The Special Pointer 'this'
When various instances of a class come into existence, it naturally follows that each instance has its own copy of member variables. If this were not the case, then for obvious reasons it would be impossible to make more than one instance of the class. On the other hand, even though the class member functions are encapsulated with the data members inside the class definition, it would be very inefficient in terms of memory usage to duplicate all these member functions and store the code for them within each instance. thus, only one copy of each member function per class is stored in memory, and must be shared by all of the instances of the class.
But this poses a big problem for the compiler: How can any given member function of a class knows which instance it is supposed to be working on ? In other words, up to now in a class member function you have easily been referring to the members directly without regard to the fact that when the instantiations occur each data member will have a dissimilar memory address. In other words, all the compiler knows is the offset of each data member from the start of the class.