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| 24 | </div> | |
| 25 | <div class="section"> | |
| 26 | <div class="titlepage"><div><div><h3 class="title"> | |
| 27 | <a name="boost_typetraits.reference.has_minus_assign"></a><a class="link" href="has_minus_assign.html" title="has_minus_assign">has_minus_assign</a> | |
| 28 | </h3></div></div></div> | |
| 29 | <pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">Lhs</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">Rhs</span><span class="special">=</span><span class="identifier">Lhs</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">Ret</span><span class="special">=</span><span class="identifier">dont_care</span><span class="special">></span> | |
| 30 | <span class="keyword">struct</span> <span class="identifier">has_minus_assign</span> <span class="special">:</span> <span class="keyword">public</span> <em class="replaceable"><code><a class="link" href="integral_constant.html" title="integral_constant">true_type</a>-or-<a class="link" href="integral_constant.html" title="integral_constant">false_type</a></code></em> <span class="special">{};</span> | |
| 31 | </pre> | |
| 32 | <p> | |
| 33 | <span class="bold"><strong>Inherits:</strong></span> If (i) <code class="computeroutput"><span class="identifier">lhs</span></code> | |
| 34 | of type <code class="computeroutput"><span class="identifier">Lhs</span></code> and <code class="computeroutput"><span class="identifier">rhs</span></code> of type <code class="computeroutput"><span class="identifier">Rhs</span></code> | |
| 35 | can be used in expression <code class="computeroutput"><span class="identifier">lhs</span><span class="special">-=</span><span class="identifier">rhs</span></code>, | |
| 36 | and (ii) <code class="computeroutput"><span class="identifier">Ret</span><span class="special">=</span><span class="identifier">dont_care</span></code> or the result of expression | |
| 37 | <code class="computeroutput"><span class="identifier">lhs</span><span class="special">-=</span><span class="identifier">rhs</span></code> is convertible to <code class="computeroutput"><span class="identifier">Ret</span></code> | |
| 38 | then inherits from <a class="link" href="integral_constant.html" title="integral_constant">true_type</a>, | |
| 39 | otherwise inherits from <a class="link" href="integral_constant.html" title="integral_constant">false_type</a>. | |
| 40 | </p> | |
| 41 | <p> | |
| 42 | The default behaviour (<code class="computeroutput"><span class="identifier">Ret</span><span class="special">=</span><span class="identifier">dont_care</span></code>) | |
| 43 | is to not check for the return value of binary <code class="computeroutput"><span class="keyword">operator</span><span class="special">-=</span></code>. If <code class="computeroutput"><span class="identifier">Ret</span></code> | |
| 44 | is different from the default <code class="computeroutput"><span class="identifier">dont_care</span></code> | |
| 45 | type, the return value is checked to be convertible to <code class="computeroutput"><span class="identifier">Ret</span></code>. | |
| 46 | Convertible to <code class="computeroutput"><span class="identifier">Ret</span></code> means | |
| 47 | that the return value of the operator can be used as argument to a function | |
| 48 | expecting <code class="computeroutput"><span class="identifier">Ret</span></code>: | |
| 49 | </p> | |
| 50 | <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> | |
| 51 | <span class="identifier">Lhs</span> <span class="identifier">lhs</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">lhs</span><span class="special">-=</span><span class="identifier">rhs</span><span class="special">);</span> <span class="comment">// is valid if has_minus_assign<Lhs, 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_minus_assign</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_minus_assign</span><span class="special"><</span><span class="identifier">Lhs</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_minus_assign</span><span class="special"><</span><span class="identifier">Lhs</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_minus_assign</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_minus_assign</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_minus_assign</span><span class="special"><</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">int</span><span class="special">,</span> <span class="keyword">int</span><span class="special">></span></code> | |
| 90 | inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">true_type</a></code>. | |
| 91 | </p></blockquote></div> | |
| 92 | <div class="blockquote"><blockquote class="blockquote"><p> | |
| 93 | <code class="computeroutput"><span class="identifier">has_minus_assign</span><span class="special"><</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">int</span><span class="special">,</span> <span class="keyword">long</span><span class="special">></span></code> | |
| 94 | inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">true_type</a></code>. | |
| 95 | </p></blockquote></div> | |
| 96 | <div class="blockquote"><blockquote class="blockquote"><p> | |
| 97 | <code class="computeroutput"><span class="identifier">has_minus_assign</span><span class="special"><</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">double</span><span class="special">,</span> <span class="keyword">double</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>. | |
| 98 | </p></blockquote></div> | |
| 99 | <div class="blockquote"><blockquote class="blockquote"><p> | |
| 100 | <code class="computeroutput"><span class="identifier">has_minus_assign</span><span class="special"><</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">double</span><span class="special">,</span> <span class="keyword">int</span><span class="special">></span></code> | |
| 101 | inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">true_type</a></code>. | |
| 102 | </p></blockquote></div> | |
| 103 | <div class="blockquote"><blockquote class="blockquote"><p> | |
| 104 | <code class="computeroutput"><span class="identifier">has_minus_assign</span><span class="special"><</span><span class="keyword">const</span> <span class="keyword">int</span><span class="special">,</span> <span class="keyword">int</span><span class="special">>::</span><span class="identifier">value</span></code> inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">false_type</a></code>. | |
| 105 | </p></blockquote></div> | |
| 106 | <div class="blockquote"><blockquote class="blockquote"><p> | |
| 107 | <code class="computeroutput"><span class="identifier">has_minus_assign</span><span class="special"><</span><span class="keyword">int</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>. | |
| 108 | </p></blockquote></div> | |
| 109 | <p> | |
| 110 | <span class="bold"><strong>See also:</strong></span> <a class="link" href="../category/value_traits/operators.html" title="Operator Type Traits">Operator | |
| 111 | Type Traits</a> | |
| 112 | </p> | |
| 113 | <p> | |
| 114 | <span class="bold"><strong>Known issues:</strong></span> | |
| 115 | </p> | |
| 116 | <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> | |
| 117 | <li class="listitem"> | |
| 118 | This trait cannot detect whether binary <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> | |
| 119 | is defined as a private member of <code class="computeroutput"><span class="identifier">Lhs</span></code> | |
| 120 | then instantiating <code class="computeroutput"><span class="identifier">has_minus_assign</span><span class="special"><</span><span class="identifier">Lhs</span><span class="special">></span></code> will produce a compiler error. For | |
| 121 | this reason <code class="computeroutput"><span class="identifier">has_minus_assign</span></code> | |
| 122 | cannot be used to determine whether a type has a public <code class="computeroutput"><span class="keyword">operator</span><span class="special">-=</span></code> | |
| 123 | or not. | |
| 124 | <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="keyword">const</span> <span class="identifier">A</span><span class="special">&);</span> <span class="special">};</span> | |
| 125 | <span class="identifier">boost</span><span class="special">::</span><span class="identifier">has_minus_assign</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-=(const A&) is private</span> | |
| 126 | </pre> | |
| 127 | </li> | |
| 128 | <li class="listitem"> | |
| 129 | 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> | |
| 130 | is convertible to <code class="computeroutput"><span class="identifier">A</span></code>. | |
| 131 | In this case, the compiler will report an ambiguous overload. | |
| 132 | <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">A</span> <span class="special">{</span> <span class="special">};</span> | |
| 133 | <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> <span class="keyword">const</span> <span class="identifier">A</span><span class="special">&);</span> | |
| 134 | <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> | |
| 135 | <span class="identifier">boost</span><span class="special">::</span><span class="identifier">has_minus_assign</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> | |
| 136 | <span class="identifier">boost</span><span class="special">::</span><span class="identifier">has_minus_assign</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> | |
| 137 | </pre> | |
| 138 | </li> | |
| 139 | <li class="listitem"> | |
| 140 | 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> | |
| 141 | is defined but does not bind for a given template type, it is still detected | |
| 142 | by the trait which returns <code class="computeroutput"><span class="keyword">true</span></code> | |
| 143 | instead of <code class="computeroutput"><span class="keyword">false</span></code>. Example: | |
| 144 | <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_minus_assign</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> | |
| 145 | <span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span> | |
| 146 | ||
| 147 | <span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> | |
| 148 | <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> | |
| 149 | ||
| 150 | <span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> | |
| 151 | <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">lhs</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> | |
| 152 | <span class="keyword">return</span> <span class="identifier">f</span><span class="special">(</span><span class="identifier">lhs</span><span class="special">.</span><span class="identifier">data</span><span class="special">,</span> <span class="identifier">rhs</span><span class="special">.</span><span class="identifier">data</span><span class="special">);</span> | |
| 153 | <span class="special">}</span> | |
| 154 | ||
| 155 | <span class="keyword">class</span> <span class="identifier">bad</span> <span class="special">{</span> <span class="special">};</span> | |
| 156 | <span class="keyword">class</span> <span class="identifier">good</span> <span class="special">{</span> <span class="special">};</span> | |
| 157 | <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="keyword">const</span> <span class="identifier">good</span><span class="special">&)</span> <span class="special">{</span> <span class="special">}</span> | |
| 158 | ||
| 159 | <span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span> <span class="special">{</span> | |
| 160 | <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> | |
| 161 | <span class="comment">// works fine for contains<good></span> | |
| 162 | <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_minus_assign</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> | |
| 163 | <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> | |
| 164 | <span class="identifier">g</span><span class="special">-=</span><span class="identifier">g</span><span class="special">;</span> <span class="comment">// ok</span> | |
| 165 | <span class="comment">// does not work for contains<bad></span> | |
| 166 | <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_minus_assign</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> | |
| 167 | <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> | |
| 168 | <span class="identifier">b</span><span class="special">-=</span><span class="identifier">b</span><span class="special">;</span> <span class="comment">// compile time error</span> | |
| 169 | <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span> | |
| 170 | <span class="special">}</span> | |
| 171 | </pre> | |
| 172 | </li> | |
| 173 | <li class="listitem"> | |
| 174 | <code class="computeroutput"><span class="keyword">volatile</span></code> qualifier is not | |
| 175 | properly handled and would lead to undefined behavior | |
| 176 | </li> | |
| 177 | </ul></div> | |
| 178 | </div> | |
| 179 | <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> | |
| 180 | <td align="left"></td> | |
| 181 | <td align="right"><div class="copyright-footer">Copyright © 2000, 2011 Adobe Systems Inc, David Abrahams, | |
| 182 | Frederic Bron, Steve Cleary, Beman Dawes, Aleksey Gurtovoy, Howard Hinnant, | |
| 183 | Jesse Jones, Mat Marcus, Itay Maman, John Maddock, Alexander Nasonov, Thorsten | |
| 184 | Ottosen, Roman Perepelitsa, Robert Ramey, Jeremy Siek, Robert Stewart and Steven | |
| 185 | Watanabe<p> | |
| 186 | Distributed under the Boost Software License, Version 1.0. (See accompanying | |
| 187 | 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>) | |
| 188 | </p> | |
| 189 | </div></td> | |
| 190 | </tr></table> | |
| 191 | <hr> | |
| 192 | <div class="spirit-nav"> | |
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