1、子类重写了getter/setter

@interface SmithPerson : Person

@end

@implementation SmithPerson

- (void)setLastName:(NSString *)lastName {
    [super setLastName:lastName];
}

@end

2、

self.lastName = @"abc";
//该方法等价于 [self setLastName:@"abc"];
//self 的类型为SmithPerson，所以会调用SmithPerson 类的 -setLastName: 方法