devn00b
/
EQ2EMu


			
				
					
						
						
							1234567891011121314151617181920212223242526272829303132333435363738394041424344454647484950515253545556575859606162636465666768697071727374757677787980818283848586878889909192939495
							<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
<title>About Tag Types</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&#160;1.&#160;Boost.FunctionTypes 2.5">
<link rel="up" href="../index.html" title="Chapter&#160;1.&#160;Boost.FunctionTypes 2.5">
<link rel="prev" href="use_cases.html" title="Use Cases">
<link rel="next" href="reference.html" title="Reference">
</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="use_cases.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.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="reference.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a>
</div>
<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="boost_functiontypes.about_tag_types"></a><a class="link" href="about_tag_types.html" title="About Tag Types">About Tag Types</a>
</h2></div></div></div>
<p>
      Boost.FunctionTypes uses tag types to encode properties that are not types
      per se, such as calling convention or whether a function is variadic or cv-
      qualified.
    </p>
<p>
      These tags can be used to determine whether one property of a type has a particular
      value.
    </p>
<pre class="programlisting"><span class="identifier">is_function</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">(...),</span> <span class="identifier">variadic</span><span class="special">&gt;::</span><span class="identifier">value</span> <span class="comment">// == true</span>
<span class="identifier">is_function</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">()</span>   <span class="special">,</span> <span class="identifier">variadic</span><span class="special">&gt;::</span><span class="identifier">value</span> <span class="comment">// == false</span>
</pre>
<p>
      A compound property tag describes a combination of possible values of different
      properties. The type <code class="literal">components&lt;F&gt;</code>, where <code class="literal">F</code>
      is a callable builtin type, is a compound property tag that describes <code class="literal">F</code>.
      The <code class="literal">tag</code> class template can be used to combine property tags.
    </p>
<pre class="programlisting"><span class="identifier">tag</span><span class="special">&lt;</span><span class="identifier">non_const</span><span class="special">,</span><span class="identifier">default_cc</span><span class="special">&gt;</span> <span class="comment">// combination of two properties</span>
</pre>
<p>
      When several values for the same property are specified in <code class="literal">tag</code>'s
      argument list, only the rightmost one is used; others are ignored.
    </p>
<pre class="programlisting"><span class="identifier">tag</span><span class="special">&lt;</span><span class="identifier">components</span><span class="special">&lt;</span><span class="identifier">F</span><span class="special">&gt;,</span> <span class="identifier">default_cc</span><span class="special">&gt;</span> <span class="comment">// overrides F's calling convention property</span>
</pre>
<p>
      When compound property tag is specified to analyse a type, all of its component
      properties must match.
    </p>
<pre class="programlisting"><span class="identifier">is_member_function_pointer</span><span class="special">&lt;</span> <span class="identifier">F</span><span class="special">,</span> <span class="identifier">tag</span><span class="special">&lt;</span><span class="identifier">const_qualified</span><span class="special">,</span><span class="identifier">default_cc</span><span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">value</span>
<span class="comment">// true for </span>
<span class="comment">//   F = void(a_class::*)() const</span>
<span class="comment">// false for</span>
<span class="comment">//   F = void(a_class::*)()</span>
<span class="comment">//   F = void(__fastcall a_class::*)() const</span>
</pre>
<p>
      Default values are selected for properties not specified by the tag in the
      context of type synthesis.
    </p>
<pre class="programlisting"><span class="comment">// given S = mpl::vector&lt;int,a_class const &amp;&gt;</span>

<span class="identifier">member_function_pointer</span><span class="special">&lt;</span><span class="identifier">S</span><span class="special">&gt;::</span><span class="identifier">type</span> <span class="comment">// is int (a_class::*)() const</span>
<span class="comment">// note: the cv-qualification is picked based on the class type,</span>
<span class="comment">// a nonvariadic signature and the default calling convention </span>
<span class="comment">// are used</span>

<span class="identifier">member_function_pointer</span><span class="special">&lt;</span><span class="identifier">S</span><span class="special">,</span><span class="identifier">non_const</span><span class="special">&gt;::</span><span class="identifier">type</span> <span class="comment">// is int (a_class::*)()</span>
<span class="comment">// no const qualification, as explicitly specified by the tag type</span>
</pre>
</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 &#169; 2004-2007 Tobias
      Schwinger<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="use_cases.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.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="reference.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a>
</div>
</body>
</html>