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Posted on: September 26, 2008 at 12:00 AM

When programmer wants to add some more functionality to the class, typically extend the class.

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When programmer wants to add some more functionality to the class, typically extend the class. But this is not a right way everywhere, so like ruby Objective-C also provides categories to achieve this. Categories allows programmer to add functionality to already existing classes without extending them.

In the example given below we have a class BaseClass that has some methods and the second class SubClass that is used to add a method to the BaseClass. In the main, we have created object of base class and use the method defined in the sub class.

Example:
This is code of primary class.

BaseClass.h

BaseClass.m

#import<Foundation/NSObject.h>

@interface BaseClass : NSObject {
    int num1, num2;
  }
  -(void)set :(int) x and: (int) y;
  -(int)add;
  -(int)sub;
@end

#import"BaseClass.h"

@implementation BaseClass
-(void)set :(int) x and: (int) y {
num1 = x;
num2 = y;
}
-(int)add {
return num1+num2;
}
-(int)sub {
if(num1>num2){
return num1-num2;
}
else
return num2-num1;
}
@end

This is code of sub class that is used to add method in the primary class.

SubClass.h

SubClass.m

#import"BaseClass.h"

@interface BaseClass(Category)
  -(void)show:(int)x;
@end

#import"SubClass.h"

@implementation BaseClass(BaseClass)

  -(void)show:(int)x {
      printf("Result is : %d \n",x);
  }
@end

main.m

#import"BaseClass.m"
#import"SubClass.m"
#import<stdio.h>

int main(){
   BaseClass *obj = [[BaseClass alloc] init];
   [obj set:10 and:8];
   [obj show:[obj add]];
   [obj show:[obj sub]];

   [obj release];
   return 0;
}

Output:

Result is : 18 
Result is : 2

Download Source Code

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