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| <div class="section"> |
| <div class="titlepage"><div><div><h5 class="title"> |
| <a name="spirit.qi.reference.operator.alternative"></a><a class="link" href="alternative.html" title="Alternative (a | b)">Alternative |
| (<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.qi.reference.operator.alternative.description"></a><h6> |
| <a name="id920470"></a> |
| <a class="link" href="alternative.html#spirit.qi.reference.operator.alternative.description">Description</a> |
| </h6> |
| <p> |
| The alternative operator, <code class="computeroutput"><span class="identifier">a</span> |
| <span class="special">|</span> <span class="identifier">b</span></code>, |
| matches one of two or more operands (<code class="computeroutput"><span class="identifier">a</span></code>, |
| <code class="computeroutput"><span class="identifier">b</span></code>, ... etc.): |
| </p> |
| <pre class="programlisting"><span class="identifier">a</span> <span class="special">|</span> <span class="identifier">b</span> <span class="special">|</span> <span class="special">...</span> |
| </pre> |
| <p> |
| Alternative operands are tried one by one on a first-match-wins basis |
| starting from the leftmost operand. After a successfully matched alternative |
| is found, the parser concludes its search, essentially short-circuiting |
| the search for other potentially viable candidates. This short-circuiting |
| implicitly gives the highest priority to the leftmost alternative. |
| </p> |
| <p> |
| Short-circuiting is done in the same manner as C or C++'s logical expressions; |
| e.g. <code class="computeroutput"><span class="keyword">if</span> <span class="special">(</span><span class="identifier">x</span> <span class="special"><</span> <span class="number">3</span> <span class="special">||</span> <span class="identifier">y</span> |
| <span class="special"><</span> <span class="number">2</span><span class="special">)</span></code> where, if <code class="computeroutput"><span class="identifier">x</span> |
| <span class="special"><</span> <span class="number">3</span></code>, |
| the <code class="computeroutput"><span class="identifier">y</span> <span class="special"><</span> |
| <span class="number">2</span></code> test is not done at all. In addition |
| to providing an implicit priority rule for alternatives which is necessary, |
| given its non-deterministic nature, short-circuiting improves the execution |
| time. If the order of your alternatives is logically irrelevant, strive |
| to put the (expected) most common choice first for maximum efficiency. |
| </p> |
| <a name="spirit.qi.reference.operator.alternative.header"></a><h6> |
| <a name="id920634"></a> |
| <a class="link" href="alternative.html#spirit.qi.reference.operator.alternative.header">Header</a> |
| </h6> |
| <pre class="programlisting"><span class="comment">// forwards to <boost/spirit/home/qi/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">qi_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.qi.reference.operator.alternative.model_of"></a><h6> |
| <a name="id920709"></a> |
| <a class="link" href="alternative.html#spirit.qi.reference.operator.alternative.model_of">Model |
| of</a> |
| </h6> |
| <div class="blockquote"><blockquote class="blockquote"><p> |
| <a class="link" href="../parser_concepts/naryparser.html" title="NaryParser"><code class="computeroutput"><span class="identifier">NaryParser</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">b</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.operator.alternative.expression_semantics"></a><h6> |
| <a name="id920784"></a> |
| <a class="link" href="alternative.html#spirit.qi.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="../parser_concepts/naryparser.html" title="NaryParser"><code class="computeroutput"><span class="identifier">NaryParser</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> |
| Match <code class="computeroutput"><span class="identifier">a</span></code> or |
| <code class="computeroutput"><span class="identifier">b</span></code>. |
| </p> |
| </td> |
| </tr></tbody> |
| </table></div> |
| <a name="spirit.qi.reference.operator.alternative.attributes"></a><h6> |
| <a name="id920897"></a> |
| <a class="link" href="alternative.html#spirit.qi.reference.operator.alternative.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">a</span> <span class="special">|</span> |
| <span class="identifier">b</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">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">optional</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">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="identifier">c</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">c</span><span class="special">):</span> <span class="identifier">optional</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="special">></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">optional</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">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> |
| <a name="spirit.qi.reference.operator.alternative.complexity"></a><h6> |
| <a name="id922012"></a> |
| <a class="link" href="alternative.html#spirit.qi.reference.operator.alternative.complexity">Complexity</a> |
| </h6> |
| <div class="blockquote"><blockquote class="blockquote"><p> |
| The overall complexity of the alternative parser is defined by the |
| sum of the complexities of its elements. The complexity of the alternative |
| parser itself is O(N), where N is the number of alternatives. |
| </p></blockquote></div> |
| <a name="spirit.qi.reference.operator.alternative.example"></a><h6> |
| <a name="id922033"></a> |
| <a class="link" href="alternative.html#spirit.qi.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.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">ascii</span><span class="special">::</span><span class="identifier">string</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">_1</span><span class="special">;</span> |
| <span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">variant</span><span class="special">;</span> |
| </pre> |
| <p> |
| </p> |
| <p> |
| Simple usage: |
| </p> |
| <p> |
| |
| </p> |
| <pre class="programlisting"><span class="identifier">test_parser</span><span class="special">(</span><span class="string">"Hello"</span><span class="special">,</span> <span class="identifier">string</span><span class="special">(</span><span class="string">"Hello"</span><span class="special">)</span> <span class="special">|</span> <span class="identifier">int_</span><span class="special">);</span> |
| <span class="identifier">test_parser</span><span class="special">(</span><span class="string">"123"</span><span class="special">,</span> <span class="identifier">string</span><span class="special">(</span><span class="string">"Hello"</span><span class="special">)</span> <span class="special">|</span> <span class="identifier">int_</span><span class="special">);</span></pre> |
| <p> |
| </p> |
| <p> |
| Extracting the attribute variant (using <a href="http://www.boost.org/doc/html/variant.html" target="_top">Boost.Variant</a>): |
| </p> |
| <p> |
| |
| </p> |
| <pre class="programlisting"><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">int</span><span class="special">></span> <span class="identifier">attr</span><span class="special">;</span> |
| <span class="identifier">test_parser_attr</span><span class="special">(</span><span class="string">"Hello"</span><span class="special">,</span> <span class="identifier">string</span><span class="special">(</span><span class="string">"Hello"</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></pre> |
| <p> |
| </p> |
| <p> |
| This should print <code class="computeroutput"><span class="string">"Hello"</span></code>. |
| Note: There are better ways to extract the value from the variant. See |
| <a href="http://www.boost.org/doc/html/variant.html" target="_top">Boost.Variant</a> |
| visitation. This code is solely for demonstration. |
| </p> |
| <p> |
| |
| </p> |
| <pre class="programlisting"><span class="keyword">if</span> <span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">get</span><span class="special"><</span><span class="keyword">int</span><span class="special">>(&</span><span class="identifier">attr</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">boost</span><span class="special">::</span><span class="identifier">get</span><span class="special"><</span><span class="keyword">int</span><span class="special">>(</span><span class="identifier">attr</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="keyword">else</span> |
| <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">get</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">attr</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></pre> |
| <p> |
| </p> |
| <p> |
| Extracting the attributes using Semantic Actions with <a href="../../../../../../phoenix/doc/html/index.html" target="_top">Phoenix</a> |
| (this should print <code class="computeroutput"><span class="number">123</span></code>): |
| </p> |
| <p> |
| |
| </p> |
| <pre class="programlisting"><span class="identifier">test_parser</span><span class="special">(</span><span class="string">"123"</span><span class="special">,</span> <span class="special">(</span><span class="identifier">string</span><span class="special">(</span><span class="string">"Hello"</span><span class="special">)</span> <span class="special">|</span> <span class="identifier">int_</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">_1</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</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 © 2001-2010 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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