1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>Custom Terminals</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="../inside.html" title="Inside Phoenix"> 9<link rel="prev" href="rules.html" title="Predefined Expressions and Rules"> 10<link rel="next" href="placeholder_unification.html" title="Placeholder Unification"> 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="rules.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../inside.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="placeholder_unification.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.inside.custom_terminals"></a><a class="link" href="custom_terminals.html" title="Custom Terminals">Custom Terminals</a> 28</h3></div></div></div> 29<p> 30 Custom Terminals are used in Phoenix to handle special values transparently. 31 For example, as Phoenix captures everything by value, we needed to use <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">reference_wrapper</span></code> to bring reference semantics 32 into Phoenix. 33 </p> 34<p> 35 Custom terminals could be any wrapper class: 36 </p> 37<pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">></span> 38<span class="keyword">struct</span> <span class="identifier">is_custom_terminal</span><span class="special">;</span> 39</pre> 40<p> 41 needs to be specialized in order for Phoenix to recognize this wrapper type. 42 <code class="computeroutput"><span class="identifier">default_action</span></code> calls <code class="computeroutput"><span class="identifier">custom_terminal</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>. 43 </p> 44<p> 45 Example: 46 </p> 47<pre class="programlisting"><span class="comment">// Call out boost::reference_wrapper for special handling</span> 48<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">></span> 49<span class="keyword">struct</span> <span class="identifier">is_custom_terminal</span><span class="special"><</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">reference_wrapper</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="special">></span> 50 <span class="special">:</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">true_</span> 51<span class="special">{};</span> 52 53<span class="comment">// Special handling for boost::reference_wrapper</span> 54<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">></span> 55<span class="keyword">struct</span> <span class="identifier">custom_terminal</span><span class="special"><</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">reference_wrapper</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="special">></span> 56<span class="special">{</span> 57 <span class="keyword">typedef</span> <span class="identifier">T</span> <span class="special">&</span><span class="identifier">result_type</span><span class="special">;</span> 58 59 <span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Context</span><span class="special">></span> 60 <span class="identifier">T</span> <span class="special">&</span><span class="keyword">operator</span><span class="special">()(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">reference_wrapper</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">r</span><span class="special">,</span> <span class="identifier">Context</span> <span class="special">&)</span> <span class="keyword">const</span> 61 <span class="special">{</span> 62 <span class="keyword">return</span> <span class="identifier">r</span><span class="special">;</span> 63 <span class="special">}</span> 64<span class="special">};</span> 65</pre> 66</div> 67<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 68<td align="left"></td> 69<td align="right"><div class="copyright-footer">Copyright © 2002-2005, 2010, 2014, 2015 Joel de Guzman, Dan Marsden, Thomas 70 Heller, John Fletcher<p> 71 Distributed under the Boost Software License, Version 1.0. (See accompanying 72 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>) 73 </p> 74</div></td> 75</tr></table> 76<hr> 77<div class="spirit-nav"> 78<a accesskey="p" href="rules.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../inside.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="placeholder_unification.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a> 79</div> 80</body> 81</html> 82
