Teach Yourself Borland C++ in 14 Days - portal

116 Day 4
NOTE
Another way of declaring constants is to declare a variable using the
const modifier. Here’s an example:
const int airliner = 0;
Using a const variable is probably the more modern method of
defining constants.
The next thing to note in the header is that the class includes some inline functions. These
functions are so small that it makes sense to inline them. You might also notice that the
Airplane class has one overloaded function called GetStatus(). When called with a character
array parameter, it will return a status string as well as the status data member. When called
without a parameter, it just returns status. Note that there is only one public data member.
The rest of the data members are kept private. The only way to access the private data
members is via the public functions. For instance, you can change the speed, altitude, and
heading of an airplane only by sending it a message. To use an analogy, consider that an air
traffic controller cannot physically change an aircraft’s heading. The best he can do is send
a message to the pilot and tell him to change to a new heading.
Now turn your attention to Listing 4.2. This is the definition of the Airplane class. The
constructor performs initialization, including dynamically allocating storage for the char
array that holds the name of the airplane. That memory is freed in the destructor. The
SendMessage() function does most of the work. A switch statement determines which
message was sent and takes the appropriate action. Notice that the TakeOff() and Land()
functions cannot be called directly (they are protected), but rather are called through the
SendMessage() function. Again, you can’t make an aircraft take off or land; you can only send
it a message telling it what you want it to do. The ReportStatus() function calls GetStatus()
to get a status string, which it outputs.
The main program is shown in Listing 4.3. The program first sets up an array of Airplane
pointers and creates three instances of the Airplane class. Then a loop starts. You can send
messages to any of the airplanes by calling the object’s SendMessage() function. When you
send a message, you get a response back from the airplane. The do-while loop cheats a little
in this program. Notice that the test condition is simply 1. This means that the loop will keep
running indefinitely. In this case it’s not a problem because I am using the break statement
to break out of the loop rather than relying on the test condition. Run the program and play
with it to get a feel for how it works.
Inheritance
One of the most powerful features of classes in C++ is that they can be extended through
inheritance.