<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 V1.75.2"> <link rel="home" href="../index.html" title="Chapter 1. Boost.FunctionTypes 2.5"> <link rel="up" href="../index.html" title="Chapter 1. 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"><</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> </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>