test 
 1 //
 2 //  Square.m
 3 //  Inherit
 4 //
 5 //  Created by zz cienet on 1/10/12.
 6 //  Copyright 2012 __MyCompanyName__. All rights reserved.
 7 //
 8 
 9 #import "Square.h"
10 
11 
12 @implementation Square : Rectangle
13 
14 -(void) setSide:(int)s
15 {
16     [self setWidth:s andHeight: s];
17 }
18 
19 -(int) side
20 {
21     return width;
22 }
23 
24 @end

看看效果