Teach Yourself Borland C++ in 14 Days - portal

98 Day 4
Part of a well-designed class includes hiding anything from public view that the user of the
class doesn’t need to know.
Data abstraction is the hiding of internal implementations within the class from
outside view.
NEW TERM
Data abstraction prevents the user from knowing more than he or she needs to know about
the class, but also prevents the user from messing with things that shouldn’t be messed with.
For instance, when you get in your car and turn the key to start the car, do you want to know
every detail about how the car operates? Of course not. You only want to know as much as
you need to know to operate the car safely. So in this analogy the steering wheel, pedals, gear
shift lever, speedometer, and so on represent the public interface between the car and the
driver. The driver knows which of those components to manipulate in order to make the car
perform the way he wants.
Conversely, the engine, drive train, and electrical system of the car are hidden from public
view. The engine is tucked neatly away where you never have to look at it if you don’t want
to. (That’s what service stations are for!) It’s a detail that you don’t need to know about, so
it is hidden from you—kept private, if you prefer. Imagine how much trouble driving would
be if you had to keep track of everything the car was doing at all times: Is the carburetor getting
enough gas? Does the differential have enough grease? Is the alternator producing adequate
voltage for both the ignition and the radio to operate? Are the intake valves opening properly?
Arggghhhh!!! Who needs it! In the same way, a class keeps its internal implementation private
so the user of the class doesn’t have to worry about what’s going on under the hood. The
internal workings of the class are kept private, and the user interface is public.
The protected access level is a little harder to explain. Protected class members, like private
class members, cannot be accessed by users of the class. They can, however, be accessed by
classes that are derived from this class. I will talk about protected access more a little later, in
the section “Member Functions.”
The C++ language has three keywords that pertain to class access. The keywords are (not
surprisingly) public, private, and protected. You specify a class member’s access level when
you declare the class. A class is declared with the class keyword. A class declaration looks like
a structure declaration with the access modifiers added:
class Vehicle {
public:
bool haveKey;
bool Start();
void SetGear(int gear);
void Accelerate(int acceleration);
void Break(int factor);
void Turn(int direction);