Another way to associate one object with another is to store in one object a pointer to the other object. This differs from including an instance in that the pointer might be null, and the type of the pointer might be different than the actual type of object to which it points (polymorphism). Data members of pointer types that have insertion and extraction operators for object streams behave the same as simple types. For example, real_property can have more than one type of heating_system, such as baseboard or forced_air.