1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>BOOST_DECLARE_HAS_MEMBER</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. Boost.Convert 2.0"> 8<link rel="up" href="../supporting_tools.html" title="Supporting Tools"> 9<link rel="prev" href="boost__cnv__is_range.html" title="boost::cnv::is_range"> 10<link rel="next" href="boost_declare_is_callable.html" title="BOOST_DECLARE_IS_CALLABLE"> 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="boost__cnv__is_range.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../supporting_tools.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_declare_is_callable.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="boost_convert.supporting_tools.boost_declare_has_member"></a><a class="link" href="boost_declare_has_member.html" title="BOOST_DECLARE_HAS_MEMBER">BOOST_DECLARE_HAS_MEMBER</a> 28</h3></div></div></div> 29<p> 30 The BOOST_DECLARE_HAS_MEMBER macro allows to declare a trait which would 31 then let introspect the existence of a class member (a data member or a member 32 function) with the specified name. 33 </p> 34<p> 35 For example, the following declarations introduce <code class="computeroutput"><span class="identifier">local</span><span class="special">::</span><span class="identifier">has_begin</span></code> 36 and <code class="computeroutput"><span class="identifier">local</span><span class="special">::</span><span class="identifier">has_funop</span></code> traits which then allow to test 37 if the supplied class has respectively <code class="computeroutput"><span class="identifier">begin</span></code> 38 and <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code> 39 members: 40 </p> 41<p> 42</p> 43<pre class="programlisting"><span class="keyword">namespace</span> <span class="special">{</span> <span class="keyword">namespace</span> <span class="identifier">local</span> 44<span class="special">{</span> 45 <span class="identifier">BOOST_DECLARE_HAS_MEMBER</span><span class="special">(</span><span class="identifier">has_begin</span><span class="special">,</span> <span class="identifier">begin</span><span class="special">);</span> 46 <span class="identifier">BOOST_DECLARE_HAS_MEMBER</span><span class="special">(</span><span class="identifier">has_funop</span><span class="special">,</span> <span class="keyword">operator</span><span class="special">());</span> 47<span class="special">}}</span> 48</pre> 49<p> 50 </p> 51<p> 52 Now these traits can be used as follows: 53 </p> 54<p> 55</p> 56<pre class="programlisting"><span class="keyword">namespace</span> <span class="special">{</span> <span class="keyword">namespace</span> <span class="identifier">local</span> 57<span class="special">{</span> 58 <span class="keyword">struct</span> <span class="identifier">test01</span> <span class="special">{</span> <span class="keyword">int</span> <span class="identifier">begin</span><span class="special">;</span> <span class="special">};</span> 59 <span class="keyword">struct</span> <span class="identifier">test02</span> <span class="special">{</span> <span class="keyword">char</span><span class="special">*</span> <span class="identifier">begin</span><span class="special">()</span> <span class="special">{</span> <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span> <span class="special">}</span> <span class="special">};</span> 60 <span class="keyword">struct</span> <span class="identifier">test22</span> <span class="special">{</span> <span class="keyword">void</span> <span class="keyword">operator</span><span class="special">()()</span> <span class="special">{}</span> <span class="special">};</span> 61<span class="special">}}</span> 62</pre> 63<p> 64 </p> 65<p> 66</p> 67<pre class="programlisting"><span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">local</span><span class="special">::</span><span class="identifier">has_begin</span><span class="special"><</span><span class="identifier">local</span><span class="special">::</span><span class="identifier">test01</span><span class="special">>::</span><span class="identifier">value</span> <span class="special">==</span> <span class="keyword">true</span><span class="special">);</span> 68<span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">local</span><span class="special">::</span><span class="identifier">has_begin</span><span class="special"><</span><span class="identifier">local</span><span class="special">::</span><span class="identifier">test02</span><span class="special">>::</span><span class="identifier">value</span> <span class="special">==</span> <span class="keyword">true</span><span class="special">);</span> 69<span class="identifier">BOOST_TEST</span><span class="special">(</span><span class="identifier">local</span><span class="special">::</span><span class="identifier">has_funop</span><span class="special"><</span><span class="identifier">local</span><span class="special">::</span><span class="identifier">test22</span><span class="special">>::</span><span class="identifier">value</span> <span class="special">==</span> <span class="keyword">true</span><span class="special">);</span> 70</pre> 71<p> 72 </p> 73<p> 74 As it can be seen from the example the traits only check for the existence 75 of a member (be that a data member or a member function) with the specified 76 name. 77 </p> 78</div> 79<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 80<td align="left"></td> 81<td align="right"><div class="copyright-footer">Copyright © 2009-2016 Vladimir Batov<p> 82 Distributed under the Boost Software License, Version 1.0. See copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>. 83 </p> 84</div></td> 85</tr></table> 86<hr> 87<div class="spirit-nav"> 88<a accesskey="p" href="boost__cnv__is_range.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../supporting_tools.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_declare_is_callable.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a> 89</div> 90</body> 91</html> 92
