1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>Macro BOOST_TTI_TRAIT_HAS_CLASS</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. The Type Traits Introspection Library"> 8<link rel="up" href="header/boost/tti/has_class_hpp.html" title="Header <boost/tti/has_class.hpp>"> 9<link rel="prev" href="header/boost/tti/has_class_hpp.html" title="Header <boost/tti/has_class.hpp>"> 10<link rel="next" href="BOOST_TTI_HAS_CLASS.html" title="Macro BOOST_TTI_HAS_CLASS"> 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="header/boost/tti/has_class_hpp.html"><img src="../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="header/boost/tti/has_class_hpp.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="BOOST_TTI_HAS_CLASS.html"><img src="../../../../doc/src/images/next.png" alt="Next"></a> 24</div> 25<div class="refentry"> 26<a name="BOOST_TTI_TRAIT_HAS_CLASS"></a><div class="titlepage"></div> 27<div class="refnamediv"> 28<h2><span class="refentrytitle">Macro BOOST_TTI_TRAIT_HAS_CLASS</span></h2> 29<p>BOOST_TTI_TRAIT_HAS_CLASS — A macro which expands to a metafunction which tests whether an inner class/struct with a particular name exists. </p> 30</div> 31<h2 xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv-title">Synopsis</h2> 32<div xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv"><pre class="synopsis"><span class="comment">// In header: <<a class="link" href="header/boost/tti/has_class_hpp.html" title="Header <boost/tti/has_class.hpp>">boost/tti/has_class.hpp</a>> 33 34</span>BOOST_TTI_TRAIT_HAS_CLASS(trait, name)</pre></div> 35<div class="refsect1"> 36<a name="idm45658026593808"></a><h2>Description</h2> 37<p>BOOST_TTI_TRAIT_HAS_CLASS is a macro which expands to a metafunction. The metafunction tests whether an inner class/struct with a particular name exists and, optionally, whether an MPL lambda expression invoked with the inner class/struct is true or not. The macro takes the form of BOOST_TTI_TRAIT_HAS_CLASS(trait,name) where</p> 38<p>trait = the name of the metafunction <br> 39 name = the name of the inner class/struct.</p> 40<p>BOOST_TTI_TRAIT_HAS_CLASS generates a metafunction called "trait" where 'trait' is the macro parameter.</p> 41<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">BOOST_TTI_TP_T</span><span class="special">,</span><span class="keyword">class</span> <span class="identifier">BOOST_TTI_TP_U</span><span class="special">></span> 42<span class="keyword">struct</span> <span class="identifier">trait</span> 43 <span class="special">{</span> 44 <span class="keyword">static</span> <span class="keyword">const</span> <span class="identifier">value</span> <span class="special">=</span> <span class="identifier">unspecified</span><span class="special">;</span> 45 <span class="keyword">typedef</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">bool_</span><span class="special"><</span><span class="keyword">true</span><span class="special">-</span><span class="identifier">or</span><span class="special">-</span><span class="keyword">false</span><span class="special">></span> <span class="identifier">type</span><span class="special">;</span> 46 <span class="special">}</span><span class="special">;</span> 47 48<span class="identifier">The</span> <span class="identifier">metafunction</span> <span class="identifier">types</span> <span class="identifier">and</span> <span class="keyword">return</span><span class="special">:</span> 49 50 <span class="identifier">BOOST_TTI_TP_T</span> <span class="special">=</span> <span class="identifier">the</span> <span class="identifier">enclosing</span> <span class="identifier">type</span> <span class="identifier">in</span> <span class="identifier">which</span> <span class="identifier">to</span> <span class="identifier">look</span> <span class="keyword">for</span> <span class="identifier">our</span> <span class="char">'name'</span><span class="special">.</span> 51 <span class="identifier">The</span> <span class="identifier">enclosing</span> <span class="identifier">type</span> <span class="identifier">can</span> <span class="identifier">be</span> <span class="identifier">a</span> <span class="keyword">class</span><span class="special">,</span> <span class="keyword">struct</span><span class="special">,</span> <span class="identifier">or</span> <span class="keyword">union</span><span class="special">.</span> 52 53 <span class="identifier">BOOST_TTI_TP_U</span> <span class="special">=</span> <span class="special">(</span><span class="identifier">optional</span><span class="special">)</span> <span class="identifier">An</span> <span class="identifier">optional</span> <span class="keyword">template</span> <span class="identifier">parameter</span><span class="special">,</span> <span class="identifier">defaulting</span> <span class="identifier">to</span> <span class="identifier">a</span> <span class="identifier">marker</span> <span class="identifier">type</span><span class="special">.</span> 54 <span class="identifier">If</span> <span class="identifier">specified</span> <span class="identifier">it</span> <span class="identifier">is</span> <span class="identifier">an</span> <span class="identifier">MPL</span> <span class="identifier">lambda</span> <span class="identifier">expression</span> <span class="identifier">which</span> <span class="identifier">is</span> <span class="identifier">invoked</span> 55 <span class="identifier">with</span> <span class="identifier">the</span> <span class="identifier">inner</span> <span class="keyword">class</span><span class="special">/</span><span class="keyword">struct</span> <span class="identifier">found</span> <span class="identifier">and</span> <span class="identifier">must</span> <span class="keyword">return</span> <span class="identifier">a</span> <span class="identifier">constant</span> <span class="identifier">boolean</span> 56 <span class="identifier">value</span><span class="special">.</span> 57 58 <span class="identifier">returns</span> <span class="special">=</span> <span class="char">'value'</span> <span class="identifier">depends</span> <span class="identifier">on</span> <span class="identifier">whether</span> <span class="identifier">or</span> <span class="identifier">not</span> <span class="identifier">the</span> <span class="identifier">optional</span> <span class="identifier">BOOST_TTI_TP_U</span> <span class="identifier">is</span> <span class="identifier">specified</span><span class="special">.</span> 59 60 <span class="identifier">If</span> <span class="identifier">BOOST_TTI_TP_U</span> <span class="identifier">is</span> <span class="identifier">not</span> <span class="identifier">specified</span><span class="special">,</span> <span class="identifier">then</span> <span class="char">'value'</span> <span class="identifier">is</span> <span class="keyword">true</span> <span class="keyword">if</span> <span class="identifier">the</span> <span class="char">'name'</span> <span class="keyword">class</span><span class="special">/</span><span class="keyword">struct</span> 61 <span class="identifier">exists</span> <span class="identifier">within</span> <span class="identifier">the</span> <span class="identifier">enclosing</span> <span class="identifier">type</span> <span class="identifier">BOOST_TTI_TP_T</span><span class="special">;</span> <span class="identifier">otherwise</span> <span class="char">'value'</span> <span class="identifier">is</span> <span class="keyword">false</span><span class="special">.</span> 62 63 <span class="identifier">If</span> <span class="identifier">BOOST_TTI_TP_U</span> <span class="identifier">is</span> <span class="identifier">specified</span> <span class="special">,</span> <span class="identifier">then</span> <span class="char">'value'</span> <span class="identifier">is</span> <span class="keyword">true</span> <span class="keyword">if</span> <span class="identifier">the</span> <span class="char">'name'</span> <span class="keyword">class</span><span class="special">/</span><span class="keyword">struct</span> <span class="identifier">exists</span> 64 <span class="identifier">within</span> <span class="identifier">the</span> <span class="identifier">enclosing</span> <span class="identifier">type</span> <span class="identifier">BOOST_TTI_TP_T</span> <span class="identifier">and</span> <span class="identifier">the</span> <span class="identifier">MPL</span> <span class="identifier">lambda</span> <span class="identifier">expression</span> <span class="identifier">as</span> <span class="identifier">specified</span> 65 <span class="identifier">by</span> <span class="identifier">BOOST_TTI_TP_U</span><span class="special">,</span> <span class="identifier">invoked</span> <span class="identifier">by</span> <span class="identifier">passing</span> <span class="identifier">the</span> <span class="identifier">actual</span> <span class="identifier">inner</span> <span class="keyword">class</span><span class="special">/</span><span class="keyword">struct</span> <span class="identifier">of</span> <span class="char">'name'</span><span class="special">,</span> <span class="identifier">returns</span> 66 <span class="identifier">a</span> <span class="char">'value'</span> <span class="identifier">of</span> <span class="keyword">true</span><span class="special">;</span> <span class="identifier">otherwise</span> <span class="char">'value'</span> <span class="identifier">is</span> <span class="keyword">false</span><span class="special">.</span> 67 68 <span class="identifier">The</span> <span class="identifier">action</span> <span class="identifier">taken</span> <span class="identifier">with</span> <span class="identifier">BOOST_TTI_TP_U</span> <span class="identifier">occurs</span> <span class="identifier">only</span> <span class="identifier">when</span> <span class="identifier">the</span> <span class="char">'name'</span> <span class="keyword">class</span><span class="special">/</span><span class="keyword">struct</span> <span class="identifier">exists</span> 69 <span class="identifier">within</span> <span class="identifier">the</span> <span class="identifier">enclosing</span> <span class="identifier">type</span> <span class="identifier">BOOST_TTI_TP_T</span><span class="special">.</span> 70</pre> 71<p>Example usage:</p> 72<pre class="programlisting"><span class="identifier">BOOST_TTI_TRAIT_HAS_CLASS</span><span class="special">(</span><span class="identifier">LookFor</span><span class="special">,</span><span class="identifier">MyType</span><span class="special">)</span> <span class="identifier">generates</span> <span class="identifier">the</span> <span class="identifier">metafunction</span> <span class="string">"LookFor"</span> <span class="identifier">in</span> <span class="identifier">the</span> <span class="identifier">current</span> <span class="identifier">scope</span> 73<span class="identifier">to</span> <span class="identifier">look</span> <span class="keyword">for</span> <span class="identifier">an</span> <span class="identifier">inner</span> <span class="keyword">class</span><span class="special">/</span><span class="keyword">struct</span> <span class="identifier">called</span> <span class="identifier">MyType</span><span class="special">.</span> 74 75<span class="identifier">LookFor</span><span class="special"><</span><span class="identifier">EnclosingType</span><span class="special">></span><span class="special">::</span><span class="identifier">value</span> <span class="identifier">is</span> <span class="keyword">true</span> <span class="keyword">if</span> <span class="identifier">MyType</span> <span class="identifier">is</span> <span class="identifier">an</span> <span class="identifier">inner</span> <span class="keyword">class</span><span class="special">/</span><span class="keyword">struct</span> <span class="identifier">of</span> <span class="identifier">EnclosingType</span><span class="special">,</span> <span class="identifier">otherwise</span> <span class="keyword">false</span><span class="special">.</span> 76 77<span class="identifier">LookFor</span><span class="special"><</span><span class="identifier">EnclosingType</span><span class="special">,</span><span class="identifier">ALambdaExpression</span><span class="special">></span><span class="special">::</span><span class="identifier">value</span> <span class="identifier">is</span> <span class="keyword">true</span> <span class="keyword">if</span> <span class="identifier">MyType</span> <span class="identifier">is</span> <span class="identifier">an</span> <span class="identifier">inner</span> <span class="keyword">class</span><span class="special">/</span><span class="keyword">struct</span> <span class="identifier">of</span> <span class="identifier">EnclosingType</span> 78 <span class="identifier">and</span> <span class="identifier">invoking</span> <span class="identifier">ALambdaExpression</span> <span class="identifier">with</span> <span class="identifier">the</span> <span class="identifier">inner</span> <span class="keyword">class</span><span class="special">/</span><span class="keyword">struct</span> <span class="identifier">returns</span> <span class="identifier">a</span> <span class="identifier">value</span> <span class="identifier">of</span> <span class="keyword">true</span><span class="special">,</span> <span class="identifier">otherwise</span> <span class="keyword">false</span><span class="special">.</span> 79 80<span class="identifier">A</span> <span class="identifier">popular</span> <span class="identifier">use</span> <span class="identifier">of</span> <span class="identifier">the</span> <span class="identifier">optional</span> <span class="identifier">MPL</span> <span class="identifier">lambda</span> <span class="identifier">expression</span> <span class="identifier">is</span> <span class="identifier">to</span> <span class="identifier">check</span> <span class="identifier">whether</span> <span class="identifier">the</span> <span class="keyword">class</span><span class="special">/</span><span class="keyword">struct</span> <span class="identifier">found</span> <span class="identifier">is</span> <span class="identifier">the</span> <span class="identifier">same</span> 81<span class="identifier">as</span> <span class="identifier">another</span> <span class="identifier">type</span><span class="special">,</span> <span class="identifier">when</span> <span class="identifier">the</span> <span class="keyword">class</span><span class="special">/</span><span class="keyword">struct</span> <span class="identifier">found</span> <span class="identifier">is</span> <span class="identifier">a</span> <span class="keyword">typedef</span><span class="special">.</span> <span class="identifier">In</span> <span class="identifier">that</span> <span class="keyword">case</span> <span class="identifier">our</span> <span class="identifier">example</span> <span class="identifier">would</span> <span class="identifier">be</span><span class="special">:</span> 82 83<span class="identifier">LookFor</span><span class="special"><</span><span class="identifier">EnclosingType</span><span class="special">,</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">is_same</span><span class="special"><</span><span class="identifier">_</span><span class="special">,</span><span class="identifier">SomeOtherType</span><span class="special">></span> <span class="special">></span><span class="special">::</span><span class="identifier">value</span> <span class="identifier">is</span> <span class="keyword">true</span> <span class="keyword">if</span> <span class="identifier">MyType</span> <span class="identifier">is</span> <span class="identifier">an</span> <span class="identifier">inner</span> <span class="keyword">class</span><span class="special">/</span><span class="keyword">struct</span> 84 <span class="identifier">of</span> <span class="identifier">EnclosingType</span> <span class="identifier">and</span> <span class="identifier">is</span> <span class="identifier">the</span> <span class="identifier">same</span> <span class="identifier">type</span> <span class="identifier">as</span> <span class="identifier">SomeOtherType</span><span class="special">.</span> 85</pre> 86<p> </p> 87</div> 88</div> 89<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 90<td align="left"></td> 91<td align="right"><div class="copyright-footer">Copyright © 2011-2013 Tropic Software 92 East Inc<p> 93 Distributed under the Boost Software License, Version 1.0. (See accompanying 94 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>) 95 </p> 96</div></td> 97</tr></table> 98<hr> 99<div class="spirit-nav"> 100<a accesskey="p" href="header/boost/tti/has_class_hpp.html"><img src="../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="header/boost/tti/has_class_hpp.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="BOOST_TTI_HAS_CLASS.html"><img src="../../../../doc/src/images/next.png" alt="Next"></a> 101</div> 102</body> 103</html> 104
