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- <div class="section">
- <div class="titlepage"><div><div><h3 class="title">
- <a name="spirit_x3.tutorials.semantic_actions"></a><a class="link" href="semantic_actions.html" title="Parser Semantic Actions">Parser Semantic
- Actions</a>
- </h3></div></div></div>
- <p>
- The example in the previous section was very simplistic. It only recognized
- data, but did nothing with it. It answered the question: "Did the input
- match?". Now, we want to extract information from what was parsed. For
- example, we would want to store the parsed number after a successful match.
- To do this, you will need <span class="emphasis"><em>semantic actions</em></span>.
- </p>
- <p>
- Semantic actions may be attached to any point in the grammar specification.
- These actions are polymorphic function objects that are called whenever a
- part of the parser successfully recognizes a portion of the input. Say you
- have a parser <code class="computeroutput"><span class="identifier">p</span></code>, and a polymorphic
- C++ function object <code class="computeroutput"><span class="identifier">f</span></code>. You
- can make the parser call <code class="computeroutput"><span class="identifier">f</span></code>
- whenever it matches an input by attaching <code class="computeroutput"><span class="identifier">f</span></code>:
- </p>
- <pre class="programlisting"><span class="identifier">p</span><span class="special">[</span><span class="identifier">f</span><span class="special">]</span>
- </pre>
- <p>
- The expression above links <code class="computeroutput"><span class="identifier">f</span></code>
- to the parser, <code class="computeroutput"><span class="identifier">p</span></code>. <code class="computeroutput"><span class="identifier">f</span></code> is expected to be a polymorphic function
- object with the signature:
- </p>
- <pre class="programlisting"><span class="keyword">template</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="keyword">operator</span><span class="special">()(</span><span class="identifier">Context</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">ctx</span><span class="special">)</span> <span class="keyword">const</span><span class="special">;</span>
- </pre>
- <p>
- We can also use C++14 generic lambdas of the form:
- </p>
- <pre class="programlisting"><span class="special">[](</span><span class="keyword">auto</span><span class="special">&</span> <span class="identifier">ctx</span><span class="special">)</span> <span class="special">{</span> <span class="comment">/*...*/</span> <span class="special">}</span>
- </pre>
- <p>
- From the context, we can extract relevant information:
- </p>
- <div class="table">
- <a name="spirit_x3.tutorials.semantic_actions.parse_context_access_functions"></a><p class="title"><b>Table 2. Parse Context Access Functions</b></p>
- <div class="table-contents"><table class="table" summary="Parse Context Access Functions">
- <colgroup>
- <col>
- <col>
- <col>
- </colgroup>
- <thead><tr>
- <th>
- <p>
- Function
- </p>
- </th>
- <th>
- <p>
- Description
- </p>
- </th>
- <th>
- <p>
- Example
- </p>
- </th>
- </tr></thead>
- <tbody>
- <tr>
- <td>
- <p>
- <code class="computeroutput"><span class="identifier">_val</span></code>
- </p>
- </td>
- <td>
- <p>
- A reference to the attribute of the innermost rule that directly
- or indirectly invokes the parser <code class="computeroutput"><span class="identifier">p</span></code>
- </p>
- </td>
- <td>
- <p>
- <code class="computeroutput"><span class="identifier">_val</span><span class="special">(</span><span class="identifier">ctx</span><span class="special">)</span>
- <span class="special">=</span> <span class="string">"Gotya!"</span></code>
- </p>
- </td>
- </tr>
- <tr>
- <td>
- <p>
- <code class="computeroutput"><span class="identifier">_where</span></code>
- </p>
- </td>
- <td>
- <p>
- Iterator range to the input stream
- </p>
- </td>
- <td>
- <p>
- <code class="computeroutput"><span class="identifier">_where</span><span class="special">(</span><span class="identifier">ctx</span><span class="special">).</span><span class="identifier">begin</span><span class="special">()</span></code>
- </p>
- </td>
- </tr>
- <tr>
- <td>
- <p>
- <code class="computeroutput"><span class="identifier">_attr</span></code>
- </p>
- </td>
- <td>
- <p>
- A reference to the attribute of the parser <code class="computeroutput"><span class="identifier">p</span></code>
- </p>
- </td>
- <td>
- <p>
- <code class="computeroutput"><span class="identifier">_val</span><span class="special">(</span><span class="identifier">ctx</span><span class="special">)</span>
- <span class="special">+=</span> <span class="identifier">_attr</span><span class="special">(</span><span class="identifier">ctx</span><span class="special">)</span></code>
- </p>
- </td>
- </tr>
- <tr>
- <td>
- <p>
- <code class="computeroutput"><span class="identifier">_pass</span></code>
- </p>
- </td>
- <td>
- <p>
- A reference to a <code class="computeroutput"><span class="keyword">bool</span></code>
- flag that can be used to force the <code class="computeroutput"><span class="identifier">p</span></code>
- to fail
- </p>
- </td>
- <td>
- <p>
- <code class="computeroutput"><span class="identifier">_pass</span><span class="special">(</span><span class="identifier">ctx</span><span class="special">)</span>
- <span class="special">=</span> <span class="keyword">false</span></code>
- </p>
- </td>
- </tr>
- </tbody>
- </table></div>
- </div>
- <br class="table-break"><h5>
- <a name="spirit_x3.tutorials.semantic_actions.h0"></a>
- <span class="phrase"><a name="spirit_x3.tutorials.semantic_actions.examples_of_semantic_actions"></a></span><a class="link" href="semantic_actions.html#spirit_x3.tutorials.semantic_actions.examples_of_semantic_actions">Examples
- of Semantic Actions</a>
- </h5>
- <p>
- Given:
- </p>
- <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">print_action</span>
- <span class="special">{</span>
- <span class="keyword">template</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="keyword">operator</span><span class="special">()(</span><span class="identifier">Context</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">ctx</span><span class="special">)</span> <span class="keyword">const</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">_attr</span><span class="special">(</span><span class="identifier">ctx</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="special">}</span>
- <span class="special">};</span>
- </pre>
- <p>
- Take note that with function objects, we need to have an <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code> with the Context argument. If we don't
- care about the context, we can use <code class="computeroutput"><span class="identifier">unused_type</span></code>.
- We'll see more of <code class="computeroutput"><span class="identifier">unused_type</span></code>
- elsewhere. <code class="computeroutput"><span class="identifier">unused_type</span></code> is
- a Spirit supplied support class.
- </p>
- <p>
- All examples parse inputs of the form:
- </p>
- <pre class="programlisting"><span class="string">"{NNN}"</span>
- </pre>
- <p>
- Where NNN is an integer inside the curly braces (e.g. {44}).
- </p>
- <p>
- The first example shows how to attach a function object:
- </p>
- <pre class="programlisting"><span class="identifier">parse</span><span class="special">(</span><span class="identifier">first</span><span class="special">,</span> <span class="identifier">last</span><span class="special">,</span> <span class="char">'{'</span> <span class="special">>></span> <span class="identifier">int_</span><span class="special">[</span><span class="identifier">print_action</span><span class="special">()]</span> <span class="special">>></span> <span class="char">'}'</span><span class="special">);</span>
- </pre>
- <p>
- What's new? Well <code class="computeroutput"><span class="identifier">int_</span></code> is
- the sibling of <code class="computeroutput"><span class="identifier">double_</span></code>. I'm
- sure you can guess what this parser does.
- </p>
- <p>
- The next example shows how use C++14 lambda:
- </p>
- <pre class="programlisting"><span class="keyword">auto</span> <span class="identifier">f</span> <span class="special">=</span> <span class="special">[](</span><span class="keyword">auto</span><span class="special">&</span> <span class="identifier">ctx</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">_attr</span><span class="special">(</span><span class="identifier">ctx</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="special">};</span>
- <span class="identifier">parse</span><span class="special">(</span><span class="identifier">first</span><span class="special">,</span> <span class="identifier">last</span><span class="special">,</span> <span class="char">'{'</span> <span class="special">>></span> <span class="identifier">int_</span><span class="special">[</span><span class="identifier">f</span><span class="special">]</span> <span class="special">>></span> <span class="char">'}'</span><span class="special">);</span>
- </pre>
- <p>
- Attaching semantic actions is the first hurdle one has to tackle when getting
- started with parsing with Spirit. Familiarize yourself with this task.
- </p>
- <p>
- The examples above can be found here: <a href="../../../../../example/x3/actions.cpp" target="_top">actions.cpp</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 © 2001-2018 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>
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