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- <div class="titlepage"><div><div><h3 class="title">
- <a name="boost_typetraits.reference.common_type"></a><a class="link" href="common_type.html" title="common_type">common_type</a>
- </h3></div></div></div>
- <p>
- <span class="bold"><strong>Header:</strong></span> <code class="computeroutput"> <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">common_type</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></code>
- or <code class="computeroutput"> <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">hpp</span><span class="special">></span></code>
- </p>
- <pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span>
- <span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span><span class="special">...</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">struct</span> <span class="identifier">common_type</span><span class="special">;</span>
- <span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span><span class="special">...</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">using</span> <span class="identifier">common_type_t</span> <span class="special">=</span> <span class="keyword">typename</span> <span class="identifier">common_type</span><span class="special"><</span><span class="identifier">T</span><span class="special">...>::</span><span class="identifier">type</span><span class="special">;</span> <span class="comment">// C++11 and above</span>
- <span class="special">}</span>
- </pre>
- <p>
- <code class="computeroutput"><span class="identifier">common_type</span></code> is a traits class
- used to deduce a type common to a several types, useful as the return type
- of functions operating on multiple input types such as in mixed-mode arithmetic..
- </p>
- <p>
- The nested typedef <code class="computeroutput"><span class="special">::</span><span class="identifier">type</span></code>
- could be defined as follows:
- </p>
- <pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span><span class="special">...</span> <span class="identifier">T</span><span class="special">></span>
- <span class="keyword">struct</span> <span class="identifier">common_type</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">class</span> <span class="identifier">U</span><span class="special">,</span> <span class="keyword">class</span><span class="special">...</span> <span class="identifier">V</span><span class="special">></span>
- <span class="keyword">struct</span> <span class="identifier">common_type</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">U</span><span class="special">,</span> <span class="identifier">V</span><span class="special">...></span> <span class="special">{</span>
- <span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">common_type</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">common_type</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">U</span><span class="special">>::</span><span class="identifier">type</span><span class="special">,</span> <span class="identifier">V</span><span class="special">...>::</span><span class="identifier">type</span> <span class="identifier">type</span><span class="special">;</span>
- <span class="special">};</span>
- <span class="keyword">template</span> <span class="special"><></span>
- <span class="keyword">struct</span> <span class="identifier">common_type</span><span class="special"><></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">struct</span> <span class="identifier">common_type</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="special">{</span>
- <span class="keyword">typedef</span> <span class="keyword">typename</span> <a class="link" href="decay.html" title="decay">decay</a><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">type</span> <span class="identifier">type</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">class</span> <span class="identifier">U</span><span class="special">></span>
- <span class="keyword">struct</span> <span class="identifier">common_type</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">U</span><span class="special">></span> <span class="special">{</span>
- <span class="keyword">typedef</span> <span class="keyword">typename</span> <a class="link" href="decay.html" title="decay">decay</a><span class="special"><</span>
- <span class="keyword">decltype</span><span class="special">(</span> <a class="link" href="declval.html" title="declval">declval</a><span class="special"><</span><span class="keyword">bool</span><span class="special">>()?</span>
- <a class="link" href="declval.html" title="declval">declval</a><span class="special"><</span><span class="keyword">typename</span> <a class="link" href="decay.html" title="decay">decay</a><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">type</span><span class="special">>():</span>
- <a class="link" href="declval.html" title="declval">declval</a><span class="special"><</span><span class="keyword">typename</span> <a class="link" href="decay.html" title="decay">decay</a><span class="special"><</span><span class="identifier">U</span><span class="special">>::</span><span class="identifier">type</span><span class="special">>()</span> <span class="special">)</span>
- <span class="special">>::</span><span class="identifier">type</span> <span class="identifier">type</span><span class="special">;</span>
- <span class="special">};</span>
- </pre>
- <p>
- All parameter types must be complete. This trait is permitted to be specialized
- by a user if at least one template parameter is a user-defined type. <span class="bold"><strong>Note:</strong></span> Such specializations are required when only
- explicit conversions are desired among the <code class="computeroutput"><span class="identifier">common_type</span></code>
- arguments.
- </p>
- <p>
- Note that when the compiler does not support variadic templates (and the
- macro <code class="computeroutput"><span class="identifier">BOOST_NO_CXX11_VARIADIC_TEMPLATES</span></code>
- is defined) then the maximum number of template arguments is 9.
- </p>
- <h5>
- <a name="boost_typetraits.reference.common_type.h0"></a>
- <span class="phrase"><a name="boost_typetraits.reference.common_type.tutorial"></a></span><a class="link" href="common_type.html#boost_typetraits.reference.common_type.tutorial">Tutorial</a>
- </h5>
- <p>
- In a nutshell, <code class="computeroutput"><span class="identifier">common_type</span></code>
- is a trait that takes 1 or more types, and returns a type which all of the
- types will convert to. The default definition demands this conversion be
- implicit. However the trait can be specialized for user-defined types which
- want to limit their inter-type conversions to explicit, and yet still want
- to interoperate with the <code class="computeroutput"><span class="identifier">common_type</span></code>
- facility.
- </p>
- <p>
- <span class="bold"><strong>Example:</strong></span>
- </p>
- <pre class="programlisting"><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">class</span> <span class="identifier">U</span><span class="special">></span>
- <span class="identifier">complex</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">common_type</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">U</span><span class="special">>::</span><span class="identifier">type</span><span class="special">></span>
- <span class="keyword">operator</span><span class="special">+(</span><span class="identifier">complex</span><span class="special"><</span><span class="identifier">T</span><span class="special">>,</span> <span class="identifier">complex</span><span class="special"><</span><span class="identifier">U</span><span class="special">>);</span>
- </pre>
- <p>
- In the above example, "mixed-mode" complex arithmetic is allowed.
- The return type is described by <code class="computeroutput"><span class="identifier">common_type</span></code>.
- For example the resulting type of adding a <code class="computeroutput"><span class="identifier">complex</span><span class="special"><</span><span class="keyword">float</span><span class="special">></span></code> and <code class="computeroutput"><span class="identifier">complex</span><span class="special"><</span><span class="keyword">double</span><span class="special">></span></code> might be a <code class="computeroutput"><span class="identifier">complex</span><span class="special"><</span><span class="keyword">double</span><span class="special">></span></code>.
- </p>
- <p>
- Here is how someone might produce a variadic comparison function:
- </p>
- <pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="special">...</span><span class="identifier">T</span><span class="special">></span>
- <span class="keyword">typename</span> <span class="identifier">common_type</span><span class="special"><</span><span class="identifier">T</span><span class="special">...>::</span><span class="identifier">type</span>
- <span class="identifier">min</span><span class="special">(</span><span class="identifier">T</span><span class="special">...</span> <span class="identifier">t</span><span class="special">);</span>
- </pre>
- <p>
- This is a very useful and broadly applicable utility.
- </p>
- <h5>
- <a name="boost_typetraits.reference.common_type.h1"></a>
- <span class="phrase"><a name="boost_typetraits.reference.common_type.how_to_get_the_common_type_of_types_with_explicit_conversions_"></a></span><a class="link" href="common_type.html#boost_typetraits.reference.common_type.how_to_get_the_common_type_of_types_with_explicit_conversions_">How
- to get the common type of types with explicit conversions?</a>
- </h5>
- <p>
- Another choice for the author of the preceding operator could be
- </p>
- <pre class="programlisting"><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">class</span> <span class="identifier">U</span><span class="special">></span>
- <span class="keyword">typename</span> <span class="identifier">common_type</span><span class="special"><</span><span class="identifier">complex</span><span class="special"><</span><span class="identifier">T</span><span class="special">>,</span> <span class="identifier">complex</span><span class="special"><</span><span class="identifier">U</span><span class="special">></span> <span class="special">>::</span><span class="identifier">type</span>
- <span class="keyword">operator</span><span class="special">+(</span><span class="identifier">complex</span><span class="special"><</span><span class="identifier">T</span><span class="special">>,</span> <span class="identifier">complex</span><span class="special"><</span><span class="identifier">U</span><span class="special">>);</span>
- </pre>
- <p>
- As the default definition of <code class="computeroutput"><span class="identifier">common_type</span></code>
- demands the conversion be implicit, we need to specialize the trait for complex
- types as follows.
- </p>
- <pre class="programlisting"><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">class</span> <span class="identifier">U</span><span class="special">></span>
- <span class="keyword">struct</span> <span class="identifier">common_type</span><span class="special"><</span><span class="identifier">complex</span><span class="special"><</span><span class="identifier">T</span><span class="special">>,</span> <span class="identifier">complex</span><span class="special"><</span><span class="identifier">U</span><span class="special">></span> <span class="special">></span> <span class="special">{</span>
- <span class="keyword">typedef</span> <span class="identifier">complex</span><span class="special"><</span> <span class="identifier">common_type</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">U</span><span class="special">></span> <span class="special">></span> <span class="identifier">type</span><span class="special">;</span>
- <span class="special">};</span>
- </pre>
- <h5>
- <a name="boost_typetraits.reference.common_type.h2"></a>
- <span class="phrase"><a name="boost_typetraits.reference.common_type.how_important_is_the_order_of_the__code__phrase_role__identifier__common_type__phrase__phrase_role__special___lt__gt___phrase___code__template_arguments_"></a></span><a class="link" href="common_type.html#boost_typetraits.reference.common_type.how_important_is_the_order_of_the__code__phrase_role__identifier__common_type__phrase__phrase_role__special___lt__gt___phrase___code__template_arguments_">How
- important is the order of the <code class="computeroutput"><span class="identifier">common_type</span><span class="special"><></span></code> template arguments?</a>
- </h5>
- <p>
- The order of the template parameters is important.
- </p>
- <p>
- <code class="computeroutput"><span class="identifier">common_type</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span><span class="identifier">B</span><span class="special">,</span><span class="identifier">C</span><span class="special">>::</span><span class="identifier">type</span></code> is not equivalent to <code class="computeroutput"><span class="identifier">common_type</span><span class="special"><</span><span class="identifier">C</span><span class="special">,</span><span class="identifier">A</span><span class="special">,</span><span class="identifier">B</span><span class="special">>::</span><span class="identifier">type</span></code>, but to <code class="computeroutput"><span class="identifier">common_type</span><span class="special"><</span><span class="identifier">common_type</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span><span class="identifier">B</span><span class="special">>::</span><span class="identifier">type</span><span class="special">,</span> <span class="identifier">C</span><span class="special">>::</span><span class="identifier">type</span></code>.
- </p>
- <p>
- Consider
- </p>
- <pre class="programlisting"><span class="keyword">struct</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">struct</span> <span class="identifier">C</span> <span class="special">{</span>
- <span class="identifier">C</span><span class="special">()</span> <span class="special">{}</span>
- <span class="identifier">C</span><span class="special">(</span><span class="identifier">A</span> <span class="keyword">const</span><span class="special">&)</span> <span class="special">{}</span>
- <span class="identifier">C</span><span class="special">(</span><span class="identifier">B</span> <span class="keyword">const</span><span class="special">&)</span> <span class="special">{}</span>
- <span class="identifier">C</span><span class="special">&</span> <span class="keyword">operator</span><span class="special">=(</span><span class="identifier">C</span> <span class="keyword">const</span><span class="special">&)</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="special">};</span>
- </pre>
- <p>
- The following doesn't compile
- </p>
- <pre class="programlisting"><span class="keyword">typedef</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">common_type</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span> <span class="identifier">B</span><span class="special">,</span> <span class="identifier">C</span><span class="special">>::</span><span class="identifier">type</span> <span class="identifier">ABC</span><span class="special">;</span> <span class="comment">// Does not compile</span>
- </pre>
- <p>
- while
- </p>
- <pre class="programlisting"><span class="keyword">typedef</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">common_type</span><span class="special"><</span><span class="identifier">C</span><span class="special">,</span> <span class="identifier">A</span><span class="special">,</span> <span class="identifier">B</span><span class="special">>::</span><span class="identifier">type</span> <span class="identifier">ABC</span><span class="special">;</span>
- </pre>
- <p>
- compiles.
- </p>
- <p>
- Thus, as <code class="computeroutput"><span class="identifier">common_type</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span><span class="identifier">B</span><span class="special">>::</span><span class="identifier">type</span></code>
- is undefined, <code class="computeroutput"><span class="identifier">common_type</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span><span class="identifier">B</span><span class="special">,</span><span class="identifier">C</span><span class="special">>::</span><span class="identifier">type</span></code>
- is also undefined.
- </p>
- <p>
- It is intended that clients who wish for <code class="computeroutput"><span class="identifier">common_type</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span>
- <span class="identifier">B</span><span class="special">></span></code>
- to be well defined to define it themselves:
- </p>
- <pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span>
- <span class="special">{</span>
- <span class="keyword">template</span> <span class="special"><></span>
- <span class="keyword">struct</span> <span class="identifier">common_type</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span> <span class="identifier">B</span><span class="special">></span> <span class="special">{</span><span class="keyword">typedef</span> <span class="identifier">C</span> <span class="identifier">type</span><span class="special">;};</span>
- <span class="special">}</span>
- </pre>
- <p>
- Now this client can ask for <code class="computeroutput"><span class="identifier">common_type</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span>
- <span class="identifier">B</span><span class="special">,</span> <span class="identifier">C</span><span class="special">></span></code> (and
- get the same answer).
- </p>
- <p>
- Clients wanting to ask <code class="computeroutput"><span class="identifier">common_type</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span>
- <span class="identifier">B</span><span class="special">,</span> <span class="identifier">C</span><span class="special">></span></code> in
- any order and get the same result need to add in addition:
- </p>
- <pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span>
- <span class="special">{</span>
- <span class="keyword">template</span> <span class="special"><></span> <span class="keyword">struct</span> <span class="identifier">common_type</span><span class="special"><</span><span class="identifier">B</span><span class="special">,</span> <span class="identifier">A</span><span class="special">></span>
- <span class="special">:</span> <span class="keyword">public</span> <span class="identifier">common_type</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span> <span class="identifier">B</span><span class="special">></span> <span class="special">{};</span>
- <span class="special">}</span>
- </pre>
- <p>
- This is needed as the specialization of <code class="computeroutput"><span class="identifier">common_type</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span>
- <span class="identifier">B</span><span class="special">></span></code>
- is not be used implicitly for <code class="computeroutput"><span class="identifier">common_type</span><span class="special"><</span><span class="identifier">B</span><span class="special">,</span>
- <span class="identifier">A</span><span class="special">></span></code>.
- </p>
- <h5>
- <a name="boost_typetraits.reference.common_type.h3"></a>
- <span class="phrase"><a name="boost_typetraits.reference.common_type.can_the__code__phrase_role__identifier__common_type__phrase___code__of_two_types_be_a_third_type_"></a></span><a class="link" href="common_type.html#boost_typetraits.reference.common_type.can_the__code__phrase_role__identifier__common_type__phrase___code__of_two_types_be_a_third_type_">Can
- the <code class="computeroutput"><span class="identifier">common_type</span></code> of two types
- be a third type?</a>
- </h5>
- <p>
- Given the preceding example, one might expect <code class="computeroutput"><span class="identifier">common_type</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span><span class="identifier">B</span><span class="special">>::</span><span class="identifier">type</span></code> to be <code class="computeroutput"><span class="identifier">C</span></code>
- without any intervention from the user. But the default <code class="computeroutput"><span class="identifier">common_type</span><span class="special"><></span></code> implementation doesn't grant that.
- It is intended that clients who wish for <code class="computeroutput"><span class="identifier">common_type</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span>
- <span class="identifier">B</span><span class="special">></span></code>
- to be well defined to define it themselves:
- </p>
- <pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span>
- <span class="special">{</span>
- <span class="keyword">template</span> <span class="special"><></span>
- <span class="keyword">struct</span> <span class="identifier">common_type</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span> <span class="identifier">B</span><span class="special">></span> <span class="special">{</span><span class="keyword">typedef</span> <span class="identifier">C</span> <span class="identifier">type</span><span class="special">;};</span>
- <span class="keyword">template</span> <span class="special"><></span> <span class="keyword">struct</span> <span class="identifier">common_type</span><span class="special"><</span><span class="identifier">B</span><span class="special">,</span> <span class="identifier">A</span><span class="special">></span>
- <span class="special">:</span> <span class="keyword">public</span> <span class="identifier">common_type</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span> <span class="identifier">B</span><span class="special">></span> <span class="special">{};</span>
- <span class="special">}</span>
- </pre>
- <p>
- Now this client can ask for <code class="computeroutput"><span class="identifier">common_type</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span>
- <span class="identifier">B</span><span class="special">></span></code>.
- </p>
- <h5>
- <a name="boost_typetraits.reference.common_type.h4"></a>
- <span class="phrase"><a name="boost_typetraits.reference.common_type.how_does__code__phrase_role__identifier__common_type__phrase___code__behave_with_pointers_"></a></span><a class="link" href="common_type.html#boost_typetraits.reference.common_type.how_does__code__phrase_role__identifier__common_type__phrase___code__behave_with_pointers_">How
- does <code class="computeroutput"><span class="identifier">common_type</span></code> behave with
- pointers?</a>
- </h5>
- <p>
- Consider
- </p>
- <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">C</span> <span class="special">{</span> <span class="special">}:</span>
- <span class="keyword">struct</span> <span class="identifier">B</span> <span class="special">:</span> <span class="identifier">C</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">C</span> <span class="special">{</span> <span class="special">};</span>
- </pre>
- <p>
- Shouldn't <code class="computeroutput"><span class="identifier">common_type</span><span class="special"><</span><span class="identifier">A</span><span class="special">*,</span><span class="identifier">B</span><span class="special">*>::</span><span class="identifier">type</span></code>
- be <code class="computeroutput"><span class="identifier">C</span><span class="special">*</span></code>?
- I would say yes, but the default implementation will make it ill-formed.
- </p>
- <p>
- The library could add a specialization for pointers, as
- </p>
- <pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span>
- <span class="special">{</span>
- <span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">A</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">B</span><span class="special">></span>
- <span class="keyword">struct</span> <span class="identifier">common_type</span><span class="special"><</span><span class="identifier">A</span><span class="special">*,</span> <span class="identifier">B</span><span class="special">*></span> <span class="special">{</span>
- <span class="keyword">typedef</span> <span class="identifier">common_type</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span> <span class="identifier">B</span><span class="special">>*</span> <span class="identifier">type</span><span class="special">;</span>
- <span class="special">};</span>
- <span class="special">}</span>
- </pre>
- <p>
- But in the absence of a motivating use cases, we prefer not to add more than
- the standard specifies.
- </p>
- <p>
- Of course the user can always make this specialization.
- </p>
- <h5>
- <a name="boost_typetraits.reference.common_type.h5"></a>
- <span class="phrase"><a name="boost_typetraits.reference.common_type.can_you_explain_the_pros_cons_of__code__phrase_role__identifier__common_type__phrase___code__against_boost_typeof_"></a></span><a class="link" href="common_type.html#boost_typetraits.reference.common_type.can_you_explain_the_pros_cons_of__code__phrase_role__identifier__common_type__phrase___code__against_boost_typeof_">Can
- you explain the pros/cons of <code class="computeroutput"><span class="identifier">common_type</span></code>
- against Boost.Typeof?</a>
- </h5>
- <p>
- Even if they appear to be close, <code class="computeroutput"><span class="identifier">common_type</span></code>
- and <code class="computeroutput"><span class="identifier">typeof</span></code> have different
- purposes. You use <code class="computeroutput"><span class="identifier">typeof</span></code>
- to get the type of an expression, while you use <code class="computeroutput"><span class="identifier">common_type</span></code>
- to set explicitly the type returned of a template function. Both are complementary,
- and indeed <code class="computeroutput"><span class="identifier">common_type</span></code> is
- approximately equivalent to <code class="computeroutput"><span class="keyword">decltype</span><span class="special">(</span><a class="link" href="declval.html" title="declval">declval</a><span class="special"><</span><span class="keyword">bool</span><span class="special">>()</span>
- <span class="special">?</span> <a class="link" href="declval.html" title="declval">declval</a><span class="special"><</span><span class="identifier">T</span><span class="special">>()</span>
- <span class="special">:</span> <a class="link" href="declval.html" title="declval">declval</a><span class="special"><</span><span class="identifier">U</span><span class="special">>())</span></code>.
- </p>
- <p>
- <code class="computeroutput"><span class="identifier">common_type</span></code> is also similar
- to <code class="computeroutput"><span class="identifier">promote_args</span><span class="special"><</span><span class="keyword">class</span> <span class="special">...</span><span class="identifier">T</span><span class="special">></span></code> in
- <code class="computeroutput"><span class="identifier">boost</span><span class="special">/</span><span class="identifier">math</span><span class="special">/</span><span class="identifier">tools</span><span class="special">/</span><span class="identifier">promotion</span><span class="special">.</span><span class="identifier">hpp</span></code>, though
- it is not exactly the same as <code class="computeroutput"><span class="identifier">promote_args</span></code>
- either. <code class="computeroutput"><span class="identifier">common_type</span><span class="special"><</span><span class="identifier">T1</span><span class="special">,</span> <span class="identifier">T2</span><span class="special">>::</span><span class="identifier">type</span></code>
- simply represents the result of some operation on <code class="computeroutput"><span class="identifier">T1</span></code>
- and <code class="computeroutput"><span class="identifier">T2</span></code>, and defaults to the
- type obtained by putting <code class="computeroutput"><span class="identifier">T1</span></code>
- and <code class="computeroutput"><span class="identifier">T2</span></code> into a conditional
- statement.
- </p>
- <p>
- It is meant to be customizable (via specialization) if this default is not
- appropriate.
- </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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