1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>Function</title> 5<link rel="stylesheet" href="../../../../../../doc/src/boostbook.css" type="text/css"> 6<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 7<link rel="home" href="../../index.html" title="Chapter 1. Phoenix 3.2.0"> 8<link rel="up" href="../modules.html" title="Modules"> 9<link rel="prev" href="core/nothing.html" title="Nothing"> 10<link rel="next" href="function/adapting_functions.html" title="Adapting Functions"> 11</head> 12<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 13<table cellpadding="2" width="100%"><tr> 14<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../boost.png"></td> 15<td align="center"><a href="../../../../../../index.html">Home</a></td> 16<td align="center"><a href="../../../../../../libs/libraries.htm">Libraries</a></td> 17<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> 18<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> 19<td align="center"><a href="../../../../../../more/index.htm">More</a></td> 20</tr></table> 21<hr> 22<div class="spirit-nav"> 23<a accesskey="p" href="core/nothing.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../modules.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="function/adapting_functions.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a> 24</div> 25<div class="section"> 26<div class="titlepage"><div><div><h3 class="title"> 27<a name="phoenix.modules.function"></a><a class="link" href="function.html" title="Function">Function</a> 28</h3></div></div></div> 29<div class="toc"><dl class="toc"><dt><span class="section"><a href="function/adapting_functions.html">Adapting 30 Functions</a></span></dt></dl></div> 31<p> 32 The <code class="computeroutput"><span class="identifier">function</span></code> class template 33 provides a mechanism for implementing lazily evaluated functions. Syntactically, 34 a lazy function looks like an ordinary C/C++ function. The function call 35 looks familiar and feels the same as ordinary C++ functions. However, unlike 36 ordinary functions, the actual function execution is deferred. 37 </p> 38<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> 39</pre> 40<p> 41 Unlike ordinary function pointers or functor objects that need to be explicitly 42 bound through the bind function (see <a class="link" href="bind.html" title="Bind">Bind</a>), 43 the argument types of these functions are automatically lazily bound. 44 </p> 45<p> 46 In order to create a lazy function, we need to implement a model of the 47 <a href="http://www.boost.org/doc/libs/release/libs/fusion/doc/html/fusion/functional/concepts/poly.html" target="_top">Polymorphic 48 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 49 Function Object</a> must provide: 50 </p> 51<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 52<li class="listitem"> 53 An <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code> 54 that takes <code class="computeroutput"><span class="identifier">N</span></code> arguments, 55 and implements the function logic. This is also true for ordinary function 56 pointers. 57 </li> 58<li class="listitem"> 59 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>, 60 following the <a href="http://www.boost.org/doc/libs/release/libs/utility/utility.htm#result_of" target="_top">Boost.Result 61 Of</a> Protocol 62 </li> 63</ul></div> 64<p> 65 For example, the following type implements the FunctionEval concept, in order 66 to provide a lazy factorial function: 67 </p> 68<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">factorial_impl</span> 69<span class="special">{</span> 70 <span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Sig</span><span class="special">></span> 71 <span class="keyword">struct</span> <span class="identifier">result</span><span class="special">;</span> 72 73 <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> 74 <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> 75 <span class="special">{</span> 76 <span class="keyword">typedef</span> <span class="identifier">Arg</span> <span class="identifier">type</span><span class="special">;</span> 77 <span class="special">};</span> 78 79 <span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Arg</span><span class="special">></span> 80 <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> 81 <span class="special">{</span> 82 <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> 83 <span class="special">}</span> 84<span class="special">};</span> 85</pre> 86<p> 87 (See <a href="../../../../example/factorial.cpp" target="_top">factorial.cpp</a>) 88 </p> 89<p> 90 Having implemented the <code class="computeroutput"><span class="identifier">factorial_impl</span></code> 91 type, we can declare and instantiate a lazy <code class="computeroutput"><span class="identifier">factorial</span></code> 92 function this way: 93 </p> 94<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> 95</pre> 96<p> 97 Invoking a lazy function such as <code class="computeroutput"><span class="identifier">factorial</span></code> 98 does not immediately execute the function object <code class="computeroutput"><span class="identifier">factorial_impl</span></code>. 99 Instead, an <a class="link" href="../actor.html" title="Actor">actor</a> object is created 100 and returned to the caller. Example: 101 </p> 102<pre class="programlisting"><span class="identifier">factorial</span><span class="special">(</span><span class="identifier">arg1</span><span class="special">)</span> 103</pre> 104<p> 105 does nothing more than return an actor. A second function call will invoke 106 the actual factorial function. Example: 107 </p> 108<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> 109</pre> 110<p> 111 will print out "24". 112 </p> 113<p> 114 Take note that in certain cases (e.g. for function objects with state), an 115 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 116 Function Object</a> may be passed on to the constructor. Example: 117 </p> 118<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> 119</pre> 120<p> 121 where ftor is an instance of factorial_impl (this is not necessary in this 122 case as <code class="computeroutput"><span class="identifier">factorial_impl</span></code> does 123 not require any state). 124 </p> 125<div class="important"><table border="0" summary="Important"> 126<tr> 127<td rowspan="2" align="center" valign="top" width="25"><img alt="[Important]" src="../../../../../../doc/src/images/important.png"></td> 128<th align="left">Important</th> 129</tr> 130<tr><td align="left" valign="top"><p> 131 Take care though when using function objects with state because they are 132 often copied repeatedly, and state may change in one of the copies, rather 133 than the original. 134 </p></td></tr> 135</table></div> 136</div> 137<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 138<td align="left"></td> 139<td align="right"><div class="copyright-footer">Copyright © 2002-2005, 2010, 2014, 2015 Joel de Guzman, Dan Marsden, Thomas 140 Heller, John Fletcher<p> 141 Distributed under the Boost Software License, Version 1.0. (See accompanying 142 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>) 143 </p> 144</div></td> 145</tr></table> 146<hr> 147<div class="spirit-nav"> 148<a accesskey="p" href="core/nothing.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../modules.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="function/adapting_functions.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a> 149</div> 150</body> 151</html> 152
