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

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