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<div class="titlepage"><div><div><h3 class="title">
<a name="boost_typetraits.reference.has_logical_or"></a><a class="link" href="has_logical_or.html" title="has_logical_or">has_logical_or</a>
</h3></div></div></div>
<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>
<span class="keyword">struct</span> <span class="identifier">has_logical_or</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>
</pre>
<p>
        <span class="bold"><strong>Inherits:</strong></span> If (i) <code class="computeroutput"><span class="identifier">lhs</span></code>
        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>
        can be used in expression <code class="computeroutput"><span class="identifier">lhs</span><span class="special">||</span><span class="identifier">rhs</span></code>,
        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
        <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>
        then inherits from <a class="link" href="integral_constant.html" title="integral_constant">true_type</a>,
        otherwise inherits from <a class="link" href="integral_constant.html" title="integral_constant">false_type</a>.
      </p>
<p>
        The default behaviour (<code class="computeroutput"><span class="identifier">Ret</span><span class="special">=</span><span class="identifier">dont_care</span></code>)
        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>
        is different from the default <code class="computeroutput"><span class="identifier">dont_care</span></code>
        type, the return value is checked to be convertible to <code class="computeroutput"><span class="identifier">Ret</span></code>.
        Convertible to <code class="computeroutput"><span class="identifier">Ret</span></code> means
        that the return value of the operator can be used as argument to a function
        expecting <code class="computeroutput"><span class="identifier">Ret</span></code>:
</p>
<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>
<span class="identifier">Lhs</span> <span class="identifier">lhs</span><span class="special">;</span>
<span class="identifier">Rhs</span> <span class="identifier">rhs</span><span class="special">;</span>
<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_logical_or&lt;Lhs, Rhs, Ret&gt;::value==true</span>
</pre>
<p>
        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
        <code class="computeroutput"><span class="keyword">void</span></code>.
      </p>
<p>
        <span class="bold"><strong>Header:</strong></span> <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_logical_or</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span></code>
        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>
        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>
      </p>
<p>
        <span class="bold"><strong>Compiler Compatibility:</strong></span> Requires working
        SFINAE (i.e. BOOST_NO_SFINAE is not set). Only a minority of rather old compilers
        do not support this.
      </p>
<p>
        <span class="bold"><strong>Examples:</strong></span>
      </p>
<div class="blockquote"><blockquote class="blockquote"><p>
          <code class="computeroutput"><span class="identifier">has_logical_or</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>.
        </p></blockquote></div>
<div class="blockquote"><blockquote class="blockquote"><p>
          <code class="computeroutput"><span class="identifier">has_logical_or</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>
          integral constant expression.
        </p></blockquote></div>
<div class="blockquote"><blockquote class="blockquote"><p>
          <code class="computeroutput"><span class="identifier">has_logical_or</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>
          integral constant expression that evaluates to <code class="computeroutput"><span class="keyword">true</span></code>.
        </p></blockquote></div>
<div class="blockquote"><blockquote class="blockquote"><p>
          <code class="computeroutput"><span class="identifier">has_logical_or</span><span class="special">&lt;</span><span class="keyword">bool</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>.
        </p></blockquote></div>
<div class="blockquote"><blockquote class="blockquote"><p>
          <code class="computeroutput"><span class="identifier">has_logical_or</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>
          inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">true_type</a></code>.
        </p></blockquote></div>
<div class="blockquote"><blockquote class="blockquote"><p>
          <code class="computeroutput"><span class="identifier">has_logical_or</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>
          inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">true_type</a></code>.
        </p></blockquote></div>
<div class="blockquote"><blockquote class="blockquote"><p>
          <code class="computeroutput"><span class="identifier">has_logical_or</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>
          inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">true_type</a></code>.
        </p></blockquote></div>
<div class="blockquote"><blockquote class="blockquote"><p>
          <code class="computeroutput"><span class="identifier">has_logical_or</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">true_type</a></code>.
        </p></blockquote></div>
<div class="blockquote"><blockquote class="blockquote"><p>
          <code class="computeroutput"><span class="identifier">has_logical_or</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>.
        </p></blockquote></div>
<p>
        <span class="bold"><strong>See also:</strong></span> <a class="link" href="../category/value_traits/operators.html" title="Operator Type Traits">Operator
        Type Traits</a>
      </p>
<p>
        <span class="bold"><strong>Known issues:</strong></span>
      </p>
<p>
        For modern compilers (those that support arbitrary SFINAE-expressions and
        decltype/declval) this trait offers near perfect detection. In this situation
        the macro <code class="computeroutput"><span class="identifier">BOOST_TT_HAS_ACCURATE_BINARY_OPERATOR_DETECTION</span></code>
        will be defined after including <code class="literal">&lt;boost/type_traits/has_logical_or.hpp&gt;</code>.
        Please note however, that detection is based on function signature only,
        in the case that the operator is a function template then has_logical_or
        cannot perform introspection of the template function body to ensure that
        the type meets all of the conceptual requirements of the actual code.
      </p>
<p>
        Example:
      </p>
<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_logical_or<span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">iostream</span><span class="special">&gt;</span>

<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>
<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="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>
<span class="special">};</span>

<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>
bool <span class="keyword">operator</span> || <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>
    <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>
<span class="special">}</span>

<span class="keyword">class</span> <span class="identifier">bad</span> <span class="special">{</span> <span class="special">};</span>
<span class="keyword">class</span> <span class="identifier">good</span> <span class="special">{</span> <span class="special">};</span>
<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>

<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span> <span class="special">{</span>
    <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>
    <span class="comment">// works fine for contains&lt;good&gt;</span>
    <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_logical_or<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>
    <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>
    <span class="identifier">g</span><span class="special">&amp;</span><span class="identifier">g</span><span class="special">;</span> <span class="comment">// ok</span>
    <span class="comment">// does not work for contains&lt;bad&gt;</span>
    <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_logical_or<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>
    <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>
    <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>
    <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
        For older compilers (<code class="computeroutput"><span class="identifier">BOOST_TT_HAS_ACCURATE_BINARY_OPERATOR_DETECTION</span></code>
        not defined) then there are a number of issues:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
            This trait cannot detect whether binary <code class="computeroutput"><span class="keyword">operator</span></code>||
            is public or not: if <code class="computeroutput"><span class="keyword">operator</span></code>||
            is defined as a private member of <code class="computeroutput"><span class="identifier">Lhs</span></code>
            then instantiating <code class="literal">has_logical_or&lt;Lhs&gt;</code> will
            produce a compiler error. For this reason <code class="literal">has_logical_or</code>
            cannot be used to determine whether a type has a public <code class="computeroutput"><span class="keyword">operator</span></code>|| or not.
          </li></ul></div>
<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> || <span class="special">(</span><span class="keyword">const</span> <span class="identifier">A</span><span class="special">&amp;);</span> <span class="special">};</span>
<span class="identifier">boost</span><span class="special">::</span>has_logical_or<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>
</pre>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
            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>
            is convertible to <code class="computeroutput"><span class="identifier">A</span></code>.
            In this case, the compiler will report an ambiguous overload.
          </li></ul></div>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">A</span> <span class="special">{</span> <span class="special">};</span>
bool <span class="keyword">operator</span> || <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>
<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>
<span class="identifier">boost</span><span class="special">::</span>has_logical_or<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>
<span class="identifier">boost</span><span class="special">::</span>has_logical_or<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>
</pre>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
            <code class="computeroutput"><span class="keyword">volatile</span></code> qualifier is not
            properly handled and would lead to undefined behavior
          </li></ul></div>
</div>
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