Identity operators (=== and !==) in Swift

Introduction

Identity operators check if two variables point to the exact same object in memory. They help you know if two references are identical, not just equal.

When you want to check if two variables refer to the same instance of a class.

When comparing objects to avoid duplicate processing of the same instance.

When you want to ensure two references are not just equal in value but actually the same object.

When managing resources or caches where object identity matters.

When debugging to confirm if two references point to the same object.

Syntax

Swift

object1 === object2
object1 !== object2

Use === to check if two references point to the same object.

Use !== to check if two references point to different objects.

Examples

personA and personB point to the same object, so === returns true. personC is a different object.

Swift

class Person {}

let personA = Person()
let personB = personA
let personC = Person()

print(personA === personB)  // true
print(personA === personC)  // false

car1 and car2 are different objects, so !== returns true and prints the message.

Swift

class Car {}

let car1 = Car()
let car2 = Car()

if car1 !== car2 {
    print("Different cars")
}

Sample Program

This program creates three book references. book1 and book2 point to the same object, so === is true. book3 is a different object with the same content, so === is false and !== is true.

Swift

class Book {
    let title: String
    init(title: String) {
        self.title = title
    }
}

let book1 = Book(title: "Swift Basics")
let book2 = book1
let book3 = Book(title: "Swift Basics")

print(book1 === book2)  // true
print(book1 === book3)  // false
print(book1 !== book3)  // true

Important Notes

Identity operators only work with class instances, not with structs or enums.

Use equality operators (==) to compare values, identity operators (===) to compare references.

Summary

=== checks if two references point to the same object.

!== checks if two references point to different objects.

Use identity operators only with class instances, not value types.

Practice

1. What does the identity operator === check in Swift?

easy

A. If two variables are of the same type

B. If two variables have the same value

C. If two variables refer to the exact same class instance

D. If two variables are both nil

Solution

1. Step 1: Understand the purpose of ===

   The === operator checks if two references point to the same object in memory.

2. Step 2: Differentiate from value equality

   Unlike ==, which checks value equality, === checks identity (same instance).

3. Final Answer:

   If two variables refer to the exact same class instance -> Option C

4. Quick Check:

   === checks identity [OK]

Hint: Remember: === means same object, not just same value [OK]

Common Mistakes:

• Confusing === with == for value equality
• Using === with structs or value types
• Thinking === checks type equality

2. Which of the following is the correct syntax to check if two class instances a and b are NOT the same object in Swift?

easy

A. if a != b

B. if a !==== b

C. if a !=== b

D. if a !== b

Solution

1. Step 1: Identify the correct identity operator for inequality

   The operator to check if two references are NOT the same object is !==.

2. Step 2: Verify syntax correctness

   Options C and D have invalid operators. if a != b uses != which checks value inequality, not identity.

3. Final Answer:

   if a !== b -> Option D

4. Quick Check:

   Use !== for identity inequality [OK]

Hint: Use !== to check different objects, not != [OK]

Common Mistakes:

• Using != instead of !== for identity check
• Typing invalid operators like !=== or !====
• Confusing identity operators with value operators

3. Consider the following Swift code:

class Person {}

let p1 = Person()
let p2 = p1
let p3 = Person()

print(p1 === p2)
print(p1 === p3)

What will be the output?

medium

A. true\nfalse

B. false\ntrue

C. true\ntrue

D. false\nfalse

Solution

1. Step 1: Analyze the references

   p2 is assigned to p1, so both point to the same object. p3 is a new instance.

2. Step 2: Evaluate identity comparisons

   p1 === p2 is true because they are the same instance. p1 === p3 is false because they are different instances.

3. Final Answer:

   true\nfalse -> Option A

4. Quick Check:

   Same instance = true, different instance = false [OK]

Hint: === true if same object, false if different [OK]

Common Mistakes:

• Assuming p1 and p3 are same because same class
• Confusing value equality with identity
• Forgetting p2 points to p1's instance

4. The following Swift code produces an error. What is the problem?

struct Point {
  var x: Int
  var y: Int
}

let p1 = Point(x: 1, y: 2)
let p2 = Point(x: 1, y: 2)

if p1 === p2 {
  print("Same point")
}

medium

A. p1 and p2 are not initialized

B. Cannot use === with structs because they are value types

C. Missing parentheses in if condition

D. === operator requires optional types

Solution

1. Step 1: Identify the types used

   Point is a struct, which is a value type in Swift.

2. Step 2: Understand identity operator restrictions

   Identity operators === and !== only work with class instances (reference types), not value types like structs.

3. Final Answer:

   Cannot use === with structs because they are value types -> Option B

4. Quick Check:

   === only for class instances [OK]

Hint: Use === only with classes, not structs or enums [OK]

Common Mistakes:

• Trying to use === with value types
• Assuming structs have identity
• Ignoring compiler error message

5. You have a class Car and an array of Car instances. You want to find if a specific Car instance myCar is already in the array by identity (same object), not by value. Which Swift code correctly checks this?

hard

A. if cars.contains(where: { $0 === myCar }) { print("Found") }

B. if cars.contains(myCar) { print("Found") }

C. if cars.firstIndex(of: myCar) != nil { print("Found") }

D. if cars.filter({ $0 == myCar }).count > 0 { print("Found") }

Solution

1. Step 1: Understand identity check in collections

   To check if an instance exists by identity, use === inside a closure with contains(where:).

2. Step 2: Analyze options

   if cars.contains(where: { $0 === myCar }) { print("Found") } uses === correctly. Options B and C use == or of: which check value equality or require Equatable conformance. if cars.filter({ $0 == myCar }).count > 0 { print("Found") } uses == which is value equality.

3. Final Answer:

   if cars.contains(where: { $0 === myCar }) { print("Found") } -> Option A

4. Quick Check:

   Use contains(where:) with === for identity [OK]

Hint: Use contains(where:) with === to check object identity [OK]

Common Mistakes:

• Using contains(myCar) which checks value equality
• Assuming == checks identity
• Not using closure for identity check