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| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="phoenix.inside.expression"></a><a class="link" href="expression.html" title="Phoenix Expressions">Phoenix Expressions</a> |
| </h3></div></div></div> |
| <div class="toc"><dl class="toc"><dt><span class="section"><a href="expression/boilerplate_macros.html">Boilerplate |
| Macros</a></span></dt></dl></div> |
| <p> |
| A Phoenix Expression is a model of the <a href="http://www.boost.org/doc/libs/release/doc/html/Expr.html" target="_top">Proto |
| Expression</a> Concept. These expressions are wrapped inside an <a class="link" href="actor.html" title="Actors in Detail">Actor</a> template. The <code class="computeroutput"><span class="identifier">actor</span></code> |
| provides the function call operator which evaluates the expressions. The |
| <code class="computeroutput"><span class="identifier">actor</span></code> is the domain specific |
| wrapper around Phoenix expressions. |
| </p> |
| <p> |
| By design, Phoenix Expressions do not carry any information on how they will |
| be evaluated later on. They are the data structure on which the <code class="computeroutput"><span class="identifier">Actions</span></code> will work. |
| </p> |
| <p> |
| The library provides a convenience template to define expressions: |
| </p> |
| <pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span><span class="special">></span> <span class="identifier">Actor</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Tag</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">A0</span><span class="special">,</span> <span class="special">...,</span> <span class="keyword">typename</span> <span class="identifier">A1</span><span class="special">></span> |
| <span class="keyword">struct</span> <span class="identifier">expr_ext</span> |
| <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">transform</span><span class="special"><</span><span class="identifier">expr_ext</span><span class="special"><</span><span class="identifier">Actor</span><span class="special">,</span> <span class="identifier">Tag</span><span class="special">,</span> <span class="identifier">A0</span><span class="special">,</span> <span class="special">...,</span> <span class="identifier">A1</span><span class="special">></span> <span class="special">></span> |
| <span class="special">{</span> |
| <span class="keyword">typedef</span> <span class="emphasis"><em>unspecified</em></span> <span class="identifier">base_expr</span><span class="special">;</span> |
| <span class="keyword">typedef</span> <span class="identifier">Actor</span><span class="special"><</span><span class="identifier">base_expr</span><span class="special">></span> <span class="identifier">type</span><span class="special">;</span> |
| |
| <span class="keyword">typedef</span> <span class="emphasis"><em>unspecified</em></span> <span class="identifier">proto_grammar</span><span class="special">;</span> |
| |
| <span class="keyword">static</span> <span class="identifier">type</span> <span class="identifier">make</span><span class="special">(</span><span class="identifier">A0</span> <span class="identifier">a0</span><span class="special">,</span> <span class="special">...,</span> <span class="identifier">A1</span> <span class="identifier">a1</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">Tag</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">A0</span><span class="special">,</span> <span class="special">...,</span> <span class="keyword">typename</span> <span class="identifier">A1</span><span class="special">></span> |
| <span class="keyword">struct</span> <span class="identifier">expr</span> <span class="special">:</span> <span class="identifier">expr_ext</span><span class="special"><</span><span class="identifier">actor</span><span class="special">,</span> <span class="identifier">Tag</span><span class="special">,</span> <span class="identifier">A0</span><span class="special">,</span> <span class="special">...,</span> <span class="identifier">A1</span><span class="special">></span> <span class="special">{};</span> |
| </pre> |
| <p> |
| <span class="bold"><strong>Notation</strong></span> |
| </p> |
| <div class="variablelist"> |
| <p class="title"><b></b></p> |
| <dl class="variablelist"> |
| <dt><span class="term"><code class="computeroutput"><span class="identifier">A0</span><span class="special">...</span><span class="identifier">AN</span></code></span></dt> |
| <dd><p> |
| Child node types |
| </p></dd> |
| <dt><span class="term"><code class="computeroutput"><span class="identifier">a0</span><span class="special">...</span><span class="identifier">aN</span></code></span></dt> |
| <dd><p> |
| Child node objects |
| </p></dd> |
| <dt><span class="term"><code class="computeroutput"><span class="identifier">G0</span><span class="special">...</span><span class="identifier">GN</span></code></span></dt> |
| <dd><p> |
| <a href="http://www.boost.org/doc/libs/release/libs/proto/index.html" target="_top">Boost.Proto</a> |
| grammar types |
| </p></dd> |
| </dl> |
| </div> |
| <p> |
| <span class="bold"><strong>Expression Semantics</strong></span> |
| </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">expr</span><span class="special"><</span><span class="identifier">Tag</span><span class="special">,</span> |
| <span class="identifier">A0</span><span class="special">...</span><span class="identifier">AN</span><span class="special">>::</span><span class="identifier">type</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| The type of Expression having tag <code class="computeroutput"><span class="identifier">Tag</span></code> |
| and <code class="computeroutput"><span class="identifier">A0</span><span class="special">...</span><span class="identifier">AN</span></code> children |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="identifier">expr</span><span class="special"><</span><span class="identifier">Tag</span><span class="special">,</span> |
| <span class="identifier">G0</span><span class="special">...</span><span class="identifier">GN</span><span class="special">></span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| A <a href="http://www.boost.org/doc/libs/release/libs/proto/index.html" target="_top">Boost.Proto</a> |
| grammar and <a href="http://www.boost.org/doc/libs/release/doc/html/boost/proto/pass_through.html" target="_top">Proto |
| Pass Through Transform</a> |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="identifier">expr</span><span class="special"><</span><span class="identifier">Tag</span><span class="special">,</span> |
| <span class="identifier">A0</span><span class="special">...</span><span class="identifier">AN</span><span class="special">>::</span><span class="identifier">make</span><span class="special">(</span><span class="identifier">a0</span><span class="special">...</span><span class="identifier">aN</span><span class="special">)</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| Returns a Phoenix Expression |
| </p> |
| </td> |
| </tr> |
| </tbody> |
| </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 might have noticed the template template argument <code class="computeroutput"><span class="identifier">Actor</span></code> |
| used in <code class="computeroutput"><span class="identifier">expr_ext</span></code>. This |
| can be a user supplied custom Actor adding other member functions or objects |
| than the default <code class="computeroutput"><span class="identifier">actor</span></code> |
| template. See <a class="link" href="../examples/extending_actors.html" title="Extending Actors">Extending |
| Actors</a> for more details. |
| </p></td></tr> |
| </table></div> |
| <h5> |
| <a name="phoenix.inside.expression.h0"></a> |
| <span class="phrase"><a name="phoenix.inside.expression.meta_grammar"></a></span><a class="link" href="expression.html#phoenix.inside.expression.meta_grammar">meta_grammar</a> |
| </h5> |
| <p> |
| Defining expressions is only part of the game to make it a valid Phoenix |
| Expression. In order to use the expressions in the Phoenix domain, we need |
| to "register" them to our grammar. |
| </p> |
| <p> |
| The <code class="computeroutput"><span class="identifier">meta_grammar</span></code> is a struct |
| for exactly that purpose. It is an openly extendable <a href="http://www.boost.org/doc/libs/release/libs/proto/index.html" target="_top">Boost.Proto</a> |
| Grammar: |
| </p> |
| <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">meta_grammar</span> |
| <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">switch_</span><span class="special"><</span><span class="identifier">meta_grammar</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">Tag</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Dummy</span><span class="special">></span> |
| <span class="keyword">struct</span> <span class="identifier">case_</span> |
| <span class="special">:</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">not_</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">_</span><span class="special">></span> |
| <span class="special">{};</span> |
| <span class="special">};</span> |
| </pre> |
| <p> |
| As you can see, by default the <code class="computeroutput"><span class="identifier">meta_grammar</span></code> |
| matches nothing. With every <a class="link" href="../modules.html" title="Modules">Module</a> |
| you include this grammar gets extended by various expressions. |
| </p> |
| <h5> |
| <a name="phoenix.inside.expression.h1"></a> |
| <span class="phrase"><a name="phoenix.inside.expression.example"></a></span><a class="link" href="expression.html#phoenix.inside.expression.example">Example</a> |
| </h5> |
| <p> |
| Define an expression: |
| </p> |
| <pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Lhs</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Rhs</span><span class="special">></span> |
| <span class="keyword">struct</span> <span class="identifier">plus</span> |
| <span class="special">:</span> <span class="identifier">expr</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">plus</span><span class="special">,</span> <span class="identifier">Lhs</span><span class="special">,</span> <span class="identifier">Rhs</span><span class="special">></span> |
| <span class="special">{};</span> |
| </pre> |
| <p> |
| And add it to the grammar: |
| </p> |
| <pre class="programlisting"><span class="keyword">template</span> <span class="special"><></span> |
| <span class="keyword">struct</span> <span class="identifier">meta_grammar</span><span class="special">::</span><span class="identifier">case_</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">plus</span><span class="special">></span> |
| <span class="special">:</span> <span class="identifier">enable_rule</span><span class="special"><</span> |
| <span class="identifier">plus</span><span class="special"><</span> |
| <span class="identifier">meta_grammar</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> |
| Define a generator function to make the life of our potential users easier: |
| </p> |
| <pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Lhs</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Rhs</span><span class="special">></span> |
| <span class="keyword">typename</span> <span class="identifier">plus</span><span class="special"><</span><span class="identifier">Lhs</span><span class="special">,</span> <span class="identifier">Rhs</span><span class="special">>::</span><span class="identifier">type</span> |
| <span class="identifier">plus</span><span class="special">(</span><span class="identifier">Lhs</span> <span class="keyword">const</span> <span class="special">&</span> <span class="identifier">lhs</span><span class="special">,</span> <span class="identifier">Rhs</span> <span class="keyword">const</span> <span class="special">&</span> <span class="identifier">rhs</span><span class="special">)</span> |
| <span class="special">{</span> |
| <span class="keyword">return</span> <span class="identifier">expression</span><span class="special">::</span><span class="identifier">plus</span><span class="special"><</span><span class="identifier">Lhs</span><span class="special">,</span> <span class="identifier">Rhs</span><span class="special">>::</span><span class="identifier">make</span><span class="special">(</span><span class="identifier">lhs</span><span class="special">,</span> <span class="identifier">rhs</span><span class="special">);</span> |
| <span class="special">}</span> |
| </pre> |
| <p> |
| Look if it really works: |
| </p> |
| <pre class="programlisting"><span class="identifier">plus</span><span class="special">(</span><span class="number">6</span><span class="special">,</span> <span class="number">5</span><span class="special">)();</span> |
| </pre> |
| <p> |
| returns 11! |
| </p> |
| <pre class="programlisting"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">display_expr</span><span class="special">(</span><span class="identifier">plus</span><span class="special">(</span><span class="number">5</span><span class="special">,</span> <span class="number">6</span><span class="special">));</span> |
| </pre> |
| <p> |
| prints: |
| </p> |
| <pre class="programlisting"><span class="identifier">plus</span><span class="special">(</span> |
| <span class="identifier">terminal</span><span class="special">(</span><span class="number">6</span><span class="special">)</span> |
| <span class="special">,</span> <span class="identifier">terminal</span><span class="special">(</span><span class="number">5</span><span class="special">)</span> |
| <span class="special">)</span> |
| </pre> |
| <p> |
| See <a href="../../../../example/define_expression.cpp" target="_top">define_expression.cpp</a> |
| for the full example. |
| </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> |
| The example shown here only works because <code class="computeroutput"><span class="identifier">default_actions</span></code> |
| knows how to handle an expression having the <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">tag</span><span class="special">::</span><span class="identifier">plus</span></code> |
| and two children. This is because <code class="computeroutput"><span class="identifier">default_actions</span></code> |
| uses the <code class="computeroutput"><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></code> |
| transform to evaluate operators and functions. Learn more about actions |
| <a class="link" href="actions.html" title="More on Actions">here</a>. |
| </p></td></tr> |
| </table></div> |
| </div> |
| <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> |
| <td align="left"></td> |
| <td align="right"><div class="copyright-footer">Copyright © 2002-2005, 2010, 2014, 2015 Joel de Guzman, Dan Marsden, Thomas |
| Heller, John Fletcher<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> |
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