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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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