Thinking in C++ 2nd ed Volume 1 Revision 6

forced to use the idioms of the underlying machine. These concepts
are expressed as fundamental units in the programming language
by using the class keyword.
It seems a pity, however, to go to all the trouble to create a class and
then be forced to create a brand new one that might have similar
functionality. It’s nicer if we can take the existing class, clone it and
make additions and modifications to the clone. This is effectively
what you get with inheritance, with the exception that if the original
class (called the base or super or parent class) is changed, the
modified “clone” (called the derived or inherited or sub or child
class) also reflects those changes.
Base
Derived
(The arrow in the above UML diagram points from the derived class
to the base class. As you shall see, there can be more than one
derived class.)
A type does more than describe the constraints on a set of objects; it
also has a relationship with other types. Two types can have
characteristics and behaviors in common, but one type may contain
more characteristics than another and may also handle more
messages (or handle them differently). Inheritance expresses this
similarity between types with the concept of base types and derived
types. A base type contains all the characteristics and behaviors that
are shared among the types derived from it. You create a base type
to represent the core of your ideas about some objects in your
system. From the base type, you derive other types to express the
different ways that core can be realized.
For example, a trash-recycling machine sorts pieces of trash. The
base type is “trash,” and each piece of trash has a weight, a value,
and so on and can be shredded, melted, or decomposed. From this,
1: Introduction to Objects 49