Creating Function Object

Here is the code,

Code:

class IntSequence
{
public:
	IntSequence(int initialValue) : value(initialValue) {}

	int operator()()
	{
		return value++;
	}

private:
	int value;
};

int main()
{		
 	list<int> coll;

	generate_n(back_inserter(coll), 9, IntSequence(1));
  	return 0;
}

In the call of generate_n, generator function IntSequence(1) with an int argument will call operator() but operator() doesn't have any arguments. I wander why? Also if I define operator() as the follows,

Code:

int operator()(int elem)
{
     return value+elem;
}

How can I call generate_n?