三种setter方法

assign:
// 基本数据类型
 - (void) setAge:(int)age{
         
         _age=age;
              
         }
copy:
// 针对对象
 - (void)setDog:(Dog)dog{
         
         if(_dog==dog)
         return;
         [_dog release];
         _dog=[dog copy];
         }
 retain
- (void)setDog:(Dog)dog{
         
         if(_dog==dog)
         return;
          [_dog release];
           _dog=[dog retain];
            }