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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"></a><a class="link" href="has_minus.html" title="has_minus">has_minus</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</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>, and | |
36 | (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<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</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</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</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</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</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</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</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</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</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</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">true_type</a></code>. | |
105 | </p></blockquote></div> | |
106 | <div class="blockquote"><blockquote class="blockquote"><p> | |
107 | <code class="computeroutput"><span class="identifier">has_minus</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> is defined as a private member of <code class="computeroutput"><span class="identifier">Lhs</span></code> then instantiating <code class="computeroutput"><span class="identifier">has_minus</span><span class="special"><</span><span class="identifier">Lhs</span><span class="special">></span></code> | |
119 | will produce a compiler error. For this reason <code class="computeroutput"><span class="identifier">has_minus</span></code> | |
120 | cannot be used to determine whether a type has a public <code class="computeroutput"><span class="keyword">operator</span><span class="special">-</span></code> | |
121 | or not. | |
122 | <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> | |
123 | <span class="identifier">boost</span><span class="special">::</span><span class="identifier">has_minus</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> | |
124 | </pre> | |
125 | </li> | |
126 | <li class="listitem"> | |
127 | 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> | |
128 | is convertible to <code class="computeroutput"><span class="identifier">A</span></code>. | |
129 | In this case, the compiler will report an ambiguous overload. | |
130 | <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">A</span> <span class="special">{</span> <span class="special">};</span> | |
131 | <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> | |
132 | <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> | |
133 | <span class="identifier">boost</span><span class="special">::</span><span class="identifier">has_minus</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> | |
134 | <span class="identifier">boost</span><span class="special">::</span><span class="identifier">has_minus</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> | |
135 | </pre> | |
136 | </li> | |
137 | <li class="listitem"> | |
138 | 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> | |
139 | is defined but does not bind for a given template type, it is still detected | |
140 | by the trait which returns <code class="computeroutput"><span class="keyword">true</span></code> | |
141 | instead of <code class="computeroutput"><span class="keyword">false</span></code>. Example: | |
142 | <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</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> | |
143 | <span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span> | |
144 | ||
145 | <span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> | |
146 | <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> | |
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">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> | |
150 | <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> | |
151 | <span class="special">}</span> | |
152 | ||
153 | <span class="keyword">class</span> <span class="identifier">bad</span> <span class="special">{</span> <span class="special">};</span> | |
154 | <span class="keyword">class</span> <span class="identifier">good</span> <span class="special">{</span> <span class="special">};</span> | |
155 | <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> | |
156 | ||
157 | <span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span> <span class="special">{</span> | |
158 | <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> | |
159 | <span class="comment">// works fine for contains<good></span> | |
160 | <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</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> | |
161 | <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> | |
162 | <span class="identifier">g</span><span class="special">-</span><span class="identifier">g</span><span class="special">;</span> <span class="comment">// ok</span> | |
163 | <span class="comment">// does not work for contains<bad></span> | |
164 | <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</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> | |
165 | <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> | |
166 | <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> | |
167 | <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span> | |
168 | <span class="special">}</span> | |
169 | </pre> | |
170 | </li> | |
171 | <li class="listitem"> | |
172 | <code class="computeroutput"><span class="keyword">volatile</span></code> qualifier is not | |
173 | properly handled and would lead to undefined behavior | |
174 | </li> | |
175 | </ul></div> | |
176 | </div> | |
177 | <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> | |
178 | <td align="left"></td> | |
179 | <td align="right"><div class="copyright-footer">Copyright © 2000, 2011 Adobe Systems Inc, David Abrahams, | |
180 | Frederic Bron, Steve Cleary, Beman Dawes, Aleksey Gurtovoy, Howard Hinnant, | |
181 | Jesse Jones, Mat Marcus, Itay Maman, John Maddock, Alexander Nasonov, Thorsten | |
182 | Ottosen, Roman Perepelitsa, Robert Ramey, Jeremy Siek, Robert Stewart and Steven | |
183 | Watanabe<p> | |
184 | Distributed under the Boost Software License, Version 1.0. (See accompanying | |
185 | 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>) | |
186 | </p> | |
187 | </div></td> | |
188 | </tr></table> | |
189 | <hr> | |
190 | <div class="spirit-nav"> | |
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