="special">[</span><span class="identifier">a</span><span class="special">]:</span> <span class="identifier">Unused</span></pre> |
| <p> |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="identifier">skip</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">skip</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">skip</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> |
| <a name="spirit.karma.reference.directive.omit.complexity"></a><h6> |
| <a name="id1053300"></a> |
| <a class="link" href="omit.html#spirit.karma.reference.directive.omit.complexity">Complexity</a> |
| </h6> |
| <div class="blockquote"><blockquote class="blockquote"><p> |
| The overall complexity of the <code class="computeroutput"><span class="identifier">omit</span><span class="special">[]</span></code> directive depends on the complexity |
| of the embedded generator. The overall complexity of the <code class="computeroutput"><span class="identifier">skip</span><span class="special">[]</span></code> |
| generator directive is O(1) as it does not generate any output. |
| </p></blockquote></div> |
| <a name="spirit.karma.reference.directive.omit.example"></a><h6> |
| <a name="id1053342"></a> |
| <a class="link" href="omit.html#spirit.karma.reference.directive.omit.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="../karma_basics.html#spirit.karma.reference.karma_basics.examples">Basics Examples</a> |
| section. |
| </p></td></tr> |
| </table></div> |
| <p> |
| Some includes: |
| </p> |
| <p> |
| |
| </p> |
| <pre class="programlisting"><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">karma</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></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">phoenix_core</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></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">phoenix_operator</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> |
| <span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">fusion</span><span class="special">/</span><span class="identifier">include</span><span class="special">/</span><span class="identifier">std_pair</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> |
| <span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span> |
| <span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">string</span><span class="special">></span> |
| </pre> |
| <p> |
| </p> |
| <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">karma</span><span class="special">::</span><span class="identifier">double_</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">karma</span><span class="special">::</span><span class="identifier">omit</span><span class="special">;</span> |
| </pre> |
| <p> |
| </p> |
| <p> |
| Basic usage of a <code class="computeroutput"><span class="identifier">omit</span></code> |
| generator directive. It shows how it consumes the first element of the |
| provided attribute without generating anything, leaving the second element |
| of the attribute to the non-wrapped <code class="computeroutput"><span class="identifier">double_</span></code> |
| generator. |
| </p> |
| <p> |
| |
| </p> |
| <pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="keyword">double</span><span class="special">,</span> <span class="keyword">double</span><span class="special">></span> <span class="identifier">p</span> <span class="special">(</span><span class="number">1.0</span><span class="special">,</span> <span class="number">2.0</span><span class="special">);</span> |
| <span class="identifier">test_generator_attr</span><span class="special">(</span><span class="string">"2.0"</span><span class="special">,</span> <span class="identifier">omit</span><span class="special">[</span><span class="identifier">double_</span><span class="special">]</span> <span class="special"><<</span> <span class="identifier">double_</span><span class="special">,</span> <span class="identifier">p</span><span class="special">);</span> |
| </pre> |
| <p> |
| </p> |
| <p> |
| Generally, this directive is helpful in situations, where the attribute |
| type contains more information (elements) than need to be used to generate |
| the required output. Normally in such situations we would resolve to |
| use semantic actions to explicitly pass the correct parts of the overall |
| attribute to the generators. The <code class="computeroutput"><span class="identifier">omit</span></code> |
| directive helps achieving the same without having to use semantic actions. |
| </p> |
| <p> |
| Consider the attribute type: |
| </p> |
| <pre class="programlisting"><span class="keyword">typedef</span> <span class="identifier">fusion</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="keyword">double</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">attribute_type</span><span class="special">;</span> |
| </pre> |
| <p> |
| where we need to generate output only from the first and last element: |
| </p> |
| <pre class="programlisting"><span class="keyword">typedef</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">back_insert</span><span class="special">:</span><span class="identifier">iterator</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">iterator_type</span><span class="special">;</span> |
| |
| <span class="identifier">karma</span><span class="special">::</span><span class="identifier">rule</span><span class="special"><</span><span class="identifier">iterator_type</span><span class="special">,</span> <span class="identifier">attribute_type</span><span class="special">()></span> <span class="identifier">r</span><span class="special">;</span> |
| <span class="identifier">r</span> <span class="special">=</span> <span class="identifier">int_</span><span class="special">[</span><span class="identifier">_1</span> <span class="special">=</span> <span class="identifier">phoenix</span><span class="special">::</span><span class="identifier">at_c</span><span class="special"><</span><span class="number">0</span><span class="special">>(</span><span class="identifier">_val</span><span class="special">)]</span> <span class="special"><<</span> <span class="identifier">string</span><span class="special">[</span><span class="identifier">_1</span> <span class="special">=</span> <span class="identifier">phoenix</span><span class="special">::</span><span class="identifier">at_c</span><span class="special"><</span><span class="number">2</span><span class="special">>(</span><span class="identifier">_val</span><span class="special">)];</span> |
| |
| <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">str</span><span class="special">;</span> |
| <span class="identifier">iterator_type</span> <span class="identifier">sink</span><span class="special">(</span><span class="identifier">str</span><span class="special">);</span> |
| <span class="identifier">generate</span><span class="special">(</span><span class="identifier">sink</span><span class="special">,</span> <span class="identifier">r</span><span class="special">,</span> <span class="identifier">attribute_type</span><span class="special">(</span><span class="number">1</span><span class="special">,</span> <span class="number">2.0</span><span class="special">,</span> <span class="string">"example"</span><span class="special">));</span> <span class="comment">// will generate: '1example' |
| </span></pre> |
| <p> |
| This is error prone and not really readable. The same can be achieved |
| by using the <code class="computeroutput"><span class="identifier">omit</span></code> directive: |
| </p> |
| <pre class="programlisting"><span class="identifier">r</span> <span class="special">=</span> <span class="identifier">int_</span> <span class="special"><<</span> <span class="identifier">omit</span><span class="special">[</span><span class="identifier">double_</span><span class="special">]</span> <span class="special"><<</span> <span class="identifier">string</span><span class="special">;</span> |
| </pre> |
| <p> |
| which is at the same time more readable and more efficient as we don't |
| have to use semantic actions. |
| </p> |
| <p> |
| The semantics of using the <code class="computeroutput"><span class="identifier">skip</span><span class="special">[]</span></code> directive are identical to the <code class="computeroutput"><span class="identifier">omit</span><span class="special">[]</span></code> |
| directive, except that it does not actually execute the embedded generator. |
| For this reason it is usually preferable to utilize the <code class="computeroutput"><span class="identifier">skip</span><span class="special">[]</span></code> |
| directive instead of the <code class="computeroutput"><span class="identifier">omit</span><span class="special">[]</span></code> directive. On the other hand, the |
| <code class="computeroutput"><span class="identifier">omit</span><span class="special">[]</span></code> |
| directive is very useful whenever the embedded generator produces side |
| effects (has semantic actions which need to be executed). |
| </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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