Swift 协议扩展中变量的重新声明无效

Question

在Swift中，据我了解，协议描述了可以应用于数据结构的属性。然后，协议扩展允许根据它们应用的数据结构定义这些属性。

如果是这样，为什么会出现以下错误：

invalid redeclaration of 'invalid'

这一行：

extension CausesError where Self: Example { var invalid: Bool { return true } }

在此代码中：

struct Example: CausesError { }

protocol CausesError { var invalid: Bool { get } }

extension CausesError where Self: Example { var invalid: Bool { return true } }

Answer 1

只是综合@dfri 对那些 see this later 实际错误是由以下原因引起的人所说的话：

extension CausesError where Self: Example

因为 Example 是一个结构，因此没有 Self 属性。

问题是我对协议扩展有一个根本性的误解。

A struct is concrete type, so defining a "default implementation" of invalid would simply corespond to conforming to the protocol. [One] could choose to let Example conform to CausesError by extension rather than at declaration:

extension Example: CausesError { var invalid: Bool { return true } }

but this is mostly semantics (w.r.t. direct conformance). This is not the same as supplying a default implementation (for a group of objects e.g. derived from a class type or conforming to some protocol), however, but simply conformance by a specific type to a given protocol.

因此，我应该做的（为了在协议级别提供默认实现，甚至是数据类型）很简单：

extension CausesError { var invalid: Bool { return false } }

Swift 协议扩展中变量的重新声明无效

Invalid Redeclaration of Variable in Swift Protocol Extension

swift

swift-protocols