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| <div class="section"> |
| <div class="titlepage"><div><div><h4 class="title"> |
| <a name="spirit.karma.tutorials.semantic_actions"></a><a class="link" href="semantic_actions.html" title="Semantic Actions">Semantic Actions</a> |
| </h4></div></div></div> |
| <p> |
| In the previous section we mentioned a very important difference between |
| parsers and generators. While parsers may be used without 'producing' any |
| data, generators always need data to generate the output from. We mentioned |
| one way of passing data to the generator by supplying it as a parameter |
| to one of the main API functions (for instance <code class="computeroutput"><span class="identifier">generate</span><span class="special">()</span></code> or <code class="computeroutput"><span class="identifier">generate_delimited</span><span class="special">()</span></code>). But sometimes this is not possible |
| or not desirable. |
| </p> |
| <p> |
| Very much like for <span class="emphasis"><em>Spirit.Qi</em></span> we have semantic actions |
| in <span class="emphasis"><em>Spirit.Karma</em></span> as well. 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 generator is |
| about to be invoked. Say you have a generator <code class="computeroutput"><span class="identifier">G</span></code>, |
| and a C++ function <code class="computeroutput"><span class="identifier">F</span></code>, you |
| can make the generator call <code class="computeroutput"><span class="identifier">F</span></code> |
| just before it gets invoked by attaching <code class="computeroutput"><span class="identifier">F</span></code>: |
| </p> |
| <pre class="programlisting"><span class="identifier">G</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 generator, <code class="computeroutput"><span class="identifier">G</span></code>. |
| </p> |
| <p> |
| Semantic actions in <span class="emphasis"><em>Spirit.Qi</em></span> are invoked after a |
| parser successfully matches its input and the matched value is passed into |
| the semantic action. In <span class="emphasis"><em>Spirit.Karma</em></span> the opposite |
| happens. Semantic actions are called before its associated generator is |
| invoked. They may provide the data required by the generator. |
| </p> |
| <p> |
| The function/function object signature depends on the type of the generator |
| to which it is attached. The generator <code class="computeroutput"><span class="identifier">double_</span></code> |
| expects the number to generate. 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="special">&</span> <span class="identifier">n</span><span class="special">);</span> |
| </pre> |
| <p> |
| where the function is expected to initialize the parameter <code class="computeroutput"><span class="identifier">n</span></code> with the value to generate. |
| </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> |
| Generally, and more formally, the semantic action <code class="computeroutput"><span class="identifier">F</span></code> |
| attached to a generator <code class="computeroutput"><span class="identifier">G</span></code> |
| needs to take a reference to the generator's attribute type as its first |
| parameter. For more information about generator attributes please see |
| the section Generator Attributes. |
| </p> |
| <p> |
| In the example above the function F takes a <code class="computeroutput"><span class="keyword">double</span><span class="special">&</span></code> as its first parameter as the attribute |
| of the <code class="computeroutput"><span class="identifier">double_</span></code> generator |
| happens to be a <code class="computeroutput"><span class="keyword">double</span></code>. |
| </p> |
| </td></tr> |
| </table></div> |
| <p> |
| There are actually 2 more arguments being passed (the generator context |
| and a reference to a boolean 'pass' parameter). We don't need these, for |
| now, but we'll see more on these other arguments later. <span class="emphasis"><em>Spirit.Karma</em></span> |
| allows us to bind a single argument function, like above. The other arguments |
| are simply ignored. |
| </p> |
| <p> |
| To sum up, the possible signatures for semantic actions are: |
| </p> |
| <pre class="programlisting"><span class="keyword">void</span> <span class="identifier">f</span><span class="special">(</span><span class="identifier">Attrib</span><span class="special">&);</span> |
| <span class="keyword">void</span> <span class="identifier">f</span><span class="special">(</span><span class="identifier">Attrib</span><span class="special">&,</span> <span class="identifier">Context</span><span class="special">&);</span> |
| <span class="keyword">void</span> <span class="identifier">f</span><span class="special">(</span><span class="identifier">Attrib</span><span class="special">&,</span> <span class="identifier">Context</span><span class="special">&,</span> <span class="keyword">bool</span><span class="special">&);</span> |
| </pre> |
| <a name="spirit.karma.tutorials.semantic_actions.examples_of_semantic_actions"></a><h6> |
| <a name="id956470"></a> |
| <a class="link" href="semantic_actions.html#spirit.karma.tutorials.semantic_actions.examples_of_semantic_actions">Examples |
| of Semantic Actions</a> |
| </h6> |
| <p> |
| In the following example we present various ways to attach semantic actions: |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" type="disc"> |
| <li class="listitem"> |
| Using a plain function pointer |
| </li> |
| <li class="listitem"> |
| Using a 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> |
| Let's assume we have: |
| </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">karma</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">karma</span><span class="special">;</span> |
| |
| <span class="comment">// A plain function |
| </span> <span class="keyword">void</span> <span class="identifier">read_function</span><span class="special">(</span><span class="keyword">int</span><span class="special">&</span> <span class="identifier">i</span><span class="special">)</span> |
| <span class="special">{</span> |
| <span class="identifier">i</span> <span class="special">=</span> <span class="number">42</span><span class="special">;</span> |
| <span class="special">}</span> |
| |
| <span class="comment">// A member function |
| </span> <span class="keyword">struct</span> <span class="identifier">reader</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="special">&</span> <span class="identifier">i</span><span class="special">)</span> <span class="keyword">const</span> |
| <span class="special">{</span> |
| <span class="identifier">i</span> <span class="special">=</span> <span class="number">42</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">read_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="special">&</span> <span class="identifier">i</span><span class="special">,</span> <span class="identifier">unused_type</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">i</span> <span class="special">=</span> <span class="number">42</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. Get used to it. <code class="computeroutput"><span class="identifier">unused_type</span></code> |
| is a Spirit supplied support class. Most of the time it stands for 'I don't |
| care, just use the appropriate default'. |
| </p> |
| <p> |
| All following examples generate outputs 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">generate</span><span class="special">(</span><span class="identifier">outiter</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">read_function</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 generator does and what type of attribute |
| it expects. |
| </p> |
| <p> |
| The next example shows how to attach a simple function object: |
| </p> |
| <p> |
| |
| </p> |
| <pre class="programlisting"><span class="identifier">generate</span><span class="special">(</span><span class="identifier">outiter</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">read_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">reader</span> <span class="identifier">r</span><span class="special">;</span> |
| <span class="identifier">generate</span><span class="special">(</span><span class="identifier">outiter</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">reader</span><span class="special">::</span><span class="identifier">print</span><span class="special">,</span> <span class="special">&</span><span class="identifier">r</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">generate</span><span class="special">(</span><span class="identifier">outiter</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">read_function</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> |
| And last but not least, we can also use <a href="../../../../../../../libs/lambda/index.html" target="_top">Boost.Lambda</a>: |
| </p> |
| <p> |
| |
| </p> |
| <pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">stringstream</span> <span class="identifier">strm</span><span class="special">(</span><span class="string">"42"</span><span class="special">);</span> |
| <span class="identifier">generate</span><span class="special">(</span><span class="identifier">outiter</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">strm</span> <span class="special">>></span> <span class="identifier">lambda</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> |
| 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 generating with Spirit. If |
| you didn't do so yet, it is probably a good idea to familiarize yourself |
| 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/karma/actions.cpp" target="_top">actions.cpp</a> |
| </p> |
| <a name="spirit.karma.tutorials.semantic_actions.phoenix"></a><h6> |
| <a name="id958407"></a> |
| <a class="link" href="semantic_actions.html#spirit.karma.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 generation, I suggest you use this library. Examples presented |
| henceforth shall be using the Phoenix library exclusively. |
| </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.Karma</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">karma</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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