std::result_of,std::invoke_result(3)

NAME
std::result_of,std::invoke_result - std::result_of,std::invoke_result

Synopsis
   Defined in header <type_traits>
   template< class >
   class result_of; // not defined (since C++11)
                        (1) (deprecated in C++17)
   template< class F, class... ArgTypes > (removed in C++20)
   class result_of<F(ArgTypes...)>;
   template< class F, class... ArgTypes > (2) (since C++17)
   class invoke_result;
   Deduces the return type of an INVOKE expression at compile time.
   F must be a callable type, reference to function, or reference to (since C++11)
   callable type. Invoking F with ArgTypes... must be a well-formed (until C++14)
   expression.
   F and all types in ArgTypes can be any complete type, array of unknown (since C++14)
   bound, or (possibly cv-qualified) void.
   If the program adds specializations for any of the templates described on this page,
   the behavior is undefined.

Member types
   Member type Definition
               the return type of the Callable type F if invoked with the arguments
               ArgTypes....
   type Only defined if F can be called with the arguments ArgTypes... in
               unevaluated context.
               (since C++14)

Helper types
   template< class T > (since C++14)
   using result_of_t = typename result_of<T>::type; (1) (deprecated in C++17)
                        (removed in C++20)
   template< class F, class... ArgTypes >
   using invoke_result_t = typename invoke_result<F, (2) (since C++17)
   ArgTypes...>::type;

Possible implementation
 namespace detail
 {
     template<class T>
     struct is_reference_wrapper : std::false_type {};
     template<class U>
     struct is_reference_wrapper<std::reference_wrapper<U>> : std::true_type {};
     template<class T>
     struct invoke_impl
     {
         template<class F, class... Args>
         static auto call(F&& f, Args&&... args)
             -> decltype(std::forward<F>(f)(std::forward<Args>(args)...));
     };
     template<class B, class MT>
     struct invoke_impl<MT B::*>
     {
         template<class T, class Td = typename std::decay<T>::type,
             class = typename std::enable_if<std::is_base_of<B, Td>::value>::type>
         static auto get(T&& t) -> T&&;
         template<class T, class Td = typename std::decay<T>::type,
             class = typename std::enable_if<is_reference_wrapper<Td>::value>::type>
         static auto get(T&& t) -> decltype(t.get());
         template<class T, class Td = typename std::decay<T>::type,
             class = typename std::enable_if<!std::is_base_of<B, Td>::value>::type,
             class = typename std::enable_if<!is_reference_wrapper<Td>::value>::type>
         static auto get(T&& t) -> decltype(*std::forward<T>(t));
         template<class T, class... Args, class MT1,
             class = typename std::enable_if<std::is_function<MT1>::value>::type>
         static auto call(MT1 B::*pmf, T&& t, Args&&... args)
             -> decltype((invoke_impl::get(
                 std::forward<T>(t)).*pmf)(std::forward<Args>(args)...));
         template<class T>
         static auto call(MT B::*pmd, T&& t)
             -> decltype(invoke_impl::get(std::forward<T>(t)).*pmd);
     };
     template<class F, class... Args, class Fd = typename std::decay<F>::type>
     auto INVOKE(F&& f, Args&&... args)
         -> decltype(invoke_impl<Fd>::call(std::forward<F>(f),
             std::forward<Args>(args)...));
 } // namespace detail
 // Minimal C++11 implementation:
 template<class> struct result_of;
 template<class F, class... ArgTypes>
 struct result_of<F(ArgTypes...)>
 {
     using type = decltype(detail::INVOKE(std::declval<F>(), std::declval<ArgTypes>()...));
 };
 // Conforming C++14 implementation (is also a valid C++11 implementation):
 namespace detail
 {
     template<typename AlwaysVoid, typename, typename...>
     struct invoke_result {};
     template<typename F, typename...Args>
     struct invoke_result<
         decltype(void(detail::INVOKE(std::declval<F>(), std::declval<Args>()...))),
             F, Args...>
     {
         using type = decltype(detail::INVOKE(std::declval<F>(), std::declval<Args>()...));
     };
 } // namespace detail
 template<class> struct result_of;
 template<class F, class... ArgTypes>
 struct result_of<F(ArgTypes...)> : detail::invoke_result<void, F, ArgTypes...> {};
 template<class F, class... ArgTypes>
 struct invoke_result : detail::invoke_result<void, F, ArgTypes...> {};

Notes
   As formulated in C++11, the behavior of std::result_of is undefined when
   INVOKE(std::declval<F>(), std::declval<ArgTypes>()...) is ill-formed (e.g. when F is
   not a callable type at all). C++14 changes that to a SFINAE (when F is not callable,
   std::result_of<F(ArgTypes...)> simply doesn't have the type member).
   The motivation behind std::result_of is to determine the result of invoking a
   Callable, in particular if that result type is different for different sets of
   arguments.
   F(Args...) is a function type with Args... being the argument types and F being the
   return type. As such, std::result_of suffers from several quirks that led to its
   deprecation in favor of std::invoke_result in C++17:
     * F cannot be a function type or an array type (but can be a reference to them);
     * if any of the Args has type "array of T" or a function type T, it is
       automatically adjusted to T*;
     * neither F nor any of Args... can be an abstract class type;
     * if any of Args... has a top-level cv-qualifier, it is discarded;
     * none of Args... may be of type void.
   To avoid these quirks, result_of is often used with reference types as F and
   Args.... For example:
 template<class F, class... Args>
 std::result_of_t<F&&(Args&&...)> // instead of std::result_of_t<F(Args...)>, which is wrong
     my_invoke(F&& f, Args&&... args)
     {
         /* implementation */
     }

Notes
       Feature-test macro Value Std Feature
   __cpp_lib_result_of_sfinae 201210L (C++14) std::result_of and SFINAE
   __cpp_lib_is_invocable 201703L (C++17) std::is_invocable, std::invoke_result

Examples
// Run this code
 #include <iostream>
 #include <type_traits>
 struct S
 {
     double operator()(char, int&);
     float operator()(int) { return 1.0; }
 };
 template<class T>
 typename std::result_of<T(int)>::type f(T& t)
 {
     std::cout << "overload of f for callable T\n";
     return t(0);
 }
 template<class T, class U>
 int f(U u)
 {
     std::cout << "overload of f for non-callable T\n";
     return u;
 }
 int main()
 {
     // the result of invoking S with char and int& arguments is double
     std::result_of<S(char, int&)>::type d = 3.14; // d has type double
     static_assert(std::is_same<decltype(d), double>::value, "");
     // std::invoke_result uses different syntax (no parentheses)
     std::invoke_result<S,char,int&>::type b = 3.14;
     static_assert(std::is_same<decltype(b), double>::value, "");
     // the result of invoking S with int argument is float
     std::result_of<S(int)>::type x = 3.14; // x has type float
     static_assert(std::is_same<decltype(x), float>::value, "");
     // result_of can be used with a pointer to member function as follows
     struct C { double Func(char, int&); };
     std::result_of<decltype(&C::Func)(C, char, int&)>::type g = 3.14;
     static_assert(std::is_same<decltype(g), double>::value, "");
     f<C>(1); // may fail to compile in C++11; calls the non-callable overload in C++14
 }

Output:
 overload of f for non-callable T

See also
   invoke invokes any Callable object with given arguments
   invoke_r and possibility to specify return type
   (C++17) (since C++23)
   (C++23) (function template)
   is_invocable
   is_invocable_r checks if a type can be invoked (as if by std::invoke) with
   is_nothrow_invocable the given argument types
   is_nothrow_invocable_r (class template)
   (C++17)
   declval obtains a reference to its argument for use in unevaluated
   (C++11) context
                        (function template)