std:: invoke, std:: invoke_r

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C++
Utilities library
General utilities
Relational operators (deprecated in C++20)
Function objects
Function invocation
invoke invoke_r
(C++17) (C++23)
Identity function object
(C++20)
Old binders and adaptors
( until C++17* )
( until C++17* )
( until C++17* )
( until C++17* )
( until C++17* )
( until C++17* )
( until C++17* ) ( until C++17* ) ( until C++17* ) ( until C++17* )
( until C++20* )
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( until C++17* ) ( until C++17* )
( until C++17* ) ( until C++17* )

( until C++17* )
( until C++17* ) ( until C++17* ) ( until C++17* ) ( until C++17* )
( until C++20* )
( until C++20* )
Defined in header <functional>
template < class F, class ... Args >

std:: invoke_result_t < F, Args... >

invoke ( F && f, Args && ... args ) noexcept ( /* see below */ ) ;
(1) (since C++17)
(constexpr since C++20)
template < class R, class F, class ... Args >

constexpr R

invoke_r ( F && f, Args && ... args ) noexcept ( /* see below */ ) ;
(2) (since C++23)
1) Invoke the Callable object f with the parameters args as by INVOKE ( std:: forward < F > ( f ) , std:: forward < Args > ( args ) ... ) . This overload participates in overload resolution only if std:: is_invocable_v < F, Args... > is true .
2) Invoke the Callable object f with the parameters args as by INVOKE<R> ( std:: forward < F > ( f ) , std:: forward < Args > ( args ) ... ) . This overload participates in overload resolution only if std:: is_invocable_r_v < R, F, Args... > is true .

Parameters

f - Callable object to be invoked
args - arguments to pass to f

Return value

1) The value returned by f .
2) The value returned by f , implicitly converted to R , if R is not (possibly cv-qualified ) void . None otherwise.

Exceptions

1)
noexcept specification:
noexcept ( std:: is_nothrow_invocable_v < F, Args... > )
2)
noexcept specification:
noexcept ( std:: is_nothrow_invocable_r_v < R, F, Args... > )

Possible implementation

invoke (1) 
namespace detail
{
    template<class>
    constexpr bool is_reference_wrapper_v = false;
    template<class U>
    constexpr bool is_reference_wrapper_v<std::reference_wrapper<U>> = true;
 
    template<class T>
    using remove_cvref_t = std::remove_cv_t<std::remove_reference_t<T>>;
 
    template<class C, class Pointed, class Object, class... Args>
    constexpr decltype(auto) invoke_memptr(Pointed C::* member, Object&& object,
                                           Args&&... args)
    {
        using object_t = remove_cvref_t<Object>;
        constexpr bool is_member_function = std::is_function_v<Pointed>;
        constexpr bool is_wrapped = is_reference_wrapper_v<object_t>;
        constexpr bool is_derived_object = std::is_same_v<C, object_t>
                                        || std::is_base_of_v<C, object_t>;
 
        if constexpr (is_member_function)
        {
            if constexpr (is_derived_object)
                return (std::forward<Object>(object) .* member)
                           (std::forward<Args>(args)...);
            else if constexpr (is_wrapped)
                return (object.get() .* member)(std::forward<Args>(args)...);
            else
                return ((*std::forward<Object>(object)) .* member)
                           (std::forward<Args>(args)...);
        }
        else
        {
            static_assert(std::is_object_v<Pointed> && sizeof...(args) == 0);
            if constexpr (is_derived_object)
                return std::forward<Object>(object) .* member;
            else if constexpr (is_wrapped)
                return object.get() .* member;
            else
                return (*std::forward<Object>(object)) .* member;
        }
    }
} // namespace detail
 
template<class F, class... Args>
constexpr std::invoke_result_t<F, Args...> invoke(F&& f, Args&&... args)
    noexcept(std::is_nothrow_invocable_v<F, Args...>)
{
    if constexpr (std::is_member_pointer_v<detail::remove_cvref_t<F>>)
        return detail::invoke_memptr(f, std::forward<Args>(args)...);
    else
        return std::forward<F>(f)(std::forward<Args>(args)...);
}
invoke_r (2) 
template<class R, class F, class... Args>
    requires std::is_invocable_r_v<R, F, Args...>
constexpr R invoke_r(F&& f, Args&&... args)
    noexcept(std::is_nothrow_invocable_r_v<R, F, Args...>)
{
    if constexpr (std::is_void_v<R>)
        std::invoke(std::forward<F>(f), std::forward<Args>(args)...);
    else
        return std::invoke(std::forward<F>(f), std::forward<Args>(args)...);
}

Notes

Feature-test macro Value Std Feature
__cpp_lib_invoke 201411L (C++17) std::invoke , ( 1 )
__cpp_lib_invoke_r 202106L (C++23) std::invoke_r , ( 2 )

Example

Run this code 
#include <functional>
#include <iostream>
#include <type_traits>
 
struct Foo
{
    Foo(int num) : num_(num) {}
    void print_add(int i) const { std::cout << num_ + i << '\n'; }
    int num_;
};
 
void print_num(int i)
{
    std::cout << i << '\n';
}
 
struct PrintNum
{
    void operator()(int i) const
    {
        std::cout << i << '\n';
    }
};
 
int main()
{
    std::cout << "invoke a free function: ";
    std::invoke(print_num, -9);
 
    std::cout << "invoke a lambda: ";
    std::invoke([](){ print_num(42); });
 
    std::cout << "invoke a member function: ";
    const Foo foo(314159);
    std::invoke(&Foo::print_add, foo, 1);
 
    std::cout << "invoke (i.e., access) a data member num_: "
              << std::invoke(&Foo::num_, foo) << '\n';
 
    std::cout << "invoke a function object: ";
    std::invoke(PrintNum(), 18);
 
#if defined(__cpp_lib_invoke_r)
    auto add = [](int x, int y){ return x + y; };
    std::cout << "invoke a lambda converting result to float: ";
    auto ret = std::invoke_r<float>(add, 11, 22);
    static_assert(std::is_same<decltype(ret), float>());
    std::cout << std::fixed << ret << "\ninvoke print_num: ";
    std::invoke_r<void>(print_num, 44);
#endif
}

Possible output: 

invoke a free function: -9
invoke a lambda: 42
invoke a member function: 314160
invoke (i.e., access) a data member num_: 314159
invoke a function object: 18
invoke a lambda converting result to float: 33.000000
invoke print_num: 44

See also

(C++11)
creates a function object out of a pointer to a member
(function template)
(C++11) (removed in C++20) (C++17)
deduces the result type of invoking a callable object with a set of arguments
(class template)
(C++17)
checks if a type can be invoked (as if by std::invoke ) with the given argument types
(class template)
(C++17)
calls a function with a tuple of arguments
(function template)
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