What is the purpose of inheritance?

In C++, inheritance is for subtyping. It lets developers model the kind-of relationship.

Inheritance allows developers to make one class a kind-of another class. In an inheritance relationship, the new class is called the derived class and the original class from which the new class is being derived is called the base class. All the data structures and member functions that belong to the base class automatically become part of the derived class.

For example, suppose the class Stack has member functions push() and pop(), and there is a need for a class PrintableStack. PrintableStack is exactly like Stack except PrintableStack also provides the member function print(). Class PrintableStack can be built by inheriting from Stack—Stack would be the base class and PrintableStack would be the derived class. The member functions push() and pop() and any others that belong to Stack automatically become part of PrintableStack, so the only requirement for building PrintableStack is adding the print() member function to it.

To do something similar in C, the existing Stack source file would be modified (which is trouble if Stack is being used by other source files and they rely on the exact layout of Stack) or copied into another file that would then be tweaked. However, code copying is the least desirable form of reuse: it doubles the maintenance costs and duplicates any bugs in the original source file. Using C++ and inheritance, Stack remains unmodified, yet PrintableStack doesn't need to duplicate the code for the inherited member functions.