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26<div class="titlepage"><div><div><h3 class="title">
27<a name="boost_typetraits.reference.has_modulus_assign"></a><a class="link" href="has_modulus_assign.html" title="has_modulus_assign">has_modulus_assign</a>
28</h3></div></div></div>
29<pre class="programlisting"><span class="keyword">template</span> <span class="special">&lt;</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">&gt;</span>
30<span class="keyword">struct</span> <span class="identifier">has_modulus_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_modulus_assign&lt;Lhs, Rhs, Ret&gt;::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">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">type_traits</span><span class="special">/</span><span class="identifier">has_modulus_assign</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span></code>
62 or <code class="computeroutput"><span class="preprocessor">#include</span> <span class="special">&lt;</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">&gt;</span></code>
63 or <code class="computeroutput"><span class="preprocessor">#include</span> <span class="special">&lt;</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">&gt;</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_modulus_assign</span><span class="special">&lt;</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">&gt;::</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_modulus_assign</span><span class="special">&lt;</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">&gt;::</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_modulus_assign</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;::</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_modulus_assign</span><span class="special">&lt;</span><span class="keyword">long</span><span class="special">&gt;</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_modulus_assign</span><span class="special">&lt;</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">&gt;</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_modulus_assign</span><span class="special">&lt;</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">&gt;</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_modulus_assign</span><span class="special">&lt;</span><span class="keyword">const</span> <span class="keyword">int</span><span class="special">,</span> <span class="keyword">int</span><span class="special">&gt;::</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>.
98 </p></blockquote></div>
99<div class="blockquote"><blockquote class="blockquote"><p>
100 <code class="computeroutput"><span class="identifier">has_modulus_assign</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">double</span><span class="special">&gt;</span></code> inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">false_type</a></code>.
101 </p></blockquote></div>
102<p>
103 <span class="bold"><strong>See also:</strong></span> <a class="link" href="../category/value_traits/operators.html" title="Operator Type Traits">Operator
104 Type Traits</a>
105 </p>
106<p>
107 <span class="bold"><strong>Known issues:</strong></span>
108 </p>
109<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
110<li class="listitem">
111 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>
112 is defined as a private member of <code class="computeroutput"><span class="identifier">Lhs</span></code>
113 then instantiating <code class="computeroutput"><span class="identifier">has_modulus_assign</span><span class="special">&lt;</span><span class="identifier">Lhs</span><span class="special">&gt;</span></code> will produce a compiler error. For
114 this reason <code class="computeroutput"><span class="identifier">has_modulus_assign</span></code>
115 cannot be used to determine whether a type has a public <code class="computeroutput"><span class="keyword">operator</span><span class="special">%=</span></code>
116 or not.
117<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">&amp;);</span> <span class="special">};</span>
118<span class="identifier">boost</span><span class="special">::</span><span class="identifier">has_modulus_assign</span><span class="special">&lt;</span><span class="identifier">A</span><span class="special">&gt;::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// error: A::operator%=(const A&amp;) is private</span>
119</pre>
120 </li>
121<li class="listitem">
122 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>
123 is convertible to <code class="computeroutput"><span class="identifier">A</span></code>.
124 In this case, the compiler will report an ambiguous overload.
125<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">A</span> <span class="special">{</span> <span class="special">};</span>
126<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">&amp;,</span> <span class="keyword">const</span> <span class="identifier">A</span><span class="special">&amp;);</span>
127<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>
128<span class="identifier">boost</span><span class="special">::</span><span class="identifier">has_modulus_assign</span><span class="special">&lt;</span><span class="identifier">A</span><span class="special">&gt;::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// this is fine</span>
129<span class="identifier">boost</span><span class="special">::</span><span class="identifier">has_modulus_assign</span><span class="special">&lt;</span><span class="identifier">B</span><span class="special">&gt;::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// error: ambiguous overload</span>
130</pre>
131 </li>
132<li class="listitem">
133 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>
134 is defined but does not bind for a given template type, it is still detected
135 by the trait which returns <code class="computeroutput"><span class="keyword">true</span></code>
136 instead of <code class="computeroutput"><span class="keyword">false</span></code>. Example:
137<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">type_traits</span><span class="special">/</span><span class="identifier">has_modulus_assign</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
138<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">iostream</span><span class="special">&gt;</span>
139
140<span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">&gt;</span>
141<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>
142
143<span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">&gt;</span>
144<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">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="special">&amp;</span><span class="identifier">lhs</span><span class="special">,</span> <span class="keyword">const</span> <span class="identifier">contains</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="special">&amp;</span><span class="identifier">rhs</span><span class="special">)</span> <span class="special">{</span>
145 <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>
146<span class="special">}</span>
147
148<span class="keyword">class</span> <span class="identifier">bad</span> <span class="special">{</span> <span class="special">};</span>
149<span class="keyword">class</span> <span class="identifier">good</span> <span class="special">{</span> <span class="special">};</span>
150<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">&amp;,</span> <span class="keyword">const</span> <span class="identifier">good</span><span class="special">&amp;)</span> <span class="special">{</span> <span class="special">}</span>
151
152<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span> <span class="special">{</span>
153 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special">&lt;&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">boolalpha</span><span class="special">;</span>
154 <span class="comment">// works fine for contains&lt;good&gt;</span>
155 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special">&lt;&lt;</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">has_modulus_assign</span><span class="special">&lt;</span> <span class="identifier">contains</span><span class="special">&lt;</span> <span class="identifier">good</span> <span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">value</span><span class="special">&lt;&lt;</span><span class="char">'\n'</span><span class="special">;</span> <span class="comment">// true</span>
156 <span class="identifier">contains</span><span class="special">&lt;</span><span class="identifier">good</span><span class="special">&gt;</span> <span class="identifier">g</span><span class="special">;</span>
157 <span class="identifier">g</span><span class="special">%=</span><span class="identifier">g</span><span class="special">;</span> <span class="comment">// ok</span>
158 <span class="comment">// does not work for contains&lt;bad&gt;</span>
159 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special">&lt;&lt;</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">has_modulus_assign</span><span class="special">&lt;</span> <span class="identifier">contains</span><span class="special">&lt;</span> <span class="identifier">bad</span> <span class="special">&gt;</span> <span class="special">&gt;::</span><span class="identifier">value</span><span class="special">&lt;&lt;</span><span class="char">'\n'</span><span class="special">;</span> <span class="comment">// true, should be false</span>
160 <span class="identifier">contains</span><span class="special">&lt;</span><span class="identifier">bad</span><span class="special">&gt;</span> <span class="identifier">b</span><span class="special">;</span>
161 <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>
162 <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
163<span class="special">}</span>
164</pre>
165 </li>
166<li class="listitem">
167 <code class="computeroutput"><span class="keyword">volatile</span></code> qualifier is not
168 properly handled and would lead to undefined behavior
169 </li>
170</ul></div>
171</div>
172<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
173<td align="left"></td>
174<td align="right"><div class="copyright-footer">Copyright &#169; 2000, 2011 Adobe Systems Inc, David Abrahams,
175 Frederic Bron, Steve Cleary, Beman Dawes, Aleksey Gurtovoy, Howard Hinnant,
176 Jesse Jones, Mat Marcus, Itay Maman, John Maddock, Alexander Nasonov, Thorsten
177 Ottosen, Roman Perepelitsa, Robert Ramey, Jeremy Siek, Robert Stewart and Steven
178 Watanabe<p>
179 Distributed under the Boost Software License, Version 1.0. (See accompanying
180 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>)
181 </p>
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