Add use_future support for Microsoft Visual C++.

Author: chriskohlhoff

asio/include/asio/detail/config.hpp

@@ -157,6 +157,31 @@
 # endif // !defined(ASIO_DISABLE_VARIADIC_TEMPLATES)
 #endif // !defined(ASIO_HAS_VARIADIC_TEMPLATES)
 
+// Support constexpr on compilers known to allow it.
+#if !defined(ASIO_HAS_CONSTEXPR)
+# if !defined(ASIO_DISABLE_CONSTEXPR)
+#  if defined(__clang__)
+#   if __has_feature(__cxx_constexpr__)
+#    define ASIO_HAS_CONSTEXPR 1
+#   endif // __has_feature(__cxx_constexr__)
+#  endif // defined(__clang__)
+#  if defined(__GNUC__)
+#   if ((__GNUC__ == 4) && (__GNUC_MINOR__ >= 6)) || (__GNUC__ > 4)
+#    if defined(__GXX_EXPERIMENTAL_CXX0X__)
+#     define ASIO_HAS_CONSTEXPR 1
+#    endif // defined(__GXX_EXPERIMENTAL_CXX0X__)
+#   endif // ((__GNUC__ == 4) && (__GNUC_MINOR__ >= 6)) || (__GNUC__ > 4)
+#  endif // defined(__GNUC__)
+# endif // !defined(ASIO_DISABLE_CONSTEXPR)
+#endif // !defined(ASIO_HAS_CONSTEXPR)
+#if !defined(ASIO_CONSTEXPR)
+# if defined(ASIO_HAS_CONSTEXPR)
+#  define ASIO_CONSTEXPR constexpr
+# else // defined(ASIO_HAS_CONSTEXPR)
+#  define ASIO_CONSTEXPR
+# endif // defined(ASIO_HAS_CONSTEXPR)
+#endif // !defined(ASIO_CONSTEXPR)
+
 // Standard library support for system errors.
 #if !defined(ASIO_HAS_STD_SYSTEM_ERROR)
 # if !defined(ASIO_DISABLE_STD_SYSTEM_ERROR)

asio/include/asio/use_future.hpp

@@ -46,7 +46,7 @@ class use_future_t
   typedef Allocator allocator_type;
 
   /// Construct using default-constructed allocator.
-  constexpr use_future_t()
+  ASIO_CONSTEXPR use_future_t()
   {
   }
 
@@ -77,7 +77,11 @@ class use_future_t
 /**
  * See the documentation for asio::use_future_t for a usage example.
  */
+#if defined(ASIO_HAS_CONSTEXPR) || defined(GENERATING_DOCUMENTATION)
 constexpr use_future_t<> use_future;
+#elif defined(ASIO_MSVC)
+__declspec(selectany) use_future_t<> use_future;
+#endif
 
 } // namespace asio
 

asio/src/doc/reference.dox

@@ -197,6 +197,7 @@ PREDEFINED             = GENERATING_DOCUMENTATION \
                          ASIO_OS_ERROR(e1,e2)=implementation_defined \
                          ASIO_MOVE_ARG(a)=a \
                          ASIO_DECL= \
+                         ASIO_CONSTEXPR=constexpr \
                          ASIO_INITFN_RESULT_TYPE(t,a)=void_or_deduced
 EXPAND_AS_DEFINED      = 
 SKIP_FUNCTION_MACROS   = YES