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1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> 4<title>Values</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="../core.html" title="Core"> 9<link rel="prev" href="../core.html" title="Core"> 10<link rel="next" href="references.html" title="References"> 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.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="references.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.values"></a><a class="link" href="values.html" title="Values">Values</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">value</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 30</pre> 31<p> 32 Whenever we see a constant in a partially applied function, an 33 </p> 34<pre class="programlisting"><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> 35</pre> 36<p> 37 (where T is the type of the constant) is automatically created for us. 38 For instance: 39 </p> 40<pre class="programlisting"><span class="identifier">add</span><span class="special">(</span><span class="identifier">arg1</span><span class="special">,</span> <span class="number">6</span><span class="special">)</span> 41</pre> 42<p> 43 Passing a second argument, <code class="computeroutput"><span class="number">6</span></code>, 44 an <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> is implicitly created behind the 45 scenes. This is also equivalent to <code class="computeroutput"><span class="identifier">add</span><span class="special">(</span><span class="identifier">arg1</span><span class="special">,</span> <span class="identifier">val</span><span class="special">(</span><span class="number">6</span><span class="special">))</span></code>. 46 </p> 47<pre class="programlisting"><span class="identifier">val</span><span class="special">(</span><span class="identifier">v</span><span class="special">)</span> 48</pre> 49<p> 50 generates an <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> 51 where <code class="computeroutput"><span class="identifier">T</span></code> is the type of 52 <code class="computeroutput"><span class="identifier">x</span></code>. In most cases, there's 53 no need to explicitly use <code class="computeroutput"><span class="identifier">val</span></code>, 54 but, as we'll see later on, there are situations where this is unavoidable. 55 </p> 56<a name="phoenix.modules.core.values.evaluating_a_value"></a><h3> 57<a name="id807608"></a> 58 <a class="link" href="values.html#phoenix.modules.core.values.evaluating_a_value">Evaluating 59 a Value</a> 60 </h3> 61<p> 62 Like arguments, values are also actors. As such, values can be evaluated. 63 Invoking a value gives the value's identity. Example: 64 </p> 65<pre class="programlisting"><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">val</span><span class="special">(</span><span class="number">3</span><span class="special">)()</span> <span class="special"><<</span> <span class="identifier">val</span><span class="special">(</span><span class="string">"Hello World"</span><span class="special">)();</span> 66</pre> 67<p> 68 prints out "3 Hello World". 69 </p> 70</div> 71<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 72<td align="left"></td> 73<td align="right"><div class="copyright-footer">Copyright © 2002-2005, 2010 Joel de Guzman, Dan Marsden, Thomas Heller<p> 74 Distributed under the Boost Software License, Version 1.0. (See accompanying 75 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>) 76 </p> 77</div></td> 78</tr></table> 79<hr> 80<div class="spirit-nav"> 81<a accesskey="p" href="../core.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="references.html"><img src="../../../../../../../doc/src/images/next.png" alt="Next"></a> 82</div> 83</body> 84</html>