问题描述
当然，有序集是一个更具体的集合，为什么 NSOrderedSet 继承自 NSObject 而不是 NSSet ？
Surely an ordered set is a more-specific case of a set, so why does NSOrderedSet inherit from NSObject rather than NSSet?
推荐答案
我经历了<$的界面c $ c> NSSet 你是对的，有序套装似乎满足并且可以继承自 NSSet 。
I went through the interface of NSSet and you're right, ordered sets appear to satisfy the Liskov substitution principle and could therefor inherit from NSSet.
有一个小方法可以打破这个： mutableCopy 。 mutableCopy 的返回值必须是 NSMutableSet ，但 NSMutableOrderedSet 应该从 NSOrderedSet 继承。你不能同时拥有这两个。
There is one little method that breaks this: mutableCopy. The return value of mutableCopy must be an NSMutableSet, but NSMutableOrderedSet should inherit from NSOrderedSet. You can't have both.
让我解释一些代码。首先，让我们看一下 NSSet 和 NSMutableSet 的正确行为：
Let me explain with some code. First, let's look at the correct behaviour of NSSet and NSMutableSet:
NSSet* immutable = [NSSet set];
NSMutableSet* mutable = [immutable mutableCopy];
[mutable isKindOfClass:[NSSet class]]; // YES
[mutable isKindOfClass:[NSMutableSet class]]; // YES
现在，让我们假装 NSOrderedSet 继承自 NSSet ， NSMutableOrderedSet 继承自 NSOrderedSet ：
Now, let's pretend NSOrderedSet inherits from NSSet, and NSMutableOrderedSet inherits from NSOrderedSet:
//Example 1
NSOrderedSet* immutable = [NSOrderedSet orderedSet];
NSMutableOrderedSet* mutable = [immutable mutableCopy];
[mutable isKindOfClass:[NSSet class]]; // YES
[mutable isKindOfClass:[NSMutableSet class]]; // NO (this is the problem)
如果 NSMutableOrderedSet 继承自 NSMutableSet ？然后我们得到一个不同的问题：
What if NSMutableOrderedSet inherited from NSMutableSet instead? Then we get a different problem:
//Example 2
NSOrderedSet* immutable = [NSOrderedSet orderedSet];
NSMutableOrderedSet* mutable = [immutable mutableCopy];
[mutable isKindOfClass:[NSSet class]]; // YES
[mutable isKindOfClass:[NSMutableSet class]]; // YES
[mutable isKindOfClass:[NSOrderedSet class]]; // NO (this is a problem)
在例1中，你将无法通过一个 NSOrderedSet 进入一个期望 NSSet 的函数，因为行为不同。基本上，这是一个向后兼容性问题。
In Example 1, you wouldn't be able to pass an NSOrderedSet into a function expecting an NSSet because the behaviour is different. Basically, it's a backwards compatibility problem.
在示例2中，您不能将 NSMutableOrderedSet 传递给函数期待 NSOrderedSet 因为前者不会继承后者。
In Example 2, you can't pass an NSMutableOrderedSet into a function expecting an NSOrderedSet because the former doesn't inherit from the latter.
所有这一切都是因为 NSMutableOrderedSet 无法从 NSMutableSet 和 NSOrderedSet 继承，因为Objective-C没有多重继承。解决这个问题的方法是为 NSMutableSet 和 NSOrderedSet 制定协议，因为那时 NSMutableOrderedSet 可以实现这两种协议。我猜Apple开发人员只是在没有额外协议的情况下更简单。
All of this is because NSMutableOrderedSet can't inherit from both NSMutableSet and NSOrderedSet because Objective-C doesn't have multiple inheritance. The way to get around this is to make protocols for NSMutableSet and NSOrderedSet, because then NSMutableOrderedSet can implement both protocols. I guess the Apple developers just though it was simpler without the extra protocols.
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