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std::result_of, std::invoke_result

Defined in header `<type_traits>`
template< class > class result_of; // not defined template< class F, class... ArgTypes > class result_of<F(ArgTypes...)>;	(1)	(since C++11) (deprecated in C++17) (removed in C++20)
template< class F, class... ArgTypes> class invoke_result;	(2)	(since C++17)

Deduces the return type of an INVOKE expression at compile time.

`F` must be a callable type, reference to function, or reference to callable type. Invoking `F` with `ArgTypes...` must be a well-formed expression.	(since C++11) (until C++14)
`F` and all types in `ArgTypes` can be any complete type, array of unknown bound, or (possibly cv-qualified) `void`.	(since C++14)

Member types

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

Helper types

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

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 lead 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 */
}

Examples

#include <type_traits>
#include <iostream>
 
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, "");
 
    // 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