//Animal.h

#import <Foundation/Foundation.h>

@interface Animal : NSObject

{

    int _age; // 不写@public，默认只能在本类和子类当中访问.间接继承.

}

@end

//Animal.m

#import "Animal.h"

@implementation Animal

{

    int _weight; // 写在.m文件中的成员变量,叫做私有成员变量.只能在本类当中访问，子类不能访问或者设置

}

@end

//Dog.h

#import "Animal.h"

@interface Dog : Animal

- (void)lookHome;

@end

//Dog.m

#import "Dog.h"

@implementation Dog

- (void)lookHome

{

    _age = ;

    //    _weight = 9; // 私有变量,子类不能访问或者设置

    NSLog(@"看家......");

}

@end