What does quot;Protocol ... can only be used as a generic constraint because it has Self or associated type requirementsquot; mean?(“Protocol ... can only be used as a generic constraint because it has Self or associated type requirements是什么意思?意思是?)
问题描述
我正在尝试创建一个字典(实际上是一个 HashSet)键入自定义协议斯威夫特，但它给了我标题中的错误:
I am trying to create a Dictionary (actually a HashSet) keyed on a custom protocol in Swift, but it is giving me the error in the title:
Protocol 'myProtocol' 只能用作通用约束，因为它具有 Self 或关联的类型要求
Protocol 'myProtocol' can only be used as a generic constraint because it has Self or associated type requirements
而且我无法判断它的正面或反面.
and I can't make heads nor tails of it.
protocol Observing: Hashable { }
var observers = HashSet<Observing>()
推荐答案
协议 Observing 继承自协议 Hashable，而协议又继承自协议 Equatable.协议Equatable有如下要求:
Protocol Observing inherits from protocol Hashable, which in turn inherits from protocol Equatable. Protocol Equatable has the following requirement:
func ==(lhs: Self, rhs: Self) -> Bool
并且在其内部某处包含 Self 的协议不能在任何地方使用，除非在类型约束中.
And a protocol that contains Self somewhere inside it cannot be used anywhere except in a type constraint.
这里是一个类似的问题.
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