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| <div class="section"> |
| <div class="titlepage"><div><div><h5 class="title"> |
| <a name="spirit.karma.reference.operator.alternative"></a><a class="link" href="alternative.html" title="Alternative Generator (a | b)">Alternative |
| Generator (<code class="computeroutput"><span class="identifier">a</span> <span class="special">|</span> |
| <span class="identifier">b</span></code>)</a> |
| </h5></div></div></div> |
| <a name="spirit.karma.reference.operator.alternative.description"></a><h6> |
| <a name="spirit.karma.reference.operator.alternative.description-heading"></a> |
| <a class="link" href="alternative.html#spirit.karma.reference.operator.alternative.description">Description</a> |
| </h6> |
| <p> |
| Generator alternatives are used to combine different, more primitive |
| generators into alternatives. All generators in an alternative are invoked |
| from left to right until one of them succeeds. |
| </p> |
| <a name="spirit.karma.reference.operator.alternative.header"></a><h6> |
| <a name="spirit.karma.reference.operator.alternative.header-heading"></a> |
| <a class="link" href="alternative.html#spirit.karma.reference.operator.alternative.header">Header</a> |
| </h6> |
| <pre class="programlisting"><span class="comment">// forwards to <boost/spirit/home/karma/operator/alternative.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_alternative</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.karma.reference.operator.alternative.model_of"></a><h6> |
| <a name="spirit.karma.reference.operator.alternative.model_of-heading"></a> |
| <a class="link" href="alternative.html#spirit.karma.reference.operator.alternative.model_of">Model |
| of</a> |
| </h6> |
| <div class="blockquote"><blockquote class="blockquote"><p> |
| <a class="link" href="../generator_concepts/narygenerator.html" title="NaryGenerator"><code class="computeroutput"><span class="identifier">NaryGenerator</span></code></a> |
| </p></blockquote></div> |
| <a name="spirit.karma.reference.operator.alternative.expression_semantics"></a><h6> |
| <a name="spirit.karma.reference.operator.alternative.expression_semantics-heading"></a> |
| <a class="link" href="alternative.html#spirit.karma.reference.operator.alternative.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/narygenerator.html" title="NaryGenerator"><code class="computeroutput"><span class="identifier">NaryGenerator</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">a</span> <span class="special">|</span> |
| <span class="identifier">b</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| The generators <code class="computeroutput"><span class="identifier">a</span></code> |
| and <code class="computeroutput"><span class="identifier">b</span></code> are executed |
| sequentially from left to right until one of them succeeds. |
| A failed generator forces the alternative generator to try |
| the next one. The alternative fails as a whole only if all |
| elements of the alternative fail. Each element of the alternative |
| gets passed the whole attribute of the alternative. |
| </p> |
| </td> |
| </tr></tbody> |
| </table></div> |
| <p> |
| Alternatives intercept and buffer the output of the currently executed |
| element. This allows to avoid partial outputs from failing elements as |
| the buffered content will be forwarded to the actual output only after |
| an element succeeded. |
| </p> |
| <a name="spirit.karma.reference.operator.alternative.attributes"></a><h6> |
| <a name="spirit.karma.reference.operator.alternative.attributes-heading"></a> |
| <a class="link" href="alternative.html#spirit.karma.reference.operator.alternative.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">a</span> <span class="special">|</span> |
| <span class="identifier">b</span></code> (alternative) |
| </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">b</span><span class="special">:</span> <span class="identifier">B</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">variant</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">a</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">Unused</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">A</span> |
| <span class="identifier">a</span><span class="special">:</span> <span class="identifier">Unused</span><span class="special">,</span> <span class="identifier">b</span><span class="special">:</span> <span class="identifier">B</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">B</span> |
| <span class="identifier">a</span><span class="special">:</span> <span class="identifier">Unused</span><span class="special">,</span> <span class="identifier">b</span><span class="special">:</span> <span class="identifier">Unused</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">Unused</span> |
| <span class="identifier">a</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">A</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">A</span></pre> |
| <p> |
| </p> |
| </td> |
| </tr></tbody> |
| </table></div> |
| <div class="important"><table border="0" summary="Important"> |
| <tr> |
| <td rowspan="2" align="center" valign="top" width="25"><img alt="[Important]" src="../../../../images/important.png"></td> |
| <th align="left">Important</th> |
| </tr> |
| <tr><td align="left" valign="top"><p> |
| The table above uses <code class="computeroutput"><span class="identifier">variant</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span> <span class="identifier">B</span><span class="special">></span></code> as a placeholder only. The notation |
| <code class="computeroutput"><span class="identifier">variant</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span> <span class="identifier">B</span><span class="special">></span></code> |
| stands for the type <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">variant</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span> <span class="identifier">B</span><span class="special">></span></code>. |
| </p></td></tr> |
| </table></div> |
| <p> |
| The attribute handling of Alternatives is special as their behavior is |
| not completely defined at compile time. First of all the selected alternative |
| element depends on the actual type of the attribute supplied to the alternative |
| generator (i.e. what is stored in the variant). The attribute type supplied |
| at <span class="emphasis"><em>runtime</em></span> narrows the set of considered alternatives |
| to those being compatible attribute wise. The remaining alternatives |
| are tried sequentially until the first of them succeeds. See below for |
| an example of this behavior. |
| </p> |
| <a name="spirit.karma.reference.operator.alternative.complexity"></a><h6> |
| <a name="spirit.karma.reference.operator.alternative.complexity-heading"></a> |
| <a class="link" href="alternative.html#spirit.karma.reference.operator.alternative.complexity">Complexity</a> |
| </h6> |
| <div class="blockquote"><blockquote class="blockquote"><p> |
| The overall complexity of the alternative generator is defined by the |
| sum of the complexities of its elements. The complexity of the alternative |
| itself is O(N), where N is the number of elements in the alternative. |
| </p></blockquote></div> |
| <a name="spirit.karma.reference.operator.alternative.example"></a><h6> |
| <a name="spirit.karma.reference.operator.alternative.example-heading"></a> |
| <a class="link" href="alternative.html#spirit.karma.reference.operator.alternative.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.karma.reference.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">support_utree</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">ascii</span><span class="special">::</span><span class="identifier">string</span><span class="special">;</span> |
| </pre> |
| <p> |
| </p> |
| <p> |
| Basic usage of an alternative. While being only the second alternative, |
| the <code class="computeroutput"><span class="identifier">double_</span></code> generator |
| is chosen for output formatting because the supplied attribute type is |
| not compatible (i.e. not convertible) to the attribute type of the <code class="computeroutput"><span class="identifier">string</span></code> alternative. |
| </p> |
| <p> |
| |
| </p> |
| <pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">variant</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="keyword">double</span><span class="special">></span> <span class="identifier">v1</span><span class="special">(</span><span class="number">1.0</span><span class="special">);</span> |
| <span class="identifier">test_generator_attr</span><span class="special">(</span><span class="string">"1.0"</span><span class="special">,</span> <span class="identifier">string</span> <span class="special">|</span> <span class="identifier">double_</span><span class="special">,</span> <span class="identifier">v1</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">string</span> <span class="special">|</span> <span class="identifier">double_</span><span class="special">,</span> <span class="number">2.0</span><span class="special">);</span> |
| </pre> |
| <p> |
| </p> |
| <p> |
| The same formatting rules may be used to output a string. This time we |
| supply the string <code class="computeroutput"><span class="string">"example"</span></code>, |
| resulting in the first alternative to be chosen for the generated output. |
| </p> |
| <p> |
| |
| </p> |
| <pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">variant</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="keyword">double</span><span class="special">></span> <span class="identifier">v2</span><span class="special">(</span><span class="string">"example"</span><span class="special">);</span> |
| <span class="identifier">test_generator_attr</span><span class="special">(</span><span class="string">"example"</span><span class="special">,</span> <span class="identifier">string</span> <span class="special">|</span> <span class="identifier">double_</span><span class="special">,</span> <span class="identifier">v2</span><span class="special">);</span> |
| <span class="identifier">test_generator_attr</span><span class="special">(</span><span class="string">"example"</span><span class="special">,</span> <span class="identifier">string</span> <span class="special">|</span> <span class="identifier">double_</span><span class="special">,</span> <span class="string">"example"</span><span class="special">);</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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