Swift 协议一致性

Question

I want a variable that conforms to protocol, but the swift compiler tells me the protocol does not confirm.

protocol A {}
protocol B {
    var a : A { get }
}
class AA : A {}
// Type 'BB' does not conform to protocol 'B'
class BB : B { 
    let a = AA()
}

Answer 1

You are not conforming to the protocol because your a is not clearly typed as an A , but rather inferred as an AA . Make it explicit.

protocol A {}
protocol B {
    var a : A { get }
}
class AA : A {}

class BB : B {
    let a: A = AA() // Explicitly typed here.
}

The fact that you have used var in the protocol and let in the implementation is a red herring, as the var is a get only, and has no set .