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| 2 | <head> | |
| 3 | <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> | |
| 4 | <title>About Tag Types</title> | |
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| 24 | </div> | |
| 25 | <div class="section"> | |
| 26 | <div class="titlepage"><div><div><h2 class="title" style="clear: both"> | |
| 27 | <a name="boost_functiontypes.about_tag_types"></a><a class="link" href="about_tag_types.html" title="About Tag Types">About Tag Types</a> | |
| 28 | </h2></div></div></div> | |
| 29 | <p> | |
| 30 | Boost.FunctionTypes uses tag types to encode properties that are not types | |
| 31 | per se, such as calling convention or whether a function is variadic or cv- | |
| 32 | qualified. | |
| 33 | </p> | |
| 34 | <p> | |
| 35 | These tags can be used to determine whether one property of a type has a particular | |
| 36 | value. | |
| 37 | </p> | |
| 38 | <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 | |
| 39 | </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 | |
| 40 | </span></pre> | |
| 41 | <p> | |
| 42 | A compound property tag describes a combination of possible values of different | |
| 43 | properties. The type <code class="literal">components<F></code>, where <code class="literal">F</code> | |
| 44 | is a callable builtin type, is a compound property tag that describes <code class="literal">F</code>. | |
| 45 | The <code class="literal">tag</code> class template can be used to combine property tags. | |
| 46 | </p> | |
| 47 | <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 | |
| 48 | </span></pre> | |
| 49 | <p> | |
| 50 | When several values for the same property are specified in <code class="literal">tag</code>'s | |
| 51 | argument list, only the rightmost one is used; others are ignored. | |
| 52 | </p> | |
| 53 | <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 | |
| 54 | </span></pre> | |
| 55 | <p> | |
| 56 | When compound property tag is specified to analyse a type, all of its component | |
| 57 | properties must match. | |
| 58 | </p> | |
| 59 | <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> | |
| 60 | <span class="comment">// true for | |
| 61 | </span><span class="comment">// F = void(a_class::*)() const | |
| 62 | </span><span class="comment">// false for | |
| 63 | </span><span class="comment">// F = void(a_class::*)() | |
| 64 | </span><span class="comment">// F = void(__fastcall a_class::*)() const | |
| 65 | </span></pre> | |
| 66 | <p> | |
| 67 | Default values are selected for properties not specified by the tag in the | |
| 68 | context of type synthesis. | |
| 69 | </p> | |
| 70 | <pre class="programlisting"><span class="comment">// given S = mpl::vector<int,a_class const &> | |
| 71 | </span> | |
| 72 | <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 | |
| 73 | </span><span class="comment">// note: the cv-qualification is picked based on the class type, | |
| 74 | </span><span class="comment">// a nonvariadic signature and the default calling convention | |
| 75 | </span><span class="comment">// are used | |
| 76 | </span> | |
| 77 | <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::*)() | |
| 78 | </span><span class="comment">// no const qualification, as explicitly specified by the tag type | |
| 79 | </span></pre> | |
| 80 | </div> | |
| 81 | <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> | |
| 82 | <td align="left"></td> | |
| 83 | <td align="right"><div class="copyright-footer">Copyright © 2004-2007 Tobias | |
| 84 | Schwinger<p> | |
| 85 | Distributed under the Boost Software License, Version 1.0. (See accompanying | |
| 86 | 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>) | |
| 87 | </p> | |
| 88 | </div></td> | |
| 89 | </tr></table> | |
| 90 | <hr> | |
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| 93 | </div> | |
| 94 | </body> | |
| 95 | </html> |