objective-c – 为什么我只创建超类的属性才能使用子类

第一个作业要求学生创建一些课程中详细介绍的课程(卡片,PlayCard,Deck,PlayingCardDeck),并更新视图控制器以在一副扑克牌中显示随机卡片.

其中两项任务包括：

1. Add a private property of type Deck * to the CardGameViewController.
2. Use lazy instantiation to allocate and initialize this property (in the property’s getter)
   so that it starts off with a full deck of PlayingCards.

我在代码中添加了以下内容：

// CardGameViewController.m
#import "PlayingCardDeck.H"

@interface CardGameViewController ()
...
@property (strong,nonatomic) Deck *deck;
@end

@implementation CardGameViewController
- (Deck *)deck
{
    if (!_deck) _deck = [[PlayingCardDeck alloc] init];
    return _deck;
}
...
@end

提示表示以下内容：

1. Even though the type of the property you must add is required to be a Deck (not PlayingCardDeck) you’ll obviously have to lazily instantiate it using a PlayingCardDeck. This is perfectly legal in object-oriented programming because a PlayingCardDeck inherits from Deck and thus it “is a” Deck. If you are confused by this concept in object-oriented programming,this course may be rather difficult for you.

PlayingCardDeck是Deck的子类.我知道它“是一个”甲板.

我不明白为什么使用Deck的属性而不是PlyaingCardDeck.