What is a dog? When you look at your pet, what do you see? A biologist sees a network of interacting organs, a physicist sees atoms and fo...
What is a dog? When you look at your pet, what do you see? A biologist sees a network of interacting organs, a physicist sees atoms and forces at work, and a taxonomist sees a representative of the species canine domesticus.
It is that last assessment that interests us at the moment. A dog is a kind of canine, a canine is a kind of mammal, and so forth. Taxonomists divide the world of living things into Kingdom, Phylum, Class, Order, Family, Genus, and Species.
This hierarchy establishes an is-a relationship. A dog is a kind of canine. We see this relationship everywhere: A Toyota is a kind of car, which is a kind of vehicle. A sundae is a kind of dessert, which is a kind of food.
What do we mean when we say something is a kind of something else? We mean that it is a specialization of that thing. That is, a car is a special kind of vehicle.
Inheritance and Derivation
The concept dog inherits, that is, it automatically gets, all the features of a mammal. Because it is a mammal, we know that it moves and that it breathes air--all mammals move and breathe air by definition. The concept of a dog adds the idea of barking, wagging its tail, and so forth to that definition. We can further divide dogs into hunting dogs and terriers, and we can divide terriers into Yorkshire Terriers, Dandie Dinmont Terriers, and so forth.
A Yorkshire Terrier is a kind of terrier, therefore it is a kind of dog, therefore a kind of mammal, therefore a kind of animal, and therefore a kind of living thing.
C++ attempts to represent these relationships by enabling you to define classes that derive from one another. Derivation is a way of expressing the is-a relationship. You derive a new class, Dog, from the class Mammal. You don't have to state explicitly that dogs move, because they inherit that from Mammal.
New Term: A class which adds new functionality to an existing class is said to derive from that original class. The original class is said to be the new class's base class.
If the Dog class derives from the Mammal class, then Mammal is a base class of Dog. Derived classes are supersets of their base classes. Just as dog adds certain features to the idea of mammal, the Dog class will add certain methods or data to the Mammal class.
Typically, a base class will have more than one derived class. Just as dogs, cats, and horses are all types of mammals, their classes would all derive from the Mammal class.
The Syntax of Derivation
When you declare a class, you can indicate what class it derives from by writing a colon after the class name, the type of derivation (public or otherwise), and the class from which it derives. The following is an example.
class Dog : public Mammal
The type of derivation will be discussed later in this chapter. For now, always use public. The class from which you derive must have been declared earlier, or you will get a compiler error. Listing 12.1 illustrates how to declare a Dog class that is derived from a Mammal class.
