documentation and examples

example/can_invoke_constexpr_function_object.cpp

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+/*<-
+Copyright Barrett Adair 2016
+Distributed under the Boost Software License, Version 1.0.
+(See accompanying file LICENSE.md or copy at http ://boost.org/LICENSE_1_0.txt)
+->*/
+
+//[ can_invoke_constexpr_function_object
+#include <type_traits>
+#include <callable_traits/callable_traits.hpp>
+
+// NOTE: Due to non-compliance in MSVC, can_invoke_constexpr
+// always return std::false_type on that compiler, which causes
+// the static asserts below to fail.
+
+namespace ct = callable_traits;
+
+using T1 = std::integral_constant<int, 3>;
+using T2 = std::integral_constant<int, 7>;
+
+//'subtract' is a constexpr function object that
+// subtracts std::integral_constant types.
+struct subtract {
+
+    // To compile failing cases of can_invoke_constexpr, the function object
+    // must have a SFINAE-safe signature. In this case, 'subtract' is made
+    // SFINAE-safe with an explicit, trailing return type.
+    template<typename T1, typename T2>
+    constexpr auto operator()(T1, T2) const -> decltype(T1::value - T2::value) {
+        return T1::value - T2::value;
+    }
+};
+
+// can_invoke_constexpr returns std::true_type in the first case, because
+// INVOKE(subtract{}, T1{}, T2{}) is a valid expression, AND 'subtract{}' is
+// a constexpr function object.
+static_assert(ct::can_invoke_constexpr(subtract{}, T1{}, T2{}), "");
+static_assert(!ct::can_invoke_constexpr(subtract{}, 3, 7), "");
+static_assert(!ct::can_invoke_constexpr(subtract{}, T1{}), "");
+
+//this is a function object, but is NOT constexpr
+struct add {
+    template<typename T1, typename T2>
+    auto operator()(T1, T2) const -> decltype(T1::value + T2::value) {
+        return T1::value + T2::value;
+    }
+};
+
+// Even though INVOKE(add{}, T1{}, T2{}) is valid, the respective
+// can_invoke_constexpr call returns std::false_type because 'add{}'
+// is not a constexpr function object.
+static_assert(!ct::can_invoke_constexpr(add{}, T1{}, T2{}), "");
+static_assert(!ct::can_invoke_constexpr(add{}, 3, 7), "");
+
+
+// This last section demonstrates that can_invoke_constexpr will always
+// return std::false_type when any of the arguments do not decay to literal
+// types. (see http://en.cppreference.com/w/cpp/concept/LiteralType).
+// Even though 'S' below is a constexpr function object, it is incompatible
+// with can_invoke_constexpr because 'S' isn't a literal type. Additionally,
+// all arguments must be default constructible.
+
+struct S {
+    S() = delete;
+    S(int){};
+    constexpr int operator()() const { return 0; }
+};
+
+S s{0};
+static_assert(!ct::can_invoke_constexpr(s), "");
+
+
+int main() {}
+//]