What Is Composition?

Composition in programming is like composition in music: you're bringing individual components together and making them work to build something bigger. In music, you might bring together a bassoon part and an oboe part in creating a symphony. In software, you might bring together a pedal object and a tire object as part of a virtual unicycle.

In Objective-C, you compose by including pointers to objects as instance variables. So, our virtual unicycle would have a pointer to a Pedal object and a pointer to a Tire object and would look something like this:

@interface Unicycle : NSObject
{
    Pedal *pedal;
    Tire *tire;
}

@end // Unicycle

Through composition, a Unicycle consists of a Pedal and a Tire.

NOTE

You've already seen a form of composition in the Shapes-Object program: the