Objective-C Tutorial 2: Objective-C – Inheritance

Welcome to the second instalment of the series on introducing you to the world of Objective-C. In this article, we will talk about Objective-C Polymorphism concept of Inheritance. When writing classes, the importance is underlined in relationships between classes and objects,

The aim of being an efficient programmer is for code-reuse, and if you are coming from an existing OOP oriented background, like Java or C++, you know what i’m talking about, and therefore when we defined new classes, we form them in the mould of other classes, with slight differences, taking in the important properties of a parent class in addition to specialised specific code of your sub-class.


The removal of duplicated code by creating common code in the higher-up classes (superclasses), we can inherit the common code as well as specialise specific code. The inheritance syntax is:-

@interface Shape : NSObject
ShapeColor fillColor;
ShapeRect bounds;
- (void) setFillColor: (ShapeColor) fillColor;
- (void) setBounds: (ShapeRect) bounds;
- (void) draw;


@implementation Shape

- (void) setFillColor: (ShapeColor) c {
fillColor = c;

- (void) setBounds: (ShapeRect) b
bounds = b; } // setBounds

- (void) draw { }

Now, implementing the Circle, we would do:

@implementation Circle - (void) draw {
NSLog (@"drawing a circle at (%d %d %d %d) in %@", bounds.x, bounds.y,
bounds.width, bounds.height, colorName(fillColor));

} // draw
@end // Circle

As you can see, we can either override (by creating the same method as in your superclass) your parent class, or create new sub-class specific methods and inherit the superclass methods. To override, you would call [super methodName: param] to get all the properties of the parent before specialising.

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