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<html>
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<title>Actors in Detail</title>
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<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="phoenix.inside.actor"></a><a class="link" href="actor.html" title="Actors in Detail">Actors in Detail</a>
</h3></div></div></div>
<h5>
<a name="phoenix.inside.actor.h0"></a>
<span class="phrase"><a name="phoenix.inside.actor.actor"></a></span><a class="link" href="actor.html#phoenix.inside.actor.actor">Actor</a>
</h5>
<p>
The main concept is the <code class="computeroutput"><span class="identifier">Actor</span></code>.
An <code class="computeroutput"><span class="identifier">Actor</span></code> is a model of the
<a href="http://www.boost.org/doc/libs/release/libs/fusion/doc/html/fusion/functional/concepts/poly.html" target="_top">Polymorphic
Function Object</a> concept (that can accept 0 to N arguments (where
N is a predefined maximum).
</p>
<p>
An <code class="computeroutput"><span class="identifier">Actor</span></code> contains a valid
Phoenix Expression, a call to one of the function call operator overloads,
starts the evaluation process.
</p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
You can set <code class="computeroutput"><span class="identifier">BOOST_PHOENIX_LIMIT</span></code>,
the predefined maximum arity an actor can take. By default, <code class="computeroutput"><span class="identifier">BOOST_PHOENIX_LIMIT</span></code> is set to 10.
</p></td></tr>
</table></div>
<p>
The <code class="computeroutput"><span class="identifier">actor</span></code> template class
models the <code class="computeroutput"><span class="identifier">Actor</span></code> concept:
</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">actor</span>
<span class="special">{</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">typename</span> <span class="identifier">result_of</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="identifier">type</span>
<span class="keyword">operator</span><span class="special">()()</span> <span class="keyword">const</span><span class="special">;</span>
<span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T0</span><span class="special">></span>
<span class="keyword">typename</span> <span class="identifier">result_of</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="identifier">T0</span> <span class="special">&>::</span><span class="identifier">type</span>
<span class="keyword">operator</span><span class="special">()(</span><span class="identifier">T0</span><span class="special">&</span> <span class="identifier">_0</span><span class="special">)</span> <span class="keyword">const</span><span class="special">;</span>
<span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T0</span><span class="special">></span>
<span class="keyword">typename</span> <span class="identifier">result_of</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="identifier">T0</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">T0</span> <span class="keyword">const</span> <span class="special">&</span> <span class="identifier">_0</span><span class="special">)</span> <span class="keyword">const</span><span class="special">;</span>
<span class="comment">//...</span>
<span class="special">};</span>
</pre>
<div class="table">
<a name="phoenix.inside.actor.t0"></a><p class="title"><b>Table 1.9. Actor Concept Requirements</b></p>
<div class="table-contents"><table class="table" summary="Actor Concept Requirements">
<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">actor</span><span class="special">(</span><span class="identifier">arg0</span><span class="special">,</span>
<span class="identifier">arg1</span><span class="special">,</span>
<span class="special">...,</span> <span class="identifier">argN</span><span class="special">)</span></code>
</p>
</td>
<td>
<p>
Function call operators to start the evaluation
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">result_of</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="identifier">Arg0</span><span class="special">,</span>
<span class="identifier">Arg1</span><span class="special">,</span>
<span class="special">...,</span> <span class="identifier">ArgN</span><span class="special">)>::</span><span class="identifier">type</span></code>
</p>
</td>
<td>
<p>
Result of the evaluation
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">result_of</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="identifier">Arg0</span><span class="special">,</span>
<span class="identifier">Arg1</span><span class="special">,</span>
<span class="special">...,</span> <span class="identifier">ArgN</span><span class="special">>::</span><span class="identifier">type</span></code>
</p>
</td>
<td>
<p>
Result of the evaluation
</p>
</td>
</tr>
</tbody>
</table></div>
</div>
<br class="table-break"><h5>
<a name="phoenix.inside.actor.h1"></a>
<span class="phrase"><a name="phoenix.inside.actor.function_call_operators"></a></span><a class="link" href="actor.html#phoenix.inside.actor.function_call_operators">Function
Call Operators</a>
</h5>
<p>
There are 2*N function call operators for 0 to N arguments (N == <code class="computeroutput"><span class="identifier">BOOST_PHOENIX_LIMIT</span></code>). The actor class accepts
the arguments and forwards the arguments to the default evaluation action.
</p>
<p>
Additionally, there exist function call operators accepting permutations
of const and non-const references. These operators are created for all N
<= <code class="computeroutput"><span class="identifier">BOOST_PHOENIX_PERFECT_FORWARD_LIMIT</span></code>
(which defaults to 3).
</p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top">
<p>
<span class="bold"><strong>Forwarding Function Problem</strong></span>
</p>
<p>
There is a known issue with current C++ called the "<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2002/n1385.htm" target="_top">Forwarding
Function Problem</a>". The problem is that given an arbitrary
function <code class="computeroutput"><span class="identifier">F</span></code>, using current
C++ language rules, one cannot create a forwarding function <code class="computeroutput"><span class="identifier">FF</span></code> that transparently assumes the arguments
of <code class="computeroutput"><span class="identifier">F</span></code>.
</p>
</td></tr>
</table></div>
<h5>
<a name="phoenix.inside.actor.h2"></a>
<span class="phrase"><a name="phoenix.inside.actor.context"></a></span><a class="link" href="actor.html#phoenix.inside.actor.context">Context</a>
</h5>
<p>
On an actor function call, before calling the evaluation function, the actor
created a <span class="bold"><strong>context</strong></span>. This context consists
of an <code class="computeroutput"><span class="identifier">Environment</span></code> and an
<code class="computeroutput"><span class="identifier">Action</span></code> part. These contain
all information necessary to evaluate the given expression.
</p>
<div class="table">
<a name="phoenix.inside.actor.t1"></a><p class="title"><b>Table 1.10. Context Concept Requirements</b></p>
<div class="table-contents"><table class="table" summary="Context Concept Requirements">
<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">result_of</span><span class="special">::</span><span class="identifier">context</span><span class="special"><</span><span class="identifier">Env</span><span class="special">,</span>
<span class="identifier">Actions</span><span class="special">>::</span><span class="identifier">type</span></code>
</p>
</td>
<td>
<p>
Type of a Context
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">context</span><span class="special">(</span><span class="identifier">e</span><span class="special">,</span>
<span class="identifier">a</span><span class="special">)</span></code>
</p>
</td>
<td>
<p>
A Context containing environment <code class="computeroutput"><span class="identifier">e</span></code>
and actions <code class="computeroutput"><span class="identifier">a</span></code>
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">env</span><span class="special"><</span><span class="identifier">Context</span><span class="special">>::</span><span class="identifier">type</span></code>
</p>
</td>
<td>
<p>
Type of the contained Environment
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">env</span><span class="special">(</span><span class="identifier">ctx</span><span class="special">)</span></code>
</p>
</td>
<td>
<p>
The environment
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">actions</span><span class="special"><</span><span class="identifier">Context</span><span class="special">>::</span><span class="identifier">type</span></code>
</p>
</td>
<td>
<p>
Type of the contained Actions
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">actions</span><span class="special">(</span><span class="identifier">ctx</span><span class="special">)</span></code>
</p>
</td>
<td>
<p>
The actions
</p>
</td>
</tr>
</tbody>
</table></div>
</div>
<br class="table-break"><h5>
<a name="phoenix.inside.actor.h3"></a>
<span class="phrase"><a name="phoenix.inside.actor.environment"></a></span><a class="link" href="actor.html#phoenix.inside.actor.environment">Environment</a>
</h5>
<p>
The Environment is a model of <a href="http://www.boost.org/doc/libs/release/libs/fusion/doc/html/fusion/sequence/concepts/random_access_sequence.html" target="_top">Random
Access Sequence</a>.
</p>
<p>
The arguments passed to the actor's function call operator are collected
inside the Environment:
</p>
<p>
<span class="inlinemediaobject"><img src="../../images/funnel_in.png"></span>
</p>
<p>
Other parts of the library (e.g. the scope module) extends the <code class="computeroutput"><span class="identifier">Environment</span></code> concept to hold other information
such as local variables, etc.
</p>
<h5>
<a name="phoenix.inside.actor.h4"></a>
<span class="phrase"><a name="phoenix.inside.actor.actions"></a></span><a class="link" href="actor.html#phoenix.inside.actor.actions">Actions</a>
</h5>
<p>
Actions is the part of Phoenix which are responsible for giving the actual
expressions a specific behaviour. During the traversal of the Phoenix Expression
Tree these actions are called whenever a specified rule in the grammar matches.
</p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">actions</span>
<span class="special">{</span>
<span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Rule</span><span class="special">></span>
<span class="keyword">struct</span> <span class="identifier">when</span><span class="special">;</span>
<span class="special">};</span>
</pre>
<p>
The nested <code class="computeroutput"><span class="identifier">when</span></code> template
is required to be <a href="http://www.boost.org/doc/libs/release/doc/html/PrimitiveTransform.html" target="_top">Proto
Primitive Transform</a>. No worries, you don't have to learn <a href="http://www.boost.org/doc/libs/release/libs/proto/index.html" target="_top">Boost.Proto</a>
just yet! Phoenix provides some wrappers to let you define simple actions
without the need to dive deep into proto.
</p>
<p>
Phoenix ships with a predefined <code class="computeroutput"><span class="identifier">default_actions</span></code>
class that evaluates the expressions with C++ semantics:
</p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">default_actions</span>
<span class="special">{</span>
<span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Rule</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Dummy</span> <span class="special">=</span> <span class="keyword">void</span><span class="special">></span>
<span class="keyword">struct</span> <span class="identifier">when</span>
<span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">_default</span><span class="special"><</span><span class="identifier">meta_grammar</span><span class="special">></span>
<span class="special">{};</span>
<span class="special">};</span>
</pre>
<p>
For more information on how to use the default_actions class and how to attach
custom actions to the evaluation process, see <a class="link" href="actions.html" title="More on Actions">more
on actions</a>.
</p>
<h5>
<a name="phoenix.inside.actor.h5"></a>
<span class="phrase"><a name="phoenix.inside.actor.evaluation"></a></span><a class="link" href="actor.html#phoenix.inside.actor.evaluation">Evaluation</a>
</h5>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">evaluator</span>
<span class="special">{</span>
<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">typename</span> <span class="identifier">Context</span><span class="special">></span>
<span class="emphasis"><em>unspecified</em></span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">Expr</span> <span class="special">&,</span> <span class="identifier">Context</span> <span class="special">&);</span>
<span class="special">};</span>
<span class="identifier">evaluator</span> <span class="keyword">const</span> <span class="identifier">eval</span> <span class="special">=</span> <span class="special">{};</span>
</pre>
<p>
The evaluation of a Phoenix expression is started by a call to the function
call operator of <code class="computeroutput"><span class="identifier">evaluator</span></code>.
</p>
<p>
The evaluator is called by the <code class="computeroutput"><span class="identifier">actor</span></code>
function operator overloads after the context is built up. For reference,
here is a typical <code class="computeroutput"><span class="identifier">actor</span><span class="special">::</span><span class="keyword">operator</span><span class="special">()</span></code>
that accepts two arguments:
</p>
<pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T0</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">T1</span><span class="special">></span>
<span class="keyword">typename</span> <span class="identifier">result_of</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="identifier">T0</span> <span class="special">&,</span> <span class="identifier">T1</span> <span class="special">&>::</span><span class="identifier">type</span>
<span class="keyword">operator</span><span class="special">()(</span><span class="identifier">T0</span> <span class="special">&</span><span class="identifier">t0</span><span class="special">,</span> <span class="identifier">T1</span> <span class="special">&</span><span class="identifier">t1</span><span class="special">)</span> <span class="keyword">const</span>
<span class="special">{</span>
<span class="identifier">fusion</span><span class="special">::</span><span class="identifier">vector2</span><span class="special"><</span><span class="identifier">T0</span> <span class="special">&,</span> <span class="identifier">T1</span> <span class="special">&></span> <span class="identifier">env</span><span class="special">(</span><span class="identifier">t0</span><span class="special">,</span> <span class="identifier">t1</span><span class="special">);</span>
<span class="keyword">return</span> <span class="identifier">eval</span><span class="special">(*</span><span class="keyword">this</span><span class="special">,</span> <span class="identifier">context</span><span class="special">(</span><span class="identifier">env</span><span class="special">,</span> <span class="identifier">default_actions</span><span class="special">()));</span>
<span class="special">}</span>
</pre>
<h5>
<a name="phoenix.inside.actor.h6"></a>
<span class="phrase"><a name="phoenix.inside.actor.result_of__actor"></a></span><a class="link" href="actor.html#phoenix.inside.actor.result_of__actor">result_of::actor</a>
</h5>
<p>
For reasons of symmetry to the family of <code class="computeroutput"><span class="identifier">actor</span><span class="special">::</span><span class="keyword">operator</span><span class="special">()</span></code> there is a special metafunction usable
for actor result type calculation named <code class="computeroutput"><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">actor</span></code>.
This metafunction allows us to directly specify the types of the parameters
to be passed to the <code class="computeroutput"><span class="identifier">actor</span><span class="special">::</span><span class="keyword">operator</span><span class="special">()</span></code> function. Here's a typical <code class="computeroutput"><span class="identifier">actor_result</span></code> that accepts two arguments:
</p>
<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">result_of</span>
<span class="special">{</span>
<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">typename</span> <span class="identifier">T0</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">T1</span><span class="special">></span>
<span class="keyword">struct</span> <span class="identifier">actor</span>
<span class="special">{</span>
<span class="keyword">typedef</span> <span class="identifier">fusion</span><span class="special">::</span><span class="identifier">vector2</span><span class="special"><</span><span class="identifier">T0</span><span class="special">,</span> <span class="identifier">T1</span><span class="special">></span> <span class="identifier">env_tpe</span><span class="special">;</span>
<span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">result_of</span><span class="special">::</span><span class="identifier">context</span><span class="special"><</span><span class="identifier">env_type</span><span class="special">,</span> <span class="identifier">default_actions</span><span class="special">>::</span><span class="identifier">type</span> <span class="identifier">ctx_type</span>
<span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">result_of</span><span class="special"><</span><span class="identifier">evaluator</span><span class="special">(</span><span class="identifier">Expr</span> <span class="keyword">const</span><span class="special">&,</span> <span class="identifier">ctx_type</span><span class="special">)>::</span><span class="identifier">type</span> <span class="identifier">type</span><span class="special">;</span>
<span class="special">};</span>
<span class="special">}</span>
</pre>
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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>)
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