CarParts

新建工程：CarParts，修改代码如下：

//
//  main.m
//  CarParts
//
//  Created by cloud on 13-3-8.
//  Copyright (c) 2013年 cloud. All rights reserved.
//

#import <Foundation/Foundation.h>
@interface Tire:NSObject
@end

@implementation Tire

- (NSString *) description
{
    return (@"I am a tire.I last a while");
}//description

@end
@interface Engine : NSObject

@end
@implementation Engine
- (NSString *)description
{
    return (@"I am an engine.Vroom!");
}

@end
@interface Car : NSObject
{
    Engine *engine;
    Tire   *tires[4];
}
- (void) print;

@end
@implementation Car

- (id)init
{
    if (self=[super init]) {
        engine=[Engine new];
        tires[0]=[Tire new];
        tires[1]=[Tire new];
        tires[2]=[Tire new];
        tires[3]=[Tire new];
    }
    return (self);
}
- (void) print
{
    NSLog(@"%@",engine);
    
    NSLog(@"%@",tires[0]);
    NSLog(@"%@",tires[1]);
    NSLog(@"%@",tires[2]);
    NSLog(@"%@",tires[3]);
}

@end

int main(int argc, const char * argv[])
{

    @autoreleasepool {
        
        // insert code here...
        Car *car;
        car=[Car new];
        [car print];
        
    }
    return 0;
}

运行，结果如下：

注意：一、description方法为对象的描述方法，二、init为初始化方法，对象分配成功后，就会调用它。