std::function<R(Args...)>::operator() (3) - Linux Manuals

std::function<R(Args...)>::operator(): std::function<R(Args...)>::operator()

NAME

std::function<R(Args...)>::operator() - std::function<R(Args...)>::operator()

Synopsis

R operator()( Args... args ) const; (since C++11)

Invokes the stored callable function target with the parameters args.
Effectively does INVOKE<R>(f, std::forward<Args>(args)...), where f is the target object of *this and INVOKE is the operation described in Callable.

Parameters

args - parameters to pass to the stored callable function target

Return value

None if R is void. Otherwise the return value of the invocation of the stored callable object.

Exceptions

* std::bad_function_call if *this does not store a callable function target, i.e. !*this == true.

Example

The following example shows how std::function can passed to other functions by value. Also, it shows how std::function can store lambdas.
// Run this code

  #include <iostream>
  #include <functional>

  void call(std::function<int()> f) // can be passed by value
  {
      std::cout << f() << '\n';
  }

  int normal_function()
  {
      return 42;
  }

  int main()
  {
      int n = 1;
      std::function<int()> f = [&n](){ return n; };
      call(f);

      n = 2;
      call(f);

      f = normal_function;
      call(f);
  }

Output:

  1
  2
  42