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result_of uses decltype on compilers that implement N3276

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[SVN r77903]
This commit is contained in:
Eric Niebler 2012-04-11 00:05:56 +00:00
parent a7e8d28621
commit 2637dfcc59
3 changed files with 54 additions and 23 deletions
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6
include/boost/utility/detail/result_of_iterate.hpp
View File

@ -38,7 +38,7 @@ struct tr1_result_of<F(BOOST_RESULT_OF_ARGS)>
(boost::detail::has_result_type<F>::value)> >::type { };
#endif
#if !defined(BOOST_NO_DECLTYPE) && defined(BOOST_RESULT_OF_USE_DECLTYPE)
#ifdef BOOST_RESULT_OF_USE_DECLTYPE
// Uses declval following N3225 20.7.7.6 when F is not a pointer.
template<typename F BOOST_PP_COMMA_IF(BOOST_PP_ITERATION())
@ -71,7 +71,7 @@ struct cpp0x_result_of_impl<F(BOOST_PP_ENUM_PARAMS(BOOST_PP_ITERATION(),T))>
} // namespace detail
#else // defined(BOOST_NO_DECLTYPE)
#else // defined(BOOST_RESULT_OF_USE_DECLTYPE)
#if !BOOST_WORKAROUND(__BORLANDC__, BOOST_TESTED_AT(0x551))
template<typename F BOOST_PP_COMMA_IF(BOOST_PP_ITERATION())
@ -80,7 +80,7 @@ struct result_of<F(BOOST_RESULT_OF_ARGS)>
: tr1_result_of<F(BOOST_RESULT_OF_ARGS)> { };
#endif
#endif // defined(BOOST_NO_DECLTYPE)
#endif // defined(BOOST_RESULT_OF_USE_DECLTYPE)
#undef BOOST_RESULT_OF_ARGS

18
include/boost/utility/result_of.hpp
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@ -30,6 +30,24 @@
# define BOOST_RESULT_OF_NUM_ARGS 16
#endif
// Use the decltype-based version of result_of by default if the compiler
// supports N3276 <http://www.open-std.org/JTC1/SC22/WG21/docs/papers/2011/n3276.pdf>.
// The user can force the choice by defining either BOOST_RESULT_OF_USE_DECLTYPE or
// BOOST_RESULT_OF_USE_TR1, but not both!
#if defined(BOOST_RESULT_OF_USE_DECLTYPE) && defined(BOOST_RESULT_OF_USE_TR1)
# error Both BOOST_RESULT_OF_USE_DECLTYPE and BOOST_RESULT_OF_USE_TR1 cannot be defined at the same time.
#endif
#ifndef BOOST_RESULT_OF_USE_TR1
# ifndef BOOST_RESULT_OF_USE_DECLTYPE
# ifndef BOOST_NO_DECLTYPE_N3276 // this implies !defined(BOOST_NO_DECLTYPE)
# define BOOST_RESULT_OF_USE_DECLTYPE
# else
# define BOOST_RESULT_OF_USE_TR1
# endif
# endif
#endif
namespace boost {
template<typename F> struct result_of;

53
utility.htm
View File

@ -161,14 +161,12 @@ void f() {
resides in the header <code>&lt;<a
href="../../boost/utility/result_of.hpp">boost/utility/result_of.hpp</a>&gt;</code>.</p>
<p>If your compiler supports <code>decltype</code>,
then you can enable automatic result type deduction by
defining the macro <code>BOOST_RESULT_OF_USE_DECLTYPE</code>,
as in the following example.</p>
<p>If your compiler's support for <code>decltype</code> is
adequate, <code>result_of</code> automatically uses it to
deduce the result type of your callable.</p>
<blockquote>
<pre>#define BOOST_RESULT_OF_USE_DECLTYPE
#include &lt;boost/utility/result_of.hpp&gt;
<pre>#include &lt;boost/utility/result_of.hpp&gt;
struct functor {
template&lt;class T&gt;
@ -180,21 +178,29 @@ struct functor {
typedef boost::result_of&lt;
functor(int)
&gt;::type type;</pre>
&gt;::type type; // type is int</pre>
</blockquote>
<p>If <code>decltype</code> is not enabled,
<p>You can test whether <code>result_of</code> is using
<code>decltype</code> checking if the macro
<code>BOOST_RESULT_OF_USE_DECLTYPE</code> is defined after
including <code>result_of.hpp</code>. You can also force
<code>result_of</code> to use <code>decltype</code> by
defining <code>BOOST_RESULT_OF_USE_DECLTYPE</code> prior
to including <code>result_of.hpp</code>.</p>
<p>If <code>decltype</code> is not used,
then automatic result type deduction of function
objects is not possible. Instead, <code>result_of</code>
uses the following protocol to allow the programmer to
specify a type. When <code>F</code> is a class type with a
member type <code>result_type</code>,
<code>result_of&lt;F(T1, T2, ...,
T<em>N</em>)&gt;</code> is
T<em>N</em>)&gt;::type</code> is
<code>F::result_type</code>. When <code>F</code> does
not contain <code>result_type</code>,
<code>result_of&lt;F(T1, T2, ...,
T<em>N</em>)&gt;</code> is <code>F::result&lt;F(T1,
T<em>N</em>)&gt;::type</code> is <code>F::result&lt;F(T1,
T2, ..., T<em>N</em>)&gt;::type</code> when
<code><em>N</em> &gt; 0</code> or <code>void</code>
when <code><em>N</em> = 0</code>. Note that it is the
@ -221,22 +227,29 @@ typedef boost::result_of&lt;
typedef boost::result_of&lt;
functor(int)
&gt;::type type;</pre>
&gt;::type type; // type is int</pre>
</blockquote>
<p>In a future
release, <code>BOOST_RESULT_OF_USE_DECLTYPE</code>
may be enabled by default on compilers that
support <code>decltype</code>, so if you use the above
protocol please take care to ensure that
the <code>result_type</code>
and <code>result&lt;&gt;</code> members accurately
represent the result type. If you wish to continue to
<p>If you are writing a reusable function object
that should work with <code>result_of</code>, for
maximum portability, you might consider following
the above protocol even if your compiler has
proper <code>decltype</code> support. If you do,
take care to ensure that the
<code>result_type</code> and
<code>result&lt;&gt;</code> members accurately
represent the return type of
<code>operator()</code>.</p>
<p>If you wish to continue to
use the protocol on compilers that
support <code>decltype</code>,
use <code>boost::tr1_result_of</code>, which is also
defined
in <code>&lt;<a href="../../boost/utility/result_of.hpp">boost/utility/result_of.hpp</a>&gt;</code>.</p>
in <code>&lt;<a href="../../boost/utility/result_of.hpp">boost/utility/result_of.hpp</a>&gt;</code>. You can also define the macro
<code>BOOST_RESULT_OF_USE_TR1</code>, which causes
<code>result_of</code> to use the convention described
above instead of <code>decltype</code>.</p>
<a name="BOOST_NO_RESULT_OF"></a>
<p>This implementation of <code>result_of</code>