added new result_of mode that uses TR1 with a decltype fallback as suggested by Nathan Crookston; fixes #7753

[SVN r84949]

include/boost/utility/detail/result_of_iterate.hpp

@@ -38,10 +38,25 @@ struct tr1_result_of<F(BOOST_RESULT_OF_ARGS)>
 #endif
 
 #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_ENUM_TRAILING_PARAMS(BOOST_PP_ITERATION(),typename T)>
-struct result_of<F(BOOST_PP_ENUM_PARAMS(BOOST_PP_ITERATION(),T))>
+struct result_of<F(BOOST_RESULT_OF_ARGS)>
+    : detail::cpp0x_result_of<F(BOOST_RESULT_OF_ARGS)> { };
+#endif // BOOST_RESULT_OF_USE_DECLTYPE
+
+#ifdef BOOST_RESULT_OF_USE_TR1_WITH_DECLTYPE_FALLBACK
+template<typename F BOOST_PP_ENUM_TRAILING_PARAMS(BOOST_PP_ITERATION(),typename T)>
+struct result_of<F(BOOST_RESULT_OF_ARGS)>
+    : mpl::if_<mpl::or_<detail::has_result_type<F>, detail::has_result<F> >,
+               tr1_result_of<F(BOOST_RESULT_OF_ARGS)>,
+               detail::cpp0x_result_of<F(BOOST_RESULT_OF_ARGS)> >::type { };
+#endif // BOOST_RESULT_OF_USE_TR1_WITH_DECLTYPE_FALLBACK
+
+#if defined(BOOST_RESULT_OF_USE_DECLTYPE) || defined(BOOST_RESULT_OF_USE_TR1_WITH_DECLTYPE_FALLBACK)
+
+namespace detail {
+
+template<typename F BOOST_PP_ENUM_TRAILING_PARAMS(BOOST_PP_ITERATION(),typename T)>
+struct cpp0x_result_of<F(BOOST_PP_ENUM_PARAMS(BOOST_PP_ITERATION(),T))>
     : mpl::if_<
           is_member_function_pointer<F>
         , detail::tr1_result_of_impl<
@@ -54,8 +69,6 @@ struct result_of<F(BOOST_PP_ENUM_PARAMS(BOOST_PP_ITERATION(),T))>
       >::type
 {};
 
-namespace detail {
-
 #ifdef BOOST_NO_SFINAE_EXPR
 
 template<typename F>
@@ -139,7 +152,7 @@ struct cpp0x_result_of_impl<F(BOOST_PP_ENUM_PARAMS(BOOST_PP_ITERATION(),T)),
 
 } // namespace detail
 
-#else // defined(BOOST_RESULT_OF_USE_DECLTYPE)
+#else // defined(BOOST_RESULT_OF_USE_DECLTYPE) || defined(BOOST_RESULT_OF_USE_TR1_WITH_DECLTYPE_FALLBACK)
 
 #if !BOOST_WORKAROUND(__BORLANDC__, BOOST_TESTED_AT(0x551))
 template<typename F BOOST_PP_ENUM_TRAILING_PARAMS(BOOST_PP_ITERATION(),typename T)>

include/boost/utility/result_of.hpp

@@ -38,18 +38,27 @@
 
 // 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
+// The user can force the choice by defining BOOST_RESULT_OF_USE_DECLTYPE,
-// BOOST_RESULT_OF_USE_TR1, but not both!
+// BOOST_RESULT_OF_USE_TR1, or BOOST_RESULT_OF_USE_TR1_WITH_DECLTYPE_FALLBACK but not more than one!
-#if defined(BOOST_RESULT_OF_USE_DECLTYPE) && defined(BOOST_RESULT_OF_USE_TR1)
+#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.
+    (defined(BOOST_RESULT_OF_USE_DECLTYPE) && defined(BOOST_RESULT_OF_USE_TR1_WITH_DECLTYPE_FALLBACK)) || \
+    (defined(BOOST_RESULT_OF_USE_TR1) && defined(BOOST_RESULT_OF_USE_TR1_WITH_DECLTYPE_FALLBACK))
+#  error More than one of BOOST_RESULT_OF_USE_DECLTYPE, BOOST_RESULT_OF_USE_TR1 and \
+  BOOST_RESULT_OF_USE_TR1_WITH_DECLTYPE_FALLBACK cannot be defined at the same time.
+#endif
+
+#if defined(BOOST_RESULT_OF_USE_TR1_WITH_DECLTYPE_FALLBACK) && defined(BOOST_MPL_CFG_NO_HAS_XXX_TEMPLATE)
+#  error Cannot fallback to decltype if BOOST_MPL_CFG_NO_HAS_XXX_TEMPLATE is not defined.
 #endif
 
 #ifndef BOOST_RESULT_OF_USE_TR1
 #  ifndef BOOST_RESULT_OF_USE_DECLTYPE
-#    ifndef BOOST_NO_CXX11_DECLTYPE_N3276 // this implies !defined(BOOST_NO_CXX11_DECLTYPE)
+#    ifndef BOOST_RESULT_OF_USE_TR1_WITH_DECLTYPE_FALLBACK
-#      define BOOST_RESULT_OF_USE_DECLTYPE
+#      ifndef BOOST_NO_CXX11_DECLTYPE_N3276 // this implies !defined(BOOST_NO_CXX11_DECLTYPE)
-#    else
+#        define BOOST_RESULT_OF_USE_DECLTYPE
-#      define BOOST_RESULT_OF_USE_TR1
+#      else
+#        define BOOST_RESULT_OF_USE_TR1
+#      endif
 #    endif
 #  endif
 #endif
@@ -64,8 +73,12 @@ namespace detail {
 
 BOOST_MPL_HAS_XXX_TRAIT_DEF(result_type)
 
+BOOST_MPL_HAS_XXX_TEMPLATE_DEF(result)
+
 template<typename F, typename FArgs, bool HasResultType> struct tr1_result_of_impl;
 
+template<typename F> struct cpp0x_result_of;
+
 #ifdef BOOST_NO_SFINAE_EXPR
 
 // There doesn't seem to be any other way to turn this off such that the presence of

test/result_of_test.cpp

@@ -5,6 +5,12 @@
 //  1.0. (See accompanying file LICENSE_1_0.txt or copy at
 //  http://www.boost.org/LICENSE_1_0.txt)
 
+// Examples:
+//   To run the default test:
+//   $ cd libs/utility/test && bjam
+//   To test decltype on g++ 2.7:
+//   $ cd libs/utility/test && bjam cxxflags="-std=c++11 -D BOOST_RESULT_OF_USE_DECLTYPE"
+
 #include <boost/config.hpp>
 
 // For more information, see http://www.boost.org/libs/utility
@@ -190,7 +196,7 @@ int main()
   BOOST_STATIC_ASSERT((is_same<tr1_result_of<volatile int_result_of_template<void>(void)>::type, void>::value));
 
   // Prior to decltype, result_of could not deduce the return type
-  // nullary function objects unless they exposed a result_type.
+  // of nullary function objects unless they exposed a result_type.
 #if defined(BOOST_RESULT_OF_USE_DECLTYPE)
   BOOST_STATIC_ASSERT((is_same<result_of<int_result_of(void)>::type, int>::value));
   BOOST_STATIC_ASSERT((is_same<result_of<volatile int_result_of(void)>::type, int>::value));
@@ -273,7 +279,7 @@ int main()
   BOOST_STATIC_ASSERT((is_same<tr1_result_of<pf_t(int)>::type, int>::value));
   BOOST_STATIC_ASSERT((is_same<tr1_result_of<pf_t const(int)>::type,int>::value));
 
-#if defined(BOOST_RESULT_OF_USE_DECLTYPE)
+#if defined(BOOST_RESULT_OF_USE_DECLTYPE) || defined(BOOST_RESULT_OF_USE_TR1_WITH_DECLTYPE_FALLBACK)
   BOOST_STATIC_ASSERT((is_same<result_of<no_result_type_or_result(double)>::type, short>::value));
   BOOST_STATIC_ASSERT((is_same<result_of<const no_result_type_or_result(double)>::type, cv_overload_check<const short> >::value));
   BOOST_STATIC_ASSERT((is_same<result_of<volatile no_result_type_or_result(double)>::type, cv_overload_check<volatile short> >::value));
@@ -301,7 +307,7 @@ int main()
 #endif
 #endif
 
-#if defined(BOOST_RESULT_OF_USE_DECLTYPE)
+#if defined(BOOST_RESULT_OF_USE_DECLTYPE) || defined(BOOST_RESULT_OF_USE_TR1_WITH_DECLTYPE_FALLBACK)
   int i = 123;
   sfinae_test(sfinae_test_f, i);
 #endif // defined(BOOST_RESULT_OF_USE_DECLTYPE)

utility.htm

@@ -259,6 +259,46 @@ typedef boost::result_of<
                 represent the return type of
                 <code>operator()</code> given a call expression.</p>
 
+                <p>Additionally, <code>boost::result_of</code>
+                provides a third mode of operation, which some users
+                may find convenient. When
+                <code>BOOST_RESULT_OF_USE_TR1_WITH_DECLTYPE_FALLBACK</code>
+                is defined, <code>boost::result_of</code> behaves as
+                follows. If the function object has a member
+                type <code>result_type</code> or member
+                template <code>result&lt;&gt;</code>, then
+                <code>boost::result_of</code> will use the TR1
+                protocol. Otherwise,
+                <code>boost::result_of</code> will
+                use <code>decltype</code>. Using TR1 with
+                a <code>declytpe</code> fallback may workaround
+                certain problems at the cost of portability. For
+                example:
+                <ul>
+                    <li>Deficient compiler: If your code
+                    requires <code>boost::result_of</code> to work
+                    with incomplete return types but your
+                    compiler's <code>decltype</code> implementation
+                    does not support incomplete return types, then you
+                    can use the TR1 protocol as a workaround. Support
+                    for incomplete return types was added late in the
+                    C++11 standardization process
+                    (see <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2011/n3276.pdf">N3276</a>)
+                    and is not implemented by some compilers.</li>
+
+                    <li>Deficient legacy code: If your existing TR1
+                    function object advertises a different type than
+                    the actual result type deduced
+                    by <code>decltype</code>, then using TR1 with a
+                    <code>decltype</code> fallback will allow you to
+                    work with both your existing TR1 function objects
+                    and new C++11 function object. This situation
+                    could occur if your legacy function objects
+                    misused the TR1 protocol. See the documentation on
+                    known <a href="#result_of_tr1_diff">differences</a>
+                    between <code>boost::result_of</code> and TR1.</li>
+                </ul>
+
                 <a name="BOOST_NO_RESULT_OF"></a>
                 <p>This implementation of <code>result_of</code>
                 requires class template partial specialization, the