Objective-C Polymorphism

2018-02-07 by xujinzhong

The word polymorphism means having many forms. Typically, polymorphism occurs when there is a hierarchy of classes and they are related by inheritance.

Objective-C polymorphism means that a call to a member function will cause a different function to be executed depending on the type of object that invokes the function.

Consider the example, we have a class Shape that provides the basic interface for all the shapes. Square and Rectangle are derived from the base class Shape.

We have the method printArea that is going to show about the OOP feature polymorphism.

1 #import <Foundation/Foundation.h> 2 3 @interface Shape : NSObject 4 5 { 6 CGFloat area; 7 } 8 9 - (void)printArea;10 - (void)calculateArea;11 @end12 13 @implementation Shape14 15 - (void)printArea{16 NSLog(@"The area is %f", area);17 }18 19 - (void)calculateArea{20 21 }22 23 @end24 25 26 @interface Square : Shape27 {28 CGFloat length;29 }30 31 - (id)initWithSide:(CGFloat)side;32 33 - (void)calculateArea;34 35 @end36 37 @implementation Square38 39 - (id)initWithSide:(CGFloat)side{40 length = side;41 return self;42 }43 44 - (void)calculateArea{45 area = length * length;46 }47 48 - (void)printArea{49 NSLog(@"The area of square is %f", area);50 }51 52 @end53 54 @interface Rectangle : Shape55 {56 CGFloat length;57 CGFloat breadth;58 }59 60 - (id)initWithLength:(CGFloat)rLength andBreadth:(CGFloat)rBreadth;61 62 63 @end64 65 @implementation Rectangle66 67 - (id)initWithLength:(CGFloat)rLength andBreadth:(CGFloat)rBreadth{68 length = rLength;69 breadth = rBreadth;70 return self;71 }72 73 - (void)calculateArea{74 area = length * breadth;75 }76 77 @end78 79 80 int main(int argc, const char * argv[])81 {82 NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];83 Shape *square = [[Square alloc]initWithSide:10.0];84 [square calculateArea];85 [square printArea];86 Shape *rect = [[Rectangle alloc]87 initWithLength:10.0 andBreadth:5.0];88 [rect calculateArea];89 [rect printArea]; 90 [pool drain];91 return 0;92 }

When the above code is compiled and executed, it produces the following result:

1 2013-09-22 21:21:50.785 Polymorphism[358:303] The area of square is 100.0000002 2013-09-22 21:21:50.786 Polymorphism[358:303] The area is 50.000000

In the above example based on the availability of the method calculateArea and printArea, either the method in the base class or the derived class executed.

Polymorphism handles the switching of methods between the base class and derived class based on the method implementation of the two classes.

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