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<div class="titlepage"><div><div><h3 class="title">
<a name="boost_typetraits.reference.has_unary_minus"></a><a class="link" href="has_unary_minus.html" title="has_unary_minus">has_unary_minus</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">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>
<span class="keyword">struct</span> <span class="identifier">has_unary_minus</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">rhs</span></code>
        of type <code class="computeroutput"><span class="identifier">Rhs</span></code> can be used in
        expression <code class="computeroutput"><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="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 prefix <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">Rhs</span> <span class="identifier">rhs</span><span class="special">;</span>
<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_unary_minus&lt;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_unary_minus</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_unary_minus</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>.
        </p></blockquote></div>
<div class="blockquote"><blockquote class="blockquote"><p>
          <code class="computeroutput"><span class="identifier">has_unary_minus</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>
          integral constant expression.
        </p></blockquote></div>
<div class="blockquote"><blockquote class="blockquote"><p>
          <code class="computeroutput"><span class="identifier">has_unary_minus</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_unary_minus</span><span class="special">&lt;</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_unary_minus</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>.
        </p></blockquote></div>
<div class="blockquote"><blockquote class="blockquote"><p>
          <code class="computeroutput"><span class="identifier">has_unary_minus</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>.
        </p></blockquote></div>
<div class="blockquote"><blockquote class="blockquote"><p>
          <code class="computeroutput"><span class="identifier">has_unary_minus</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>
          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_unary_minus</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>
          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_unary_minus</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>.
        </p></blockquote></div>
<div class="blockquote"><blockquote class="blockquote"><p>
          <code class="computeroutput"><span class="identifier">has_unary_minus</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>.
        </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>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
            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> is defined as a private member of <code class="computeroutput"><span class="identifier">Rhs</span></code> then instantiating <code class="computeroutput"><span class="identifier">has_unary_minus</span><span class="special">&lt;</span><span class="identifier">Rhs</span><span class="special">&gt;</span></code>
            will produce a compiler error. For this reason <code class="computeroutput"><span class="identifier">has_unary_minus</span></code>
            cannot be used to determine whether a type has a public <code class="computeroutput"><span class="keyword">operator</span><span class="special">-</span></code>
            or not.
<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>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">has_unary_minus</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>
</pre>
          </li>
<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.
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">A</span> <span class="special">{</span> <span class="special">};</span>
<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>
<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><span class="identifier">has_unary_minus</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>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">has_unary_minus</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>
</pre>
          </li>
<li class="listitem">
            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>
            is defined but does not bind for a given template type, it is still detected
            by the trait which returns <code class="computeroutput"><span class="keyword">true</span></code>
            instead of <code class="computeroutput"><span class="keyword">false</span></code>. Example:
<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_unary_minus</span><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="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>
<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>
	<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>
<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="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>

<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><span class="identifier">has_unary_minus</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>
	<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="special">-</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><span class="identifier">has_unary_minus</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>
	<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="special">-</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>
          </li>
<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>
<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_unary_minus.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_unary_minus
        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>
        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 unary <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 class member of type T then instantiating <code class="literal">has_unary_minus&lt;T&gt;</code>
            will produce a compiler error. For this reason <code class="literal">has_unary_minus</code>
            cannot be used to determine whether a type has a public <code class="computeroutput"><span class="keyword">operator</span></code>- or not.
          </li>
<li class="listitem">
            <code class="computeroutput"><span class="keyword">volatile</span></code> qualifier is not
            properly handled and would lead to undefined behavior
          </li>
<li class="listitem">
            Capturless lambdas are not supported and will lead to a compiler error
            if has_unary_minus is instantiated on the lambda type.
          </li>
<li class="listitem">
            Scoped enum types are not supported and will lead to a compiler error
            if has_unary_minus is instantiated on such a type.
          </li>
</ul></div>
</div>
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<td align="right"><div class="copyright-footer">Copyright &#169; 2000, 2011 Adobe Systems Inc, David Abrahams,
      Frederic Bron, Steve Cleary, Beman Dawes, Aleksey Gurtovoy, Howard Hinnant,
      Jesse Jones, Mat Marcus, Itay Maman, John Maddock, Alexander Nasonov, Thorsten
      Ottosen, Roman Perepelitsa, Robert Ramey, Jeremy Siek, Robert Stewart and Steven
      Watanabe<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        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>)
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