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swift - 为什么我会遇到此一致性问题？

问题描述

protocol A {}
protocol B: A {}
protocol C { 
    var X: A { get } 
}
struct D: C { 
    let X: B // ERROR: does not conform to protocol "C"
}

B既然符合，这不应该没问题A吗？

我试图通过使用associatedtypein 协议来解决它C：

protocol A {}
protocol B: A {}
protocol C {
    associatedtype SomeType: A
    var X: SomeType { get }
}
struct D: C {
    let X: B // ERROR: does not conform to protocol C -- seemingly requiring a concrete type
}

标签： swift

解决方案

如果您按照以下方式考虑该错误，则该错误是有道理的。

/// It does not have any requirements
protocol A {}

/// It has to satisfy all of the requirements of A (whatever they are)
/// It does not have any of it's own requirements (currently)
/// It is expected to add some additional requirements of it's own
/// Otherwise it defeats the purpose of having this
protocol B: A {}

/// This has only one requirement, X that conforms to A
protocol C { 
    var X: A { get } 
}

/// This has only one requirement, X that conforms to B
/// The problem here is 
/// - A & B are different and can not be used interchangeably 
/// (at least in the current versions)
struct D: C { 
    let X: B // ERROR: does not conform to protocol "C"
}

从逻辑上讲，这是有道理的——它B必须满足所有的要求，A因此应该被允许。

我认为Swift 论坛可能是讨论这个问题的更好地方。

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