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  25 <div class="section boost_typetraits_reference_has_left_shift_assign">
  26 <div class="titlepage"><div><div><h3 class="title">
  27 <a name="boost_typetraits.reference.has_left_shift_assign"></a><a class="link" href="has_left_shift_assign.html" title="has_left_shift_assign">has_left_shift_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_left_shift_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">&lt;&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;&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;&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;&lt;=</span><span class="identifier">rhs</span><span class="special">);</span> <span class="comment">// is valid if has_left_shift_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_left_shift_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>Examples:</strong></span>
  67       </p>
  68 <div class="blockquote"><blockquote class="blockquote"><p>
  69           <code class="computeroutput"><span class="identifier">has_left_shift_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>.
  70         </p></blockquote></div>
  71 <div class="blockquote"><blockquote class="blockquote"><p>
  72           <code class="computeroutput"><span class="identifier">has_left_shift_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>
  73           integral constant expression.
  74         </p></blockquote></div>
  75 <div class="blockquote"><blockquote class="blockquote"><p>
  76           <code class="computeroutput"><span class="identifier">has_left_shift_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>
  77           integral constant expression that evaluates to <code class="computeroutput"><span class="keyword">true</span></code>.
  78         </p></blockquote></div>
  79 <div class="blockquote"><blockquote class="blockquote"><p>
  80           <code class="computeroutput"><span class="identifier">has_left_shift_assign</span><span class="special">&lt;</span><span class="keyword">long</span><span class="special">&gt;</span></code>
  81           inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">true_type</a></code>.
  82         </p></blockquote></div>
  83 <div class="blockquote"><blockquote class="blockquote"><p>
  84           <code class="computeroutput"><span class="identifier">has_left_shift_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>
  85           inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">true_type</a></code>.
  86         </p></blockquote></div>
  87 <div class="blockquote"><blockquote class="blockquote"><p>
  88           <code class="computeroutput"><span class="identifier">has_left_shift_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></code>
  89           inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">false_type</a></code>.
  90         </p></blockquote></div>
  91 <div class="blockquote"><blockquote class="blockquote"><p>
  92           <code class="computeroutput"><span class="identifier">has_left_shift_assign</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>
  93           inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">false_type</a></code>.
  94         </p></blockquote></div>
  95 <div class="blockquote"><blockquote class="blockquote"><p>
  96           <code class="computeroutput"><span class="identifier">has_left_shift_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="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>.
  97         </p></blockquote></div>
  98 <p>
  99         <span class="bold"><strong>See also:</strong></span> <a class="link" href="../category/value_traits/operators.html" title="Operator Type Traits">Operator
 100         Type Traits</a>
 101       </p>
 102 <p>
 103         <span class="bold"><strong>Limitation:</strong></span>
 104       </p>
 105 <div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem">
 106             Requires a compiler with working SFINAE.
 107           </li></ul></div>
 108 <p>
 109         <span class="bold"><strong>Known issues:</strong></span>
 110       </p>
 111 <div class="itemizedlist"><ul class="itemizedlist" type="disc">
 112 <li class="listitem">
 113             This trait cannot detect whether binary <code class="computeroutput"><span class="keyword">operator</span><span class="special">&lt;&lt;=</span></code> is public or not: if <code class="computeroutput"><span class="keyword">operator</span><span class="special">&lt;&lt;=</span></code>
 114             is defined as a private member of <code class="computeroutput"><span class="identifier">Lhs</span></code>
 115             then instantiating <code class="computeroutput"><span class="identifier">has_left_shift_assign</span><span class="special">&lt;</span><span class="identifier">Lhs</span><span class="special">&gt;</span></code> will produce a compiler error. For
 116             this reason <code class="computeroutput"><span class="identifier">has_left_shift_assign</span></code>
 117             cannot be used to determine whether a type has a public <code class="computeroutput"><span class="keyword">operator</span><span class="special">&lt;&lt;=</span></code>
 118             or not.
 119 <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">&lt;&lt;=(</span><span class="keyword">const</span> <span class="identifier">A</span><span class="special">&amp;);</span> <span class="special">};</span>
 120 <span class="identifier">boost</span><span class="special">::</span><span class="identifier">has_left_shift_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&lt;&lt;=(const A&amp;) is private</span>
 121 </pre>
 122           </li>
 123 <li class="listitem">
 124             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>
 125             is convertible to <code class="computeroutput"><span class="identifier">A</span></code>.
 126             In this case, the compiler will report an ambiguous overload.
 127 <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">A</span> <span class="special">{</span> <span class="special">};</span>
 128 <span class="keyword">void</span> <span class="keyword">operator</span><span class="special">&lt;&lt;=(</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>
 129 <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>
 130 <span class="identifier">boost</span><span class="special">::</span><span class="identifier">has_left_shift_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>
 131 <span class="identifier">boost</span><span class="special">::</span><span class="identifier">has_left_shift_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>
 132 </pre>
 133           </li>
 134 <li class="listitem">
 135             There is an issue when applying this trait to template classes. If <code class="computeroutput"><span class="keyword">operator</span><span class="special">&lt;&lt;=</span></code>
 136             is defined but does not bind for a given template type, it is still detected
 137             by the trait which returns <code class="computeroutput"><span class="keyword">true</span></code>
 138             instead of <code class="computeroutput"><span class="keyword">false</span></code>. Example:
 139 <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_left_shift_assign</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
 140 <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">iostream</span><span class="special">&gt;</span>
 141
 142 <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>
 143 <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>
 144
 145 <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>
 146 <span class="keyword">bool</span> <span class="keyword">operator</span><span class="special">&lt;&lt;=(</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>
 147         <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>
 148 <span class="special">}</span>
 149
 150 <span class="keyword">class</span> <span class="identifier">bad</span> <span class="special">{</span> <span class="special">};</span>
 151 <span class="keyword">class</span> <span class="identifier">good</span> <span class="special">{</span> <span class="special">};</span>
 152 <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>
 153
 154 <span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span> <span class="special">{</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">std</span><span class="special">::</span><span class="identifier">boolalpha</span><span class="special">;</span>
 156         <span class="comment">// works fine for contains&lt;good&gt;</span>
 157         <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_left_shift_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>
 158         <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>
 159         <span class="identifier">g</span><span class="special">&lt;&lt;=</span><span class="identifier">g</span><span class="special">;</span> <span class="comment">// ok</span>
 160         <span class="comment">// does not work for contains&lt;bad&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_left_shift_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>
 162         <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>
 163         <span class="identifier">b</span><span class="special">&lt;&lt;=</span><span class="identifier">b</span><span class="special">;</span> <span class="comment">// compile time error</span>
 164         <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
 165 <span class="special">}</span>
 166 </pre>
 167           </li>
 168 <li class="listitem">
 169             <code class="computeroutput"><span class="keyword">volatile</span></code> qualifier is not
 170             properly handled and would lead to undefined behavior
 171           </li>
 172 </ul></div>
 173 </div>
 174 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
 175 <td align="left"></td>
 176 <td align="right"><div class="copyright-footer">Copyright &#169; 2000, 2011 Adobe Systems Inc, David Abrahams,
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