std::result_of, std::invoke_result
std::result_of, std::invoke_result
Defined in header <type_traits>
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(1) | (since C++11) (deprecated in C++17) |
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(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.
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(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 .
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(since C++14) |
Member types
Member type | Definition |
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type
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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)
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Helper types
|
(1) | (since C++14) (deprecated in C++17) |
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(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 ofT
" or a function typeT
, it is automatically adjusted toT*
; - neither
F
nor any ofArgs...
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 typevoid
.
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
See also
(C++17) |
invokes any Callable object with given arguments (function template) |
(C++17) |
checks if a type can be invoked (as if by std::invoke ) with the given argument types(class template) |
(C++11) |
obtains a reference to its argument for use in unevaluated context (function template) |
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