Objective-C class, inheritance, protocol, delegate demo

objcFoundation.m

#import <Foundation/Foundation.h>
@interface DaGiac: NSObject {
    int a;
    int b;
}
@property int a;
@property int b;
-(float) getDienTich;
@end
    
@implementation DaGiac
@synthesize a, b;
-(float)getDienTich {
    return 0;
}
@end
/////////////////////////////////////////////////////
@protocol DaGiacDelegate 
-(void)daGiacDelegateWannaSayHello;
@end
/////////////////////////////////////////////////////
@interface HinhChuNhat: DaGiac {
    id <DaGiacDelegate> delegate;
}
@property (nonatomic, retain) id delegate;
@end
@implementation HinhChuNhat
@synthesize delegate;
-(float)getDienTich {
    [self.delegate daGiacDelegateWannaSayHello];
    return self.a * self.b;
}
@end
    
@interface TamGiac: DaGiac
@end
@implementation TamGiac
-(float)getDienTich {
    return 1;
}
@end
    
/////////////////////////////////////////////////////
@interface Application: NSObject <DaGiacDelegate>
@end
@implementation Application
-(id) init {
    HinhChuNhat *hcn = [[HinhChuNhat alloc] init];
    hcn.delegate = self;
    TamGiac *tg = [[TamGiac alloc] init];
    hcn.a = 10; hcn.b = 2;
    NSLog(@"HCN = %f ----- TG = %f",[hcn getDienTich], [tg getDienTich]);
}

-(void) daGiacDelegateWannaSayHello {
    NSLog(@"HELLO!");
}
@end
    
/////////////////////////////////////////////////////
int main()
{
    Application* app = [[Application alloc] init];
    return 0;
}

The output should be:

2014-06-24 06:51:04.765 program[229] HELLO!
2014-06-24 06:51:04.768 program[229] HCN = 20.000000 ----- TG = 1.000000