1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>Lazy Functions</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. 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When you reach that point, you'll 32 wish that ordinary functions can co-exist with phoenix functions. Unfortunately, 33 the <span class="emphasis"><em>immediate</em></span> nature of plain C++ functions make them 34 incompatible. 35 </p> 36<p> 37 Lazy functions are your friends. The library provides a facility to make 38 lazy functions. The code below is a rewrite of the <code class="computeroutput"><span class="identifier">is_odd</span></code> 39 function using the facility: 40 </p> 41<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">is_odd_impl</span> 42<span class="special">{</span> 43 <span class="keyword">typedef</span> <span class="keyword">bool</span> <span class="identifier">result_type</span><span class="special">;</span> 44 45 <span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Arg</span><span class="special">></span> 46 <span class="keyword">bool</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">Arg</span> <span class="identifier">arg1</span><span class="special">)</span> <span class="keyword">const</span> 47 <span class="special">{</span> 48 <span class="keyword">return</span> <span class="identifier">arg1</span> <span class="special">%</span> <span class="number">2</span> <span class="special">==</span> <span class="number">1</span><span class="special">;</span> 49 <span class="special">}</span> 50<span class="special">};</span> 51 52<span class="identifier">function</span><span class="special"><</span><span class="identifier">is_odd_impl</span><span class="special">></span> <span class="identifier">is_odd</span><span class="special">;</span> 53</pre> 54<h5> 55<a name="phoenix.starter_kit.lazy_functions.h0"></a> 56 <span class="phrase"><a name="phoenix.starter_kit.lazy_functions.things_to_note_"></a></span><a class="link" href="lazy_functions.html#phoenix.starter_kit.lazy_functions.things_to_note_">Things 57 to note:</a> 58 </h5> 59<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 60<li class="listitem"> 61 Result type deduction is implemented with the help of the result_of protocol. 62 For more information see <a href="http://www.boost.org/doc/libs/release/libs/utility/utility.htm#result_of" target="_top">Boost.Result 63 Of</a> 64 </li> 65<li class="listitem"> 66 <code class="computeroutput"><span class="identifier">is_odd_impl</span></code> implements 67 the function. 68 </li> 69<li class="listitem"> 70 <code class="computeroutput"><span class="identifier">is_odd</span></code>, an instance of 71 <code class="computeroutput"><span class="identifier">function</span><span class="special"><</span><span class="identifier">is_odd_impl</span><span class="special">></span></code>, 72 is the lazy function. 73 </li> 74</ul></div> 75<p> 76 Now, <code class="computeroutput"><span class="identifier">is_odd</span></code> is a truly lazy 77 function that we can use in conjunction with the rest of phoenix. Example: 78 </p> 79<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">find_if</span><span class="special">(</span><span class="identifier">c</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">c</span><span class="special">.</span><span class="identifier">end</span><span class="special">(),</span> <span class="identifier">is_odd</span><span class="special">(</span><span class="identifier">arg1</span><span class="special">));</span> 80</pre> 81<p> 82 (See <a href="../../../../example/function.cpp" target="_top">function.cpp</a>) 83 </p> 84<h5> 85<a name="phoenix.starter_kit.lazy_functions.h1"></a> 86 <span class="phrase"><a name="phoenix.starter_kit.lazy_functions.predefined_lazy_functions"></a></span><a class="link" href="lazy_functions.html#phoenix.starter_kit.lazy_functions.predefined_lazy_functions">Predefined 87 Lazy Functions</a> 88 </h5> 89<p> 90 The library is chock full of STL savvy, predefined lazy functions covering 91 the whole of the STL containers, iterators and algorithms. For example, there 92 are lazy versions of container related operations such as assign, at, back, 93 begin, pop_back, pop_front, push_back, push_front, etc. (See <a class="link" href="../modules/stl.html" title="STL">STL</a>). 94 </p> 95</div> 96<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 97<td align="left"></td> 98<td align="right"><div class="copyright-footer">Copyright © 2002-2005, 2010, 2014, 2015 Joel de Guzman, Dan Marsden, Thomas 99 Heller, John Fletcher<p> 100 Distributed under the Boost Software License, Version 1.0. (See accompanying 101 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>) 102 </p> 103</div></td> 104</tr></table> 105<hr> 106<div class="spirit-nav"> 107<a accesskey="p" href="construct__new__delete__casts.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../starter_kit.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="more.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a> 108</div> 109</body> 110</html> 111
