Derived classes

The more you use C++ the more you'll be developing classes which require greater sophistication. Often you'll need to add extra functionality to an existing class. C++ provides a mechanism to build new classes from old ones

   class More : public Base {
   int value;
   ...
   };

Here More inherits the members of Base. The public keyword means that the members of Base are as accessible to More as Base allows. By default derivation is private (which means that even Base's public members can't be accessed by More), but usually derivation should be public, privacy being control by the base class.

When an object of type More is created, first the Base constructor is called, then More's constructor. When the object is destroyed, the destructors are called in reverse order. The constructors/destructors are not inherited, but you can say

   More::More(int sz):Base(sz){}

passing arguments to a base member class constructor (or to a number of constructors using a comma-separated list).

A derived class can be assigned to any of its public base classes, so the following is possible

   class More : public Base {
   int value;
   ...
   };
   Base b;
   More m;
   b=m;

It will fill the fields of b with the corresponding values in m. However, m=b isn't possible.

You can derive new classes from a derived class, and you can derive many different classes from a base class so you can develop a family tree of related classes. As we'll see soon, the ``parent'' classes can keep parts of themselves private, not only to unrelated classes but to derived classes too.

Big classes don't necessarily imply big objects: all objects of a class share member functions, for instance.

It's possible for a class to be derived from 2 or more other classes. It's called Multiple Inheritance but it introduces complications, so don't use it unless you know what you're doing.