Question

I am trying to do a class extension and calling the extended method from a generic method. I’m just wondering if it’s possible to do it? Below is my code.

ClassA.h:

@interface ClassA : NSObject

-(void) method;

ClassA.m:

#import "ClassA.h"

-(void) method{
    NSLog(@"do A");
}

ClassB.h:

@interface ClassB : ClassA

-(void) method;

ClassB.m:

#import "ClassB.h"

-(void) method{
    NSLog(@"do B");
}

and in MainClass.m

-(void) doMethod {
    ClassA *class = [[ClassB alloc] init];
    [class method];
}

I tried to do something like this, but nothing was print out. Is it possible to do this? and will it print out “do B”??

Answer 1

Your code compiles and runs fine (minus a small typo in @interface ClassA.h that should be @interface ClassA):

cristi:tmp diciu$ cat test.m
#import <Cocoa/Cocoa.h>

@interface ClassA : NSObject
- (void) method;
@end

@interface ClassB : ClassA
-(void) method;
@end

@implementation ClassA
-(void) method{
    NSLog(@"do A");
}
@end

@implementation ClassB
-(void) method{
    NSLog(@"do B");
}
@end

int main()
{
    ClassA *class = [[ClassB alloc] init];
    [class method];
}
cristi:tmp diciu$ gcc test.m -framework Cocoa
cristi:tmp diciu$ ./a.out 
2012-06-21 15:04:22.652 a.out[14408:707] do B

Apple’s Objective-C docs contain a chapter on how inheritance works that is available online here.