3.2. Classes, Objects, Member Functions and Data Members
Let's begin with a simple analogy to help you reinforce your understanding from Section 1.17 of classes and their contents. Suppose you want to drive a car and make it go faster by pressing down on its accelerator pedal. What must happen before you can do this? Well, before you can drive a car, someone has to design it and build it. A car typically begins as engineering drawings, similar to the blueprints used to design a house. These drawings include the design for an accelerator pedal that the driver will use to make the car go faster. In a sense, the pedal "hides" the complex mechanisms that actually make the car go faster, just as the brake pedal "hides" the mechanisms that slow the car, the steering wheel "hides" the mechanisms that turn the car and so on. This enables people with little or no knowledge of how cars are engineered to drive a car easily, simply by using the accelerator pedal, the brake pedal, the steering wheel, the transmission shifting mechanism and other such simple and user-friendly "interfaces" to the car's complex internal mechanisms.
Unfortunately, you cannot drive the engineering drawings of a carbefore you can drive a car, it must be built from the engineering drawings that describe it. A completed car will have an actual accelerator pedal to make the car go faster. But even that's not enoughthe car will not accelerate on its own, so the driver must press the accelerator pedal to tell the car to go faster.
Now let's use our car example to introduce the key object-oriented programming concepts of this section. Performing a task in a program requires a function (such as main, as described in Chapter 2). The function describes the mechanisms that actually perform its tasks. The function hides from its user the complex tasks that it performs, just as the accelerator pedal of a car hides from the driver the complex mechanisms of making the car go faster. In C++, we begin by creating a program unit called a class to house a function, just as a car's engineering drawings house the design of an accelerator pedal. Recall from Section 1.17 that a function belonging to a class is called a member function. In a class, you provide one or more member functions that are designed to perform the class's tasks. For example, a class that represents a bank account might contain one member function to deposit money into the account, another to withdraw money from the account and a third to inquire what the current account balance is.
Just as you cannot drive an engineering drawing of a car, you cannot "drive" a class. Just as someone has to build a car from its engineering drawings before you can actually drive the car, you must create an object of a class before you can get a program to perform the tasks the class describes. That is one reason C++ is known as an object-oriented programming language. Note also that just as many cars can be built from the same engineering drawing, many objects can be built from the same class.
When you drive a car, pressing its gas pedal sends a message to the car to perform a taskthat is, make the car go faster. Similarly, you send messages to an objecteach message is known as a member-function call and tells a member function of the object to perform its task. This is often called requesting a service from an object.
Thus far, we have used the car analogy to introduce classes, objects and member functions. In addition to the capabilities a car provides, it also has many attributes, such as its color, the number of doors, the amount of gas in its tank, its current speed and its total miles driven (i.e., its odometer reading). Like the car's capabilities, these attributes are represented as part of a car's design in its engineering diagrams. As you drive a car, these attributes are always associated with the car. Every car maintains its own attributes. For example, each car knows how much gas is in its own gas tank, but not how much is in the tanks of other cars. Similarly, an object has attributes that are carried with the object as it is used in a program. These attributes are specified as part of the object's class. For example, a bank account object has a balance attribute that represents the amount of money in the account. Each bank account object knows the balance in the account it represents, but not the balances of the other accounts in the bank. Attributes are specified by the class's data members.