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result_of uses decltype on compilers that implement N3276

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[SVN r77905]
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Eric Niebler committed 2012-04-11 00:28:33 +00:00
1 parent df9315101e
commit 4111de6f68
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utility.htm
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@@ -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>