std::invoke

Defined in header <functional> 
template< class F, class... Args>
std::invoke_result_t<F, Args...> invoke(F&& f, Args&&... args) noexcept(/* see below */);
 (since C++17)

Invoke the Callable object f with the parameters args. As by INVOKE(std::forward<F>(f), std::forward<Args>(args)...).

where INVOKE(f, t1, t2, ..., tN) is defined as follows:

  • If f is a pointer to member function of class T:
    • If std::is_base_of<T, std::decay_t<decltype(t1)>>::value is true, then INVOKE(f, t1, t2, ..., tN) is equivalent to (t1.*f)(t2, ..., tN)
    • If std::decay_t<decltype(t1)> is a specialization of std::reference_wrapper, then INVOKE(f, t1, t2, ..., tN) is equivalent to (t1.get().*f)(t2, ..., tN)
    • If t1 does not satisfy the previous items, then INVOKE(f, t1, t2, ..., tN) is equivalent to ((*t1).*f)(t2, ..., tN).
  • Otherwise, if N == 1 and f is a pointer to data member of class T:
    • If std::is_base_of<T, std::decay_t<decltype(t1)>>::value is true, then INVOKE(f, t1) is equivalent to t1.*f
    • If std::decay_t<decltype(t1)> is a specialization of std::reference_wrapper, then INVOKE(f, t1) is equivalent to t1.get().*f
    • If t1 does not satisfy the previous items, then INVOKE(f, t1) is equivalent to (*t1).*f
  • Otherwise, INVOKE(f, t1, t2, ..., tN) is equivalent to f(t1, t2, ..., tN) (that is, f is a FunctionObject)

Parameters

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

Exceptions

noexcept specification:
noexcept(std::is_nothrow_invocable_v<F, Args...>)

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>
constexpr bool is_reference_wrapper_v = is_reference_wrapper<T>::value;
 
template <class T, class Type, class T1, class... Args>
decltype(auto) INVOKE(Type T::* f, T1&& t1, Args&&... args)
{
    if constexpr (std::is_member_function_pointer_v<decltype(f)>) {
        if constexpr (std::is_base_of_v<T, std::decay_t<T1>>)
            return (std::forward<T1>(t1).*f)(std::forward<Args>(args)...);
        else if constexpr (is_reference_wrapper_v<std::decay_t<T1>>)
            return (t1.get().*f)(std::forward<Args>(args)...);
        else
            return ((*std::forward<T1>(t1)).*f)(std::forward<Args>(args)...);
    } else {
        static_assert(std::is_member_object_pointer_v<decltype(f)>);
        static_assert(sizeof...(args) == 0);
        if constexpr (std::is_base_of_v<T, std::decay_t<T1>>)
            return std::forward<T1>(t1).*f;
        else if constexpr (is_reference_wrapper_v<std::decay_t<T1>>)
            return t1.get().*f;
        else
            return (*std::forward<T1>(t1)).*f;
    }
}
 
template <class F, class... Args>
decltype(auto) INVOKE(F&& f, Args&&... args)
{
      return std::forward<F>(f)(std::forward<Args>(args)...);
}
} // namespace detail
 
template< class F, class... Args>
std::invoke_result_t<F, Args...> invoke(F&& f, Args&&... args) 
  noexcept(std::is_nothrow_invocable_v<F, Args...>)
{
    return detail::INVOKE(std::forward<F>(f), std::forward<Args>(args)...);
}

Example

#include <functional>
#include <iostream>
 
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()
{
    // invoke a free function
    std::invoke(print_num, -9);
 
    // invoke a lambda
    std::invoke([]() { print_num(42); });
 
    // invoke a member function
    const Foo foo(314159);
    std::invoke(&Foo::print_add, foo, 1);
 
    // invoke (access) a data member
    std::cout << "num_: " << std::invoke(&Foo::num_, foo) << '\n';
 
    // invoke a function object
    std::invoke(PrintNum(), 18);
}

Output:

-9
42
314160
num_: 314159
18

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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