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std::result_of, std::invoke_result

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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)

If the program adds specializations for any of the templates described on this page, the behavior is undefined.

Contents

[edit] 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)

[edit] 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)

[edit] 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...> {};

[edit] 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 */
    }

[edit] 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

[edit] 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

[edit] See also

(C++17)(C++23)
 invokes any Callable object with given arguments and possibility to specify return type(since C++23)
(function template) [edit]
(C++17)
 checks if a type can be invoked (as if by std::invoke) with the given argument types
(class template) [edit]
(C++11)
 obtains a reference to an object of the template type argument for use in an unevaluated context
(function template) [edit]
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