| <html> |
| <head> |
| <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> |
| <title>common_type</title> |
| <link rel="stylesheet" href="../../../../../../doc/src/boostbook.css" type="text/css"> |
| <meta name="generator" content="DocBook XSL Stylesheets V1.74.0"> |
| <link rel="home" href="../../index.html" title="Chapter 1. Boost.TypeTraits"> |
| <link rel="up" href="../reference.html" title="Alphabetical Reference"> |
| <link rel="prev" href="conditional.html" title="conditional"> |
| <link rel="next" href="decay.html" title="decay"> |
| </head> |
| <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> |
| <table cellpadding="2" width="100%"><tr> |
| <td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../boost.png"></td> |
| <td align="center"><a href="../../../../../../index.html">Home</a></td> |
| <td align="center"><a href="../../../../../../libs/libraries.htm">Libraries</a></td> |
| <td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> |
| <td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> |
| <td align="center"><a href="../../../../../../more/index.htm">More</a></td> |
| </tr></table> |
| <hr> |
| <div class="spirit-nav"> |
| <a accesskey="p" href="conditional.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../reference.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="decay.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a> |
| </div> |
| <div class="section" lang="en"> |
| <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> |
| <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> |
| </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> <a class="link" href="common_type.html" title="common_type">common_type</a><span class="special">;</span> |
| <span class="special">}</span> |
| </pre> |
| <p> |
| <a class="link" href="common_type.html" title="common_type">common_type</a> |
| 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> <a class="link" href="common_type.html" title="common_type">common_type</a><span class="special"><</span><span class="keyword">typename</span> <a class="link" href="common_type.html" title="common_type">common_type</a><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">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="identifier">T</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="identifier">decltype</span><span class="special">(</span><a href="../../../../../utility/doc/html/declval.html" target="_top">declval</a><span class="special"><</span><span class="keyword">bool</span><span class="special">>()</span> <span class="special">?</span> <a href="../../../../../utility/doc/html/declval.html" target="_top">declval</a><span class="special"><</span><span class="identifier">T</span><span class="special">>()</span> <span class="special">:</span> <a href="../../../../../utility/doc/html/declval.html" target="_top">declval</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> |
| </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 <a class="link" href="common_type.html" title="common_type">common_type</a> |
| arguments. |
| </p> |
| <p> |
| Note that when the compiler does not support variadic templates (and the |
| macro BOOST_NO_VARIADIC_TEMPLATES is defined) then the maximum number of |
| template arguments is 3. |
| </p> |
| <a name="boost_typetraits.reference.common_type.configuration_macros"></a><h5> |
| <a name="id1056804"></a> |
| <a class="link" href="common_type.html#boost_typetraits.reference.common_type.configuration_macros">Configuration |
| macros</a> |
| </h5> |
| <p> |
| When the compiler does not support static assertions then the user can select |
| the way static assertions are reported. Define |
| </p> |
| <div class="itemizedlist"><ul type="disc"> |
| <li> |
| BOOST_COMMON_TYPE_USES_STATIC_ASSERT: define it if you want to use Boost.StaticAssert |
| </li> |
| <li> |
| BOOST_COMMON_TYPE_USES_MPL_ASSERT: define it if you want to use Boost.MPL |
| static asertions |
| </li> |
| </ul></div> |
| <p> |
| The default behavior is to use mpl assertions in this case, but setting BOOST_COMMON_TYPE_USES_STATIC_ASSERT |
| may reduce compile times and header dependencies somewhat. |
| </p> |
| <p> |
| Depending on the static assertion used you will have an hint of the failing |
| assertion either through the symbol or through the text. |
| </p> |
| <p> |
| When possible common_type is implemented using <code class="computeroutput"><span class="identifier">decltype</span></code>. |
| Otherwise when BOOST_COMMON_TYPE_DONT_USE_TYPEOF is not defined it uses Boost.TypeOf. |
| </p> |
| <a name="boost_typetraits.reference.common_type.tutorial"></a><h5> |
| <a name="id1056860"></a> |
| <a class="link" href="common_type.html#boost_typetraits.reference.common_type.tutorial">Tutorial</a> |
| </h5> |
| <p> |
| In a nutshell, <a class="link" href="common_type.html" title="common_type">common_type</a> |
| 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 <a class="link" href="common_type.html" title="common_type">common_type</a> |
| 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> <a class="link" href="common_type.html" title="common_type">common_type</a><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 <a class="link" href="common_type.html" title="common_type">common_type</a>. |
| 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> <a class="link" href="common_type.html" title="common_type">common_type</a><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> |
| <a name="boost_typetraits.reference.common_type.how_to_get_the_common_type_of_types_with_explicit_conversions_"></a><h5> |
| <a name="id1057162"></a> |
| <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> <a class="link" href="common_type.html" title="common_type">common_type</a><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 <a class="link" href="common_type.html" title="common_type">common_type</a> |
| 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> <a class="link" href="common_type.html" title="common_type">common_type</a><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> <a class="link" href="common_type.html" title="common_type">common_type</a><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> |
| <a name="boost_typetraits.reference.common_type.how_important_is_the_order_of_the_common_type_lt__gt__template_arguments_"></a><h5> |
| <a name="id1057462"></a> |
| <a class="link" href="common_type.html#boost_typetraits.reference.common_type.how_important_is_the_order_of_the_common_type_lt__gt__template_arguments_">How |
| important is the order of the common_type<> 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> |
| <a name="boost_typetraits.reference.common_type.can_the_common_type_of_two_types_be_a_third_type_"></a><h5> |
| <a name="id1058329"></a> |
| <a class="link" href="common_type.html#boost_typetraits.reference.common_type.can_the_common_type_of_two_types_be_a_third_type_">Can |
| the common_type 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> |
| <a name="boost_typetraits.reference.common_type.how_common_type_behaves_with_pointers_"></a><h5> |
| <a name="id1058611"></a> |
| <a class="link" href="common_type.html#boost_typetraits.reference.common_type.how_common_type_behaves_with_pointers_">How |
| common_type behaves 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> |
| <a name="boost_typetraits.reference.common_type.can_you_explain_the_pros_cons_of_common_type_against_boost_typeof_"></a><h5> |
| <a name="id1058896"></a> |
| <a class="link" href="common_type.html#boost_typetraits.reference.common_type.can_you_explain_the_pros_cons_of_common_type_against_boost_typeof_">Can |
| you explain the pros/cons of common_type against Boost.Typeof?</a> |
| </h5> |
| <p> |
| Even if they appear to be close, <code class="computeroutput"><a class="link" href="common_type.html" title="common_type">common_type</a></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 <a class="link" href="common_type.html" title="common_type">common_type</a> |
| to set explicitly the type returned of a template function. Both are complementary, |
| and indeed <a class="link" href="common_type.html" title="common_type">common_type</a> |
| is equivalent to <code class="computeroutput"><span class="identifier">decltype</span><span class="special">(</span><a href="../../../../../utility/doc/html/declval.html" target="_top">declval</a><span class="special"><</span><span class="keyword">bool</span><span class="special">>()</span> |
| <span class="special">?</span> <a href="../../../../../utility/doc/html/declval.html" target="_top">declval</a><span class="special"><</span><span class="identifier">T</span><span class="special">>()</span> |
| <span class="special">:</span> <a href="../../../../../utility/doc/html/declval.html" target="_top">declval</a><span class="special"><</span><span class="identifier">U</span><span class="special">>())</span></code> |
| </p> |
| <p> |
| <a class="link" href="common_type.html" title="common_type">common_type</a> |
| is also similar to promote_args<class ...T> in boost/math/tools/promotion.hpp, |
| though it is not exactly the same as promote_args either. <a class="link" href="common_type.html" title="common_type">common_type</a><T1, |
| T2>::type simply represents the result of some operation on T1 and T2, |
| and defaults to the type obtained by putting T1 and T2 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, 2006 Adobe Systems Inc, David Abrahams, |
| Steve Cleary, Beman Dawes, Aleksey Gurtovoy, Howard Hinnant, Jesse Jones, Mat |
| Marcus, Itay Maman, John Maddock, Alexander Nasonov, Thorsten Ottosen, Robert |
| Ramey and Jeremy Siek<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> |
| <div class="spirit-nav"> |
| <a accesskey="p" href="conditional.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../reference.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="decay.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a> |
| </div> |
| </body> |
| </html> |