C++ fully supports object-oriented programming, including the four pillars of
object-oriented development: encapsulation, data hiding, inheritance, and
polymorphism. Encapsulation and Data Hiding When an engineer needs to add a
resistor to the device she is creating, she doesn't typically build a new one
from scratch. She walks over to a bin of resistors, examines the colored bands
that indicate the properties, and picks the one she needs. The resistor is a
"black box" as far as the engineer is concerned--she doesn't much care
how it does its work as long as it conforms to her specifications; she doesn't
need to look inside the box to use it in her design.
The property of being a self-contained unit is called encapsulation. With
encapsulation, we can accomplish data hiding. Data hiding is the highly valued
characteristic that an object can be used without the user knowing or caring how
it works internally. Just as you can use a refrigerator without knowing how the
compressor works, you can use a well-designed object without knowing about its
internal data members.
Similarly, when the engineer uses the resistor, she need not know anything
about the internal state of the resistor. All the properties of the resistor are
encapsulated in the resistor object; they are not spread out through the
circuitry. It is not necessary to understand how the resistor works in order to
use it effectively. Its data is hidden inside the resistor's casing.
C++ supports the properties of encapsulation and data hiding through the
creation of user-defined types, called classes. You'll see how to create classes
on Day 6, "Basic Classes." Once created, a well-defined class acts as
a fully encapsulated entity--it is used as a whole unit. The actual inner
workings of the class should be hidden. Users of a well-defined class do not
need to know how the class works; they just need to know how to use it.
Inheritance and Reuse When the engineers at Acme Motors want to build a new car,
they have two choices: They can start from scratch, or they can modify an
existing model. Perhaps their Star model is nearly perfect, but they'd like to
add a turbocharger and a six-speed transmission. The chief engineer would prefer
not to start from the ground up, but rather to say, "Let's build another
Star, but let's add these additional capabilities. We'll call the new model a
Quasar." A Quasar is a kind of Star, but one with new features.
C++ supports the idea of reuse through inheritance. A new type, which is an
extension of an existing type, can be declared. This new subclass is said to
derive from the existing type and is sometimes called a derived type. The Quasar
is derived from the Star and thus inherits all its qualities, but can add to
them as needed. Inheritance and its application in C++ are discussed on Day 12,
"Inheritance," and Day 15, "Advanced Inheritance."
Polymorphism The new Quasar might respond differently than a Star does when you
press down on the accelerator. The Quasar might engage fuel injection and a
turbocharger, while the Star would simply let gasoline into its carburetor. A
user, however, does not have to know about these differences. He can just
"floor it," and the right thing will happen, depending on which car
C++ supports the idea that different objects do "the right thing"
through what is called function polymorphism and class polymorphism. Poly means
many, and morph means form. Polymorphism refers to the same name taking many
forms, and is discussed on Day 10, "Advanced Functions," and Day 13,