std::result...nvoke_result(3)
💖 NAME 💖
okay so bestie, welcome to the <pink>std::result...nvoke_result(3)</pink> page from the <purple>C++ Standard Libary</purple> manual ✨ This one is literally about std::result_of,std::invoke_result — yep, that’s the name: 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;Okay so the whole point: it deduces the return type of an INVOKE expression at compile time. Literally the compile-time detective 🕵️♀️
F must be a callable type, reference to function, or reference to callable type. (since C++11) Invoking F with ArgTypes... must be a well-formed expression. (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. 💀
🦋 Member types 🦋
type— the return type of the Callable typeFif invoked with the argumentsArgTypes.... It’s only defined ifFcan be called with the argumentsArgTypes...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:
Fcannot be a function type or an array type (but can be a reference to them);- if any of the
Argshas type "array ofT" or a function typeT, it is automatically adjusted toT*; - neither
Fnor 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 */
}🧪 Notes 🧪 (feature-test macros)
__cpp_lib_result_of_sfinae—201210L(C++14) —std::result_ofand 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(C++17),invoke_r(since C++23) — invokes any Callable object with given arguments and possibility to specify return type (function template)is_invocable,is_invocable_r,is_nothrow_invocable,is_nothrow_invocable_r(C++17) — checks if a type can be invoked (as if bystd::invoke) with the given argument types (class template)declval(C++11) — obtains a reference to its argument for use in unevaluated context (function template)
And that’s the whole page, bestie 💖 source: http://cppreference.com, version/date: 2024.06.10, manual page: std::result...nvoke_result(3) — go slay some template metaprogramming 🫶✨
📦 where this page comes from
- nixpkgs unstable: stdman 2024.07.05
- opensuse-leap 16.0: stdman 2024.07.05-bp160.1.9 · MIT
- 💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash-1017 on : a language model's rewrite of the original, under its terms