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)
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)
`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)
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 Base, class T, class Derived, class... Args>
auto INVOKE(T Base::*pmf, Derived&& ref, Args&&... args)
 -> typename std::enable_if<std::is_function<T>::value &&
                     std::is_base_of<Base, typename std::decay<Derived>::type>::value,
    decltype((std::forward<Derived>(ref).*pmf)(std::forward<Args>(args)...))>::type;
 
template <class Base, class T, class RefWrap, class... Args>
auto INVOKE(T Base::*pmf, RefWrap&& ref, Args&&... args)
 -> typename std::enable_if<std::is_function<T>::value &&
                     is_reference_wrapper<typename std::decay<RefWrap>::type>::value,
    decltype((ref.get().*pmf)(std::forward<Args>(args)...))>::type;
 
template <class Base, class T, class Pointer, class... Args>
auto INVOKE(T Base::*pmf, Pointer&& ptr, Args&&... args)
 -> typename std::enable_if<std::is_function<T>::value &&
                     !is_reference_wrapper<typename std::decay<Pointer>::type>::value &&
                     !std::is_base_of<Base, typename std::decay<Pointer>::type>::value,
    decltype(((*std::forward<Pointer>(ptr)).*pmf)(std::forward<Args>(args)...))>::type;
 
template <class Base, class T, class Derived>
auto INVOKE(T Base::*pmd, Derived&& ref)
 -> typename std::enable_if<!std::is_function<T>::value &&
                     std::is_base_of<Base, typename std::decay<Derived>::type>::value,
    decltype(std::forward<Derived>(ref).*pmd)>::type;
 
template <class Base, class T, class RefWrap>
auto INVOKE(T Base::*pmd, RefWrap&& ref)
 -> typename std::enable_if<!std::is_function<T>::value &&
                     is_reference_wrapper<typename std::decay<RefWrap>::type>::value,
    decltype(ref.get().*pmd)>::type;
 
template <class Base, class T, class Pointer>
auto INVOKE(T Base::*pmd, Pointer&& ptr)
 -> typename std::enable_if<!std::is_function<T>::value &&
                     !is_reference_wrapper<typename std::decay<Pointer>::type>::value &&
                     !std::is_base_of<Base, typename std::decay<Pointer>::type>::value,
    decltype((*std::forward<Pointer>(ptr)).*pmd)>::type;
 
template <class F, class... Args>
auto INVOKE(F&& f, Args&&... args)
 -> typename std::enable_if<!std::is_member_pointer<typename std::decay<F>::type>::value,
    decltype(std::forward<F>(f)(std::forward<Args>(args)...))>::type;
} // 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

Language
Standard library headers
Concepts
Utilities library
Strings library
Containers library
Algorithms library
Iterators library
Numerics library
Input/output library
Localizations library
Regular expressions library (C++11)
Atomic operations library (C++11)
Thread support library (C++11)
Filesystem library (C++17)
Technical Specifications

invoke (C++17)	invokes any `Callable` object with given arguments (function template)
is_invocableis_invocable_ris_nothrow_invocableis_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)