Programming in Objective-C, Sixth Edition

The @class Directive

Now you can work with rectangles (and squares as well!) with the ability to set their widths, heights, and origins. First, let’s take a complete look at your Rectangle.h interface file:

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#import <Foundation/Foundation.h>@class XYPoint;@interface Rectangle: NSObject@property int width, height;-(XYPoint *) origin;-(void)  setOrigin: (XYPoint *) pt;-(void)  setWidth: (int) w andHeight: (int) h;-(int)   area;-(int)   perimeter;@end

You used a new directive in the Rectangle.h header file:

@class XYPoint;

You needed this because the compiler needs to know what an XYPoint is when it encounters it as one of the instance variables defined for a Rectangle. The class name is also used in the argument and ...