std::invoke (3) Linux Manual Page

std::invoke – std::invoke

Synopsis

Defined in header <functional>

template< class F, class... Args>                                                          (since C++17)

std::invoke_result_t<F, Args...> invoke(F&& f, Args&&... args) noexcept(/* see below */);

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

// Run this code

#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

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