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  26 <div class="titlepage"><div><div><h3 class="title">
  27 <a name="boost_typetraits.reference.has_less"></a><a class="link" href="has_less.html" title="has_less">has_less</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_less</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">&lt;</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">&lt;</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">&lt;</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">&lt;</span><span class="identifier">rhs</span><span class="special">);</span> <span class="comment">// is valid if has_less&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_less</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_less</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_less</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_less</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_less</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_less</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_less</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_less</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_less</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_less</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_less</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 <p>
 116         For modern compilers (those that support arbitrary SFINAE-expressions and
 117         decltype/declval) this trait offers near perfect detection. In this situation
 118         the macro <code class="computeroutput"><span class="identifier">BOOST_TT_HAS_ACCURATE_BINARY_OPERATOR_DETECTION</span></code>
 119         will be defined after including <code class="literal">&lt;boost/type_traits/has_less.hpp&gt;</code>.
 120         Please note however, that detection is based on function signature only,
 121         in the case that the operator is a function template then has_less cannot
 122         perform introspection of the template function body to ensure that the type
 123         meets all of the conceptual requirements of the actual code.
 124       </p>
 125 <p>
 126         Example:
 127       </p>
 128 <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>has_less<span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
 129 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">iostream</span><span class="special">&gt;</span>
 130
 131 <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>
 132 <span class="keyword">struct</span> <span class="identifier">contains</span>
 133 <span class="special">{</span>
 134    <span class="identifier">T</span> <span class="identifier">data</span><span class="special">;</span>
 135    <span class="identifier">contains</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">T</span><span class="special">&amp;</span> <span class="identifier">d</span><span class="special">)</span> <span class="special">:</span> <span class="identifier">data</span><span class="special">(</span><span class="identifier">d</span><span class="special">)</span> <span class="special">{}</span>
 136 <span class="special">};</span>
 137
 138 <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>
 139 bool <span class="keyword">operator</span> &lt; <span class="special">(</span>const <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>
 140     <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>
 141 <span class="special">}</span>
 142
 143 <span class="keyword">class</span> <span class="identifier">bad</span> <span class="special">{</span> <span class="special">};</span>
 144 <span class="keyword">class</span> <span class="identifier">good</span> <span class="special">{</span> <span class="special">};</span>
 145 <span class="identifier">good</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="keyword">return</span> <span class="comment">/*something*/</span><span class="special">;</span> <span class="special">}</span>
 146
 147 <span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span> <span class="special">{</span>
 148     <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>
 149     <span class="comment">// works fine for contains&lt;good&gt;</span>
 150     <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>has_less<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>
 151     <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>
 152     <span class="identifier">g</span><span class="special">&amp;</span><span class="identifier">g</span><span class="special">;</span> <span class="comment">// ok</span>
 153     <span class="comment">// does not work for contains&lt;bad&gt;</span>
 154     <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>has_less<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>
 155     <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>
 156     <span class="identifier">b</span><span class="special">&amp;</span><span class="identifier">b</span><span class="special">;</span> <span class="comment">// compile time error</span>
 157     <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
 158 <span class="special">}</span>
 159 </pre>
 160 <p>
 161         For older compilers (<code class="computeroutput"><span class="identifier">BOOST_TT_HAS_ACCURATE_BINARY_OPERATOR_DETECTION</span></code>
 162         not defined) then there are a number of issues:
 163       </p>
 164 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
 165             This trait cannot detect whether binary <code class="computeroutput"><span class="keyword">operator</span></code>&lt;
 166             is public or not: if <code class="computeroutput"><span class="keyword">operator</span></code>&lt;
 167             is defined as a private member of <code class="computeroutput"><span class="identifier">Lhs</span></code>
 168             then instantiating <code class="literal">has_less&lt;Lhs&gt;</code> will produce
 169             a compiler error. For this reason <code class="literal">has_less</code> cannot
 170             be used to determine whether a type has a public <code class="computeroutput"><span class="keyword">operator</span></code>&lt;
 171             or not.
 172           </li></ul></div>
 173 <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> bool <span class="keyword">operator</span> &lt; <span class="special">(</span><span class="keyword">const</span> <span class="identifier">A</span><span class="special">&amp;);</span> <span class="special">};</span>
 174 <span class="identifier">boost</span><span class="special">::</span>has_less<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 &lt; (const A&amp;) is private</span>
 175 </pre>
 176 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
 177             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>
 178             is convertible to <code class="computeroutput"><span class="identifier">A</span></code>.
 179             In this case, the compiler will report an ambiguous overload.
 180           </li></ul></div>
 181 <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">A</span> <span class="special">{</span> <span class="special">};</span>
 182 bool <span class="keyword">operator</span> &lt; <span class="special">(</span>const <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>
 183 <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>
 184 <span class="identifier">boost</span><span class="special">::</span>has_less<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>
 185 <span class="identifier">boost</span><span class="special">::</span>has_less<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>
 186 </pre>
 187 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
 188             <code class="computeroutput"><span class="keyword">volatile</span></code> qualifier is not
 189             properly handled and would lead to undefined behavior
 190           </li></ul></div>
 191 </div>
 192 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
 193 <td align="left"></td>
 194 <td align="right"><div class="copyright-footer">Copyright &#169; 2000, 2011 Adobe Systems Inc, David Abrahams,
 195       Frederic Bron, Steve Cleary, Beman Dawes, Aleksey Gurtovoy, Howard Hinnant,
 196       Jesse Jones, Mat Marcus, Itay Maman, John Maddock, Alexander Nasonov, Thorsten
 197       Ottosen, Roman Perepelitsa, Robert Ramey, Jeremy Siek, Robert Stewart and Steven
 198       Watanabe<p>
 199         Distributed under the Boost Software License, Version 1.0. (See accompanying
 200         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>)
 201       </p>
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