Teach Yourself Borland C++ in 14 Days - portal

96 Day 4
So, uh…What’s a Class?
A class, like a structure, is a collection of data members and functions that work together to
accomplish a specific programming task. In this way a class is said to encapsulate the task.
Classes have the following features:
■ The capability to control access
■ Constructors
■ Destructors
■ Data members
■ Member functions
■ A hidden, special pointer called this
Before diving into an examination of those features, let me give you a quick example of how
a class might work. Let’s use a typical Windows control as an example—a check box, for
instance. A class that represents a check box could have data members for the caption of the
check box and for the state (checked or unchecked). This class would also have functions that
would allow you to set and query both the check box caption and the check state. These
functions might be named GetCheck(), SetCheck(), GetCaption(), and SetCaption(). Once
the class has been written, you can create an instance of the class to control a check box in
Windows. (It’s not quite that simple, but this is just an example, after all.) If you have three
check boxes, you could have three instances of the CheckBox class that could then be used to
control each check box individually. Here’s an example:
MyCheckBox check1(ID_CHECK1);
MyCheckBox check2(ID_CHECK2);
MyCheckBox check3(ID_CHECK3);
check1.SetCaption(“Thingamabob Option”);
check1.SetCheck(true);
check2.SetCaption(“Doohickey Options”);
check2.SetCheck(false);
check3.SetCaption(“Whodyacallum Options”);
check3.SetCheck(true);
if (check1.GetCheck()) DoThingamabobTask();
if (check2.GetCheck()) DoDoohickeyTask();
// etc.
In this example, each instance of the class is a separate object. Each instance has its own data
members, and the objects operate independently of one another. They are all objects of the
same type, but are separate instances in memory. With that brief introduction, let’s roll up
our sleeves once more and go to work on understanding classes.