Function Object

A function object is an object for which the function call operator operator() is defined.

Invocation

The INVOKE(f, arg_0, arg_1, ..., arg_n) operation is defined as follows, where T is std::remove_cvref_t<decltype(arg_0)>:

• If f is a pointer to a member function of class C, then the operation is equivalent to:
  • If std::is_same_v<C, T> || std::is_base_of<C, T> is true, invoke the member function on the object.
  • If T is a specialization of std::reference_wrapper, invoke the member function on the referred object
  • Otherwise, invoke the member function on the dereferenced object.
• If n == 0 and f is a pointer to a data member of class C, then the operation is equivalent to:
  • If std::same_as_v<C, T> || std::is_base_of<C, T> is true, access the data member of the object.
  • If T is a specialization of std::reference_wrapper, access the data member of the referred object.
  • Otherwise, access the data member of the dereferenced object.
• Otherwise, the operation is equivalent to f(arg_0, arg_1, ..., arg_n).

The INVOKE<R>(f, arg_0, arg_1, ..., arg_n) operation is defined as executing INVOKE(f, arg_0, arg_1, ..., arg_n) and converting the result to R. If the conversion operation binds R to a temporary object, the operation is ill-formed.

invoke and invoke_r

invoke and invoke_r can invoke a callable object with given arguments based on the rules of INVOKE and INVOKE<R>. invoke_result_t represents the return type of an INVOKE operation deduced at compile time.

template <class F, class... Args>
  requires std::is_invocable_v<F, Args...>
std::invoke_result_t<F, Args...>
invoke(F &&f, Args &&...args) noexcept(std::is_nothrow_invocable_v<F, Args...>);

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...>);