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- </div>
- <div class="section">
- <div class="titlepage"><div><div><h3 class="title">
- <a name="phoenix.examples.extending_actors"></a><a class="link" href="extending_actors.html" title="Extending Actors">Extending Actors</a>
- </h3></div></div></div>
- <p>
- <a class="link" href="../inside/actor.html" title="Actors in Detail">Actors</a> are one of the main parts
- of the library, and one of the many customization points. The default actor
- implementation provides several operator() overloads which deal with the
- evaluation of expressions.
- </p>
- <p>
- For some use cases this might not be enough. For convenience it is thinkable
- to provide custom member functions which generate new expressions. An example
- is the <a class="link" href="../modules/statement/___if_else_____statement.html" title="if_else_ Statement">if_else_
- Statement</a>
- which provides an additional else member for generating a lazy if-else expression.
- With this the actual Phoenix expression becomes more expressive.
- </p>
- <p>
- Another scenario is to give actors the semantics of a certain well known
- interface or concept. This tutorial like section will provide information
- on how to implement a custom actor which is usable as if it were a <a href="http://www.sgi.com/tech/stl/Container.html" target="_top">STL Container</a>.
- </p>
- <a name="phoenix.examples.extending_actors.requirements"></a><h5>
- <a name="id859806"></a>
- <a class="link" href="extending_actors.html#phoenix.examples.extending_actors.requirements">Requirements</a>
- </h5>
- <p>
- Let's repeat what we want to have:
- </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">begin</span><span class="special">()</span></code>
- </p>
- </td>
- <td>
- <p>
- Returns an iterator pointing to the first element in the container.
- </p>
- </td>
- </tr>
- <tr>
- <td>
- <p>
- <code class="computeroutput"><span class="identifier">a</span><span class="special">.</span><span class="identifier">end</span><span class="special">()</span></code>
- </p>
- </td>
- <td>
- <p>
- Returns an iterator pointing one past the last element in the container.
- </p>
- </td>
- </tr>
- <tr>
- <td>
- <p>
- <code class="computeroutput"><span class="identifier">a</span><span class="special">.</span><span class="identifier">size</span><span class="special">()</span></code>
- </p>
- </td>
- <td>
- <p>
- Returns the size of the container, that is, its number of elements.
- </p>
- </td>
- </tr>
- <tr>
- <td>
- <p>
- <code class="computeroutput"><span class="identifier">a</span><span class="special">.</span><span class="identifier">max_size</span><span class="special">()</span></code>
- </p>
- </td>
- <td>
- <p>
- Returns the largest size that this container can ever have.
- </p>
- </td>
- </tr>
- <tr>
- <td>
- <p>
- <code class="computeroutput"><span class="identifier">a</span><span class="special">.</span><span class="identifier">empty</span><span class="special">()</span></code>
- </p>
- </td>
- <td>
- <p>
- Equivalent to a.size() == 0. (But possibly faster.)
- </p>
- </td>
- </tr>
- <tr>
- <td>
- <p>
- <code class="computeroutput"><span class="identifier">a</span><span class="special">.</span><span class="identifier">swap</span><span class="special">(</span><span class="identifier">b</span><span class="special">)</span></code>
- </p>
- </td>
- <td>
- <p>
- Equivalent to swap(a,b)
- </p>
- </td>
- </tr>
- </tbody>
- </table></div>
- <p>
- Additionally, we want all the operator() overloads of the regular actor.
- </p>
- <a name="phoenix.examples.extending_actors.defining_the_actor"></a><h5>
- <a name="id860119"></a>
- <a class="link" href="extending_actors.html#phoenix.examples.extending_actors.defining_the_actor">Defining
- the actor</a>
- </h5>
- <p>
- The first version of our <code class="computeroutput"><span class="identifier">container_actor</span></code>
- interface will show the general principle. This will be continually extended.
- For the sake of simplicity, every member function generator will return
- <a class="link" href="../modules/core/nothing.html" title="Nothing"><code class="computeroutput"><span class="identifier">nothing</span></code></a>
- at first.
- </p>
- <pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">></span>
- <span class="keyword">struct</span> <span class="identifier">container_actor</span>
- <span class="special">:</span> <span class="identifier">actor</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">></span>
- <span class="special">{</span>
- <span class="keyword">typedef</span> <span class="identifier">actor</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">></span> <span class="identifier">base_type</span><span class="special">;</span>
- <span class="keyword">typedef</span> <span class="identifier">container_actor</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">></span> <span class="identifier">that_type</span><span class="special">;</span>
-
- <span class="identifier">container_actor</span><span class="special">(</span> <span class="identifier">base_type</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">base</span> <span class="special">)</span>
- <span class="special">:</span> <span class="identifier">base_type</span><span class="special">(</span> <span class="identifier">base</span> <span class="special">)</span> <span class="special">{}</span>
- <span class="identifier">expression</span><span class="special">::</span><span class="identifier">null</span><span class="special"><</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">void_</span><span class="special">>::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">begin</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">{</span> <span class="keyword">return</span> <span class="identifier">nothing</span><span class="special">;</span> <span class="special">}</span>
- <span class="identifier">expression</span><span class="special">::</span><span class="identifier">null</span><span class="special"><</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">void_</span><span class="special">>::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">end</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">{</span> <span class="keyword">return</span> <span class="identifier">nothing</span><span class="special">;</span> <span class="special">}</span>
- <span class="identifier">expression</span><span class="special">::</span><span class="identifier">null</span><span class="special"><</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">void_</span><span class="special">>::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">size</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">{</span> <span class="keyword">return</span> <span class="identifier">nothing</span><span class="special">;</span> <span class="special">}</span>
- <span class="identifier">expression</span><span class="special">::</span><span class="identifier">null</span><span class="special"><</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">void_</span><span class="special">>::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">max_size</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">{</span> <span class="keyword">return</span> <span class="identifier">nothing</span><span class="special">;</span> <span class="special">}</span>
- <span class="identifier">expression</span><span class="special">::</span><span class="identifier">null</span><span class="special"><</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">void_</span><span class="special">>::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">empty</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">{</span> <span class="keyword">return</span> <span class="identifier">nothing</span><span class="special">;</span> <span class="special">}</span>
- <span class="comment">// Note that swap is the only function needing another container.
- </span> <span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Container</span><span class="special">></span>
- <span class="identifier">expression</span><span class="special">::</span><span class="identifier">null</span><span class="special"><</span><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">void_</span><span class="special">>::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">swap</span><span class="special">(</span> <span class="identifier">actor</span><span class="special"><</span><span class="identifier">Container</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="special">)</span> <span class="keyword">const</span> <span class="special">{</span> <span class="keyword">return</span> <span class="identifier">nothing</span><span class="special">;</span> <span class="special">}</span>
- <span class="special">};</span>
- </pre>
- <a name="phoenix.examples.extending_actors.using_the_actor"></a><h5>
- <a name="id861004"></a>
- <a class="link" href="extending_actors.html#phoenix.examples.extending_actors.using_the_actor">Using the
- actor</a>
- </h5>
- <p>
- Although the member functions do nothing right now, we want to test if we
- can use our new actor.
- </p>
- <p>
- First, lets create a generator which wraps the <code class="computeroutput"><span class="identifier">container_actor</span></code>
- around any other expression:
- </p>
- <pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">></span>
- <span class="identifier">container_actor</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">></span> <span class="keyword">const</span>
- <span class="identifier">container</span><span class="special">(</span> <span class="identifier">actor</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">expr</span> <span class="special">)</span>
- <span class="special">{</span>
- <span class="keyword">return</span> <span class="identifier">expr</span><span class="special">;</span>
- <span class="special">}</span>
- </pre>
- <p>
- Now let's test this:
- </p>
- <pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">v</span><span class="special">;</span>
- <span class="identifier">v</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="number">0</span><span class="special">);</span>
- <span class="identifier">v</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="number">1</span><span class="special">);</span>
- <span class="identifier">v</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="number">2</span><span class="special">);</span>
- <span class="identifier">v</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="number">3</span><span class="special">);</span>
- <span class="special">(</span><span class="identifier">container</span><span class="special">(</span><span class="identifier">arg1</span><span class="special">).</span><span class="identifier">size</span><span class="special">())(</span><span class="identifier">v</span><span class="special">);</span>
- </pre>
- <p>
- Granted, this is not really elegant and not very practical (we could have
- just used phoenix::begin(v) from the <a class="link" href="../modules/stl/algorithm.html" title="Algorithm">Phoenix
- algorithm module</a>, but we can do better.
- </p>
- <p>
- Let's have an <a class="link" href="../modules/core/arguments.html" title="Arguments">argument placeholder</a>
- which is usable as if it was a STL container:
- </p>
- <pre class="programlisting"><span class="identifier">container_actor</span><span class="special"><</span><span class="identifier">expression</span><span class="special">::</span><span class="identifier">argument</span><span class="special"><</span><span class="number">1</span><span class="special">>::</span><span class="identifier">type</span><span class="special">></span> <span class="keyword">const</span> <span class="identifier">con1</span><span class="special">;</span>
- <span class="comment">// and so on ...
- </span></pre>
- <p>
- The above example can be rewritten as:
- </p>
- <pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">v</span><span class="special">;</span>
- <span class="identifier">v</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="number">0</span><span class="special">);</span>
- <span class="identifier">v</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="number">1</span><span class="special">);</span>
- <span class="identifier">v</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="number">2</span><span class="special">);</span>
- <span class="identifier">v</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="number">3</span><span class="special">);</span>
- <span class="special">(</span><span class="identifier">con1</span><span class="special">.</span><span class="identifier">size</span><span class="special">())(</span><span class="identifier">v</span><span class="special">);</span>
- </pre>
- <p>
- Wow, that was easy!
- </p>
- <a name="phoenix.examples.extending_actors.adding_life_to_the_actor"></a><h5>
- <a name="id861671"></a>
- <a class="link" href="extending_actors.html#phoenix.examples.extending_actors.adding_life_to_the_actor">Adding
- life to the actor</a>
- </h5>
- <p>
- This one will be even easier!
- </p>
- <p>
- First, we define a <a class="link" href="../modules/function.html" title="Function">lazy function</a>
- which evaluates the expression we want to implement. Following is the implementation
- of the size function:
- </p>
- <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">size_impl</span>
- <span class="special">{</span>
- <span class="comment">// result_of protocol:
- </span> <span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Sig</span><span class="special">></span>
- <span class="keyword">struct</span> <span class="identifier">result</span><span class="special">;</span>
- <span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">This</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Container</span><span class="special">></span>
- <span class="keyword">struct</span> <span class="identifier">result</span><span class="special"><</span><span class="identifier">This</span><span class="special">(</span><span class="identifier">Container</span><span class="special">)></span>
- <span class="special">{</span>
- <span class="comment">// Note, remove reference here, because Container can be anything
- </span> <span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">remove_reference</span><span class="special"><</span><span class="identifier">Container</span><span class="special">>::</span><span class="identifier">type</span> <span class="identifier">container_type</span><span class="special">;</span>
- <span class="comment">// The result will be size_type
- </span> <span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">container_type</span><span class="special">::</span><span class="identifier">size_type</span> <span class="identifier">type</span><span class="special">;</span>
- <span class="special">};</span>
- <span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Container</span><span class="special">></span>
- <span class="keyword">typename</span> <span class="identifier">result</span><span class="special"><</span><span class="identifier">size_impl</span><span class="special">(</span><span class="identifier">Container</span> <span class="keyword">const</span><span class="special">&)>::</span><span class="identifier">type</span>
- <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">Container</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">container</span><span class="special">)</span> <span class="keyword">const</span>
- <span class="special">{</span>
- <span class="keyword">return</span> <span class="identifier">container</span><span class="special">.</span><span class="identifier">size</span><span class="special">();</span>
- <span class="special">}</span>
- <span class="special">};</span>
- </pre>
- <p>
- Good, this was the first part. The second part will be to implement the size
- member function of <code class="computeroutput"><span class="identifier">container_actor</span></code>:
- </p>
- <pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">></span>
- <span class="keyword">struct</span> <span class="identifier">container_actor</span>
- <span class="special">:</span> <span class="identifier">actor</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">></span>
- <span class="special">{</span>
- <span class="keyword">typedef</span> <span class="identifier">actor</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">></span> <span class="identifier">base_type</span><span class="special">;</span>
- <span class="keyword">typedef</span> <span class="identifier">container_actor</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">></span> <span class="identifier">that_type</span><span class="special">;</span>
-
- <span class="identifier">container_actor</span><span class="special">(</span> <span class="identifier">base_type</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">base</span> <span class="special">)</span>
- <span class="special">:</span> <span class="identifier">base_type</span><span class="special">(</span> <span class="identifier">base</span> <span class="special">)</span> <span class="special">{}</span>
- <span class="keyword">typename</span> <span class="identifier">expression</span><span class="special">::</span><span class="identifier">function</span><span class="special"><</span><span class="identifier">size_impl</span><span class="special">,</span> <span class="identifier">that_type</span><span class="special">>::</span><span class="identifier">type</span> <span class="keyword">const</span>
- <span class="identifier">size</span><span class="special">()</span> <span class="keyword">const</span>
- <span class="special">{</span>
- <span class="identifier">function</span><span class="special"><</span><span class="identifier">size_impl</span><span class="special">></span> <span class="keyword">const</span> <span class="identifier">f</span> <span class="special">=</span> <span class="identifier">size_impl</span><span class="special">();</span>
- <span class="keyword">return</span> <span class="identifier">f</span><span class="special">(*</span><span class="keyword">this</span><span class="special">);</span>
- <span class="special">}</span>
- <span class="comment">// the rest ...
- </span><span class="special">};</span>
- </pre>
- <p>
- It is left as an exercise to the user to implement the missing parts by reusing
- functions from the <a class="link" href="../modules/stl/algorithm.html" title="Algorithm">Phoenix
- Algorithm Module</a> (the impatient take a look here: <a href="../../../../example/container_actor.cpp" target="_top">container_actor.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 © 2002-2005, 2010 Joel de Guzman, Dan Marsden, Thomas Heller<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>
- <hr>
- <div class="spirit-nav">
- <a accesskey="p" href="../examples.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../examples.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="adding_an_expression.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
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