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| <div class="section"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="phoenix.modules.function"></a><a class="link" href="function.html" title="Function">Function</a> |
| </h3></div></div></div> |
| <div class="toc"><dl><dt><span class="section"><a href="function/adapting_functions.html">Adapting |
| Functions</a></span></dt></dl></div> |
| <p> |
| The <code class="computeroutput"><span class="identifier">function</span></code> class template |
| provides a mechanism for implementing lazily evaluated functions. Syntactically, |
| a lazy function looks like an ordinary C/C++ function. The function call |
| looks familiar and feels the same as ordinary C++ functions. However, unlike |
| ordinary functions, the actual function execution is deferred. |
| </p> |
| <pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">phoenix</span><span class="special">/</span><span class="identifier">function</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> |
| </pre> |
| <p> |
| Unlike ordinary function pointers or functor objects that need to be explicitly |
| bound through the bind function (see <a class="link" href="bind.html" title="Bind">Bind</a>), |
| the argument types of these functions are automatically lazily bound. |
| </p> |
| <p> |
| In order to create a lazy function, we need to implement 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. For a function that takes <code class="computeroutput"><span class="identifier">N</span></code> arguments, a model of <a href="http://www.boost.org/doc/libs/release/libs/fusion/doc/html/fusion/functional/concepts/poly.html" target="_top">Polymorphic |
| Function Object</a> must provide: |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" type="disc"> |
| <li class="listitem"> |
| An <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code> |
| that takes <code class="computeroutput"><span class="identifier">N</span></code> arguments, |
| and implements the function logic. This is also true for ordinary function |
| pointers. |
| </li> |
| <li class="listitem"> |
| A nested metafunction <code class="computeroutput"><span class="identifier">result</span><span class="special"><</span><span class="identifier">Signature</span><span class="special">></span></code> or nested typedef <code class="computeroutput"><span class="identifier">result_type</span></code>, |
| following the <a href="http://www.boost.org/doc/libs/release/libs/utility/utility.htm#result_of" target="_top">Boost.Result |
| Of</a> Protocol |
| </li> |
| </ul></div> |
| <p> |
| For example, the following type implements the FunctionEval concept, in order |
| to provide a lazy factorial function: |
| </p> |
| <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">factorial_impl</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">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">This</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Arg</span><span class="special">></span> |
| <span class="keyword">struct</span> <span class="identifier">result</span><span class="special"><</span><span class="identifier">This</span><span class="special">(</span><span class="identifier">Arg</span> <span class="keyword">const</span> <span class="special">&)></span> |
| <span class="special">{</span> |
| <span class="keyword">typedef</span> <span class="identifier">Arg</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">Arg</span><span class="special">></span> |
| <span class="identifier">Arg</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">Arg</span> <span class="keyword">const</span> <span class="special">&</span> <span class="identifier">n</span><span class="special">)</span> <span class="keyword">const</span> |
| <span class="special">{</span> |
| <span class="keyword">return</span> <span class="special">(</span><span class="identifier">n</span> <span class="special"><=</span> <span class="number">0</span><span class="special">)</span> <span class="special">?</span> <span class="number">1</span> <span class="special">:</span> <span class="identifier">n</span> <span class="special">*</span> <span class="special">(*</span><span class="keyword">this</span><span class="special">)(</span><span class="identifier">n</span><span class="special">-</span><span class="number">1</span><span class="special">);</span> |
| <span class="special">}</span> |
| <span class="special">};</span> |
| </pre> |
| <p> |
| (See <a href="../../../../example/factorial.cpp" target="_top">factorial.cpp</a>) |
| </p> |
| <p> |
| Having implemented the <code class="computeroutput"><span class="identifier">factorial_impl</span></code> |
| type, we can declare and instantiate a lazy <code class="computeroutput"><span class="identifier">factorial</span></code> |
| function this way: |
| </p> |
| <pre class="programlisting"><span class="identifier">function</span><span class="special"><</span><span class="identifier">factorial_impl</span><span class="special">></span> <span class="identifier">factorial</span><span class="special">;</span> |
| </pre> |
| <p> |
| Invoking a lazy function such as <code class="computeroutput"><span class="identifier">factorial</span></code> |
| does not immediately execute the function object <code class="computeroutput"><span class="identifier">factorial_impl</span></code>. |
| Instead, an <a class="link" href="../actor.html" title="Actor">actor</a> object is created |
| and returned to the caller. Example: |
| </p> |
| <pre class="programlisting"><span class="identifier">factorial</span><span class="special">(</span><span class="identifier">arg1</span><span class="special">)</span> |
| </pre> |
| <p> |
| does nothing more than return an actor. A second function call will invoke |
| the actual factorial function. Example: |
| </p> |
| <pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">factorial</span><span class="special">(</span><span class="identifier">arg1</span><span class="special">)(</span><span class="number">4</span><span class="special">);</span> |
| </pre> |
| <p> |
| will print out "24". |
| </p> |
| <p> |
| Take note that in certain cases (e.g. for function objects with state), an |
| instance of the model of <a href="http://www.boost.org/doc/libs/release/libs/fusion/doc/html/fusion/functional/concepts/poly.html" target="_top">Polymorphic |
| Function Object</a> may be passed on to the constructor. Example: |
| </p> |
| <pre class="programlisting"><span class="identifier">function</span><span class="special"><</span><span class="identifier">factorial_impl</span><span class="special">></span> <span class="identifier">factorial</span><span class="special">(</span><span class="identifier">ftor</span><span class="special">);</span> |
| </pre> |
| <p> |
| where ftor is an instance of factorial_impl (this is not necessary in this |
| case as <code class="computeroutput"><span class="identifier">factorial_impl</span></code> does |
| not require any state). |
| </p> |
| <div class="important"><table border="0" summary="Important"> |
| <tr> |
| <td rowspan="2" align="center" valign="top" width="25"><img alt="[Important]" src="../../../../../../doc/src/images/important.png"></td> |
| <th align="left">Important</th> |
| </tr> |
| <tr><td align="left" valign="top"><p> |
| Take care though when using function objects with state because they are |
| often copied repeatedly, and state may change in one of the copies, rather |
| than the original. |
| </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 Joel de Guzman, Dan Marsden, Thomas Heller<p> |
| Distributed under the Boost Software License, Version 1.0. (See accompanying |
| file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>) |
| </p> |
| </div></td> |
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