ceph/src/boost/libs/type_traits/doc/html/boost_typetraits/reference/has_pre_increment.html

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  26 <div class="titlepage"><div><div><h3 class="title">
  27 <a name="boost_typetraits.reference.has_pre_increment"></a><a class="link" href="has_pre_increment.html" title="has_pre_increment">has_pre_increment</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">Rhs</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_pre_increment</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">rhs</span></code>
  34         of type <code class="computeroutput"><span class="identifier">Rhs</span></code> can be used in
  35         expression <code class="computeroutput"><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="special">++</span><span class="identifier">rhs</span></code>
  38         is convertible to <code class="computeroutput"><span class="identifier">Ret</span></code> then
  39         inherits from <a class="link" href="integral_constant.html" title="integral_constant">true_type</a>,
  40         otherwise inherits from <a class="link" href="integral_constant.html" title="integral_constant">false_type</a>.
  41       </p>
  42 <p>
  43         The default behaviour (<code class="computeroutput"><span class="identifier">Ret</span><span class="special">=</span><span class="identifier">dont_care</span></code>)
  44         is to not check for the return value of prefix <code class="computeroutput"><span class="keyword">operator</span><span class="special">++</span></code>. 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_pre_increment&lt;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_pre_increment</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_pre_increment</span><span class="special">&lt;</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_pre_increment</span><span class="special">&lt;</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_pre_increment</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_pre_increment</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_pre_increment</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">int</span><span class="special">&gt;</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_pre_increment</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">long</span><span class="special">&gt;</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_pre_increment</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">,</span> <span class="keyword">double</span><span class="special">&gt;</span></code>
  96           inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">true_type</a></code>.
  97         </p></blockquote></div>
  98 <div class="blockquote"><blockquote class="blockquote"><p>
  99           <code class="computeroutput"><span class="identifier">has_pre_increment</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">,</span> <span class="keyword">int</span><span class="special">&gt;</span></code>
 100           inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">true_type</a></code>.
 101         </p></blockquote></div>
 102 <div class="blockquote"><blockquote class="blockquote"><p>
 103           <code class="computeroutput"><span class="identifier">has_pre_increment</span><span class="special">&lt;</span><span class="keyword">bool</span><span class="special">&gt;</span></code>
 104           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_pre_increment</span><span class="special">&lt;</span><span class="keyword">const</span> <span class="keyword">int</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>.
 108         </p></blockquote></div>
 109 <div class="blockquote"><blockquote class="blockquote"><p>
 110           <code class="computeroutput"><span class="identifier">has_pre_increment</span><span class="special">&lt;</span><span class="keyword">void</span><span class="special">*&gt;</span></code>
 111           inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">false_type</a></code>.
 112         </p></blockquote></div>
 113 <div class="blockquote"><blockquote class="blockquote"><p>
 114           <code class="computeroutput"><span class="identifier">has_pre_increment</span><span class="special">&lt;</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">&gt;</span></code> inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">false_type</a></code>.
 115         </p></blockquote></div>
 116 <p>
 117         <span class="bold"><strong>See also:</strong></span> <a class="link" href="../category/value_traits/operators.html" title="Operator Type Traits">Operator
 118         Type Traits</a>
 119       </p>
 120 <p>
 121         <span class="bold"><strong>Known issues:</strong></span>
 122       </p>
 123 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 124 <li class="listitem">
 125             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>
 126             is defined as a private member of <code class="computeroutput"><span class="identifier">Rhs</span></code>
 127             then instantiating <code class="computeroutput"><span class="identifier">has_pre_increment</span><span class="special">&lt;</span><span class="identifier">Rhs</span><span class="special">&gt;</span></code> will produce a compiler error. For
 128             this reason <code class="computeroutput"><span class="identifier">has_pre_increment</span></code>
 129             cannot be used to determine whether a type has a public <code class="computeroutput"><span class="keyword">operator</span><span class="special">++</span></code>
 130             or not.
 131 <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>
 132 <span class="identifier">boost</span><span class="special">::</span><span class="identifier">has_pre_increment</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++() is private</span>
 133 </pre>
 134           </li>
 135 <li class="listitem">
 136             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>
 137             is convertible to <code class="computeroutput"><span class="identifier">A</span></code>.
 138             In this case, the compiler will report an ambiguous overload.
 139 <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">A</span> <span class="special">{</span> <span class="special">};</span>
 140 <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>
 141 <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>
 142 <span class="identifier">boost</span><span class="special">::</span><span class="identifier">has_pre_increment</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>
 143 <span class="identifier">boost</span><span class="special">::</span><span class="identifier">has_pre_increment</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>
 144 </pre>
 145           </li>
 146 <li class="listitem">
 147             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>
 148             is defined but does not bind for a given template type, it is still detected
 149             by the trait which returns <code class="computeroutput"><span class="keyword">true</span></code>
 150             instead of <code class="computeroutput"><span class="keyword">false</span></code>. Example:
 151 <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_pre_increment</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
 152 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">iostream</span><span class="special">&gt;</span>
 153
 154 <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>
 155 <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>
 156
 157 <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>
 158 <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">rhs</span><span class="special">)</span> <span class="special">{</span>
 159         <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>
 160 <span class="special">}</span>
 161
 162 <span class="keyword">class</span> <span class="identifier">bad</span> <span class="special">{</span> <span class="special">};</span>
 163 <span class="keyword">class</span> <span class="identifier">good</span> <span class="special">{</span> <span class="special">};</span>
 164 <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="special">{</span> <span class="special">}</span>
 165
 166 <span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span> <span class="special">{</span>
 167         <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>
 168         <span class="comment">// works fine for contains&lt;good&gt;</span>
 169         <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_pre_increment</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>
 170         <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>
 171         <span class="special">++</span><span class="identifier">g</span><span class="special">;</span> <span class="comment">// ok</span>
 172         <span class="comment">// does not work for contains&lt;bad&gt;</span>
 173         <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_pre_increment</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>
 174         <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>
 175         <span class="special">++</span><span class="identifier">b</span><span class="special">;</span> <span class="comment">// compile time error</span>
 176         <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
 177 <span class="special">}</span>
 178 </pre>
 179           </li>
 180 <li class="listitem">
 181             <code class="computeroutput"><span class="keyword">volatile</span></code> qualifier is not
 182             properly handled and would lead to undefined behavior
 183           </li>
 184 </ul></div>
 185 </div>
 186 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
 187 <td align="left"></td>
 188 <td align="right"><div class="copyright-footer">Copyright &#169; 2000, 2011 Adobe Systems Inc, David Abrahams,
 189       Frederic Bron, Steve Cleary, Beman Dawes, Aleksey Gurtovoy, Howard Hinnant,
 190       Jesse Jones, Mat Marcus, Itay Maman, John Maddock, Alexander Nasonov, Thorsten
 191       Ottosen, Roman Perepelitsa, Robert Ramey, Jeremy Siek, Robert Stewart and Steven
 192       Watanabe<p>
 193         Distributed under the Boost Software License, Version 1.0. (See accompanying
 194         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>)
 195       </p>
 196 </div></td>
 197 </tr></table>
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