==和===之间的区别

Question

在swift中似乎有两个相等运算符:double equals(==)和triple equals(===),两者之间有什么区别？

Answer 1

!==并且===是身份运算符,用于确定两个对象是否具有相同的引用.

Swift还提供了两个标识运算符(===和!==),用于测试两个对象引用是否都引用同一个对象实例.

摘录自:Apple Inc."The Swift Programming Language."iBooks.https://itun.es/us/jEUH0.l

Answer 2

简而言之:

== 运算符检查它们的实例值是否相等, "equal to"

=== operator检查引用是否指向同一个实例, "identical to"

答案很长:

类是引用类型,多个常量和变量可以在后台引用类的同一单个实例.类引用保留在运行时堆栈(RTS)中,它们的实例保留在内存的堆区域中.当你用==它控制相等时,意味着它们的实例彼此相等.它不需要是相同的实例是相等的.为此,您需要为自定义类提供相等条件.默认情况下,自定义类和结构不接收等价运算符的默认实现,称为"等于"运算符==和"不等于"运算符!=.要做到这一点,您的自定义类需要符合Equatable协议及其static func == (lhs:, rhs:) -> Bool功能

我们来看看例子:

class Person : Equatable {
    let ssn: Int
    let name: String

    init(ssn: Int, name: String) {
        self.ssn = ssn
        self.name = name
    }

    static func == (lhs: Person, rhs: Person) -> Bool {
        return lhs.ssn == rhs.ssn
    }
}

P.S.: 由于ssn(社会安全号码)是唯一号码,因此您无需比较其名称是否相等.

let person1 = Person(ssn: 5, name: "Bob")
let person2 = Person(ssn: 5, name: "Bob")

if person1 == person2 {
   print("the two instances are equal!")
}

虽然person1和person2引用指向Heap区域中的两个不同实例,但它们的实例是相同的,因为它们的ssn数相等.所以输出将是the two instance are equal!

if person1 === person2 {
   //It does not enter here
} else {
   print("the two instances are not identical!")
}

===operator检查引用是否指向同一个实例"identical to".由于person1和person2在Heap区域中有两个不同的实例,因此它们与输出不同the two instance are not identical!

let person3 = person1

P.S: 类是引用类型,person1的引用通过此赋值操作复制到person3,因此两个引用都指向堆区域中的相同实例.

if person3 === person1 {
   print("the two instances are identical!")
}

它们是相同的,输出将是 the two instances are identical!

Answer 3

在这两种Objective-C和夫特中,==和!=运营商的测试用于数值的值相等(例如,NSInteger,NSUInteger,int,在Objective-C和Int,UInt等在SWIFT).对于对象(Objective中的NSObject/NSNumber和子类以及Swift中的引用类型),==并!=测试对象/引用类型是相同的相同的东西 - 即相同的哈希值 - 或者不是相同的相同的东西,分别.

let a = NSObject()
let b = NSObject()
let c = a
a == b // false
a == c // true

Swift的身份相等运算符,===并!==检查引用相等 - 因此,应该可以称为引用相等运算符IMO.

a === b // false
a === c // true

值得指出的是,Swift中的自定义引用类型(不是符合Equatable的类的子类)不会自动实现等于运算符,但是身份相等运算符仍然适用.此外,通过实施==,!=自动实施.

class MyClass: Equatable {
  let myProperty: String

  init(s: String) {
    myProperty = s
  }
}

func ==(lhs: MyClass, rhs: MyClass) -> Bool {
  return lhs.myProperty == rhs.myProperty
}

let myClass1 = MyClass(s: "Hello")
let myClass2 = MyClass(s: "Hello")
myClass1 == myClass2 // true
myClass1 != myClass2 // false
myClass1 === myClass2 // false
myClass1 !== myClass2 // true

这些相等运算符不是为其他类型实现的,例如任何一种语言的结构.但是,可以在Swift中创建自定义运算符,例如,可以创建运算符来检查CGPoint的相等性.

infix operator <==> { precedence 130 }
func <==> (lhs: CGPoint, rhs: CGPoint) -> Bool {
  return lhs.x == rhs.x && lhs.y == rhs.y
}

let point1 = CGPoint(x: 1.0, y: 1.0)
let point2 = CGPoint(x: 1.0, y: 1.0)
point1 <==> point2 // true

Answer 4

在快速3及以上

===(或!==)

• 检查值是否相同 (都指向相同的内存地址).
• 比较参考类型.
• 就像==在Obj-C(指针相等)中一样.

==(或!=)

• 检查值是否相同.
• 比较值类型.
• 像isEqual:Obj-C行为中的默认值一样.

这里我比较三个实例 (类是引用类型)

class Person {}

let person = Person()
let person2 = person
let person3 = Person()

person === person2 // true
person === person3 // false

Answer 5

Swifts的微妙之处===不仅仅是指针算术.在Objective-C中,您可以比较任何两个指针(即NSObject *),==这在Swift中不再正确,因为类型在编译期间起着更大的作用.

游乐场会给你

1 === 2                    // false
1 === 1                    // true
let one = 1                // 1
1 === one                  // compile error: Type 'Int' does not conform to protocol 'AnyObject'
1 === (one as AnyObject)   // true (surprisingly (to me at least))

使用字符串我们将不得不习惯这个:

var st = "123"                                 // "123"
var ns = (st as NSString)                      // "123"
st == ns                                       // true, content equality
st === ns                                      // compile error
ns === (st as NSString)                        // false, new struct
ns === (st as AnyObject)                       // false, new struct
(st as NSString) === (st as NSString)          // false, new structs, bridging is not "free" (as in "lunch")
NSString(string:st) === NSString(string:st)    // false, new structs
var st1 = NSString(string:st)                  // "123"
var st2 = st1                                  // "123"
st1 === st2                                    // true
var st3 = (st as NSString)                     // "123"
st1 === st3                                    // false
(st as AnyObject) === (st as AnyObject)        // false

但是你也可以玩得如下:

var st4 = st             // "123"
st4 == st                // true
st4 += "5"               // "1235"
st4 == st                // false, not quite a reference, copy on write semantics

我相信你能想到更多有趣的案例:-)

Swift 3的更新(正如JakubTruhlář的评论所示)

1===2                                    // Compiler error: binary operator '===' cannot be applied to two 'Int' operands
(1 as AnyObject) === (2 as AnyObject)    // false
let two = 2
(2 as AnyObject) === (two as AnyObject)  // false (rather unpleasant)
(2 as AnyObject) === (2 as AnyObject)    // false (this makes it clear that there are new objects being generated)

这看起来更加一致Type 'Int' does not conform to protocol 'AnyObject',但是我们得到了

type(of:(1 as AnyObject))                // _SwiftTypePreservingNSNumber.Type

但明确的转换表明可能会发生一些事情.在字符串方面,NSString只要我们仍然可以使用import Cocoa.然后我们会有

var st = "123"                                 // "123"
var ns = (st as NSString)                      // "123"
st == ns                                       // Compile error with Fixit: 'NSString' is not implicitly convertible to 'String'; did you mean to use 'as' to explicitly convert?
st == ns as String                             // true, content equality
st === ns                                      // compile error: binary operator '===' cannot be applied to operands of type 'String' and 'NSString'
ns === (st as NSString)                        // false, new struct
ns === (st as AnyObject)                       // false, new struct
(st as NSString) === (st as NSString)          // false, new structs, bridging is not "free" (as in "lunch")
NSString(string:st) === NSString(string:st)    // false, new objects
var st1 = NSString(string:st)                  // "123"
var st2 = st1                                  // "123"
st1 === st2                                    // true
var st3 = (st as NSString)                     // "123"
st1 === st3                                    // false
(st as AnyObject) === (st as AnyObject)        // false

它仍然是混乱有两个String类,但是在丢弃隐式转换可能会使其成为一个小更溢于言表.

Answer 6

例如,如果您创建一个类的两个实例,例如myClass:

var inst1 = myClass()
var inst2 = myClass()

你可以比较这些实例,

if inst1 === inst2

引用:

用于测试两个对象引用是否都引用同一对象实例.

摘录自:Apple Inc."The Swift Programming Language."iBooks.https://itun.es/sk/jEUH0.l

Answer 7

在Swift中我们有=== simbol这意味着两个对象都指的是相同的引用相同的地址

class SomeClass {
var a: Int;

init(_ a: Int) {
    self.a = a
}

}

var someClass1 = SomeClass(4)
var someClass2 = SomeClass(4)
someClass1 === someClass2 // false
someClass2 = someClass1
someClass1 === someClass2 // true