假设我在Swift中实现了一个根类,我声明它采用了Equatable协议(我希望能够判断我的类型的数组是否包含给定的实例).
有什么区别 – 在这种特定情况下 – 在实现协议的required ==运算符之间有什么区别：
public static func ==(lhs: MyClass,rhs: MyClass) -> Bool {
return ObjectIdentifier(lhs) == ObjectIdentifier(rhs)
}
……而不是这样做：
public static func ==(lhs: MyClass,rhs: MyClass) -> Bool {
return (lhs === rhs)
}
作为参考,这是文档中关于ObjectIdentifier()的内容：
A unique identifier for a class instance or Metatype. In Swift,only
class instances and Metatypes have unique identities. There is no
notion of identity for structs,enums,functions,or tuples.
…这就是The Swift Programming Language (Swift 3)的“基本运算符”部分对===运算符的说法：
NOTE
Swift also provides two identity operators (
===and!==),which you use to test whether two object references both refer to the same object instance. For more information,see Classes and Structures.
类实例没有区别,请参阅以下内容
comments in ObjectIdentifier.swift：
comments in ObjectIdentifier.swift：
/// Creates an instance that uniquely identifies the given class instance.
///
/// The following example creates an example class `A` and compares instances
/// of the class using their object identifiers and the identical-to
/// operator (`===`):
///
/// class IntegerRef {
/// let value: Int
/// init(_ value: Int) {
/// self.value = value
/// }
/// }
///
/// let x = IntegerRef(10)
/// let y = x
///
/// print(ObjectIdentifier(x) == ObjectIdentifier(y))
/// // Prints "true"
/// print(x === y)
/// // Prints "true"
///
/// let z = IntegerRef(10)
/// print(ObjectIdentifier(x) == ObjectIdentifier(z))
/// // Prints "false"
/// print(x === z)
/// // Prints "false"
///
从中也可以看出这一点
implementation of == for ObjectIdentifier,
它只是比较指向对象存储的指针：
public static func == (x: ObjectIdentifier,y: ObjectIdentifier) -> Bool {
return Bool(Builtin.cmp_eq_RawPointer(x._value,y._value))
}
这是the === operator
也这样做：
public func === (lhs: AnyObject?,rhs: AnyObject?) -> Bool {
switch (lhs,rhs) {
case let (l?,r?):
return Bool(Builtin.cmp_eq_RawPointer(
Builtin.bridgeToRawPointer(Builtin.castToUnknownObject(l)),Builtin.bridgeToRawPointer(Builtin.castToUnknownObject(r))
))
case (nil,nil):
return true
default:
return false
}
}
ObjectIdentifier符合Hashable,因此如果您想为您的类实现该协议,它会很有用：
extension MyClass: Hashable {
var hashValue: Int {
return ObjectIdentifier(self).hashValue
}
}
还可以为元类型创建对象标识符(例如,ObjectIdentifier(Float.self)),其中未定义===.
