1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>References</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="../core.html" title="Core"> 9<link rel="prev" href="values.html" title="Values"> 10<link rel="next" href="arguments.html" title="Arguments"> 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="values.html"><img src="../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../core.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="arguments.html"><img src="../../../../../../../doc/src/images/next.png" alt="Next"></a> 24</div> 25<div class="section"> 26<div class="titlepage"><div><div><h4 class="title"> 27<a name="phoenix.modules.core.references"></a><a class="link" href="references.html" title="References">References</a> 28</h4></div></div></div> 29<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">core</span><span class="special">/</span><span class="identifier">reference</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 30</pre> 31<p> 32 Values are immutable constants. Attempting to modify a value will result 33 in a compile time error. When we want the function to modify the parameter, 34 we use a reference instead. For instance, imagine a lazy function <code class="computeroutput"><span class="identifier">add_assign</span></code>: 35 </p> 36<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">add_assign</span><span class="special">(</span><span class="identifier">T</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">T</span> <span class="identifier">y</span><span class="special">)</span> <span class="special">{</span> <span class="identifier">x</span> <span class="special">+=</span> <span class="identifier">y</span><span class="special">;</span> <span class="special">}</span> <span class="comment">// pseudo code</span> 37</pre> 38<p> 39 Here, we want the first function argument, x, to be mutable. Obviously, 40 we cannot write: 41 </p> 42<pre class="programlisting"><span class="identifier">add_assign</span><span class="special">(</span><span class="number">1</span><span class="special">,</span> <span class="number">2</span><span class="special">)</span> <span class="comment">// error first argument is immutable</span> 43</pre> 44<p> 45 In C++, we can pass in a reference to a variable as the first argument 46 in our example above. Yet, by default, the library forces arguments passed 47 to partially applied functions to be immutable values (see <a class="link" href="values.html" title="Values">Values</a>). 48 To achieve our intent, we use: 49 </p> 50<pre class="programlisting"><span class="identifier">expression</span><span class="special">::</span><span class="identifier">reference</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">type</span> 51</pre> 52<p> 53 This is similar to <code class="computeroutput"><span class="identifier">expression</span><span class="special">::</span><span class="identifier">value</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">type</span></code> 54 before but instead holds a reference to a variable. 55 </p> 56<p> 57 We normally don't instantiate <code class="computeroutput"><span class="identifier">expression</span><span class="special">::</span><span class="identifier">reference</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">type</span></code> 58 objects directly. Instead we use: 59 </p> 60<pre class="programlisting"><span class="identifier">ref</span><span class="special">(</span><span class="identifier">v</span><span class="special">)</span> 61</pre> 62<p> 63 For example (where <code class="computeroutput"><span class="identifier">i</span></code> is 64 an <code class="computeroutput"><span class="keyword">int</span></code> variable): 65 </p> 66<pre class="programlisting"><span class="identifier">add_assign</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="number">2</span><span class="special">)</span> 67</pre> 68<h6> 69<a name="phoenix.modules.core.references.h0"></a> 70 <span class="phrase"><a name="phoenix.modules.core.references.evaluating_a_reference"></a></span><a class="link" href="references.html#phoenix.modules.core.references.evaluating_a_reference">Evaluating 71 a Reference</a> 72 </h6> 73<p> 74 References are actors. Hence, references can be evaluated. Such invocation 75 gives the reference's identity. Example: 76 </p> 77<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">3</span><span class="special">;</span> 78<span class="keyword">char</span> <span class="keyword">const</span><span class="special">*</span> <span class="identifier">s</span> <span class="special">=</span> <span class="string">"Hello World"</span><span class="special">;</span> 79<span class="identifier">cout</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="special"><<</span> <span class="identifier">ref</span><span class="special">(</span><span class="identifier">s</span><span class="special">)();</span> 80</pre> 81<p> 82 prints out "3 Hello World" 83 </p> 84<h6> 85<a name="phoenix.modules.core.references.h1"></a> 86 <span class="phrase"><a name="phoenix.modules.core.references.constant_references"></a></span><a class="link" href="references.html#phoenix.modules.core.references.constant_references">Constant 87 References</a> 88 </h6> 89<p> 90 Another free function 91 </p> 92<pre class="programlisting"><span class="identifier">cref</span><span class="special">(</span><span class="identifier">cv</span><span class="special">)</span> 93</pre> 94<p> 95 may also be used. <code class="computeroutput"><span class="identifier">cref</span><span class="special">(</span><span class="identifier">cv</span><span class="special">)</span></code> 96 creates an <code class="computeroutput"><span class="identifier">expression</span><span class="special">::</span><span class="identifier">reference</span><span class="special"><</span><span class="identifier">T</span> <span class="keyword">const</span><span class="special">>::</span><span class="identifier">type</span></code> 97 object. This is similar to <code class="computeroutput"><span class="identifier">expression</span><span class="special">::</span><span class="identifier">value</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">type</span></code> 98 but when the data to be passed as argument to a function is heavy and expensive 99 to copy by value, the <code class="computeroutput"><span class="identifier">cref</span><span class="special">(</span><span class="identifier">cv</span><span class="special">)</span></code> 100 offers a lighter alternative. 101 </p> 102</div> 103<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 104<td align="left"></td> 105<td align="right"><div class="copyright-footer">Copyright © 2002-2005, 2010, 2014, 2015 Joel de Guzman, Dan Marsden, Thomas 106 Heller, John Fletcher<p> 107 Distributed under the Boost Software License, Version 1.0. (See accompanying 108 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>) 109 </p> 110</div></td> 111</tr></table> 112<hr> 113<div class="spirit-nav"> 114<a accesskey="p" href="values.html"><img src="../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../core.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="arguments.html"><img src="../../../../../../../doc/src/images/next.png" alt="Next"></a> 115</div> 116</body> 117</html> 118
