Add is_call_possible

beast/is_call_possible.h

@@ -0,0 +1,93 @@
+//------------------------------------------------------------------------------
+/*
+    This file is part of Beast: https://github.com/vinniefalco/Beast
+    Copyright 2013, Vinnie Falco <vinnie.falco@gmail.com>
+
+    Permission to use, copy, modify, and/or distribute this software for any
+    purpose  with  or without fee is hereby granted, provided that the above
+    copyright notice and this permission notice appear in all copies.
+
+    THE  SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+    WITH  REGARD  TO  THIS  SOFTWARE  INCLUDING  ALL  IMPLIED  WARRANTIES  OF
+    MERCHANTABILITY  AND  FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+    ANY  SPECIAL ,  DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+    WHATSOEVER  RESULTING  FROM  LOSS  OF USE, DATA OR PROFITS, WHETHER IN AN
+    ACTION  OF  CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+    OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+*/
+//==============================================================================
+
+#ifndef BEAST_IS_CALL_POSSIBLE_H_INCLUDED
+#define BEAST_IS_CALL_POSSIBLE_H_INCLUDED
+
+#include <beast/cxx14/type_traits.h> // <type_traits>
+
+namespace beast {
+
+namespace detail {
+
+template <class R, class C, class ...A>
+auto
+is_call_possible_test(C&& c, int, A&& ...a)
+    -> decltype(std::is_convertible<
+        decltype(c(a...)), R>::value ||
+            std::is_same<R, void>::value,
+                std::true_type());
+
+template <class R, class C, class ...A>
+std::false_type
+is_call_possible_test(C&& c, long, A&& ...a);
+
+} // detail
+
+/** Metafunction returns `true` if F callable as R(A...)
+    Example:
+        is_call_possible<T, void(std::string)>
+*/
+/** @{ */
+template <class C, class F>
+struct is_call_possible
+    : std::false_type
+{
+};
+
+template <class C, class R, class ...A>
+struct is_call_possible<C, R(A...)>
+    : decltype(detail::is_call_possible_test<R>(
+        std::declval<C>(), 1, std::declval<A>()...))
+{
+};
+/** @} */
+
+namespace test {
+
+struct is_call_possible_udt1
+{
+    void operator()(int) const;
+};
+
+struct is_call_possible_udt2
+{
+    int operator()(int) const;
+};
+
+static_assert(is_call_possible<
+    is_call_possible_udt1, void(int)>::value, "");
+
+static_assert(! is_call_possible<
+    is_call_possible_udt1, void(void)>::value, "");
+
+static_assert(is_call_possible<
+    is_call_possible_udt2, int(int)>::value, "");
+
+static_assert(! is_call_possible<
+    is_call_possible_udt2, int(void)>::value, "");
+
+static_assert(! is_call_possible<
+    is_call_possible_udt2, void(void)>::value, "");
+
+}
+
+}
+
+#endif