generic-library/boost
1
0
Fork 0
Code Issues Pull Requests Releases Wiki Activity
main
boost/libs/tti/doc/html/BOOST_TTI_HAS_UNION.html

102 lines
17 KiB
HTML
Raw Permalink Normal View History

[DEV] add v1.76.0
2021-10-05 21:37:46 +02:00
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Macro BOOST_TTI_HAS_UNION</title>
<link rel="stylesheet" href="../../../../doc/src/boostbook.css" type="text/css">
<meta name="generator" content="DocBook XSL Stylesheets V1.79.1">
<link rel="home" href="index.html" title="Chapter 1. The Type Traits Introspection Library">
<link rel="up" href="header/boost/tti/has_union_hpp.html" title="Header &lt;boost/tti/has_union.hpp&gt;">
<link rel="prev" href="BOOST_TTI_TRAIT_HAS_UNION.html" title="Macro BOOST_TTI_TRAIT_HAS_UNION">
<link rel="next" href="header/boost/tti/member_type_hpp.html" title="Header &lt;boost/tti/member_type.hpp&gt;">
</head>
<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
<table cellpadding="2" width="100%"><tr>
<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../boost.png"></td>
<td align="center"><a href="../../../../index.html">Home</a></td>
<td align="center"><a href="../../../../libs/libraries.htm">Libraries</a></td>
<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td>
<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td>
<td align="center"><a href="../../../../more/index.htm">More</a></td>
</tr></table>
<hr>
<div class="spirit-nav">
<a accesskey="p" href="BOOST_TTI_TRAIT_HAS_UNION.html"><img src="../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="header/boost/tti/has_union_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="header/boost/tti/member_type_hpp.html"><img src="../../../../doc/src/images/next.png" alt="Next"></a>
</div>
<div class="refentry">
<a name="BOOST_TTI_HAS_UNION"></a><div class="titlepage"></div>
<div class="refnamediv">
<h2><span class="refentrytitle">Macro BOOST_TTI_HAS_UNION</span></h2>
<p>BOOST_TTI_HAS_UNION — A macro which expands to a metafunction which tests whether an inner union with a particular name exists. </p>
</div>
<h2 xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv-title">Synopsis</h2>
<div xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv"><pre class="synopsis"><span class="comment">// In header: &lt;<a class="link" href="header/boost/tti/has_union_hpp.html" title="Header &lt;boost/tti/has_union.hpp&gt;">boost/tti/has_union.hpp</a>&gt;
</span>BOOST_TTI_HAS_UNION(name)</pre></div>
<div class="refsect1">
<a name="idm46191511554336"></a><h2>Description</h2>
<p>BOOST_TTI_HAS_UNION is a macro which expands to a metafunction. The metafunction tests whether an inner union with a particular name exists and, optionally, whether an MPL lambda expression invoked with the inner union is true or not. The macro takes the form of BOOST_TTI_HAS_UNION(name) where</p>
<p>name = the name of the inner union.</p>
<p>BOOST_TTI_HAS_UNION generates a metafunction called "has_union_'name'" where 'name' is the macro parameter.</p>
<pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</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">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">has_union_</span><span class="char">'name'</span>
<span class="special">{</span>
<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>
<span class="keyword">typedef</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">bool_</span><span class="special">&lt;</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">&gt;</span> <span class="identifier">type</span><span class="special">;</span>
<span class="special">}</span><span class="special">;</span>
<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>
<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>
<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>
<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>
<span class="identifier">with</span> <span class="identifier">the</span> <span class="identifier">inner</span> <span class="keyword">union</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>
<span class="identifier">value</span><span class="special">.</span>
<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>
<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">union</span>
<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>
<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">union</span> <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="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>
<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">union</span> <span class="identifier">of</span> <span class="char">'name'</span><span class="special">,</span> <span class="identifier">returns</span>
<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>
<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">union</span> <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>
</pre>
<p>Example usage:</p>
<pre class="programlisting"><span class="identifier">BOOST_TTI_HAS_UNION</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="identifier">has_union_MyType</span> <span class="identifier">in</span> <span class="identifier">the</span> <span class="identifier">current</span> <span class="identifier">scope</span>
<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">union</span> <span class="identifier">called</span> <span class="identifier">MyType</span><span class="special">.</span>
<span class="identifier">has_union_MyType</span><span class="special">&lt;</span><span class="identifier">EnclosingType</span><span class="special">&gt;</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">union</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>
<span class="identifier">has_class_MyType</span><span class="special">&lt;</span><span class="identifier">EnclosingType</span><span class="special">,</span><span class="identifier">ALambdaExpression</span><span class="special">&gt;</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">union</span> <span class="identifier">of</span> <span class="identifier">EnclosingType</span>
<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">union</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>
<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">union</span> <span class="identifier">found</span> <span class="identifier">is</span> <span class="identifier">the</span> <span class="identifier">same</span>
<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">union</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>
<span class="identifier">has_union_MyType</span><span class="special">&lt;</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">&lt;</span><span class="identifier">_</span><span class="special">,</span><span class="identifier">SomeOtherType</span><span class="special">&gt;</span> <span class="special">&gt;</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">union</span>
<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>
</pre>
<p> </p>
</div>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright © 2011-2013 Tropic Software
East Inc<p>
Distributed under the Boost Software License, Version 1.0. (See accompanying
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>)
</p>
</div></td>
</tr></table>
<hr>
<div class="spirit-nav">
<a accesskey="p" href="BOOST_TTI_TRAIT_HAS_UNION.html"><img src="../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="header/boost/tti/has_union_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="header/boost/tti/member_type_hpp.html"><img src="../../../../doc/src/images/next.png" alt="Next"></a>
</div>
</body>
</html>
Reference in New Issue Copy Permalink