1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>has_complement</title> 5<link rel="stylesheet" href="../../../../../../doc/src/boostbook.css" type="text/css"> 6<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 7<link rel="home" href="../../index.html" title="Chapter 1. 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If <code class="computeroutput"><span class="identifier">Ret</span></code> 45 is different from the default <code class="computeroutput"><span class="identifier">dont_care</span></code> 46 type, the return value is checked to be convertible to <code class="computeroutput"><span class="identifier">Ret</span></code>. 47 Convertible to <code class="computeroutput"><span class="identifier">Ret</span></code> means 48 that the return value of the operator can be used as argument to a function 49 expecting <code class="computeroutput"><span class="identifier">Ret</span></code>: 50</p> 51<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">f</span><span class="special">(</span><span class="identifier">Ret</span><span class="special">);</span> 52<span class="identifier">Rhs</span> <span class="identifier">rhs</span><span class="special">;</span> 53<span class="identifier">f</span><span class="special">(~</span><span class="identifier">rhs</span><span class="special">);</span> <span class="comment">// is valid if has_complement<Rhs, Ret>::value==true</span> 54</pre> 55<p> 56 If <code class="computeroutput"><span class="identifier">Ret</span><span class="special">=</span><span class="keyword">void</span></code>, the return type is checked to be exactly 57 <code class="computeroutput"><span class="keyword">void</span></code>. 58 </p> 59<p> 60 <span class="bold"><strong>Header:</strong></span> <code class="computeroutput"><span class="preprocessor">#include</span> 61 <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">type_traits</span><span class="special">/</span><span class="identifier">has_complement</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></code> 62 or <code class="computeroutput"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">type_traits</span><span class="special">/</span><span class="identifier">has_operator</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></code> 63 or <code class="computeroutput"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">type_traits</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></code> 64 </p> 65<p> 66 <span class="bold"><strong>Compiler Compatibility:</strong></span> Requires working 67 SFINAE (i.e. BOOST_NO_SFINAE is not set). Only a minority of rather old compilers 68 do not support this. 69 </p> 70<p> 71 <span class="bold"><strong>Examples:</strong></span> 72 </p> 73<div class="blockquote"><blockquote class="blockquote"><p> 74 <code class="computeroutput"><span class="identifier">has_complement</span><span class="special"><</span><span class="identifier">Rhs</span><span class="special">,</span> <span class="identifier">Ret</span><span class="special">>::</span><span class="identifier">value_type</span></code> is the type <code class="computeroutput"><span class="keyword">bool</span></code>. 75 </p></blockquote></div> 76<div class="blockquote"><blockquote class="blockquote"><p> 77 <code class="computeroutput"><span class="identifier">has_complement</span><span class="special"><</span><span class="identifier">Rhs</span><span class="special">,</span> <span class="identifier">Ret</span><span class="special">>::</span><span class="identifier">value</span></code> is a <code class="computeroutput"><span class="keyword">bool</span></code> 78 integral constant expression. 79 </p></blockquote></div> 80<div class="blockquote"><blockquote class="blockquote"><p> 81 <code class="computeroutput"><span class="identifier">has_complement</span><span class="special"><</span><span class="keyword">int</span><span class="special">>::</span><span class="identifier">value</span></code> is a <code class="computeroutput"><span class="keyword">bool</span></code> 82 integral constant expression that evaluates to <code class="computeroutput"><span class="keyword">true</span></code>. 83 </p></blockquote></div> 84<div class="blockquote"><blockquote class="blockquote"><p> 85 <code class="computeroutput"><span class="identifier">has_complement</span><span class="special"><</span><span class="keyword">long</span><span class="special">></span></code> 86 inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">true_type</a></code>. 87 </p></blockquote></div> 88<div class="blockquote"><blockquote class="blockquote"><p> 89 <code class="computeroutput"><span class="identifier">has_complement</span><span class="special"><</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">int</span><span class="special">></span></code> inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">true_type</a></code>. 90 </p></blockquote></div> 91<div class="blockquote"><blockquote class="blockquote"><p> 92 <code class="computeroutput"><span class="identifier">has_complement</span><span class="special"><</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">long</span><span class="special">></span></code> inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">true_type</a></code>. 93 </p></blockquote></div> 94<div class="blockquote"><blockquote class="blockquote"><p> 95 <code class="computeroutput"><span class="identifier">has_complement</span><span class="special"><</span><span class="keyword">const</span> <span class="keyword">int</span><span class="special">></span></code> inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">true_type</a></code>. 96 </p></blockquote></div> 97<div class="blockquote"><blockquote class="blockquote"><p> 98 <code class="computeroutput"><span class="identifier">has_complement</span><span class="special"><</span><span class="keyword">int</span><span class="special">*></span></code> 99 inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">false_type</a></code>. 100 </p></blockquote></div> 101<div class="blockquote"><blockquote class="blockquote"><p> 102 <code class="computeroutput"><span class="identifier">has_complement</span><span class="special"><</span><span class="keyword">double</span><span class="special">,</span> <span class="keyword">double</span><span class="special">></span></code> 103 inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">false_type</a></code>. 104 </p></blockquote></div> 105<div class="blockquote"><blockquote class="blockquote"><p> 106 <code class="computeroutput"><span class="identifier">has_complement</span><span class="special"><</span><span class="keyword">double</span><span class="special">,</span> <span class="keyword">int</span><span class="special">></span></code> 107 inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">false_type</a></code>. 108 </p></blockquote></div> 109<div class="blockquote"><blockquote class="blockquote"><p> 110 <code class="computeroutput"><span class="identifier">has_complement</span><span class="special"><</span><span class="keyword">int</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">></span></code> inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">false_type</a></code>. 111 </p></blockquote></div> 112<p> 113 <span class="bold"><strong>See also:</strong></span> <a class="link" href="../category/value_traits/operators.html" title="Operator Type Traits">Operator 114 Type Traits</a> 115 </p> 116<p> 117 <span class="bold"><strong>Known issues:</strong></span> 118 </p> 119<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 120<li class="listitem"> 121 This trait cannot detect whether prefix <code class="computeroutput"><span class="keyword">operator</span><span class="special">~</span></code> is public or not: if <code class="computeroutput"><span class="keyword">operator</span><span class="special">~</span></code> is defined as a private member of <code class="computeroutput"><span class="identifier">Rhs</span></code> then instantiating <code class="computeroutput"><span class="identifier">has_complement</span><span class="special"><</span><span class="identifier">Rhs</span><span class="special">></span></code> 122 will produce a compiler error. For this reason <code class="computeroutput"><span class="identifier">has_complement</span></code> 123 cannot be used to determine whether a type has a public <code class="computeroutput"><span class="keyword">operator</span><span class="special">~</span></code> 124 or not. 125<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">A</span> <span class="special">{</span> <span class="keyword">private</span><span class="special">:</span> <span class="keyword">void</span> <span class="keyword">operator</span><span class="special">~();</span> <span class="special">};</span> 126<span class="identifier">boost</span><span class="special">::</span><span class="identifier">has_complement</span><span class="special"><</span><span class="identifier">A</span><span class="special">>::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// error: A::operator~() is private</span> 127</pre> 128 </li> 129<li class="listitem"> 130 There is an issue if the operator exists only for type <code class="computeroutput"><span class="identifier">A</span></code> and <code class="computeroutput"><span class="identifier">B</span></code> 131 is convertible to <code class="computeroutput"><span class="identifier">A</span></code>. 132 In this case, the compiler will report an ambiguous overload. 133<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">A</span> <span class="special">{</span> <span class="special">};</span> 134<span class="keyword">void</span> <span class="keyword">operator</span><span class="special">~(</span><span class="keyword">const</span> <span class="identifier">A</span><span class="special">&);</span> 135<span class="keyword">struct</span> <span class="identifier">B</span> <span class="special">{</span> <span class="keyword">operator</span> <span class="identifier">A</span><span class="special">();</span> <span class="special">};</span> 136<span class="identifier">boost</span><span class="special">::</span><span class="identifier">has_complement</span><span class="special"><</span><span class="identifier">A</span><span class="special">>::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// this is fine</span> 137<span class="identifier">boost</span><span class="special">::</span><span class="identifier">has_complement</span><span class="special"><</span><span class="identifier">B</span><span class="special">>::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// error: ambiguous overload</span> 138</pre> 139 </li> 140<li class="listitem"> 141 There is an issue when applying this trait to template classes. If <code class="computeroutput"><span class="keyword">operator</span><span class="special">~</span></code> 142 is defined but does not bind for a given template type, it is still detected 143 by the trait which returns <code class="computeroutput"><span class="keyword">true</span></code> 144 instead of <code class="computeroutput"><span class="keyword">false</span></code>. Example: 145<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">type_traits</span><span class="special">/</span><span class="identifier">has_complement</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 146<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span> 147 148<span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> 149<span class="keyword">struct</span> <span class="identifier">contains</span> <span class="special">{</span> <span class="identifier">T</span> <span class="identifier">data</span><span class="special">;</span> <span class="special">};</span> 150 151<span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> 152<span class="keyword">bool</span> <span class="keyword">operator</span><span class="special">~(</span><span class="keyword">const</span> <span class="identifier">contains</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="special">&</span><span class="identifier">rhs</span><span class="special">)</span> <span class="special">{</span> 153 <span class="keyword">return</span> <span class="identifier">f</span><span class="special">(</span><span class="identifier">rhs</span><span class="special">.</span><span class="identifier">data</span><span class="special">);</span> 154<span class="special">}</span> 155 156<span class="keyword">class</span> <span class="identifier">bad</span> <span class="special">{</span> <span class="special">};</span> 157<span class="keyword">class</span> <span class="identifier">good</span> <span class="special">{</span> <span class="special">};</span> 158<span class="keyword">bool</span> <span class="identifier">f</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">good</span><span class="special">&)</span> <span class="special">{</span> <span class="special">}</span> 159 160<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span> <span class="special">{</span> 161 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special"><<</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">boolalpha</span><span class="special">;</span> 162 <span class="comment">// works fine for contains<good></span> 163 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special"><<</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">has_complement</span><span class="special"><</span> <span class="identifier">contains</span><span class="special"><</span> <span class="identifier">good</span> <span class="special">></span> <span class="special">>::</span><span class="identifier">value</span><span class="special"><<</span><span class="char">'\n'</span><span class="special">;</span> <span class="comment">// true</span> 164 <span class="identifier">contains</span><span class="special"><</span><span class="identifier">good</span><span class="special">></span> <span class="identifier">g</span><span class="special">;</span> 165 <span class="special">~</span><span class="identifier">g</span><span class="special">;</span> <span class="comment">// ok</span> 166 <span class="comment">// does not work for contains<bad></span> 167 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special"><<</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">has_complement</span><span class="special"><</span> <span class="identifier">contains</span><span class="special"><</span> <span class="identifier">bad</span> <span class="special">></span> <span class="special">>::</span><span class="identifier">value</span><span class="special"><<</span><span class="char">'\n'</span><span class="special">;</span> <span class="comment">// true, should be false</span> 168 <span class="identifier">contains</span><span class="special"><</span><span class="identifier">bad</span><span class="special">></span> <span class="identifier">b</span><span class="special">;</span> 169 <span class="special">~</span><span class="identifier">b</span><span class="special">;</span> <span class="comment">// compile time error</span> 170 <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span> 171<span class="special">}</span> 172</pre> 173 </li> 174<li class="listitem"> 175 <code class="computeroutput"><span class="keyword">volatile</span></code> qualifier is not 176 properly handled and would lead to undefined behavior 177 </li> 178</ul></div> 179<p> 180 <span class="bold"><strong>Known issues:</strong></span> 181 </p> 182<p> 183 For modern compilers (those that support arbitrary SFINAE-expressions and 184 decltype/declval) this trait offers near perfect detection. In this situation 185 the macro <code class="computeroutput"><span class="identifier">BOOST_TT_HAS_ACCURATE_BINARY_OPERATOR_DETECTION</span></code> 186 will be defined after including <code class="literal"><boost/type_traits/has_complement.hpp></code>. 187 Please note however, that detection is based on function signature only, 188 in the case that the operator is a function template then has_complement 189 cannot perform introspection of the template function body to ensure that 190 the type meets all of the conceptual requirements of the actual code. 191 </p> 192<p> 193 For older compilers (<code class="computeroutput"><span class="identifier">BOOST_TT_HAS_ACCURATE_BINARY_OPERATOR_DETECTION</span></code> 194 not defined) then there are a number of issues: 195 </p> 196<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 197<li class="listitem"> 198 This trait cannot detect whether unary <code class="computeroutput"><span class="keyword">operator</span></code>~ 199 is public or not: if <code class="computeroutput"><span class="keyword">operator</span></code>~ 200 is defined as a private class member of type T then instantiating <code class="literal">has_complement<T></code> 201 will produce a compiler error. For this reason <code class="literal">has_complement</code> 202 cannot be used to determine whether a type has a public <code class="computeroutput"><span class="keyword">operator</span></code>~ or not. 203 </li> 204<li class="listitem"> 205 <code class="computeroutput"><span class="keyword">volatile</span></code> qualifier is not 206 properly handled and would lead to undefined behavior 207 </li> 208<li class="listitem"> 209 Capturless lambdas are not supported and will lead to a compiler error 210 if has_complement is instantiated on the lambda type. 211 </li> 212<li class="listitem"> 213 Scoped enum types are not supported and will lead to a compiler error 214 if has_complement is instantiated on such a type. 215 </li> 216</ul></div> 217</div> 218<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 219<td align="left"></td> 220<td align="right"><div class="copyright-footer">Copyright © 2000, 2011 Adobe Systems Inc, David Abrahams, 221 Frederic Bron, Steve Cleary, Beman Dawes, Glen Fernandes, Aleksey Gurtovoy, 222 Howard Hinnant, Jesse Jones, Mat Marcus, Itay Maman, John Maddock, Alexander 223 Nasonov, Thorsten Ottosen, Roman Perepelitsa, Robert Ramey, Jeremy Siek, Robert 224 Stewart and Steven Watanabe<p> 225 Distributed under the Boost Software License, Version 1.0. (See accompanying 226 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>) 227 </p> 228</div></td> 229</tr></table> 230<hr> 231<div class="spirit-nav"> 232<a accesskey="p" href="has_bit_xor_assign.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../reference.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="has_dereference.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a> 233</div> 234</body> 235</html> 236
