std::result_of, std::invoke_result
Defined in header <type_traits>
|
||
template< class > class result_of; // not defined |
(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.
|
(since C++11) (until C++14) |
|
(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 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 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
#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) |
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) |