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1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> 4<title>Lazy Operators</title> 5<link rel="stylesheet" href="../../../../../../doc/src/boostbook.css" type="text/css"> 6<meta name="generator" content="DocBook XSL Stylesheets V1.76.1"> 7<link rel="home" href="../../index.html" title="Chapter 1. Phoenix 3.0"> 8<link rel="up" href="../starter_kit.html" title="Starter Kit"> 9<link rel="prev" href="arguments.html" title="Arguments"> 10<link rel="next" href="lazy_statements.html" title="Lazy Statements"> 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="arguments.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="lazy_statements.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.starter_kit.lazy_operators"></a><a class="link" href="lazy_operators.html" title="Lazy Operators">Lazy Operators</a> 28</h3></div></div></div> 29<p> 30 You can use the usual set of operators to form expressions. Examples: 31 </p> 32<pre class="programlisting"><span class="identifier">arg1</span> <span class="special">*</span> <span class="identifier">arg1</span> 33<span class="identifier">ref</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">=</span> <span class="identifier">arg1</span> <span class="special">+</span> <span class="identifier">ref</span><span class="special">(</span><span class="identifier">z</span><span class="special">)</span> 34<span class="identifier">arg1</span> <span class="special">=</span> <span class="identifier">arg2</span> <span class="special">+</span> <span class="special">(</span><span class="number">3</span> <span class="special">*</span> <span class="identifier">arg3</span><span class="special">)</span> 35<span class="identifier">ref</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</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="comment">// assuming arg1 is indexable and arg2 is a valid index 36</span></pre> 37<p> 38 Note the expression: <code class="computeroutput"><span class="number">3</span> <span class="special">*</span> 39 <span class="identifier">arg3</span></code>. This expression is actually 40 a short-hand equivalent to: <code class="computeroutput"><span class="identifier">val</span><span class="special">(</span><span class="number">3</span><span class="special">)</span> 41 <span class="special">*</span> <span class="identifier">arg3</span></code>. 42 In most cases, like above, you can get away with it. But in some cases, you 43 will have to explicitly wrap your values in <code class="computeroutput"><span class="identifier">val</span></code>. 44 Rules of thumb: 45 </p> 46<div class="itemizedlist"><ul class="itemizedlist" type="disc"> 47<li class="listitem"> 48 In a binary expression (e.g. <code class="computeroutput"><span class="number">3</span> 49 <span class="special">*</span> <span class="identifier">arg3</span></code>), 50 at least one of the operands must be a phoenix primitive or expression. 51 </li> 52<li class="listitem"> 53 In a unary expression (e.g. <code class="computeroutput"><span class="identifier">arg1</span><span class="special">++</span></code>), the single operand must be a phoenix 54 primitive or expression. 55 </li> 56</ul></div> 57<p> 58 If these basic rules are not followed, the result is either an error, or 59 is immediately evaluated. Some examples: 60 </p> 61<pre class="programlisting"><span class="identifier">ref</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">=</span> <span class="number">123</span> <span class="comment">// lazy 62</span><span class="identifier">x</span> <span class="special">=</span> <span class="number">123</span> <span class="comment">// immediate 63</span> 64<span class="identifier">ref</span><span class="special">(</span><span class="identifier">x</span><span class="special">)[</span><span class="number">0</span><span class="special">]</span> <span class="comment">// lazy 65</span><span class="identifier">x</span><span class="special">[</span><span class="number">0</span><span class="special">]</span> <span class="comment">// immediate 66</span> 67<span class="identifier">ref</span><span class="special">(</span><span class="identifier">x</span><span class="special">)[</span><span class="identifier">ref</span><span class="special">(</span><span class="identifier">i</span><span class="special">)]</span> <span class="comment">// lazy 68</span><span class="identifier">ref</span><span class="special">(</span><span class="identifier">x</span><span class="special">)[</span><span class="identifier">i</span><span class="special">]</span> <span class="comment">// lazy (equivalent to ref(x)[val(i)]) 69</span><span class="identifier">x</span><span class="special">[</span><span class="identifier">ref</span><span class="special">(</span><span class="identifier">i</span><span class="special">)]</span> <span class="comment">// illegal (x is not a phoenix primitive or expression) 70</span><span class="identifier">ref</span><span class="special">(</span><span class="identifier">x</span><span class="special">[</span><span class="identifier">ref</span><span class="special">(</span><span class="identifier">i</span><span class="special">)])</span> <span class="comment">// illegal (x is not a phoenix primitive or expression) 71</span></pre> 72<div class="sidebar"> 73<div class="titlepage"></div> 74<p> 75 <span class="inlinemediaobject"><img src="../../images/tip.png" alt="tip"></span> Learn more about operators <a class="link" href="../modules/operator.html" title="Operator">here.</a> 76 </p> 77</div> 78<a name="phoenix.starter_kit.lazy_operators.first_practical_example"></a><h5> 79<a name="id802885"></a> 80 <a class="link" href="lazy_operators.html#phoenix.starter_kit.lazy_operators.first_practical_example">First 81 Practical Example</a> 82 </h5> 83<p> 84 We've covered enough ground to present a real world example. We want to find 85 the first odd number in an STL container. Normally we use a functor (function 86 object) or a function pointer and pass that in to STL's <code class="computeroutput"><span class="identifier">find_if</span></code> 87 generic function: 88 </p> 89<p> 90 Write a function: 91 </p> 92<pre class="programlisting"><span class="keyword">bool</span> 93<span class="identifier">is_odd</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">arg1</span><span class="special">)</span> 94<span class="special">{</span> 95 <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> 96<span class="special">}</span> 97</pre> 98<p> 99 Pass a pointer to the function to STL's <code class="computeroutput"><span class="identifier">find_if</span></code> 100 algorithm: 101 </p> 102<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="special">&</span><span class="identifier">is_odd</span><span class="special">)</span> 103</pre> 104<p> 105 Using Phoenix, the same can be achieved directly with a one-liner: 106 </p> 107<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">arg1</span> <span class="special">%</span> <span class="number">2</span> <span class="special">==</span> <span class="number">1</span><span class="special">)</span> 108</pre> 109<p> 110 The expression <code class="computeroutput"><span class="identifier">arg1</span> <span class="special">%</span> 111 <span class="number">2</span> <span class="special">==</span> <span class="number">1</span></code> automagically creates a functor with the 112 expected behavior. In FP, this unnamed function is called a lambda function. 113 Unlike the function pointer version, which is monomorphic (expects and works 114 only with a fixed type int argument), the Phoenix version is fully polymorphic 115 and works with any container (of ints, of longs, of bignum, etc.) as long 116 as its elements can handle the <code class="computeroutput"><span class="identifier">arg1</span> 117 <span class="special">%</span> <span class="number">2</span> <span class="special">==</span> <span class="number">1</span></code> expression. 118 </p> 119<p> 120 (See <a href="../../../../example/find_if.cpp" target="_top">find_if.cpp</a>) 121 </p> 122<div class="sidebar"> 123<div class="titlepage"></div> 124<p> 125 <span class="inlinemediaobject"><img src="../../images/tip.png" alt="tip"></span> ...<span class="bold"><strong>That's it, we're done</strong></span>. 126 Well if you wish to know a little bit more, read on... 127 </p> 128</div> 129</div> 130<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 131<td align="left"></td> 132<td align="right"><div class="copyright-footer">Copyright © 2002-2005, 2010 Joel de Guzman, Dan Marsden, Thomas Heller<p> 133 Distributed under the Boost Software License, Version 1.0. (See accompanying 134 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>) 135 </p> 136</div></td> 137</tr></table> 138<hr> 139<div class="spirit-nav"> 140<a accesskey="p" href="arguments.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="lazy_statements.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a> 141</div> 142</body> 143</html>