Call a std::function class member with std::mem_fn

Question

My plan is to build several listener classes which own predefined "callback hooks".

In the example below class Foo has a "callback hook" called onChange. It will be set to a default callback function during construction. It can also be set to an arbitrary function which provides the correct interface, like shown with the object f1 and the function callback().

The problem is when I want to call the object member onChange inside the for-loop the compiler says that I provide to much arguments. i am aware of the problem that i don't provide a member function to the std::mem_fn but instead an object member which is a function wrapper.

How do I manage to pass the argument to the std::function member object of class Foo without using std::bind and use std::mem_fn instead like shown in the example?

struct Foo
{
  Foo()
  {
    // default callback
    onChange = [](int value)
               -> void { std::cerr << "Foo::onChange( " << value << " )" << std::endl; };
  }

  // class destructor
  virtual ~Foo() {}

  std::function<void(int value)> onChange;

};


void callback(int value)
{
  std::cerr << "callback( " << value << " )" << std::endl;
}


int main()
{
  Foo f0;
  Foo f1;
  f1.onChange = callback;

  auto vec = std::vector<Foo>();
  vec.push_back(f0);
  vec.push_back(f1);

  auto func_wrapper = std::mem_fn( &Foo::onChange );

  for (auto f : vec)
  {
    func_wrapper(f, 42);
  }
}