swift - 使用ObjectIdentifier()和 '==='运算符之间的区别

假设我正在Swift中实现一个根类，该类声明采用Equatable协议(protocol)(我希望能够知道我类型的数组是否包含给定的实例)。

在这种特定情况下，-(如果有)在实现协议(protocol)所需的==运算符之间有什么区别:

public static func ==(lhs: MyClass, rhs: MyClass) -> Bool {

    return ObjectIdentifier(lhs) == ObjectIdentifier(rhs)
}

...而不是仅仅这样做:

public static func ==(lhs: MyClass, rhs: MyClass) -> Bool {

    return (lhs === rhs)
}

作为引用，这是文档中有关ObjectIdentifier()的内容:



...这就是The Swift Programming Language (Swift 3)的“基本运算符”部分对===运算符的描述:

最佳答案

类实例没有区别，请参见以下内容
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
implementation of == for ObjectIdentifier ，
只是将指针与对象存储进行比较:

  public static func == (x: ObjectIdentifier, y: ObjectIdentifier) -> Bool {
    return Bool(Builtin.cmp_eq_RawPointer(x._value, y._value))
  }

这是the === operator
也可以:

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，因此如果要为您的类实现该协议(protocol)，它将非常有用:

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))未为其定义===的代码。