1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>Placeholder Unification</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="custom_terminals.html" title="Custom Terminals"> 10<link rel="next" href="../examples.html" title="Advanced Examples"> 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="custom_terminals.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="../examples.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.placeholder_unification"></a><a class="link" href="placeholder_unification.html" title="Placeholder Unification">Placeholder Unification</a> 28</h3></div></div></div> 29<p> 30 Phoenix uses <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">is_placeholder</span></code> for recognizing placeholders: 31 </p> 32<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> 33<span class="keyword">struct</span> <span class="identifier">is_placeholder</span> 34<span class="special">{</span> 35 <span class="keyword">static</span> <span class="keyword">const</span> <span class="keyword">int</span> <span class="identifier">value</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> 36<span class="special">};</span> 37</pre> 38<p> 39 To adapt your own placeholder, the nested value needs to be greater than 40 0 for your types. This is done by specializing this trait. 41 </p> 42</div> 43<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 44<td align="left"></td> 45<td align="right"><div class="copyright-footer">Copyright © 2002-2005, 2010, 2014, 2015 Joel de Guzman, Dan Marsden, Thomas 46 Heller, John Fletcher<p> 47 Distributed under the Boost Software License, Version 1.0. (See accompanying 48 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>) 49 </p> 50</div></td> 51</tr></table> 52<hr> 53<div class="spirit-nav"> 54<a accesskey="p" href="custom_terminals.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="../examples.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a> 55</div> 56</body> 57</html> 58
