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| <div class="section"> |
| <div class="titlepage"><div><div><h4 class="title"> |
| <a name="boost_typetraits.category.value_traits.operators"></a><a class="link" href="operators.html" title="Operator Type Traits">Operator |
| Type Traits</a> |
| </h4></div></div></div> |
| <h6> |
| <a name="boost_typetraits.category.value_traits.operators.h0"></a> |
| <span class="phrase"><a name="boost_typetraits.category.value_traits.operators.introduction"></a></span><a class="link" href="operators.html#boost_typetraits.category.value_traits.operators.introduction">Introduction</a> |
| </h6> |
| <p> |
| These traits are all <span class="emphasis"><em>value traits</em></span> inheriting from |
| <a class="link" href="../../reference/integral_constant.html" title="integral_constant">integral_constant</a> |
| and providing a simple <code class="computeroutput"><span class="keyword">true</span></code> |
| or <code class="computeroutput"><span class="keyword">false</span></code> boolean <code class="computeroutput"><span class="identifier">value</span></code> which reflects the fact that given |
| types can or cannot be used with given operators. |
| </p> |
| <p> |
| For example, <code class="computeroutput"><span class="identifier">has_plus</span><span class="special"><</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">double</span><span class="special">>::</span><span class="identifier">value</span></code> |
| is a <code class="computeroutput"><span class="keyword">bool</span></code> which value is |
| <code class="computeroutput"><span class="keyword">true</span></code> because it is possible |
| to add a <code class="computeroutput"><span class="keyword">double</span></code> to an <code class="computeroutput"><span class="keyword">int</span></code> like in the following code: |
| </p> |
| <pre class="programlisting"><span class="keyword">int</span> <span class="identifier">i</span><span class="special">;</span> |
| <span class="keyword">double</span> <span class="identifier">d</span><span class="special">;</span> |
| <span class="identifier">i</span><span class="special">+</span><span class="identifier">d</span><span class="special">;</span> |
| </pre> |
| <p> |
| It is also possible to know if the result of the operator can be used as |
| function argument of a given type. For example, <code class="computeroutput"><span class="identifier">has_plus</span><span class="special"><</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">double</span><span class="special">,</span> <span class="keyword">float</span><span class="special">>::</span><span class="identifier">value</span></code> |
| is <code class="computeroutput"><span class="keyword">true</span></code> because it is possible |
| to add a <code class="computeroutput"><span class="keyword">double</span></code> to an <code class="computeroutput"><span class="keyword">int</span></code> and the result (<code class="computeroutput"><span class="keyword">double</span></code>) |
| can be converted to a <code class="computeroutput"><span class="keyword">float</span></code> |
| argument like in the following code: |
| </p> |
| <pre class="programlisting"><span class="keyword">void</span> <span class="identifier">f</span><span class="special">(</span><span class="keyword">float</span><span class="special">)</span> <span class="special">{</span> <span class="special">};</span> |
| <span class="keyword">int</span> <span class="identifier">i</span><span class="special">;</span> |
| <span class="keyword">double</span> <span class="identifier">d</span><span class="special">;</span> |
| <span class="identifier">f</span><span class="special">(</span><span class="identifier">i</span><span class="special">+</span><span class="identifier">d</span><span class="special">);</span> |
| </pre> |
| <p> |
| </p> |
| <h6> |
| <a name="boost_typetraits.category.value_traits.operators.h1"></a> |
| <span class="phrase"><a name="boost_typetraits.category.value_traits.operators.example_of_application"></a></span><a class="link" href="operators.html#boost_typetraits.category.value_traits.operators.example_of_application">Example |
| of application</a> |
| </h6> |
| <p> |
| These traits can be useful to optimize the code for types supporting given |
| operations. For example a function <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">advance</span></code> |
| that increases an iterator of a given number of steps could be implemented |
| as follows: |
| </p> |
| <p> |
| </p> |
| <pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">type_traits</span><span class="special">/</span><span class="identifier">has_plus_assign</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> |
| |
| <span class="keyword">namespace</span> <span class="identifier">detail</span> <span class="special">{</span> |
| <span class="keyword">template</span> <span class="special"><</span> <span class="keyword">class</span> <span class="identifier">Iterator</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">Distance</span><span class="special">,</span> <span class="keyword">bool</span> <span class="identifier">has_plus_assign</span> <span class="special">></span> |
| <span class="keyword">struct</span> <span class="identifier">advance_impl</span><span class="special">;</span> |
| |
| <span class="comment">// this is used if += exists (efficient)</span> |
| <span class="keyword">template</span> <span class="special"><</span> <span class="keyword">class</span> <span class="identifier">Iterator</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">Distance</span> <span class="special">></span> |
| <span class="keyword">struct</span> <span class="identifier">advance_impl</span><span class="special"><</span><span class="identifier">Iterator</span><span class="special">,</span> <span class="identifier">Distance</span><span class="special">,</span> <span class="keyword">true</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="identifier">Iterator</span> <span class="special">&</span><span class="identifier">i</span><span class="special">,</span> <span class="identifier">Distance</span> <span class="identifier">n</span><span class="special">)</span> <span class="special">{</span> |
| <span class="identifier">i</span><span class="special">+=</span><span class="identifier">n</span><span class="special">;</span> |
| <span class="special">}</span> |
| <span class="special">};</span> |
| |
| <span class="comment">// this is use if += does not exists (less efficient but cannot do better)</span> |
| <span class="keyword">template</span> <span class="special"><</span> <span class="keyword">class</span> <span class="identifier">Iterator</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">Distance</span> <span class="special">></span> |
| <span class="keyword">struct</span> <span class="identifier">advance_impl</span><span class="special"><</span><span class="identifier">Iterator</span><span class="special">,</span> <span class="identifier">Distance</span><span class="special">,</span> <span class="keyword">false</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="identifier">Iterator</span> <span class="special">&</span><span class="identifier">i</span><span class="special">,</span> <span class="identifier">Distance</span> <span class="identifier">n</span><span class="special">)</span> <span class="special">{</span> |
| <span class="keyword">if</span> <span class="special">(</span><span class="identifier">n</span><span class="special">></span><span class="number">0</span><span class="special">)</span> <span class="special">{</span> |
| <span class="keyword">while</span> <span class="special">(</span><span class="identifier">n</span><span class="special">--)</span> <span class="special">++</span><span class="identifier">i</span><span class="special">;</span> |
| <span class="special">}</span> <span class="keyword">else</span> <span class="special">{</span> |
| <span class="keyword">while</span> <span class="special">(</span><span class="identifier">n</span><span class="special">++)</span> <span class="special">--</span><span class="identifier">i</span><span class="special">;</span> |
| <span class="special">}</span> |
| <span class="special">}</span> |
| <span class="special">};</span> |
| <span class="special">}</span> <span class="comment">// namespace detail</span> |
| |
| <span class="keyword">template</span> <span class="special"><</span> <span class="keyword">class</span> <span class="identifier">Iterator</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">Distance</span> <span class="special">></span> |
| <span class="keyword">inline</span> <span class="keyword">void</span> <span class="identifier">advance</span><span class="special">(</span><span class="identifier">Iterator</span> <span class="special">&</span><span class="identifier">i</span><span class="special">,</span> <span class="identifier">Distance</span> <span class="identifier">n</span><span class="special">)</span> <span class="special">{</span> |
| <span class="identifier">detail</span><span class="special">::</span><span class="identifier">advance_impl</span><span class="special"><</span> <span class="identifier">Iterator</span><span class="special">,</span> <span class="identifier">Distance</span><span class="special">,</span> <span class="special">::</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">has_plus_assign</span><span class="special"><</span><span class="identifier">Iterator</span><span class="special">>::</span><span class="identifier">value</span> <span class="special">>()(</span><span class="identifier">i</span><span class="special">,</span> <span class="identifier">n</span><span class="special">);</span> |
| <span class="special">}</span> |
| </pre> |
| <p> |
| </p> |
| <p> |
| Then the compiler chooses the most efficient implementation according to |
| the type's ability to perform <code class="computeroutput"><span class="special">+=</span></code> |
| operation: |
| </p> |
| <p> |
| </p> |
| <pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span> |
| |
| <span class="keyword">class</span> <span class="identifier">with</span> <span class="special">{</span> |
| <span class="keyword">int</span> <span class="identifier">m_i</span><span class="special">;</span> |
| <span class="keyword">public</span><span class="special">:</span> |
| <span class="identifier">with</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">i</span><span class="special">=</span><span class="number">0</span><span class="special">)</span> <span class="special">:</span> <span class="identifier">m_i</span><span class="special">(</span><span class="identifier">i</span><span class="special">)</span> <span class="special">{</span> <span class="special">}</span> |
| <span class="identifier">with</span> <span class="special">&</span><span class="keyword">operator</span><span class="special">+=(</span><span class="keyword">int</span> <span class="identifier">rhs</span><span class="special">)</span> <span class="special">{</span> <span class="identifier">m_i</span><span class="special">+=</span><span class="identifier">rhs</span><span class="special">;</span> <span class="keyword">return</span> <span class="special">*</span><span class="keyword">this</span><span class="special">;</span> <span class="special">}</span> |
| <span class="keyword">operator</span> <span class="keyword">int</span> <span class="keyword">const</span> <span class="special">()</span> <span class="special">{</span> <span class="keyword">return</span> <span class="identifier">m_i</span><span class="special">;</span> <span class="special">}</span> |
| <span class="special">};</span> |
| |
| <span class="keyword">class</span> <span class="identifier">without</span> <span class="special">{</span> |
| <span class="keyword">int</span> <span class="identifier">m_i</span><span class="special">;</span> |
| <span class="keyword">public</span><span class="special">:</span> |
| <span class="identifier">without</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">i</span><span class="special">=</span><span class="number">0</span><span class="special">)</span> <span class="special">:</span> <span class="identifier">m_i</span><span class="special">(</span><span class="identifier">i</span><span class="special">)</span> <span class="special">{</span> <span class="special">}</span> |
| <span class="identifier">without</span> <span class="special">&</span><span class="keyword">operator</span><span class="special">++()</span> <span class="special">{</span> <span class="special">++</span><span class="identifier">m_i</span><span class="special">;</span> <span class="keyword">return</span> <span class="special">*</span><span class="keyword">this</span><span class="special">;</span> <span class="special">}</span> |
| <span class="identifier">without</span> <span class="special">&</span><span class="keyword">operator</span><span class="special">--()</span> <span class="special">{</span> <span class="special">--</span><span class="identifier">m_i</span><span class="special">;</span> <span class="keyword">return</span> <span class="special">*</span><span class="keyword">this</span><span class="special">;</span> <span class="special">}</span> |
| <span class="keyword">operator</span> <span class="keyword">int</span> <span class="keyword">const</span> <span class="special">()</span> <span class="special">{</span> <span class="keyword">return</span> <span class="identifier">m_i</span><span class="special">;</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">with</span> <span class="identifier">i</span><span class="special">=</span><span class="number">0</span><span class="special">;</span> |
| <span class="identifier">advance</span><span class="special">(</span><span class="identifier">i</span><span class="special">,</span> <span class="number">10</span><span class="special">);</span> <span class="comment">// uses +=</span> |
| <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special"><<</span><span class="string">"with: "</span><span class="special"><<</span><span class="identifier">i</span><span class="special"><<</span><span class="char">'\n'</span><span class="special">;</span> |
| <span class="identifier">without</span> <span class="identifier">j</span><span class="special">=</span><span class="number">0</span><span class="special">;</span> |
| <span class="identifier">advance</span><span class="special">(</span><span class="identifier">j</span><span class="special">,</span> <span class="number">10</span><span class="special">);</span> <span class="comment">// uses ++</span> |
| <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special"><<</span><span class="string">"without: "</span><span class="special"><<</span><span class="identifier">j</span><span class="special"><<</span><span class="char">'\n'</span><span class="special">;</span> |
| <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span> |
| <span class="special">}</span> |
| </pre> |
| <p> |
| </p> |
| <h6> |
| <a name="boost_typetraits.category.value_traits.operators.h2"></a> |
| <span class="phrase"><a name="boost_typetraits.category.value_traits.operators.description"></a></span><a class="link" href="operators.html#boost_typetraits.category.value_traits.operators.description">Description</a> |
| </h6> |
| <p> |
| The syntax is the following: |
| </p> |
| <pre class="programlisting"><span class="keyword">template</span> <code class="computeroutput"><span class="special"><</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">></span></code> <span class="identifier">has_op</span><span class="special">;</span> <span class="comment">// prefix operator</span> |
| <span class="keyword">template</span> <code class="computeroutput"><span class="special"><</span> <span class="keyword">class</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">></span></code> <span class="identifier">has_op</span><span class="special">;</span> <span class="comment">// postfix operator</span> |
| <span class="keyword">template</span> <code class="computeroutput"><span class="special"><</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">></span></code> <span class="identifier">has_op</span><span class="special">;</span> <span class="comment">// binary operator</span> |
| </pre> |
| <p> |
| where: |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> |
| <li class="listitem"> |
| op represents the operator name |
| </li> |
| <li class="listitem"> |
| <code class="computeroutput"><span class="identifier">Lhs</span></code> is the type used |
| at the left hand side of <code class="computeroutput"><span class="keyword">operator</span> |
| <span class="identifier">op</span></code>, |
| </li> |
| <li class="listitem"> |
| <code class="computeroutput"><span class="identifier">Rhs</span></code> is the type used |
| at the right hand side of <code class="computeroutput"><span class="keyword">operator</span> |
| <span class="identifier">op</span></code>, |
| </li> |
| <li class="listitem"> |
| <code class="computeroutput"><span class="identifier">Ret</span></code> is the type for |
| which we want to know if the result of <code class="computeroutput"><span class="keyword">operator</span> |
| <span class="identifier">op</span></code> can be converted to. |
| </li> |
| </ul></div> |
| <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 the operator. 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>, 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 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_plus<Lhs, Rhs, Ret>::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> |
| The following tables give the list of supported binary, prefix and postfix |
| operators. |
| </p> |
| <div class="table"> |
| <a name="boost_typetraits.category.value_traits.operators.supported_prefix_operators"></a><p class="title"><b>Table 1.4. Supported prefix operators</b></p> |
| <div class="table-contents"><table class="table" summary="Supported prefix operators"> |
| <colgroup> |
| <col> |
| <col> |
| </colgroup> |
| <thead><tr> |
| <th> |
| <p> |
| prefix operator |
| </p> |
| </th> |
| <th> |
| <p> |
| trait name |
| </p> |
| </th> |
| </tr></thead> |
| <tbody> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="special">!</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| <a class="link" href="../../reference/has_logical_not.html" title="has_logical_not"><code class="computeroutput"><span class="identifier">has_logical_not</span></code></a> <code class="computeroutput"><span class="special"><</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">></span></code> |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="special">+</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| <a class="link" href="../../reference/has_unary_plus.html" title="has_unary_plus"><code class="computeroutput"><span class="identifier">has_unary_plus</span></code></a> |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="special">-</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| <a class="link" href="../../reference/has_unary_minus.html" title="has_unary_minus"><code class="computeroutput"><span class="identifier">has_unary_minus</span></code></a> and |
| <a class="link" href="../../reference/has_negate.html" title="has_negate"><code class="computeroutput"><span class="identifier">has_negate</span></code></a> |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="special">~</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| <a class="link" href="../../reference/has_complement.html" title="has_complement"><code class="computeroutput"><span class="identifier">has_complement</span></code></a> |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="special">*</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| <a class="link" href="../../reference/has_dereference.html" title="has_dereference"><code class="computeroutput"><span class="identifier">has_dereference</span></code></a> |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="special">++</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| <a class="link" href="../../reference/has_pre_increment.html" title="has_pre_increment"><code class="computeroutput"><span class="identifier">has_pre_increment</span></code></a> |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="special">--</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| <a class="link" href="../../reference/has_pre_decrement.html" title="has_pre_decrement"><code class="computeroutput"><span class="identifier">has_pre_decrement</span></code></a> |
| </p> |
| </td> |
| </tr> |
| </tbody> |
| </table></div> |
| </div> |
| <br class="table-break"><div class="table"> |
| <a name="boost_typetraits.category.value_traits.operators.supported_postfix_operators"></a><p class="title"><b>Table 1.5. Supported postfix operators</b></p> |
| <div class="table-contents"><table class="table" summary="Supported postfix operators"> |
| <colgroup> |
| <col> |
| <col> |
| </colgroup> |
| <thead><tr> |
| <th> |
| <p> |
| postfix operator |
| </p> |
| </th> |
| <th> |
| <p> |
| trait name |
| </p> |
| </th> |
| </tr></thead> |
| <tbody> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="special">++</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| <a class="link" href="../../reference/has_post_increment.html" title="has_post_increment"><code class="computeroutput"><span class="identifier">has_post_increment</span></code></a> |
| <code class="computeroutput"><span class="special"><</span> <span class="keyword">class</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">></span></code> |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="special">--</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| <a class="link" href="../../reference/has_post_decrement.html" title="has_post_decrement"><code class="computeroutput"><span class="identifier">has_post_decrement</span></code></a> |
| </p> |
| </td> |
| </tr> |
| </tbody> |
| </table></div> |
| </div> |
| <br class="table-break"><div class="table"> |
| <a name="boost_typetraits.category.value_traits.operators.supported_binary_operators"></a><p class="title"><b>Table 1.6. Supported binary operators</b></p> |
| <div class="table-contents"><table class="table" summary="Supported binary operators"> |
| <colgroup> |
| <col> |
| <col> |
| </colgroup> |
| <thead><tr> |
| <th> |
| <p> |
| binary operator |
| </p> |
| </th> |
| <th> |
| <p> |
| trait name |
| </p> |
| </th> |
| </tr></thead> |
| <tbody> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="special">+</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| <a class="link" href="../../reference/has_plus.html" title="has_plus"><code class="computeroutput"><span class="identifier">has_plus</span></code></a> <code class="computeroutput"><span class="special"><</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">></span></code> |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="special">-</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| <a class="link" href="../../reference/has_minus.html" title="has_minus"><code class="computeroutput"><span class="identifier">has_minus</span></code></a> |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="special">*</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| <a class="link" href="../../reference/has_multiplies.html" title="has_multiplies"><code class="computeroutput"><span class="identifier">has_multiplies</span></code></a> |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="special">/</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| <a class="link" href="../../reference/has_divides.html" title="has_divides"><code class="computeroutput"><span class="identifier">has_divides</span></code></a> |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="special">%</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| <a class="link" href="../../reference/has_modulus.html" title="has_modulus"><code class="computeroutput"><span class="identifier">has_modulus</span></code></a> |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="special">+=</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| <a class="link" href="../../reference/has_plus_assign.html" title="has_plus_assign"><code class="computeroutput"><span class="identifier">has_plus_assign</span></code></a> |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="special">-=</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| <a class="link" href="../../reference/has_minus_assign.html" title="has_minus_assign"><code class="computeroutput"><span class="identifier">has_minus_assign</span></code></a> |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="special">*=</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| <a class="link" href="../../reference/has_multiplies_assign.html" title="has_multiplies_assign"><code class="computeroutput"><span class="identifier">has_multiplies_assign</span></code></a> |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="special">/=</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| <a class="link" href="../../reference/has_divides_assign.html" title="has_divides_assign"><code class="computeroutput"><span class="identifier">has_divides_assign</span></code></a> |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="special">%=</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| <a class="link" href="../../reference/has_modulus_assign.html" title="has_modulus_assign"><code class="computeroutput"><span class="identifier">has_modulus_assign</span></code></a> |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="special">&</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| <a class="link" href="../../reference/has_bit_and.html" title="has_bit_and"><code class="computeroutput"><span class="identifier">has_bit_and</span></code></a> |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="special">|</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| <a class="link" href="../../reference/has_bit_or.html" title="has_bit_or"><code class="computeroutput"><span class="identifier">has_bit_or</span></code></a> |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="special">^</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| <a class="link" href="../../reference/has_bit_xor.html" title="has_bit_xor"><code class="computeroutput"><span class="identifier">has_bit_xor</span></code></a> |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="special">&=</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| <a class="link" href="../../reference/has_bit_and_assign.html" title="has_bit_and_assign"><code class="computeroutput"><span class="identifier">has_bit_and_assign</span></code></a> |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="special">|=</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| <a class="link" href="../../reference/has_bit_or_assign.html" title="has_bit_or_assign"><code class="computeroutput"><span class="identifier">has_bit_or_assign</span></code></a> |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="special">^=</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| <a class="link" href="../../reference/has_bit_xor_assign.html" title="has_bit_xor_assign"><code class="computeroutput"><span class="identifier">has_bit_xor_assign</span></code></a> |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="special"><<</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| <a class="link" href="../../reference/has_left_shift.html" title="has_left_shift"><code class="computeroutput"><span class="identifier">has_left_shift</span></code></a> |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="special">>></span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| <a class="link" href="../../reference/has_right_shift.html" title="has_right_shift"><code class="computeroutput"><span class="identifier">has_right_shift</span></code></a> |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="special"><<=</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| <a class="link" href="../../reference/has_left_shift_assign.html" title="has_left_shift_assign"><code class="computeroutput"><span class="identifier">has_left_shift_assign</span></code></a> |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="special">>>=</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| <a class="link" href="../../reference/has_right_shift_assign.html" title="has_right_shift_assign"><code class="computeroutput"><span class="identifier">has_right_shift_assign</span></code></a> |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="special">==</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| <a class="link" href="../../reference/has_equal_to.html" title="has_equal_to"><code class="computeroutput"><span class="identifier">has_equal_to</span></code></a> |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="special">!=</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| <a class="link" href="../../reference/has_not_equal_to.html" title="has_not_equal_to"><code class="computeroutput"><span class="identifier">has_not_equal_to</span></code></a> |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="special"><</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| <a class="link" href="../../reference/has_less.html" title="has_less"><code class="computeroutput"><span class="identifier">has_less</span></code></a> |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="special"><=</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| <a class="link" href="../../reference/has_less_equal.html" title="has_less_equal"><code class="computeroutput"><span class="identifier">has_less_equal</span></code></a> |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="special">></span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| <a class="link" href="../../reference/has_greater.html" title="has_greater"><code class="computeroutput"><span class="identifier">has_greater</span></code></a> |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="special">>=</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| <a class="link" href="../../reference/has_greater_equal.html" title="has_greater_equal"><code class="computeroutput"><span class="identifier">has_greater_equal</span></code></a> |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="special">&&</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| <a class="link" href="../../reference/has_logical_and.html" title="has_logical_and"><code class="computeroutput"><span class="identifier">has_logical_and</span></code></a> |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="special">||</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| <a class="link" href="../../reference/has_logical_or.html" title="has_logical_or"><code class="computeroutput"><span class="identifier">has_logical_or</span></code></a> |
| </p> |
| </td> |
| </tr> |
| </tbody> |
| </table></div> |
| </div> |
| <br class="table-break"><p> |
| The following operators are not supported because they could not be implemented |
| using the same technique: <code class="computeroutput"><span class="keyword">operator</span><span class="special">=</span></code>, <code class="computeroutput"><span class="keyword">operator</span><span class="special">-></span></code>, <code class="computeroutput"><span class="keyword">operator</span><span class="special">&</span></code>, <code class="computeroutput"><span class="keyword">operator</span><span class="special">[]</span></code>, <code class="computeroutput"><span class="keyword">operator</span><span class="special">,</span></code>, <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code>, <code class="computeroutput"><span class="keyword">operator</span> |
| <span class="keyword">new</span></code>. |
| </p> |
| <h6> |
| <a name="boost_typetraits.category.value_traits.operators.h3"></a> |
| <span class="phrase"><a name="boost_typetraits.category.value_traits.operators.cv_qualifiers_and_references"></a></span><a class="link" href="operators.html#boost_typetraits.category.value_traits.operators.cv_qualifiers_and_references">cv |
| qualifiers and references</a> |
| </h6> |
| <p> |
| A reference sign <code class="computeroutput"><span class="special">&</span></code> in |
| the operator argument is ignored so that <code class="computeroutput"><span class="identifier">has_plus</span><span class="special"><</span> <span class="keyword">int</span><span class="special">&,</span> <span class="keyword">double</span><span class="special">&</span> <span class="special">>::</span><span class="identifier">value</span><span class="special">==</span><span class="identifier">has_plus</span><span class="special"><</span> |
| <span class="keyword">int</span><span class="special">,</span> <span class="keyword">double</span> <span class="special">>::</span><span class="identifier">value</span></code>. This has been chosen because if |
| the following code works (does not work): |
| </p> |
| <pre class="programlisting"><span class="keyword">int</span> <span class="identifier">i</span><span class="special">;</span> |
| <span class="keyword">double</span> <span class="identifier">d</span><span class="special">;</span> |
| <span class="identifier">i</span><span class="special">+</span><span class="identifier">d</span><span class="special">;</span> |
| </pre> |
| <p> |
| the following code also works (does not work): |
| </p> |
| <pre class="programlisting"><span class="keyword">int</span> <span class="special">&</span><span class="identifier">ir</span><span class="special">=</span><span class="identifier">i</span><span class="special">;</span> |
| <span class="keyword">double</span> <span class="special">&</span><span class="identifier">dr</span><span class="special">=</span><span class="identifier">d</span><span class="special">;</span> |
| <span class="identifier">ir</span><span class="special">+</span><span class="identifier">dr</span><span class="special">;</span> |
| </pre> |
| <p> |
| </p> |
| <p> |
| It was not possible to handle properly the <code class="computeroutput"><span class="keyword">volatile</span></code> |
| qualifier so that any construct using this qualifier has undefined behavior. |
| </p> |
| <p> |
| As a help, the following tables give the necessary conditions over each |
| trait template argument for the trait <code class="computeroutput"><span class="identifier">value</span></code> |
| to be <code class="computeroutput"><span class="keyword">true</span></code>. They are non sufficient |
| conditions because the conditions must be <code class="computeroutput"><span class="keyword">true</span></code> |
| for all arguments and return type for <code class="computeroutput"><span class="identifier">value</span></code> |
| to be <code class="computeroutput"><span class="keyword">true</span></code>. |
| </p> |
| <div class="table"> |
| <a name="boost_typetraits.category.value_traits.operators.necessary_and_non_sufficient_condition_on_operator_argument_for_value_to_be_true"></a><p class="title"><b>Table 1.7. necessary and non sufficient condition on operator argument for |
| value to be true</b></p> |
| <div class="table-contents"><table class="table" summary="necessary and non sufficient condition on operator argument for |
| value to be true"> |
| <colgroup> |
| <col> |
| <col> |
| <col> |
| <col> |
| </colgroup> |
| <thead><tr> |
| <th> |
| <p> |
| operator declaration |
| </p> |
| </th> |
| <th> |
| <p> |
| <code class="computeroutput"><span class="identifier">has_op</span><span class="special"><</span> |
| <span class="keyword">void</span> <span class="special">></span></code> |
| </p> |
| </th> |
| <th> |
| <p> |
| <code class="computeroutput"><span class="identifier">has_op</span><span class="special"><</span> |
| <span class="identifier">Arg</span> <span class="special">></span></code> |
| and <code class="computeroutput"><span class="identifier">has_op</span><span class="special"><</span> |
| <span class="identifier">Arg</span><span class="special">&</span> |
| <span class="special">></span></code> |
| </p> |
| </th> |
| <th> |
| <p> |
| <code class="computeroutput"><span class="identifier">has_op</span><span class="special"><</span> |
| <span class="identifier">Arg</span> <span class="keyword">const</span> |
| <span class="special">></span></code> and <code class="computeroutput"><span class="identifier">has_op</span><span class="special"><</span> |
| <span class="identifier">Arg</span> <span class="keyword">const</span><span class="special">&</span> <span class="special">></span></code> |
| </p> |
| </th> |
| </tr></thead> |
| <tbody> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="keyword">operator</span></code>@<code class="computeroutput"><span class="special">(</span><span class="identifier">Arg</span><span class="special">)</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| false |
| </p> |
| </td> |
| <td> |
| <p> |
| true |
| </p> |
| </td> |
| <td> |
| <p> |
| true |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="keyword">operator</span></code>@<code class="computeroutput"><span class="special">(</span><span class="identifier">Arg</span> |
| <span class="keyword">const</span><span class="special">)</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| false |
| </p> |
| </td> |
| <td> |
| <p> |
| true |
| </p> |
| </td> |
| <td> |
| <p> |
| true |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="keyword">operator</span></code>@<code class="computeroutput"><span class="special">(</span><span class="identifier">Arg</span> |
| <span class="special">&)</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| false |
| </p> |
| </td> |
| <td> |
| <p> |
| true |
| </p> |
| </td> |
| <td> |
| <p> |
| false |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="keyword">operator</span></code>@<code class="computeroutput"><span class="special">(</span><span class="identifier">Arg</span> |
| <span class="keyword">const</span> <span class="special">&)</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| false |
| </p> |
| </td> |
| <td> |
| <p> |
| true |
| </p> |
| </td> |
| <td> |
| <p> |
| true |
| </p> |
| </td> |
| </tr> |
| </tbody> |
| </table></div> |
| </div> |
| <br class="table-break"><div class="table"> |
| <a name="boost_typetraits.category.value_traits.operators.necessary_and_non_sufficient_condition_on_operator_return_type_for_value_to_be_true"></a><p class="title"><b>Table 1.8. necessary and non sufficient condition on operator return type for |
| value to be true</b></p> |
| <div class="table-contents"><table class="table" summary="necessary and non sufficient condition on operator return type for |
| value to be true"> |
| <colgroup> |
| <col> |
| <col> |
| <col> |
| <col> |
| <col> |
| <col> |
| </colgroup> |
| <thead><tr> |
| <th> |
| <p> |
| operator declaration |
| </p> |
| </th> |
| <th> |
| <p> |
| <code class="computeroutput"><span class="identifier">has_op</span><span class="special"><</span> |
| <span class="special">...,</span> <span class="keyword">void</span> |
| <span class="special">></span></code> |
| </p> |
| </th> |
| <th> |
| <p> |
| <code class="computeroutput"><span class="identifier">has_op</span><span class="special"><</span> |
| <span class="special">...,</span> <span class="identifier">Ret</span> |
| <span class="special">></span></code> |
| </p> |
| </th> |
| <th> |
| <p> |
| <code class="computeroutput"><span class="identifier">has_op</span><span class="special"><</span> |
| <span class="special">...,</span> <span class="identifier">Ret</span> |
| <span class="keyword">const</span> <span class="special">></span></code> |
| </p> |
| </th> |
| <th> |
| <p> |
| <code class="computeroutput"><span class="identifier">has_op</span><span class="special"><</span> |
| <span class="special">...,</span> <span class="identifier">Ret</span> |
| <span class="special">&</span> <span class="special">></span></code> |
| </p> |
| </th> |
| <th> |
| <p> |
| <code class="computeroutput"><span class="identifier">has_op</span><span class="special"><</span> |
| <span class="special">...,</span> <span class="identifier">Ret</span> |
| <span class="keyword">const</span> <span class="special">&</span> |
| <span class="special">></span></code> |
| </p> |
| </th> |
| </tr></thead> |
| <tbody> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="keyword">void</span> <span class="keyword">operator</span></code>@<code class="computeroutput"><span class="special">(...)</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| true |
| </p> |
| </td> |
| <td> |
| <p> |
| false |
| </p> |
| </td> |
| <td> |
| <p> |
| false |
| </p> |
| </td> |
| <td> |
| <p> |
| false |
| </p> |
| </td> |
| <td> |
| <p> |
| false |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="identifier">Ret</span> <span class="keyword">operator</span></code>@<code class="computeroutput"><span class="special">(...)</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| false |
| </p> |
| </td> |
| <td> |
| <p> |
| true |
| </p> |
| </td> |
| <td> |
| <p> |
| true |
| </p> |
| </td> |
| <td> |
| <p> |
| false |
| </p> |
| </td> |
| <td> |
| <p> |
| true |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="identifier">Ret</span> <span class="keyword">const</span> |
| <span class="keyword">operator</span></code>@<code class="computeroutput"><span class="special">(...)</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| false |
| </p> |
| </td> |
| <td> |
| <p> |
| true |
| </p> |
| </td> |
| <td> |
| <p> |
| true |
| </p> |
| </td> |
| <td> |
| <p> |
| false |
| </p> |
| </td> |
| <td> |
| <p> |
| true |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="identifier">Ret</span> <span class="special">&</span> |
| <span class="keyword">operator</span></code>@<code class="computeroutput"><span class="special">(...)</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| false |
| </p> |
| </td> |
| <td> |
| <p> |
| true |
| </p> |
| </td> |
| <td> |
| <p> |
| true |
| </p> |
| </td> |
| <td> |
| <p> |
| true |
| </p> |
| </td> |
| <td> |
| <p> |
| true |
| </p> |
| </td> |
| </tr> |
| <tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="identifier">Ret</span> <span class="keyword">const</span> |
| <span class="special">&</span> <span class="keyword">operator</span></code>@<code class="computeroutput"><span class="special">(...)</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| false |
| </p> |
| </td> |
| <td> |
| <p> |
| true |
| </p> |
| </td> |
| <td> |
| <p> |
| true |
| </p> |
| </td> |
| <td> |
| <p> |
| false |
| </p> |
| </td> |
| <td> |
| <p> |
| true |
| </p> |
| </td> |
| </tr> |
| </tbody> |
| </table></div> |
| </div> |
| <br class="table-break"><h6> |
| <a name="boost_typetraits.category.value_traits.operators.h4"></a> |
| <span class="phrase"><a name="boost_typetraits.category.value_traits.operators.implementation"></a></span><a class="link" href="operators.html#boost_typetraits.category.value_traits.operators.implementation">Implementation</a> |
| </h6> |
| <p> |
| The implementation consists in only header files. The following headers |
| should included first: |
| </p> |
| <pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</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">></span></pre> |
| <p> |
| or |
| </p> |
| <pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">type_traits</span><span class="special">/</span><span class="identifier">has_op</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></pre> |
| <p> |
| where <code class="literal">op</code> is the textual name chosen for the wanted operator. |
| The first method includes all operator traits. |
| </p> |
| <p> |
| All traits are implemented the same way using preprocessor macros to avoid |
| code duplication. The main files are in <code class="literal">boost/type_traits/detail</code>: |
| <code class="literal">has_binary_operator.hpp</code>, <code class="literal">has_prefix_operator.hpp</code> |
| and <code class="literal">has_postfix_operator.hpp</code>. The example of prefix |
| <code class="computeroutput"><span class="keyword">operator</span><span class="special">-</span></code> |
| is presented below: |
| </p> |
| <p> |
| </p> |
| <pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span> |
| <span class="keyword">namespace</span> <span class="identifier">detail</span> <span class="special">{</span> |
| |
| <span class="comment">// This namespace ensures that argument-dependent name lookup does not mess things up.</span> |
| <span class="keyword">namespace</span> <span class="identifier">has_unary_minus_impl</span> <span class="special">{</span> |
| |
| <span class="comment">// 1. a function to have an instance of type T without requiring T to be default</span> |
| <span class="comment">// constructible</span> |
| <span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">></span> <span class="identifier">T</span> <span class="special">&</span><span class="identifier">make</span><span class="special">();</span> |
| |
| |
| <span class="comment">// 2. we provide our operator definition for types that do not have one already</span> |
| |
| <span class="comment">// a type returned from operator- when no such operator is</span> |
| <span class="comment">// found in the type's own namespace (our own operator is used) so that we have</span> |
| <span class="comment">// a means to know that our operator was used</span> |
| <span class="keyword">struct</span> <span class="identifier">no_operator</span> <span class="special">{</span> <span class="special">};</span> |
| |
| <span class="comment">// this class allows implicit conversions and makes the following operator</span> |
| <span class="comment">// definition less-preferred than any other such operators that might be found</span> |
| <span class="comment">// via argument-dependent name lookup</span> |
| <span class="keyword">struct</span> <span class="identifier">any</span> <span class="special">{</span> <span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="identifier">any</span><span class="special">(</span><span class="identifier">T</span> <span class="keyword">const</span><span class="special">&);</span> <span class="special">};</span> |
| |
| <span class="comment">// when operator- is not available, this one is used</span> |
| <span class="identifier">no_operator</span> <span class="keyword">operator</span><span class="special">-(</span><span class="keyword">const</span> <span class="identifier">any</span><span class="special">&);</span> |
| |
| |
| <span class="comment">// 3. checks if the operator returns void or not</span> |
| <span class="comment">// conditions: Rhs!=void</span> |
| |
| <span class="comment">// we first redefine "operator," so that we have no compilation error if</span> |
| <span class="comment">// operator- returns void and we can use the return type of</span> |
| <span class="comment">// (-rhs, returns_void_t()) to deduce if operator- returns void or not:</span> |
| <span class="comment">// - operator- returns void -> (-rhs, returns_void_t()) returns returns_void_t</span> |
| <span class="comment">// - operator- returns !=void -> (-rhs, returns_void_t()) returns int</span> |
| <span class="keyword">struct</span> <span class="identifier">returns_void_t</span> <span class="special">{</span> <span class="special">};</span> |
| <span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">int</span> <span class="keyword">operator</span><span class="special">,(</span><span class="keyword">const</span> <span class="identifier">T</span><span class="special">&,</span> <span class="identifier">returns_void_t</span><span class="special">);</span> |
| <span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">int</span> <span class="keyword">operator</span><span class="special">,(</span><span class="keyword">const</span> <span class="keyword">volatile</span> <span class="identifier">T</span><span class="special">&,</span> <span class="identifier">returns_void_t</span><span class="special">);</span> |
| |
| <span class="comment">// this intermediate trait has member value of type bool:</span> |
| <span class="comment">// - value==true -> operator- returns void</span> |
| <span class="comment">// - value==false -> operator- does not return void</span> |
| <span class="keyword">template</span> <span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">Rhs</span> <span class="special">></span> |
| <span class="keyword">struct</span> <span class="identifier">operator_returns_void</span> <span class="special">{</span> |
| <span class="comment">// overloads of function returns_void make the difference</span> |
| <span class="comment">// yes_type and no_type have different size by construction</span> |
| <span class="keyword">static</span> <span class="special">::</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">type_traits</span><span class="special">::</span><span class="identifier">yes_type</span> <span class="identifier">returns_void</span><span class="special">(</span><span class="identifier">returns_void_t</span><span class="special">);</span> |
| <span class="keyword">static</span> <span class="special">::</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">type_traits</span><span class="special">::</span><span class="identifier">no_type</span> <span class="identifier">returns_void</span><span class="special">(</span><span class="keyword">int</span><span class="special">);</span> |
| <span class="keyword">static</span> <span class="keyword">const</span> <span class="keyword">bool</span> <span class="identifier">value</span> <span class="special">=</span> <span class="keyword">sizeof</span><span class="special">(::</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">type_traits</span><span class="special">::</span><span class="identifier">yes_type</span><span class="special">)==</span><span class="keyword">sizeof</span><span class="special">(</span><span class="identifier">returns_void</span><span class="special">((-</span><span class="identifier">make</span><span class="special"><</span><span class="identifier">Rhs</span><span class="special">>(),</span><span class="identifier">returns_void_t</span><span class="special">())));</span> |
| <span class="special">};</span> |
| |
| |
| <span class="comment">// 4. checks if the return type is Ret or Ret==dont_care</span> |
| <span class="comment">// conditions: Rhs!=void</span> |
| |
| <span class="keyword">struct</span> <span class="identifier">dont_care</span> <span class="special">{</span> <span class="special">};</span> |
| |
| <span class="keyword">template</span> <span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">Rhs</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Ret</span><span class="special">,</span> <span class="keyword">bool</span> <span class="identifier">Returns_void</span> <span class="special">></span> |
| <span class="keyword">struct</span> <span class="identifier">operator_returns_Ret</span><span class="special">;</span> |
| |
| <span class="keyword">template</span> <span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">Rhs</span> <span class="special">></span> |
| <span class="keyword">struct</span> <span class="identifier">operator_returns_Ret</span> <span class="special"><</span> <span class="identifier">Rhs</span><span class="special">,</span> <span class="identifier">dont_care</span><span class="special">,</span> <span class="keyword">true</span> <span class="special">></span> <span class="special">{</span> |
| <span class="keyword">static</span> <span class="keyword">const</span> <span class="keyword">bool</span> <span class="identifier">value</span> <span class="special">=</span> <span class="keyword">true</span><span class="special">;</span> |
| <span class="special">};</span> |
| |
| <span class="keyword">template</span> <span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">Rhs</span> <span class="special">></span> |
| <span class="keyword">struct</span> <span class="identifier">operator_returns_Ret</span> <span class="special"><</span> <span class="identifier">Rhs</span><span class="special">,</span> <span class="identifier">dont_care</span><span class="special">,</span> <span class="keyword">false</span> <span class="special">></span> <span class="special">{</span> |
| <span class="keyword">static</span> <span class="keyword">const</span> <span class="keyword">bool</span> <span class="identifier">value</span> <span class="special">=</span> <span class="keyword">true</span><span class="special">;</span> |
| <span class="special">};</span> |
| |
| <span class="keyword">template</span> <span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">Rhs</span> <span class="special">></span> |
| <span class="keyword">struct</span> <span class="identifier">operator_returns_Ret</span> <span class="special"><</span> <span class="identifier">Rhs</span><span class="special">,</span> <span class="keyword">void</span><span class="special">,</span> <span class="keyword">true</span> <span class="special">></span> <span class="special">{</span> |
| <span class="keyword">static</span> <span class="keyword">const</span> <span class="keyword">bool</span> <span class="identifier">value</span> <span class="special">=</span> <span class="keyword">true</span><span class="special">;</span> |
| <span class="special">};</span> |
| |
| <span class="keyword">template</span> <span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">Rhs</span> <span class="special">></span> |
| <span class="keyword">struct</span> <span class="identifier">operator_returns_Ret</span> <span class="special"><</span> <span class="identifier">Rhs</span><span class="special">,</span> <span class="keyword">void</span><span class="special">,</span> <span class="keyword">false</span> <span class="special">></span> <span class="special">{</span> |
| <span class="keyword">static</span> <span class="keyword">const</span> <span class="keyword">bool</span> <span class="identifier">value</span> <span class="special">=</span> <span class="keyword">false</span><span class="special">;</span> |
| <span class="special">};</span> |
| |
| <span class="keyword">template</span> <span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">Rhs</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Ret</span> <span class="special">></span> |
| <span class="keyword">struct</span> <span class="identifier">operator_returns_Ret</span> <span class="special"><</span> <span class="identifier">Rhs</span><span class="special">,</span> <span class="identifier">Ret</span><span class="special">,</span> <span class="keyword">true</span> <span class="special">></span> <span class="special">{</span> |
| <span class="keyword">static</span> <span class="keyword">const</span> <span class="keyword">bool</span> <span class="identifier">value</span> <span class="special">=</span> <span class="keyword">false</span><span class="special">;</span> |
| <span class="special">};</span> |
| |
| <span class="comment">// otherwise checks if it is convertible to Ret using the sizeof trick</span> |
| <span class="comment">// based on overload resolution</span> |
| <span class="comment">// condition: Ret!=void and Ret!=dont_care and the operator does not return void</span> |
| <span class="keyword">template</span> <span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">Rhs</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Ret</span> <span class="special">></span> |
| <span class="keyword">struct</span> <span class="identifier">operator_returns_Ret</span> <span class="special"><</span> <span class="identifier">Rhs</span><span class="special">,</span> <span class="identifier">Ret</span><span class="special">,</span> <span class="keyword">false</span> <span class="special">></span> <span class="special">{</span> |
| <span class="keyword">static</span> <span class="special">::</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">type_traits</span><span class="special">::</span><span class="identifier">yes_type</span> <span class="identifier">is_convertible_to_Ret</span><span class="special">(</span><span class="identifier">Ret</span><span class="special">);</span> <span class="comment">// this version is preferred for types convertible to Ret</span> |
| <span class="keyword">static</span> <span class="special">::</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">type_traits</span><span class="special">::</span><span class="identifier">no_type</span> <span class="identifier">is_convertible_to_Ret</span><span class="special">(...);</span> <span class="comment">// this version is used otherwise</span> |
| |
| <span class="keyword">static</span> <span class="keyword">const</span> <span class="keyword">bool</span> <span class="identifier">value</span> <span class="special">=</span> <span class="keyword">sizeof</span><span class="special">(</span><span class="identifier">is_convertible_to_Ret</span><span class="special">(-</span><span class="identifier">make</span><span class="special"><</span><span class="identifier">Rhs</span><span class="special">>()))==</span><span class="keyword">sizeof</span><span class="special">(::</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">type_traits</span><span class="special">::</span><span class="identifier">yes_type</span><span class="special">);</span> |
| <span class="special">};</span> |
| |
| |
| <span class="comment">// 5. checks for operator existence</span> |
| <span class="comment">// condition: Rhs!=void</span> |
| |
| <span class="comment">// checks if our definition of operator- is used or an other</span> |
| <span class="comment">// existing one;</span> |
| <span class="comment">// this is done with redefinition of "operator," that returns no_operator or has_operator</span> |
| <span class="keyword">struct</span> <span class="identifier">has_operator</span> <span class="special">{</span> <span class="special">};</span> |
| <span class="identifier">no_operator</span> <span class="keyword">operator</span><span class="special">,(</span><span class="identifier">no_operator</span><span class="special">,</span> <span class="identifier">has_operator</span><span class="special">);</span> |
| |
| <span class="keyword">template</span> <span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">Rhs</span> <span class="special">></span> |
| <span class="keyword">struct</span> <span class="identifier">operator_exists</span> <span class="special">{</span> |
| <span class="keyword">static</span> <span class="special">::</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">type_traits</span><span class="special">::</span><span class="identifier">yes_type</span> <span class="identifier">check</span><span class="special">(</span><span class="identifier">has_operator</span><span class="special">);</span> <span class="comment">// this version is preferred when operator exists</span> |
| <span class="keyword">static</span> <span class="special">::</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">type_traits</span><span class="special">::</span><span class="identifier">no_type</span> <span class="identifier">check</span><span class="special">(</span><span class="identifier">no_operator</span><span class="special">);</span> <span class="comment">// this version is used otherwise</span> |
| |
| <span class="keyword">static</span> <span class="keyword">const</span> <span class="keyword">bool</span> <span class="identifier">value</span> <span class="special">=</span> <span class="keyword">sizeof</span><span class="special">(</span><span class="identifier">check</span><span class="special">(((-</span><span class="identifier">make</span><span class="special"><</span><span class="identifier">Rhs</span><span class="special">>()),</span><span class="identifier">make</span><span class="special"><</span><span class="identifier">has_operator</span><span class="special">>())))==</span><span class="keyword">sizeof</span><span class="special">(::</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">type_traits</span><span class="special">::</span><span class="identifier">yes_type</span><span class="special">);</span> |
| <span class="special">};</span> |
| |
| |
| <span class="comment">// 6. main trait: to avoid any compilation error, this class behaves</span> |
| <span class="comment">// differently when operator-(Rhs) is forbidden by the standard.</span> |
| <span class="comment">// Forbidden_if is a bool that is:</span> |
| <span class="comment">// - true when the operator-(Rhs) is forbidden by the standard</span> |
| <span class="comment">// (would yield compilation error if used)</span> |
| <span class="comment">// - false otherwise</span> |
| <span class="keyword">template</span> <span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">Rhs</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Ret</span><span class="special">,</span> <span class="keyword">bool</span> <span class="identifier">Forbidden_if</span> <span class="special">></span> |
| <span class="keyword">struct</span> <span class="identifier">trait_impl1</span><span class="special">;</span> |
| |
| <span class="keyword">template</span> <span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">Rhs</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Ret</span> <span class="special">></span> |
| <span class="keyword">struct</span> <span class="identifier">trait_impl1</span> <span class="special"><</span> <span class="identifier">Rhs</span><span class="special">,</span> <span class="identifier">Ret</span><span class="special">,</span> <span class="keyword">true</span> <span class="special">></span> <span class="special">{</span> |
| <span class="keyword">static</span> <span class="keyword">const</span> <span class="keyword">bool</span> <span class="identifier">value</span> <span class="special">=</span> <span class="keyword">false</span><span class="special">;</span> |
| <span class="special">};</span> |
| |
| <span class="keyword">template</span> <span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">Rhs</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Ret</span> <span class="special">></span> |
| <span class="keyword">struct</span> <span class="identifier">trait_impl1</span> <span class="special"><</span> <span class="identifier">Rhs</span><span class="special">,</span> <span class="identifier">Ret</span><span class="special">,</span> <span class="keyword">false</span> <span class="special">></span> <span class="special">{</span> |
| <span class="keyword">static</span> <span class="keyword">const</span> <span class="keyword">bool</span> <span class="identifier">value</span> <span class="special">=</span> |
| <span class="special">::</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">type_traits</span><span class="special">::</span><span class="identifier">ice_and</span><span class="special"><</span> |
| <span class="identifier">operator_exists</span> <span class="special"><</span> <span class="identifier">Rhs</span> <span class="special">>::</span><span class="identifier">value</span><span class="special">,</span> |
| <span class="identifier">operator_returns_Ret</span> <span class="special"><</span> <span class="identifier">Rhs</span><span class="special">,</span> <span class="identifier">Ret</span><span class="special">,</span> <span class="identifier">operator_returns_void</span> <span class="special"><</span> <span class="identifier">Rhs</span> <span class="special">>::</span><span class="identifier">value</span> <span class="special">>::</span><span class="identifier">value</span> |
| <span class="special">>::</span><span class="identifier">value</span> |
| <span class="special">;</span> |
| <span class="special">};</span> |
| |
| <span class="comment">// specialization needs to be declared for the special void case</span> |
| <span class="keyword">template</span> <span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">Ret</span> <span class="special">></span> |
| <span class="keyword">struct</span> <span class="identifier">trait_impl1</span> <span class="special"><</span> <span class="keyword">void</span><span class="special">,</span> <span class="identifier">Ret</span><span class="special">,</span> <span class="keyword">false</span> <span class="special">></span> <span class="special">{</span> |
| <span class="keyword">static</span> <span class="keyword">const</span> <span class="keyword">bool</span> <span class="identifier">value</span> <span class="special">=</span> <span class="keyword">false</span><span class="special">;</span> |
| <span class="special">};</span> |
| |
| <span class="comment">// defines some typedef for convenience</span> |
| <span class="keyword">template</span> <span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">Rhs</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Ret</span> <span class="special">></span> |
| <span class="keyword">struct</span> <span class="identifier">trait_impl</span> <span class="special">{</span> |
| <span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="special">::</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">remove_reference</span><span class="special"><</span><span class="identifier">Rhs</span><span class="special">>::</span><span class="identifier">type</span> <span class="identifier">Rhs_noref</span><span class="special">;</span> |
| <span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="special">::</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">remove_cv</span><span class="special"><</span><span class="identifier">Rhs_noref</span><span class="special">>::</span><span class="identifier">type</span> <span class="identifier">Rhs_nocv</span><span class="special">;</span> |
| <span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="special">::</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">remove_cv</span><span class="special"><</span> <span class="keyword">typename</span> <span class="special">::</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">remove_reference</span><span class="special"><</span> <span class="keyword">typename</span> <span class="special">::</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">remove_pointer</span><span class="special"><</span><span class="identifier">Rhs_noref</span><span class="special">>::</span><span class="identifier">type</span> <span class="special">>::</span><span class="identifier">type</span> <span class="special">>::</span><span class="identifier">type</span> <span class="identifier">Rhs_noptr</span><span class="special">;</span> |
| <span class="keyword">static</span> <span class="keyword">const</span> <span class="keyword">bool</span> <span class="identifier">value</span> <span class="special">=</span> <span class="identifier">trait_impl1</span> <span class="special"><</span> <span class="identifier">Rhs_noref</span><span class="special">,</span> <span class="identifier">Ret</span><span class="special">,</span> <span class="special">::</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">is_pointer</span><span class="special"><</span> <span class="identifier">Rhs_noref</span> <span class="special">>::</span><span class="identifier">value</span> <span class="special">>::</span><span class="identifier">value</span><span class="special">;</span> |
| <span class="special">};</span> |
| |
| <span class="special">}</span> <span class="comment">// namespace impl</span> |
| <span class="special">}</span> <span class="comment">// namespace detail</span> |
| |
| <span class="comment">// this is the accessible definition of the trait to end user</span> |
| <span class="keyword">template</span> <span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">Rhs</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Ret</span><span class="special">=::</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">detail</span><span class="special">::</span><span class="identifier">has_unary_minus_impl</span><span class="special">::</span><span class="identifier">dont_care</span> <span class="special">></span> |
| <span class="keyword">struct</span> <span class="identifier">has_unary_minus</span> <span class="special">:</span> <span class="special">::</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">integral_constant</span><span class="special"><</span><span class="keyword">bool</span><span class="special">,(::</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">detail</span><span class="special">::</span><span class="identifier">has_unary_minus_impl</span><span class="special">::</span><span class="identifier">trait_impl</span> <span class="special"><</span> <span class="identifier">Rhs</span><span class="special">,</span> <span class="identifier">Ret</span> <span class="special">>::</span><span class="identifier">value</span><span class="special">)></span> <span class="special">{</span> <span class="special">};</span> |
| |
| <span class="special">}</span> <span class="comment">// namespace boost</span> |
| </pre> |
| <p> |
| </p> |
| <h6> |
| <a name="boost_typetraits.category.value_traits.operators.h5"></a> |
| <span class="phrase"><a name="boost_typetraits.category.value_traits.operators.limitation"></a></span><a class="link" href="operators.html#boost_typetraits.category.value_traits.operators.limitation">Limitation</a> |
| </h6> |
| <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"> |
| Requires a compiler with working SFINAE. |
| </li></ul></div> |
| <h6> |
| <a name="boost_typetraits.category.value_traits.operators.h6"></a> |
| <span class="phrase"><a name="boost_typetraits.category.value_traits.operators.known_issues"></a></span><a class="link" href="operators.html#boost_typetraits.category.value_traits.operators.known_issues">Known |
| issues</a> |
| </h6> |
| <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> |
| <li class="listitem"> |
| These traits cannot detect whether the operators are public or not: |
| if an operator is defined as a private member of type <code class="computeroutput"><span class="identifier">T</span></code> then the instantiation of the corresponding |
| trait will produce a compiler error. For this reason these traits cannot |
| be used to determine whether a type has a public operator 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="identifier">A</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"><</span><span class="identifier">A</span><span class="special">>::</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 because |
| both the existing operator and the one we provide (with argument of |
| type <code class="computeroutput"><span class="identifier">any</span></code>) need type |
| conversion, so that none is preferred. |
| <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">&);</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"><</span><span class="identifier">A</span><span class="special">>::</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"><</span><span class="identifier">B</span><span class="special">>::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// error: ambiguous overload between</span> |
| <span class="comment">// operator-(const any&) and</span> |
| <span class="comment">// operator-(const A&)</span> |
| <span class="comment">// both need type conversion</span> |
| </pre> |
| <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">B</span> <span class="special">{</span> <span class="special">};</span> |
| <span class="keyword">struct</span> <span class="identifier">A</span> <span class="special">{</span> <span class="identifier">A</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">B</span><span class="special">&)</span> <span class="special">{</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">&);</span> |
| <span class="identifier">boost</span><span class="special">::</span><span class="identifier">has_unary_minus</span><span class="special"><</span><span class="identifier">A</span><span class="special">>::</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"><</span><span class="identifier">B</span><span class="special">>::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// error: ambiguous overload between</span> |
| <span class="comment">// operator-(const any&) and</span> |
| <span class="comment">// operator-(const A&)</span> |
| <span class="comment">// both need type conversion</span> |
| </pre> |
| </li> |
| <li class="listitem"> |
| There is an issue when applying these traits to template classes. If |
| the operator 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>. This |
| applies in particular to the containers of the standard library and |
| <code class="computeroutput"><span class="keyword">operator</span><span class="special">==</span></code>. |
| Example: |
| <pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">type_traits</span><span class="special">/</span><span class="identifier">has_equal_to</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> |
| <span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span> |
| |
| <span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></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"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></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"><</span><span class="identifier">T</span><span class="special">></span> <span class="special">&</span><span class="identifier">lhs</span><span class="special">,</span> <span class="keyword">const</span> <span class="identifier">contains</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="special">&</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="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">&,</span> <span class="keyword">const</span> <span class="identifier">good</span><span class="special">&)</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"><<</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<good></span> |
| <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special"><<</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">has_equal_to</span><span class="special"><</span> <span class="identifier">contains</span><span class="special"><</span> <span class="identifier">good</span> <span class="special">></span> <span class="special">>::</span><span class="identifier">value</span><span class="special"><<</span><span class="char">'\n'</span><span class="special">;</span> <span class="comment">// true</span> |
| <span class="identifier">contains</span><span class="special"><</span><span class="identifier">good</span><span class="special">></span> <span class="identifier">g</span><span class="special">;</span> |
| <span class="identifier">g</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<bad></span> |
| <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special"><<</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">has_equal_to</span><span class="special"><</span> <span class="identifier">contains</span><span class="special"><</span> <span class="identifier">bad</span> <span class="special">></span> <span class="special">>::</span><span class="identifier">value</span><span class="special"><<</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"><</span><span class="identifier">bad</span><span class="special">></span> <span class="identifier">b</span><span class="special">;</span> |
| <span class="identifier">b</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> |
| <h6> |
| <a name="boost_typetraits.category.value_traits.operators.h7"></a> |
| <span class="phrase"><a name="boost_typetraits.category.value_traits.operators.acknowledgments"></a></span><a class="link" href="operators.html#boost_typetraits.category.value_traits.operators.acknowledgments">Acknowledgments</a> |
| </h6> |
| <p> |
| Frederic Bron is very thankful to numerous people from the boost mailing |
| list for their kind help and patience. In particular, the following persons |
| have been very helpful for the implementation: Edward Diener, Eric Niebler, |
| Jeffrey Lee Hellrung (Jr.), Robert Stewart, Roman Perepelitsa, Steven Watanabe, |
| Vicente Botet. |
| </p> |
| </div> |
| <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> |
| <td align="left"></td> |
| <td align="right"><div class="copyright-footer">Copyright © 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>) |
| </p> |
| </div></td> |
| </tr></table> |
| <hr> |
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