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)