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- <title>About Tag Types</title>
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- <link rel="home" href="../index.html" title="Chapter 1. Boost.FunctionTypes 2.5">
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- <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"><</span><span class="keyword">int</span><span class="special">(...),</span> <span class="identifier">variadic</span><span class="special">>::</span><span class="identifier">value</span> <span class="comment">// == true</span>
- <span class="identifier">is_function</span><span class="special"><</span><span class="keyword">int</span><span class="special">()</span> <span class="special">,</span> <span class="identifier">variadic</span><span class="special">>::</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<F></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"><</span><span class="identifier">non_const</span><span class="special">,</span><span class="identifier">default_cc</span><span class="special">></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"><</span><span class="identifier">components</span><span class="special"><</span><span class="identifier">F</span><span class="special">>,</span> <span class="identifier">default_cc</span><span class="special">></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"><</span> <span class="identifier">F</span><span class="special">,</span> <span class="identifier">tag</span><span class="special"><</span><span class="identifier">const_qualified</span><span class="special">,</span><span class="identifier">default_cc</span><span class="special">></span> <span class="special">>::</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<int,a_class const &></span>
- <span class="identifier">member_function_pointer</span><span class="special"><</span><span class="identifier">S</span><span class="special">>::</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"><</span><span class="identifier">S</span><span class="special">,</span><span class="identifier">non_const</span><span class="special">>::</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 © 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>
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