# std::result_of, std::invoke_result

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< cpp‎ | types

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

Run this code

    #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

### See also

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

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