//长方形的类的声明与实现
#import <Foundation/Foundation.h>@interface Rect2 : NSObject{//    float chang;//    float w;}@property float chang;@property float w;- (float)sizeOfRect;@end#import "Rect2.h"@implementation Rect2//@synthesize chang;//@synthesize w;- (float)sizeOfRect{    NSLog(@"%@", self);    return  (self.chang)*(self.w);}@end//长方体的类的声明与定义#import <Foundation/Foundation.h>#import "Rect2.h"@interface LiFangTi : Rect2{//    float height;}@property float height;- (float)tiJiFor;@end#import "LiFangTi.h"@implementation LiFangTi-(float)tiJiFor{    return self.w*self.height*self.chang;}@end#import "ViewController.h"#import "LiFangTi.h"#import "Rect2.h"@interface ViewController ()            @end@implementation ViewController            - (void)viewDidLoad {    [super viewDidLoad];    Rect2 *rect=[[Rect2 alloc]init];    rect.chang=1;    rect.w    =2;    NSLog(@"%@", rect);    NSLog(@"%f",[rect sizeOfRect]);        LiFangTi *liFangTi=[[LiFangTi alloc]init];    liFangTi.chang=1.0;    liFangTi.w    =3.0;    liFangTi.height=3.0;    NSLog(@"%f",[liFangTi tiJiFor]);}