| <html> |
| <head> |
| <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> |
| <title>Inside Phoenix</title> |
| <link rel="stylesheet" href="../../../../../../doc/src/boostbook.css" type="text/css"> |
| <meta name="generator" content="DocBook XSL Stylesheets V1.75.0"> |
| <link rel="home" href="../index.html" title="Chapter 1. Phoenix 2.0"> |
| <link rel="up" href="../index.html" title="Chapter 1. Phoenix 2.0"> |
| <link rel="prev" href="algorithm.html" title="Algorithm"> |
| <link rel="next" href="wrap_up.html" title="Wrap Up"> |
| </head> |
| <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> |
| <table cellpadding="2" width="100%"><tr> |
| <td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../boost.png"></td> |
| <td align="center"><a href="../../../../../../index.html">Home</a></td> |
| <td align="center"><a href="../../../../../../libs/libraries.htm">Libraries</a></td> |
| <td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> |
| <td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> |
| <td align="center"><a href="../../../../../../more/index.htm">More</a></td> |
| </tr></table> |
| <hr> |
| <div class="spirit-nav"> |
| <a accesskey="p" href="algorithm.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.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="wrap_up.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a> |
| </div> |
| <div class="section"> |
| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <a name="phoenix.inside_phoenix"></a><a class="link" href="inside_phoenix.html" title="Inside Phoenix">Inside Phoenix</a> |
| </h2></div></div></div> |
| <div class="toc"><dl> |
| <dt><span class="section"><a href="inside_phoenix.html#phoenix.inside_phoenix.actors_in_detail">Actors In Detail</a></span></dt> |
| <dt><span class="section"><a href="inside_phoenix.html#phoenix.inside_phoenix.actor_example">Actor Example</a></span></dt> |
| <dt><span class="section"><a href="inside_phoenix.html#phoenix.inside_phoenix.composites_in_detail">Composites |
| In Detail</a></span></dt> |
| <dt><span class="section"><a href="inside_phoenix.html#phoenix.inside_phoenix.composing">Composing</a></span></dt> |
| <dt><span class="section"><a href="inside_phoenix.html#phoenix.inside_phoenix.extending">Extending</a></span></dt> |
| </dl></div> |
| <p> |
| This chapter explains in more detail how the library operates. The information |
| henceforth should not be necessary to those who are interested in just using |
| the library. However, a microscopic view might prove to be beneficial to moderate |
| to advanced programmers who wish to extend the library. |
| </p> |
| <div class="section"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="phoenix.inside_phoenix.actors_in_detail"></a><a class="link" href="inside_phoenix.html#phoenix.inside_phoenix.actors_in_detail" title="Actors In Detail">Actors In Detail</a> |
| </h3></div></div></div> |
| <a name="phoenix.inside_phoenix.actors_in_detail.actor_concept"></a><h4> |
| <a name="id868040"></a> |
| <a class="link" href="inside_phoenix.html#phoenix.inside_phoenix.actors_in_detail.actor_concept">Actor |
| Concept</a> |
| </h4> |
| <p> |
| The main concept is the <code class="computeroutput"><span class="identifier">Actor</span></code>. |
| Actors are function objects (that can accept 0 to N arguments (where N is |
| a predefined maximum). |
| </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">PHOENIX_LIMIT</span></code>, |
| the predefined maximum arity an actor can take. By default, <code class="computeroutput"><span class="identifier">PHOENIX_LIMIT</span></code> is set to 10. |
| </p></td></tr> |
| </table></div> |
| <a name="phoenix.inside_phoenix.actors_in_detail.actor_template_class"></a><h4> |
| <a name="id868088"></a> |
| <a class="link" href="inside_phoenix.html#phoenix.inside_phoenix.actors_in_detail.actor_template_class">actor |
| template class</a> |
| </h4> |
| <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">Eval</span><span class="special">></span> |
| <span class="keyword">struct</span> <span class="identifier">actor</span> <span class="special">:</span> <span class="identifier">Eval</span> |
| <span class="special">{</span> |
| <span class="keyword">typedef</span> <span class="identifier">Eval</span> <span class="identifier">eval_type</span><span class="special">;</span> |
| |
| <span class="identifier">actor</span><span class="special">();</span> |
| <span class="identifier">actor</span><span class="special">(</span><span class="identifier">Eval</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">base</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">explicit</span> <span class="identifier">actor</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">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="identifier">actor</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="identifier">T1</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">_1</span><span class="special">);</span> |
| |
| <span class="comment">// more constructors |
| </span> |
| <span class="keyword">typename</span> <span class="identifier">apply_actor</span><span class="special"><</span><span class="identifier">eval_type</span><span class="special">,</span> <span class="identifier">basic_environment</span><span class="special"><></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">apply_actor</span><span class="special"><</span><span class="identifier">eval_type</span><span class="special">,</span> <span class="identifier">basic_environment</span><span class="special"><</span><span class="identifier">T0</span><span class="special">></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">T1</span><span class="special">></span> |
| <span class="keyword">typename</span> <span class="identifier">apply_actor</span><span class="special"><</span><span class="identifier">eval_type</span><span class="special">,</span> <span class="identifier">basic_environment</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="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="identifier">T1</span><span class="special">&</span> <span class="identifier">_1</span><span class="special">)</span> <span class="keyword">const</span><span class="special">;</span> |
| |
| <span class="comment">// function call operators |
| </span><span class="special">};</span> |
| </pre> |
| <div class="table"> |
| <a name="id869218"></a><p class="title"><b>Table 1.10. 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> |
| Result/Semantics |
| </p> |
| </th> |
| </tr></thead> |
| <tbody> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="identifier">T</span><span class="special">::</span><span class="identifier">eval_type</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| The actor's Eval type |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="identifier">T</span><span class="special">()</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| Default Constructor |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="identifier">T</span><span class="special">(</span><span class="identifier">base</span><span class="special">)</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| Constructor from Eval |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="identifier">T</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> |
| Pass through constructors |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="identifier">x</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 |
| </p> |
| </td> |
| </tr> |
| </tbody> |
| </table></div> |
| </div> |
| <br class="table-break"><a name="phoenix.inside_phoenix.actors_in_detail.eval_concept"></a><h4> |
| <a name="id869477"></a> |
| <a class="link" href="inside_phoenix.html#phoenix.inside_phoenix.actors_in_detail.eval_concept">Eval |
| Concept</a> |
| </h4> |
| <p> |
| The <code class="computeroutput"><span class="identifier">actor</span></code> template class |
| has a single template parameter, <code class="computeroutput"><span class="identifier">Eval</span></code>, |
| from which it derives from. While the <code class="computeroutput"><span class="identifier">Actor</span></code> |
| concept represents a function, the <code class="computeroutput"><span class="identifier">Eval</span></code> |
| concept represents the function body. The requirements for <code class="computeroutput"><span class="identifier">Eval</span></code> are intentionally kept simple, to |
| make it easy to write models of the concept. We shall see an example in the |
| <a class="link" href="inside_phoenix.html#phoenix.inside_phoenix.actor_example" title="Actor Example">next section</a>. |
| </p> |
| <div class="table"> |
| <a name="id869532"></a><p class="title"><b>Table 1.11. Eval Concept Requirements</b></p> |
| <div class="table-contents"><table class="table" summary="Eval Concept Requirements"> |
| <colgroup> |
| <col> |
| <col> |
| </colgroup> |
| <thead><tr> |
| <th> |
| <p> |
| Expression |
| </p> |
| </th> |
| <th> |
| <p> |
| Result/Semantics |
| </p> |
| </th> |
| </tr></thead> |
| <tbody> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="keyword">return</span> <span class="identifier">x</span><span class="special">.</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">env</span><span class="special">)</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| Evaluates the function (see Environment below) |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="identifier">T</span><span class="special">::</span><span class="identifier">result</span><span class="special"><</span><span class="identifier">Env</span><span class="special">>::</span><span class="identifier">type</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| The return type of eval (see Environment below) |
| </p> |
| </td> |
| </tr> |
| </tbody> |
| </table></div> |
| </div> |
| <br class="table-break"><a name="phoenix.inside_phoenix.actors_in_detail.constructors"></a><h4> |
| <a name="id869669"></a> |
| <a class="link" href="inside_phoenix.html#phoenix.inside_phoenix.actors_in_detail.constructors">Constructors</a> |
| </h4> |
| <p> |
| In addition to a default constructor and an constructor from a Eval object, |
| there are templated (pass through) constructors for 1 to N arguments (N == |
| <code class="computeroutput"><span class="identifier">PHOENIX_LIMIT</span></code>). These constructors |
| simply forward the arguments to the <code class="computeroutput"><span class="identifier">base</span></code>. |
| </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>Parametric Base Class Pattern</strong></span> |
| </p> |
| <p> |
| Notice that actor derives from its template argument Eval. This is the |
| inverse of the curiously recurring template pattern (CRTP). With the CRTP, |
| a class, T, has a Derived template parameter that is assumed to be its |
| subclass. The "parametric base class pattern" (PBCP), on the |
| other hand, inverses the inheritance and makes a class, T, the derived |
| class. Both CRTP and PBCP techniques have its pros and cons, which is outside |
| the scope of this document. CRTP should really be renamed "parametric |
| subclass pattern (PSCP), but again, that's another story. |
| </p> |
| </td></tr> |
| </table></div> |
| <a name="phoenix.inside_phoenix.actors_in_detail.function_call_operators"></a><h4> |
| <a name="id869718"></a> |
| <a class="link" href="inside_phoenix.html#phoenix.inside_phoenix.actors_in_detail.function_call_operators">Function |
| Call Operators</a> |
| </h4> |
| <p> |
| There are N function call operators for 0 to N arguments (N == <code class="computeroutput"><span class="identifier">PHOENIX_LIMIT</span></code>). The actor class accepts |
| the arguments and forwards the arguments to the actor's base <code class="computeroutput"><span class="identifier">Eval</span></code> for evaluation. |
| </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> |
| The function call operators cannot accept non-const temporaries and literal |
| constants. 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>. Disallowing non-const |
| rvalues arguments partially solves the problem but prohibits code such |
| as <code class="computeroutput"><span class="identifier">f</span><span class="special">(</span><span class="number">1</span><span class="special">,</span> <span class="number">2</span><span class="special">,</span> <span class="number">3</span><span class="special">);</span></code>. |
| </p> |
| </td></tr> |
| </table></div> |
| <a name="phoenix.inside_phoenix.actors_in_detail.environment"></a><h4> |
| <a name="id869830"></a> |
| <a class="link" href="inside_phoenix.html#phoenix.inside_phoenix.actors_in_detail.environment">Environment</a> |
| </h4> |
| <p> |
| On an actor function call, before calling the actor's <code class="computeroutput"><span class="identifier">Eval</span><span class="special">::</span><span class="identifier">eval</span></code> |
| for evaluation, the actor creates an <span class="emphasis"><em><span class="bold"><strong>environment</strong></span></em></span>. |
| Basically, the environment packages the arguments in a tuple. The <code class="computeroutput"><span class="identifier">Environment</span></code> is a concept, of which, the |
| <code class="computeroutput"><span class="identifier">basic_environment</span></code> template |
| class is a model of. |
| </p> |
| <div class="table"> |
| <a name="id869879"></a><p class="title"><b>Table 1.12. Environment Concept Requirements</b></p> |
| <div class="table-contents"><table class="table" summary="Environment Concept Requirements"> |
| <colgroup> |
| <col> |
| <col> |
| </colgroup> |
| <thead><tr> |
| <th> |
| <p> |
| Expression |
| </p> |
| </th> |
| <th> |
| <p> |
| Result/Semantics |
| </p> |
| </th> |
| </tr></thead> |
| <tbody> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="identifier">x</span><span class="special">.</span><span class="identifier">args</span><span class="special">()</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| The arguments in a tie (a tuple of references) |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="identifier">T</span><span class="special">::</span><span class="identifier">args_type</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| The arguments' types in an MPL sequence |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="identifier">T</span><span class="special">::</span><span class="identifier">tie_type</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| The tie (tuple of references) type |
| </p> |
| </td> |
| </tr> |
| </tbody> |
| </table></div> |
| </div> |
| <br class="table-break"><p> |
| Schematically: |
| </p> |
| <p> |
| <span class="inlinemediaobject"><img src="../images/funnel_in.png" alt="funnel_in"></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> |
| <a name="phoenix.inside_phoenix.actors_in_detail.apply_actor"></a><h4> |
| <a name="id870054"></a> |
| <a class="link" href="inside_phoenix.html#phoenix.inside_phoenix.actors_in_detail.apply_actor">apply_actor</a> |
| </h4> |
| <p> |
| <code class="computeroutput"><span class="identifier">apply_actor</span></code> is a standard |
| MPL style metafunction that simply calls the Action's <code class="computeroutput"><span class="identifier">result</span></code> |
| nested metafunction: |
| </p> |
| <pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Action</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Env</span><span class="special">></span> |
| <span class="keyword">struct</span> <span class="identifier">apply_actor</span> |
| <span class="special">{</span> |
| <span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">Action</span><span class="special">::</span><span class="keyword">template</span> <span class="identifier">result</span><span class="special"><</span><span class="identifier">Env</span><span class="special">>::</span><span class="identifier">type</span> <span class="identifier">type</span><span class="special">;</span> |
| <span class="special">};</span> |
| </pre> |
| <p> |
| After evaluating the arguments and doing some computation, the <code class="computeroutput"><span class="identifier">eval</span></code> member function returns something |
| back to the client. To do this, the forwarding function (the actor's <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code>) |
| needs to know the return type of the eval member function that it is calling. |
| For this purpose, models of <code class="computeroutput"><span class="identifier">Eval</span></code> |
| are required to provide a nested template class: |
| </p> |
| <pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Env</span><span class="special">></span> |
| <span class="keyword">struct</span> <span class="identifier">result</span><span class="special">;</span> |
| </pre> |
| <p> |
| This nested class provides the result type information returned by the <code class="computeroutput"><span class="identifier">Eval</span></code>'s <code class="computeroutput"><span class="identifier">eval</span></code> |
| member function. The nested template class <code class="computeroutput"><span class="identifier">result</span></code> |
| should have a typedef <code class="computeroutput"><span class="identifier">type</span></code> |
| that reflects the return type of its member function <code class="computeroutput"><span class="identifier">eval</span></code>. |
| </p> |
| <p> |
| For reference, here's 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">apply_actor</span><span class="special"><</span><span class="identifier">eval_type</span><span class="special">,</span> <span class="identifier">basic_environment</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="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="identifier">T1</span><span class="special">&</span> <span class="identifier">_1</span><span class="special">)</span> <span class="keyword">const</span> |
| <span class="special">{</span> |
| <span class="keyword">return</span> <span class="identifier">eval_type</span><span class="special">::</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">basic_environment</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">_0</span><span class="special">,</span> <span class="identifier">_1</span><span class="special">));</span> |
| <span class="special">}</span> |
| </pre> |
| <a name="phoenix.inside_phoenix.actors_in_detail.actor_result"></a><h4> |
| <a name="id870527"></a> |
| <a class="link" href="inside_phoenix.html#phoenix.inside_phoenix.actors_in_detail.actor_result">actor_result</a> |
| </h4> |
| <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">actor_result</span></code>. |
| This metafunction allows us to directly to 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">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Action</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_result</span> |
| <span class="special">{</span> |
| <span class="keyword">typedef</span> <span class="identifier">basic_environment</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_type</span><span class="special">;</span> |
| <span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">Action</span><span class="special">::</span><span class="keyword">template</span> <span class="identifier">result</span><span class="special"><</span><span class="identifier">env_type</span><span class="special">>::</span><span class="identifier">type</span> <span class="identifier">type</span><span class="special">;</span> |
| <span class="special">};</span> |
| </pre> |
| </div> |
| <div class="section"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="phoenix.inside_phoenix.actor_example"></a><a class="link" href="inside_phoenix.html#phoenix.inside_phoenix.actor_example" title="Actor Example">Actor Example</a> |
| </h3></div></div></div> |
| <p> |
| Let us see a very simple prototypical example of an actor. This is not a |
| toy example. This is actually part of the library. Remember the <a class="link" href="primitives.html#phoenix.primitives.references" title="References"><code class="computeroutput"><span class="identifier">reference</span></code></a>?. |
| </p> |
| <p> |
| First, we have a model of the <code class="computeroutput"><span class="identifier">Eval</span></code> |
| concept: the <code class="computeroutput"><span class="identifier">reference</span></code>: |
| </p> |
| <pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">></span> |
| <span class="keyword">struct</span> <span class="identifier">reference</span> |
| <span class="special">{</span> |
| <span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Env</span><span class="special">></span> |
| <span class="keyword">struct</span> <span class="identifier">result</span> |
| <span class="special">{</span> |
| <span class="keyword">typedef</span> <span class="identifier">T</span><span class="special">&</span> <span class="identifier">type</span><span class="special">;</span> |
| <span class="special">};</span> |
| |
| <span class="identifier">reference</span><span class="special">(</span><span class="identifier">T</span><span class="special">&</span> <span class="identifier">arg</span><span class="special">)</span> |
| <span class="special">:</span> <span class="identifier">ref</span><span class="special">(</span><span class="identifier">arg</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">Env</span><span class="special">></span> |
| <span class="identifier">T</span><span class="special">&</span> <span class="identifier">eval</span><span class="special">(</span><span class="identifier">Env</span> <span class="keyword">const</span><span class="special">&)</span> <span class="keyword">const</span> |
| <span class="special">{</span> |
| <span class="keyword">return</span> <span class="identifier">ref</span><span class="special">;</span> |
| <span class="special">}</span> |
| |
| <span class="identifier">T</span><span class="special">&</span> <span class="identifier">ref</span><span class="special">;</span> |
| <span class="special">};</span> |
| </pre> |
| <p> |
| Models of <code class="computeroutput"><span class="identifier">Eval</span></code> are never |
| created directly and its instances never exist alone. We have to wrap it |
| inside the <code class="computeroutput"><span class="identifier">actor</span></code> template |
| class to be useful. The <code class="computeroutput"><span class="identifier">ref</span></code> |
| template function does this for us: |
| </p> |
| <pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">></span> |
| <span class="identifier">actor</span><span class="special"><</span><span class="identifier">reference</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="special">></span> <span class="keyword">const</span> |
| <span class="identifier">ref</span><span class="special">(</span><span class="identifier">T</span><span class="special">&</span> <span class="identifier">v</span><span class="special">)</span> |
| <span class="special">{</span> |
| <span class="keyword">return</span> <span class="identifier">reference</span><span class="special"><</span><span class="identifier">T</span><span class="special">>(</span><span class="identifier">v</span><span class="special">);</span> |
| <span class="special">}</span> |
| </pre> |
| <p> |
| The <code class="computeroutput"><span class="identifier">reference</span></code> template class |
| conforms to the <a class="link" href="inside_phoenix.html#phoenix.inside_phoenix.actors_in_detail.eval_concept"><code class="computeroutput"><span class="identifier">Eval</span></code></a> concept. It has a nested <code class="computeroutput"><span class="identifier">result</span></code> metafunction that reflects the return |
| type of its <code class="computeroutput"><span class="identifier">eval</span></code> member function, |
| which performs the actual function. <code class="computeroutput"><span class="identifier">reference</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> |
| stores a reference to a <code class="computeroutput"><span class="identifier">T</span></code>. |
| Its <code class="computeroutput"><span class="identifier">eval</span></code> member function |
| simply returns the reference. It does not make use of the environment <code class="computeroutput"><span class="identifier">Env</span></code>. |
| </p> |
| <p> |
| <span class="emphasis"><em>Pretty simple...</em></span> |
| </p> |
| </div> |
| <div class="section"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="phoenix.inside_phoenix.composites_in_detail"></a><a class="link" href="inside_phoenix.html#phoenix.inside_phoenix.composites_in_detail" title="Composites In Detail">Composites |
| In Detail</a> |
| </h3></div></div></div> |
| <p> |
| We stated before that composites are actors that are composed of zero or |
| more actors (see <a class="link" href="composite.html" title="Composite">Composite</a>). This |
| is not quite accurate. The definition was sufficient at that point where |
| we opted to keep things simple and not bury the reader with details which |
| she might not need anyway. |
| </p> |
| <p> |
| Actually, a composite is a model of the <a class="link" href="inside_phoenix.html#phoenix.inside_phoenix.actors_in_detail.eval_concept"><code class="computeroutput"><span class="identifier">Eval</span></code></a> concept (more on this later). |
| At the same time, it is also composed of 0..N (where N is a predefined maximum) |
| <a class="link" href="inside_phoenix.html#phoenix.inside_phoenix.actors_in_detail.eval_concept"><code class="computeroutput"><span class="identifier">Eval</span></code></a> instances and an eval policy. |
| The individual <a class="link" href="inside_phoenix.html#phoenix.inside_phoenix.actors_in_detail.eval_concept"><code class="computeroutput"><span class="identifier">Eval</span></code></a> instances are stored in a tuple. |
| </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> |
| In a sense, the original definition of "composite", more or less, |
| will do just fine because <a class="link" href="inside_phoenix.html#phoenix.inside_phoenix.actors_in_detail.eval_concept"><code class="computeroutput"><span class="identifier">Eval</span></code></a> instances never exist alone |
| and are always wrapped in an <code class="computeroutput"><span class="identifier">actor</span></code> |
| template class which inherits from it anyway. The resulting actor IS-AN |
| <a class="link" href="inside_phoenix.html#phoenix.inside_phoenix.actors_in_detail.eval_concept"><code class="computeroutput"><span class="identifier">Eval</span></code></a>. |
| </p></td></tr> |
| </table></div> |
| <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">PHOENIX_COMPOSITE_LIMIT</span></code>, |
| the predefined maximum <code class="computeroutput"><span class="identifier">Eval</span></code>s |
| (actors) a composite can take. By default, <code class="computeroutput"><span class="identifier">PHOENIX_COMPOSITE_LIMIT</span></code> |
| is set to <code class="computeroutput"><span class="identifier">PHOENIX_LIMIT</span></code> |
| (See <a class="link" href="actors.html" title="Actors">Actors</a>). |
| </p></td></tr> |
| </table></div> |
| <a name="phoenix.inside_phoenix.composites_in_detail.composite_template_class"></a><h3> |
| <a name="id872570"></a> |
| <a class="link" href="inside_phoenix.html#phoenix.inside_phoenix.composites_in_detail.composite_template_class">composite |
| template class</a> |
| </h3> |
| <pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">EvalPolicy</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">EvalTuple</span><span class="special">></span> |
| <span class="keyword">struct</span> <span class="identifier">composite</span> <span class="special">:</span> <span class="identifier">EvalTuple</span> |
| <span class="special">{</span> |
| <span class="keyword">typedef</span> <span class="identifier">EvalTuple</span> <span class="identifier">base_type</span><span class="special">;</span> |
| <span class="keyword">typedef</span> <span class="identifier">EvalPolicy</span> <span class="identifier">eval_policy_type</span><span class="special">;</span> |
| |
| <span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Env</span><span class="special">></span> |
| <span class="keyword">struct</span> <span class="identifier">result</span> |
| <span class="special">{</span> |
| <span class="keyword">typedef</span> <span class="identifier">implementation</span><span class="special">-</span><span class="identifier">defined</span> <span class="identifier">type</span><span class="special">;</span> |
| <span class="special">};</span> |
| |
| <span class="identifier">composite</span><span class="special">();</span> |
| <span class="identifier">composite</span><span class="special">(</span><span class="identifier">base_type</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">actors</span><span class="special">);</span> |
| |
| <span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">U0</span><span class="special">></span> |
| <span class="identifier">composite</span><span class="special">(</span><span class="identifier">U0</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">_0</span><span class="special">);</span> |
| |
| <span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">U0</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">U1</span><span class="special">></span> |
| <span class="identifier">composite</span><span class="special">(</span><span class="identifier">U0</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">_0</span><span class="special">,</span> <span class="identifier">U1</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">_1</span><span class="special">);</span> |
| |
| <span class="comment">// more constructors |
| </span> |
| <span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Env</span><span class="special">></span> |
| <span class="keyword">typename</span> <span class="identifier">result</span><span class="special"><</span><span class="identifier">Env</span><span class="special">>::</span><span class="identifier">type</span> |
| <span class="identifier">eval</span><span class="special">(</span><span class="identifier">Env</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">env</span><span class="special">)</span> <span class="keyword">const</span><span class="special">;</span> |
| <span class="special">};</span> |
| </pre> |
| <a name="phoenix.inside_phoenix.composites_in_detail.evaltuple"></a><h3> |
| <a name="id873013"></a> |
| <a class="link" href="inside_phoenix.html#phoenix.inside_phoenix.composites_in_detail.evaltuple">EvalTuple</a> |
| </h3> |
| <p> |
| <code class="computeroutput"><span class="identifier">EvalTuple</span></code>, holds all the |
| <a class="link" href="inside_phoenix.html#phoenix.inside_phoenix.actors_in_detail.eval_concept"><code class="computeroutput"><span class="identifier">Eval</span></code></a> instances. The <code class="computeroutput"><span class="identifier">composite</span></code> template class inherits from |
| it. In addition to a default constructor and a constructor from an <code class="computeroutput"><span class="identifier">EvalTuple</span></code> object, there are templated (pass |
| through) constructors for 1 to N arguments (again, where N == <code class="computeroutput"><span class="identifier">PHOENIX_COMPOSITE_LIMIT</span></code>). These constructors |
| simply forward the arguments to the <code class="computeroutput"><span class="identifier">EvalTuple</span></code> |
| base class. |
| </p> |
| <a name="phoenix.inside_phoenix.composites_in_detail.evalpolicy"></a><h3> |
| <a name="id873075"></a> |
| <a class="link" href="inside_phoenix.html#phoenix.inside_phoenix.composites_in_detail.evalpolicy">EvalPolicy</a> |
| </h3> |
| <p> |
| The composite's <code class="computeroutput"><span class="identifier">eval</span></code> member |
| function calls its <code class="computeroutput"><span class="identifier">EvalPolicy</span></code>'s |
| <code class="computeroutput"><span class="identifier">eval</span></code> member function (a static |
| member function) passing in the <a class="link" href="inside_phoenix.html#phoenix.inside_phoenix.actors_in_detail.environment">environment</a> |
| and each of its actors, in parallel. The following diagram illustrates what's |
| happening: |
| </p> |
| <p> |
| <span class="inlinemediaobject"><img src="../images/funnel_out.png" alt="funnel_out"></span> |
| </p> |
| <div class="table"> |
| <a name="id873133"></a><p class="title"><b>Table 1.13. EvalPolicy Requirements</b></p> |
| <div class="table-contents"><table class="table" summary="EvalPolicy Requirements"> |
| <colgroup> |
| <col> |
| <col> |
| </colgroup> |
| <thead><tr> |
| <th> |
| <p> |
| Expression |
| </p> |
| </th> |
| <th> |
| <p> |
| Result/Semantics |
| </p> |
| </th> |
| </tr></thead> |
| <tbody> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="identifier">x</span><span class="special">.</span><span class="identifier">eval</span><span class="special"><</span><span class="identifier">RT</span><span class="special">>(</span><span class="identifier">env</span><span class="special">,</span> |
| <span class="identifier">eval0</span><span class="special">,</span> |
| <span class="identifier">eval1</span><span class="special">,</span> |
| <span class="special">...,</span> <span class="identifier">evalN</span><span class="special">)</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| Evaluate the composite |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="identifier">T</span><span class="special">::</span><span class="identifier">result</span><span class="special"><</span><span class="identifier">Env</span><span class="special">,</span> |
| <span class="identifier">Eval0</span><span class="special">,</span> |
| <span class="identifier">Eval1</span><span class="special">,</span> |
| <span class="identifier">Eval2</span><span class="special">,</span> |
| <span class="special">...,</span> <span class="identifier">EvalN</span><span class="special">>::</span><span class="identifier">type</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| The return type of eval |
| </p> |
| </td> |
| </tr> |
| </tbody> |
| </table></div> |
| </div> |
| <br class="table-break"><p> |
| The <code class="computeroutput"><span class="identifier">EvalPolicy</span></code> is expected |
| to have a nested template class <code class="computeroutput"><span class="identifier">result</span></code> |
| which has a typedef <code class="computeroutput"><span class="identifier">type</span></code> |
| that reflects the return type of its member function <code class="computeroutput"><span class="identifier">eval</span></code>. |
| Here's a typical example of the composite's eval member function for a 2-actor |
| composite: |
| </p> |
| <pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Env</span><span class="special">></span> |
| <span class="keyword">typename</span> <span class="identifier">result</span><span class="special"><</span><span class="identifier">Env</span><span class="special">>::</span><span class="identifier">type</span> |
| <span class="identifier">eval</span><span class="special">(</span><span class="identifier">Env</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">env</span><span class="special">)</span> <span class="keyword">const</span> |
| <span class="special">{</span> |
| <span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">result</span><span class="special"><</span><span class="identifier">Env</span><span class="special">>::</span><span class="identifier">type</span> <span class="identifier">return_type</span><span class="special">;</span> |
| <span class="keyword">return</span> <span class="identifier">EvalPolicy</span><span class="special">::</span><span class="keyword">template</span> |
| <span class="identifier">eval</span><span class="special"><</span><span class="identifier">return_type</span><span class="special">>(</span> |
| <span class="identifier">env</span> |
| <span class="special">,</span> <span class="identifier">get</span><span class="special"><</span><span class="number">0</span><span class="special">>(*</span><span class="keyword">this</span><span class="special">)</span> <span class="comment">// gets the 0th element from EvalTuple |
| </span> <span class="special">,</span> <span class="identifier">get</span><span class="special"><</span><span class="number">1</span><span class="special">>(*</span><span class="keyword">this</span><span class="special">));</span> <span class="comment">// gets the 1st element from EvalTuple |
| </span><span class="special">}</span> |
| </pre> |
| </div> |
| <div class="section"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="phoenix.inside_phoenix.composing"></a><a class="link" href="inside_phoenix.html#phoenix.inside_phoenix.composing" title="Composing">Composing</a> |
| </h3></div></div></div> |
| <div class="toc"><dl> |
| <dt><span class="section"><a href="inside_phoenix.html#phoenix.inside_phoenix.composing.compose">compose</a></span></dt> |
| <dt><span class="section"><a href="inside_phoenix.html#phoenix.inside_phoenix.composing.as_composite">as_composite</a></span></dt> |
| <dt><span class="section"><a href="inside_phoenix.html#phoenix.inside_phoenix.composing.composite_example">Composite |
| Example</a></span></dt> |
| </dl></div> |
| <p> |
| Composites are never instantiated directly. Front end expression templates |
| are used to generate the composites. Using expression templates, we implement |
| a DSEL (Domain Specific Embedded Language) that mimics native C++. You've |
| seen this DSEL in action in the preceding sections. It is most evident in |
| the <a class="link" href="composite.html#phoenix.composite.statement" title="Statement">Statement</a> section. |
| </p> |
| <p> |
| There are some facilities in the library to make composition of composites |
| easier. We have a set of overloaded <code class="computeroutput"><span class="identifier">compose</span></code> |
| functions and an <code class="computeroutput"><span class="identifier">as_composite</span></code> |
| metafunction. Together, these helpers make composing a breeze. We'll provide |
| an <a class="link" href="inside_phoenix.html#phoenix.inside_phoenix.composing.composite_example" title="Composite Example">example |
| of a composite</a> later to see why. |
| </p> |
| <div class="section"> |
| <div class="titlepage"><div><div><h4 class="title"> |
| <a name="phoenix.inside_phoenix.composing.compose"></a><a class="link" href="inside_phoenix.html#phoenix.inside_phoenix.composing.compose" title="compose">compose</a> |
| </h4></div></div></div> |
| <pre class="programlisting"><span class="identifier">compose</span><span class="special"><</span><span class="identifier">EvalPolicy</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="identifier">arg2</span><span class="special">,</span> <span class="special">...,</span> <span class="identifier">argN</span><span class="special">);</span> |
| </pre> |
| <p> |
| Given an <a class="link" href="inside_phoenix.html#phoenix.inside_phoenix.composites_in_detail.evalpolicy"><code class="computeroutput"><span class="identifier">EvalPolicy</span></code></a> and some arguments |
| <code class="computeroutput"><span class="identifier">arg0</span></code>...argN, returns a |
| proper <code class="computeroutput"><span class="identifier">composite</span></code>. The arguments |
| may or may not be phoenix actors (primitives of composites). If not, the |
| arguments are converted to actors appropriately. For example: |
| </p> |
| <pre class="programlisting"><span class="identifier">compose</span><span class="special"><</span><span class="identifier">X</span><span class="special">>(</span><span class="number">3</span><span class="special">)</span> |
| </pre> |
| <p> |
| converts the argument <code class="computeroutput"><span class="number">3</span></code> to |
| an <code class="computeroutput"><span class="identifier">actor</span><span class="special"><</span><span class="identifier">value</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="special">>(</span><span class="number">3</span><span class="special">)</span></code>. |
| </p> |
| </div> |
| <div class="section"> |
| <div class="titlepage"><div><div><h4 class="title"> |
| <a name="phoenix.inside_phoenix.composing.as_composite"></a><a class="link" href="inside_phoenix.html#phoenix.inside_phoenix.composing.as_composite" title="as_composite">as_composite</a> |
| </h4></div></div></div> |
| <pre class="programlisting"><span class="identifier">as_composite</span><span class="special"><</span><span class="identifier">EvalPolicy</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="identifier">Arg2</span><span class="special">,</span> <span class="special">...,</span> <span class="identifier">ArgN</span><span class="special">>::</span><span class="identifier">type</span> |
| </pre> |
| <p> |
| This is the metafunction counterpart of <code class="computeroutput"><span class="identifier">compose</span></code>. |
| Given an <a class="link" href="inside_phoenix.html#phoenix.inside_phoenix.composites_in_detail.evalpolicy"><code class="computeroutput"><span class="identifier">EvalPolicy</span></code></a> and some argument types |
| <code class="computeroutput"><span class="identifier">Arg0</span></code>...ArgN, returns a |
| proper <code class="computeroutput"><span class="identifier">composite</span></code> type. |
| For example: |
| </p> |
| <pre class="programlisting"><span class="identifier">as_composite</span><span class="special"><</span><span class="identifier">X</span><span class="special">,</span> <span class="keyword">int</span><span class="special">>::</span><span class="identifier">type</span> |
| </pre> |
| <p> |
| is the composite type of the <code class="computeroutput"><span class="identifier">compose</span><span class="special"><</span><span class="identifier">X</span><span class="special">>(</span><span class="number">3</span><span class="special">)</span></code> |
| expression above. |
| </p> |
| </div> |
| <div class="section"> |
| <div class="titlepage"><div><div><h4 class="title"> |
| <a name="phoenix.inside_phoenix.composing.composite_example"></a><a class="link" href="inside_phoenix.html#phoenix.inside_phoenix.composing.composite_example" title="Composite Example">Composite |
| Example</a> |
| </h4></div></div></div> |
| <p> |
| Now, let's examine an example. Again, this is not a toy example. This is |
| actually part of the library. Remember the <a class="link" href="composite.html#phoenix.composite.statement.while__statement" title="while_ Statement"><code class="computeroutput"><span class="identifier">while_</span></code></a> lazy statement? Putting |
| together everything we've learned so far, we will present it here in its |
| entirety (verbatim): |
| </p> |
| <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">while_eval</span> |
| <span class="special">{</span> |
| <span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Env</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Cond</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Do</span><span class="special">></span> |
| <span class="keyword">struct</span> <span class="identifier">result</span> |
| <span class="special">{</span> |
| <span class="keyword">typedef</span> <span class="keyword">void</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">RT</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Env</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Cond</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Do</span><span class="special">></span> |
| <span class="keyword">static</span> <span class="keyword">void</span> |
| <span class="identifier">eval</span><span class="special">(</span><span class="identifier">Env</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">env</span><span class="special">,</span> <span class="identifier">Cond</span><span class="special">&</span> <span class="identifier">cond</span><span class="special">,</span> <span class="identifier">Do</span><span class="special">&</span> <span class="identifier">do_</span><span class="special">)</span> |
| <span class="special">{</span> |
| <span class="keyword">while</span> <span class="special">(</span><span class="identifier">cond</span><span class="special">.</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">env</span><span class="special">))</span> |
| <span class="identifier">do_</span><span class="special">.</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">env</span><span class="special">);</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">Cond</span><span class="special">></span> |
| <span class="keyword">struct</span> <span class="identifier">while_gen</span> |
| <span class="special">{</span> |
| <span class="identifier">while_gen</span><span class="special">(</span><span class="identifier">Cond</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">cond</span><span class="special">)</span> |
| <span class="special">:</span> <span class="identifier">cond</span><span class="special">(</span><span class="identifier">cond</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">Do</span><span class="special">></span> |
| <span class="identifier">actor</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">as_composite</span><span class="special"><</span><span class="identifier">while_eval</span><span class="special">,</span> <span class="identifier">Cond</span><span class="special">,</span> <span class="identifier">Do</span><span class="special">>::</span><span class="identifier">type</span><span class="special">></span> |
| <span class="keyword">operator</span><span class="special">[](</span><span class="identifier">Do</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">do_</span><span class="special">)</span> <span class="keyword">const</span> |
| <span class="special">{</span> |
| <span class="keyword">return</span> <span class="identifier">compose</span><span class="special"><</span><span class="identifier">while_eval</span><span class="special">>(</span><span class="identifier">cond</span><span class="special">,</span> <span class="identifier">do_</span><span class="special">);</span> |
| <span class="special">}</span> |
| |
| <span class="identifier">Cond</span> <span class="identifier">cond</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">Cond</span><span class="special">></span> |
| <span class="identifier">while_gen</span><span class="special"><</span><span class="identifier">Cond</span><span class="special">></span> |
| <span class="identifier">while_</span><span class="special">(</span><span class="identifier">Cond</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">cond</span><span class="special">)</span> |
| <span class="special">{</span> |
| <span class="keyword">return</span> <span class="identifier">while_gen</span><span class="special"><</span><span class="identifier">Cond</span><span class="special">>(</span><span class="identifier">cond</span><span class="special">);</span> |
| <span class="special">}</span> |
| </pre> |
| <p> |
| <code class="computeroutput"><span class="identifier">while_eval</span></code> is an example |
| of an <a class="link" href="inside_phoenix.html#phoenix.inside_phoenix.composites_in_detail.evalpolicy"><code class="computeroutput"><span class="identifier">EvalPolicy</span></code></a>. <code class="computeroutput"><span class="identifier">while_gen</span></code> |
| and <code class="computeroutput"><span class="identifier">while_</span></code> are the expression |
| template front ends. Let's break this apart to understand what's happening. |
| Let's start at the bottom. It's easier that way. |
| </p> |
| <p> |
| When you write: |
| </p> |
| <pre class="programlisting"><span class="identifier">while_</span><span class="special">(</span><span class="identifier">cond</span><span class="special">)</span> |
| </pre> |
| <p> |
| we generate an instance of <code class="computeroutput"><span class="identifier">while_gen</span><span class="special"><</span><span class="identifier">Cond</span><span class="special">></span></code>, where <code class="computeroutput"><span class="identifier">Cond</span></code> |
| is the type of <code class="computeroutput"><span class="identifier">cond</span></code>. <code class="computeroutput"><span class="identifier">cond</span></code> can be an arbitrarily complex actor |
| expression. The <code class="computeroutput"><span class="identifier">while_gen</span></code> |
| template class has an <code class="computeroutput"><span class="keyword">operator</span><span class="special">[]</span></code> accepting another expression. If we write: |
| </p> |
| <pre class="programlisting"><span class="identifier">while_</span><span class="special">(</span><span class="identifier">cond</span><span class="special">)</span> |
| <span class="special">[</span> |
| <span class="identifier">do_</span> |
| <span class="special">]</span> |
| </pre> |
| <p> |
| it will generate a proper composite with the type: |
| </p> |
| <pre class="programlisting"><span class="identifier">as_composite</span><span class="special"><</span><span class="identifier">while_eval</span><span class="special">,</span> <span class="identifier">Cond</span><span class="special">,</span> <span class="identifier">Do</span><span class="special">>::</span><span class="identifier">type</span> |
| </pre> |
| <p> |
| where <code class="computeroutput"><span class="identifier">Cond</span></code> is the type |
| of <code class="computeroutput"><span class="identifier">cond</span></code> and <code class="computeroutput"><span class="identifier">Do</span></code> is the type of <code class="computeroutput"><span class="identifier">do_</span></code>. |
| Notice how we are using phoenix's <a class="link" href="inside_phoenix.html#phoenix.inside_phoenix.composing" title="Composing">composition</a> |
| (<code class="computeroutput"><span class="identifier">compose</span></code> and <code class="computeroutput"><span class="identifier">as_composite</span></code>) mechanisms here |
| </p> |
| <pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Do</span><span class="special">></span> |
| <span class="identifier">actor</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">as_composite</span><span class="special"><</span><span class="identifier">while_eval</span><span class="special">,</span> <span class="identifier">Cond</span><span class="special">,</span> <span class="identifier">Do</span><span class="special">>::</span><span class="identifier">type</span><span class="special">></span> |
| <span class="keyword">operator</span><span class="special">[](</span><span class="identifier">Do</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">do_</span><span class="special">)</span> <span class="keyword">const</span> |
| <span class="special">{</span> |
| <span class="keyword">return</span> <span class="identifier">compose</span><span class="special"><</span><span class="identifier">while_eval</span><span class="special">>(</span><span class="identifier">cond</span><span class="special">,</span> <span class="identifier">do_</span><span class="special">);</span> |
| <span class="special">}</span> |
| </pre> |
| <p> |
| Finally, the <code class="computeroutput"><span class="identifier">while_eval</span></code> |
| does its thing: |
| </p> |
| <pre class="programlisting"><span class="keyword">while</span> <span class="special">(</span><span class="identifier">cond</span><span class="special">.</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">env</span><span class="special">))</span> |
| <span class="identifier">do_</span><span class="special">.</span><span class="identifier">eval</span><span class="special">(</span><span class="identifier">env</span><span class="special">);</span> |
| </pre> |
| <p> |
| <code class="computeroutput"><span class="identifier">cond</span></code> and <code class="computeroutput"><span class="identifier">do_</span></code>, at this point, are instances of |
| <a class="link" href="inside_phoenix.html#phoenix.inside_phoenix.actors_in_detail.eval_concept"><code class="computeroutput"><span class="identifier">Eval</span></code></a>. <code class="computeroutput"><span class="identifier">cond</span></code> |
| and <code class="computeroutput"><span class="identifier">do_</span></code> are the <a class="link" href="inside_phoenix.html#phoenix.inside_phoenix.actors_in_detail.eval_concept"><code class="computeroutput"><span class="identifier">Eval</span></code></a> elements held by the composite's |
| <a class="link" href="inside_phoenix.html#phoenix.inside_phoenix.composites_in_detail.evaltuple"><code class="computeroutput"><span class="identifier">EvalTuple</span></code></a>. <code class="computeroutput"><span class="identifier">env</span></code> |
| is the <a class="link" href="inside_phoenix.html#phoenix.inside_phoenix.actors_in_detail.environment"><code class="computeroutput"><span class="identifier">Environment</span></code></a>. |
| </p> |
| </div> |
| </div> |
| <div class="section"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="phoenix.inside_phoenix.extending"></a><a class="link" href="inside_phoenix.html#phoenix.inside_phoenix.extending" title="Extending">Extending</a> |
| </h3></div></div></div> |
| <p> |
| We've shown how it is very easy to extend phoenix by writing new primitives |
| and composites. The modular design of Phoenix makes it extremely extensible. |
| We have seen that layer upon layer, the whole library is built on a solid |
| foundation. There are only a few simple well designed concepts that are laid |
| out like bricks. Overall, the library is designed to be extended. Everything |
| above the core layer can in fact be considered just as extensions to the |
| library. This modular design was inherited from the <a href="http://spirit.sourceforge.net" target="_top">Spirit</a> |
| inline parser library. |
| </p> |
| <p> |
| Extension is non-intrusive. And, whenever a component or module is extended, |
| the new extension automatically becomes a first class citizen and is automatically |
| recognized by all modules and components in the library. |
| </p> |
| </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 © 2002-2005 Joel |
| de Guzman, Dan Marsden<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="algorithm.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.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="wrap_up.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a> |
| </div> |
| </body> |
| </html> |
