Concepts of Programming Languages

12.6 Support for Object-Oriented Programming in Objective-C 551

There is another way to provide some of the benefits of multiple inheri-

tance in Objective-C, protocols. Although Objective-C does not provide

abstract classes, as in C++, protocols are related to them. A protocol is a list of

method declarations. The syntax of a protocol is exemplified with the following:

@protocol MatrixOps

-(Matrix *) add: (Matrix *) mat;

-(Matrix *) subtract: (Matrix *) mat;

@optional

-(Matrix *) multiply: (Matrix *) mat;

@end

In this example, MatrixOps is the name of the protocol. The add and

subtract methods must be implemented by a class that uses the protocol.

This use is called implementing or adopting the protocol. The optional part

specifies that the multiply method may or may not be implemented by an

adopting class.

A class that adopts a protocol lists the name of the protocol in angle brackets

after the name of the class on the interface directive, as in the following:

@interface MyClass: NSObject <YourProtocol>

12.6.3 Dynamic Binding

In Objective-C, polymorphism is implemented in a way that differs from the

way it is done in most other common programming languages. A polymorphic

variable is created by declaring it to be of type id. Such a variable can reference

any object. The run-time system keeps track of the class of the object to which

an id type variable refers. If a call to a method is made through such a vari-

able, the call is dynamically bound to the correct method, assuming one exists.

For example, suppose that a program has classes defined named Circle

and Square and both have methods named draw. Consider the following

skeletal code:

// Create the objects

Circle *myCircle = [[Circle alloc] init];

Square *mySquare = [[Square alloc] init];

// Initialize the objects

[myCircle setCircumference: 5];

[mySquare setSide: 5];

// Create the id variable

id shapeRef;

//Set the id to reference the circle and draw it