Identity comparison (===) in Swift

Introduction

Identity comparison checks if two variables point to the exact same object in memory. It helps us know if they are truly the same instance, not just equal in value.

When you want to check if two class instances are the same object.

When comparing references to see if they share the same memory address.

When you want to avoid duplicating work by confirming if two variables refer to one object.

When debugging to understand if two variables point to the same instance.

When managing resources that should only exist once in memory.

Syntax

Swift

object1 === object2

The === operator works only with class instances (reference types).

It returns true if both variables refer to the same object, otherwise false.

Examples

personA and personB point to the same object, so identity comparison is 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

All variables point to the same Car instance, so identity comparison returns true.

Swift

class Car {}

let car1 = Car()
let car2 = car1
let car3 = car2

print(car1 === car3)  // true

book1 and book2 are two different instances, so identity comparison returns false.

Swift

class Book {}

let book1 = Book()
let book2 = Book()

print(book1 === book2)  // false

Sample Program

This program creates three Dog instances. dog1 and dog2 point to the same object, so identity comparison is true. dog3 is a different object even though the name is the same.

Swift

class Dog {
    var name: String
    init(name: String) {
        self.name = name
    }
}

let dog1 = Dog(name: "Buddy")
let dog2 = dog1
let dog3 = Dog(name: "Buddy")

print(dog1 === dog2)  // true
print(dog1 === dog3)  // false

Important Notes

Identity comparison (===) is different from equality comparison (==), which checks if values are equal.

Only class instances can be compared with ===, not structs or enums.

Summary

Identity comparison checks if two variables point to the exact same object.

Use === with class instances to compare their identity.

It returns true if both refer to the same object, otherwise false.

Practice

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

easy

A. If two variables refer to the exact same object instance

B. If two variables have the same value

C. If two variables are of the same type

D. If two variables are both nil

Solution

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

   The === operator in Swift is used to check if two references point to the exact same object in memory.

2. Step 2: Differentiate from value equality

   Unlike ==, which checks if values are equal, === checks identity, meaning the same instance.

3. Final Answer:

   If two variables refer to the exact same object instance -> Option A

4. Quick Check:

   Identity comparison = If two variables refer to the exact same object instance [OK]

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

Common Mistakes:

• Confusing === with == for value equality
• Thinking === checks type equality
• Assuming === works with structs or enums

2. Which of the following is the correct syntax to check if two class instances a and b are identical in Swift?

easy

A. if a === b { }

B. if a == b { }

C. if a = b { }

D. if a is b { }

Solution

1. Step 1: Recall identity operator syntax

   In Swift, the identity operator to check if two references point to the same object is ===.

2. Step 2: Identify correct usage

   The correct syntax is if a === b { }. The other options are either value equality, assignment, or invalid syntax.

3. Final Answer:

   if a === b { } -> Option A

4. Quick Check:

   Identity check syntax = if a === b { } [OK]

Hint: Use three equals signs (===) for identity check [OK]

Common Mistakes:

• Using == instead of === for identity
• Using single = which is assignment
• Typing invalid operators like ===

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. false\nfalse

B. false\ntrue

C. true\ntrue

D. true\nfalse

Solution

1. Step 1: Analyze object references

   Variable 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 D

4. Quick Check:

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

Hint: Same variable means true; new instance means false [OK]

Common Mistakes:

• Assuming different variables always mean false
• Confusing === with == and expecting value equality
• Ignoring that p2 points to p1's instance

4. The following Swift code gives a compile error. What is the problem?

class Car {}

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

if car1 == car2 {
    print("Same car")
}

medium

A. Cannot create instances of class Car

B. Missing parentheses in if statement

C. Cannot use == to compare class instances without Equatable conformance

D. Using === instead of ==

Solution

1. Step 1: Understand == operator requirements

   By default, class instances cannot be compared with == unless the class conforms to Equatable.

2. Step 2: Identify the error cause

   The code tries to use == without Car conforming to Equatable, causing a compile error.

3. Final Answer:

   Cannot use == to compare class instances without Equatable conformance -> Option C

4. Quick Check:

   == needs Equatable for classes [OK]

Hint: Use === for identity or add Equatable for == [OK]

Common Mistakes:

• Thinking === causes error here
• Assuming == works automatically for classes
• Ignoring Equatable protocol requirement

5. You have a cache dictionary storing class instances by keys. You want to check if a newly created instance newObj is already stored in the cache by identity. Which Swift code correctly checks this?

class Data {}

var cache: [String: Data] = [:]
let newObj = Data()

// Check if newObj is already in cache by identity

hard

A. if cache.values.contains(newObj) { print("Found") }

B. if cache.values.contains(where: { $0 === newObj }) { print("Found") }

C. if cache.keys.contains(where: { $0 === newObj }) { print("Found") }

D. if cache.values.contains(where: { $0 == newObj }) { 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 cache.values.contains(where: { $0 === newObj }) { print("Found") } correctly uses cache.values.contains(where: { $0 === newObj }). if cache.values.contains(newObj) { print("Found") } uses contains without identity check, which requires Equatable. if cache.keys.contains(where: { $0 === newObj }) { print("Found") } incorrectly checks keys (strings) with identity. if cache.values.contains(where: { $0 == newObj }) { print("Found") } uses == which needs Equatable.

3. Final Answer:

   if cache.values.contains(where: { $0 === newObj }) { print("Found") } -> Option B

4. Quick Check:

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

Hint: Use contains(where:) and === to check identity in collections [OK]

Common Mistakes:

• Using contains without where and identity check
• Checking keys instead of values
• Using == without Equatable conformance