Overloading C++ Insertion Operator (<<)

You probably put your operator<< inside a class declaration. That means it takes an extra hidden parameter (the this parameter). You need to put it outside of any class declaration.

The insertion operator (<<) can be used as a member function or a friend function.

operator << used as a member function

ostream& operator<<(ostream& os);

This function should be invoked as :

dom << cout;

In general if you are using the operator as a member function, the left hand side of the operator should be an object. Then this object is implicitly passed as an argument to the member function. But the invocation confuses the user and it does not look nice.

operator << used as a friend function

friend ostream& operator<<(ostream& os, const Domino& obj);

This function should be invoked as :

cout << dom;

In this case the object dom is explicitly passed as a reference. This invocation is more traditional and user can easily understand the meaning of the code.