swift - 引用作为 swift 字典中的键

Question

字典键需要符合Hashable:

class Test {}
var dictionary = [Test: String]() // Type 'Test' dies not conform to protocol 'Hashable'

class Test: NSObject {}
var dictionary = [Test: String]() // Works

如何获取纯 Swift 类实例的地址以用作 hashValue？

Answer 1

平等可以作为对象标识来实现，即 a == b iff a 和 b 引用该类的同一实例，哈希值可以从 ObjectIdentifier 构建(对于相同的对象来说是相同的，例如 Difference between using ObjectIdentifier() and '===' Operator 进行比较):

对于 Swift 4.2 及更高版本:

class Test : Hashable {
    static func ==(lhs: Test, rhs: Test) -> Bool {
        return lhs === rhs
    }

    public func hash(into hasher: inout Hasher) {
        hasher.combine(ObjectIdentifier(self))
    }
}

对于 Swift 3:

class Test : Hashable {
    var hashValue: Int { return ObjectIdentifier(self).hashValue }
}

func ==(lhs: Test, rhs: Test) -> Bool {
    return lhs === rhs
}

对于 Swift 2.3 及更早版本，您可以使用

/// Return an UnsafePointer to the storage used for `object`.  There's
/// not much you can do with this other than use it to identify the
/// object
func unsafeAddressOf(object: AnyObject) -> UnsafePointer<Void>

即

class Test : Hashable {
    var hashValue: Int { return unsafeAddressOf(self).hashValue }
}

func ==(lhs: Test, rhs: Test) -> Bool {
    return lhs === rhs
}

示例:

var dictionary = [Test: String]()
let a = Test()
let b = Test()
dictionary[a] = "A"
print(dictionary[a]) // Optional("A")
print(dictionary[b]) // nil

实现Equatable协议(protocol)。