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| <div class="section"> |
| <div class="titlepage"><div><div><h4 class="title"> |
| <a name="spirit.qi.tutorials.semantic_actions"></a><a class="link" href="semantic_actions.html" title="Semantic Actions">Semantic Actions</a> |
| </h4></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 C++ functions or 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 C++ function <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>. |
| </p> |
| <p> |
| The function/function object signature depends on the type of the parser |
| to which it is attached. The parser <code class="computeroutput"><span class="identifier">double_</span></code> |
| passes the parsed number. Thus, if we were to attach a function <code class="computeroutput"><span class="identifier">F</span></code> to <code class="computeroutput"><span class="identifier">double_</span></code>, |
| we need <code class="computeroutput"><span class="identifier">F</span></code> to be declared |
| as: |
| </p> |
| <pre class="programlisting"><span class="keyword">void</span> <span class="identifier">F</span><span class="special">(</span><span class="keyword">double</span> <span class="identifier">n</span><span class="special">);</span> |
| </pre> |
| <p> |
| There are actually 2 more arguments being passed (the parser context and |
| a reference to a boolean 'hit' parameter). We don't need these, for now, |
| but we'll see more on these other arguments later. Spirit.Qi allows us |
| to bind a single argument function, like above. The other arguments are |
| simply ignored. |
| </p> |
| <a name="spirit.qi.tutorials.semantic_actions.examples_of_semantic_actions"></a><h6> |
| <a name="id799304"></a> |
| <a class="link" href="semantic_actions.html#spirit.qi.tutorials.semantic_actions.examples_of_semantic_actions">Examples |
| of Semantic Actions</a> |
| </h6> |
| <p> |
| Presented are various ways to attach semantic actions: |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" type="disc"> |
| <li class="listitem"> |
| Using plain function pointer |
| </li> |
| <li class="listitem"> |
| Using simple function object |
| </li> |
| <li class="listitem"> |
| Using <a href="../../../../../../../libs/bind/index.html" target="_top">Boost.Bind</a> with |
| a plain function |
| </li> |
| <li class="listitem"> |
| Using <a href="../../../../../../../libs/bind/index.html" target="_top">Boost.Bind</a> with |
| a member function |
| </li> |
| <li class="listitem"> |
| Using <a href="../../../../../../../libs/lambda/index.html" target="_top">Boost.Lambda</a> |
| </li> |
| </ul></div> |
| <p> |
| Given: |
| </p> |
| <p> |
| |
| </p> |
| <pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">client</span> |
| <span class="special">{</span> |
| <span class="keyword">namespace</span> <span class="identifier">qi</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">qi</span><span class="special">;</span> |
| |
| <span class="comment">// A plain function |
| </span> <span class="keyword">void</span> <span class="identifier">print</span><span class="special">(</span><span class="keyword">int</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">i</span><span class="special">)</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">i</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="comment">// A member function |
| </span> <span class="keyword">struct</span> <span class="identifier">writer</span> |
| <span class="special">{</span> |
| <span class="keyword">void</span> <span class="identifier">print</span><span class="special">(</span><span class="keyword">int</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">i</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">i</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> |
| |
| <span class="comment">// A function object |
| </span> <span class="keyword">struct</span> <span class="identifier">print_action</span> |
| <span class="special">{</span> |
| <span class="keyword">void</span> <span class="keyword">operator</span><span class="special">()(</span><span class="keyword">int</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">i</span><span class="special">,</span> <span class="identifier">qi</span><span class="special">::</span><span class="identifier">unused_type</span><span class="special">,</span> <span class="identifier">qi</span><span class="special">::</span><span class="identifier">unused_type</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">i</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> |
| <span class="special">}</span> |
| </pre> |
| <p> |
| </p> |
| <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 3 arguments. Since we don't care about the other two, we can use |
| <code class="computeroutput"><span class="identifier">unused_type</span></code> for these. |
| 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">"{integer}"</span> |
| </pre> |
| <p> |
| An integer inside the curly braces. |
| </p> |
| <p> |
| The first example shows how to attach a plain function: |
| </p> |
| <p> |
| |
| </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</span><span class="special">]</span> <span class="special">>></span> <span class="char">'}'</span><span class="special">);</span> |
| </pre> |
| <p> |
| </p> |
| <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 to attach a simple function object: |
| </p> |
| <p> |
| |
| </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> |
| </p> |
| <p> |
| We can use <a href="../../../../../../../libs/bind/index.html" target="_top">Boost.Bind</a> |
| to 'bind' member functions: |
| </p> |
| <p> |
| |
| </p> |
| <pre class="programlisting"><span class="identifier">writer</span> <span class="identifier">w</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">boost</span><span class="special">::</span><span class="identifier">bind</span><span class="special">(&</span><span class="identifier">writer</span><span class="special">::</span><span class="identifier">print</span><span class="special">,</span> <span class="special">&</span><span class="identifier">w</span><span class="special">,</span> <span class="identifier">_1</span><span class="special">)]</span> <span class="special">>></span> <span class="char">'}'</span><span class="special">);</span> |
| </pre> |
| <p> |
| </p> |
| <p> |
| Likewise, we can also use <a href="../../../../../../../libs/bind/index.html" target="_top">Boost.Bind</a> |
| to 'bind' plain functions: |
| </p> |
| <p> |
| |
| </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">boost</span><span class="special">::</span><span class="identifier">bind</span><span class="special">(&</span><span class="identifier">print</span><span class="special">,</span> <span class="identifier">_1</span><span class="special">)]</span> <span class="special">>></span> <span class="char">'}'</span><span class="special">);</span> |
| </pre> |
| <p> |
| </p> |
| <p> |
| Yep, we can also use <a href="../../../../../../../libs/lambda/index.html" target="_top">Boost.Lambda</a>: |
| </p> |
| <p> |
| |
| </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">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="char">'\n'</span><span class="special">]</span> <span class="special">>></span> <span class="char">'}'</span><span class="special">);</span> |
| </pre> |
| <p> |
| </p> |
| <p> |
| There are more ways to bind semantic action functions, but the examples |
| above are the most common. Attaching semantic actions is the first hurdle |
| one has to tackle when getting started with parsing with Spirit. Familiarize |
| yourself with this task and get intimate with the tools behind it such |
| as <a href="../../../../../../../libs/bind/index.html" target="_top">Boost.Bind</a> and <a href="../../../../../../../libs/lambda/index.html" target="_top">Boost.Lambda</a>. |
| </p> |
| <p> |
| The examples above can be found here: <a href="../../../../../example/qi/actions.cpp" target="_top">../../example/qi/actions.cpp</a> |
| </p> |
| <a name="spirit.qi.tutorials.semantic_actions.phoenix"></a><h6> |
| <a name="id800989"></a> |
| <a class="link" href="semantic_actions.html#spirit.qi.tutorials.semantic_actions.phoenix">Phoenix</a> |
| </h6> |
| <p> |
| <a href="../../../../../phoenix/doc/html/index.html" target="_top">Phoenix</a>, a companion |
| library bundled with Spirit, is specifically suited for binding semantic |
| actions. It is like <a href="../../../../../../../libs/lambda/index.html" target="_top">Boost.Lambda</a> |
| on steroids, with special custom features that make it easy to integrate |
| semantic actions with Spirit. If your requirements go beyond simple to |
| moderate parsing, it is suggested that you use this library. All the following |
| examples in this tutorial will use <a href="../../../../../phoenix/doc/html/index.html" target="_top">Phoenix</a> |
| for semantic actions. |
| </p> |
| <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> |
| There are different ways to write semantic actions for <span class="emphasis"><em>Spirit.Qi</em></span>: |
| using plain functions, <a href="../../../../../../../libs/bind/index.html" target="_top">Boost.Bind</a>, |
| <a href="../../../../../../../libs/lambda/index.html" target="_top">Boost.Lambda</a>, or |
| <a href="../../../../../phoenix/doc/html/index.html" target="_top">Phoenix</a>. The latter |
| three allow you to use special placeholders to control parameter placement |
| (<code class="computeroutput"><span class="identifier">_1</span></code>, <code class="computeroutput"><span class="identifier">_2</span></code>, |
| etc.). Each of those libraries has it's own implementation of the placeholders, |
| all in different namespaces. You have to make sure not to mix placeholders |
| with a library they don't belong to and not to use different libraries |
| while writing a semantic action. |
| </p> |
| <p> |
| Generally, for <a href="../../../../../../../libs/bind/index.html" target="_top">Boost.Bind</a>, |
| use <code class="computeroutput"><span class="special">::</span><span class="identifier">_1</span></code>, |
| <code class="computeroutput"><span class="special">::</span><span class="identifier">_2</span></code>, |
| etc. (yes, these placeholders are defined in the global namespace). |
| </p> |
| <p> |
| For <a href="../../../../../../../libs/lambda/index.html" target="_top">Boost.Lambda</a> use |
| the placeholders defined in the namespace <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">lambda</span></code>. |
| </p> |
| <p> |
| For semantic actions written using <a href="../../../../../phoenix/doc/html/index.html" target="_top">Phoenix</a> |
| use the placeholders defined in the namespace <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span></code>. |
| Please note that all existing placeholders for your convenience are also |
| available from the namespace <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">qi</span></code>. |
| </p> |
| </td></tr> |
| </table></div> |
| </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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