Identity comparison (===) in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of identity comparison with class instances?

Consider the following Swift code. What will be printed when it runs?

Swift

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

let p1 = Person(name: "Alice")
let p2 = Person(name: "Alice")
let p3 = p1

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

A false\ntrue

B true\nfalse

C false\nfalse

D true\ntrue

💡 Hint

Remember that === checks if two references point to the exact same object in memory.

The operator === checks if two variables refer to the same instance.

Here, p1 and p2 are two different instances with the same content, so p1 === p2 is false.

p3 is assigned to p1, so both refer to the same instance, making p1 === p3 true.

🧠 Conceptual

intermediate

Which statement about === in Swift is true?

Choose the correct statement about the identity operator === in Swift.

A <code>===</code> compares the values of two structs.

B <code>===</code> checks if two strings have the same characters.

C <code>===</code> can be used to compare two integers.

D <code>===</code> compares if two class instances are the same object.

💡 Hint

Think about what kinds of types === works with in Swift.

The identity operator === only works with class instances (reference types) to check if they point to the same object.

It does not work with value types like structs, integers, or strings.

🔧 Debug

advanced

Why does this code cause a compile error?

Examine the code below. Why does the line with if a === b cause a compile error?

Swift

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

let a = Point(x: 1, y: 2)
let b = Point(x: 1, y: 2)

if a === b {
    print("Same")
} else {
    print("Different")
}

A Because <code>Point</code> does not implement the Equatable protocol.

B Because <code>a</code> and <code>b</code> are constants, not variables.

C Because <code>===</code> cannot be used with structs, only with class instances.

D Because <code>===</code> requires both operands to be optional types.

💡 Hint

Recall the difference between value types and reference types in Swift.

The identity operator === only works with reference types (classes).

Structs are value types, so you cannot use === with them.

❓ Predict Output

advanced

What is the output when comparing optional class instances with ===?

What will this Swift code print?

Swift

class Box {
    let value: Int
    init(_ value: Int) { self.value = value }
}

let box1: Box? = Box(10)
let box2: Box? = Box(10)
let box3: Box? = box1
let box4: Box? = nil

print(box1 === box2)
print(box1 === box3)
print(box1 === box4)

A false\ntrue\nfalse

B true\ntrue\nfalse

C false\nfalse\ntrue

D true\nfalse\nfalse

💡 Hint

Remember that === works with optionals by unwrapping them and comparing references.

box1 and box2 are different instances, so box1 === box2 is false.

box3 points to the same instance as box1, so box1 === box3 is true.

box4 is nil, so box1 === box4 is false.

🚀 Application

expert

How many unique instances are in this array?

Given the following Swift code, how many unique class instances does the array arr contain?

Swift

class Node {}

let n1 = Node()
let n2 = Node()
let n3 = n1
let n4 = n2
let n5 = Node()

let arr = [n1, n2, n3, n4, n5]

A 5

B 3

C 4

D 2

💡 Hint

Use identity comparison to find which variables point to the same instance.

n1 and n3 refer to the same instance.

n2 and n4 refer to the same instance.

n5 is a new instance.

So, the unique instances are: n1/n3, n2/n4, and n5 — total 3.

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