Readonly classes in PHP - Time & Space Complexity

Time Complexity: Readonly classes

O(n)

Understanding Time Complexity

Let's see how using readonly classes affects the time it takes for a program to run.

We want to know how the program's steps grow when working with readonly classes.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

final readonly class UserData {
    public function __construct(
        public string $name,
        public int $age
    ) {}
}

$users = [];
for ($i = 0; $i < $n; $i++) {
    $users[] = new UserData("User$i", $i);
}

This code creates many readonly UserData objects and stores them in an array.

Identify Repeating Operations

Look for loops or repeated actions.

• Primary operation: Creating a new UserData object inside a loop.
• How many times: The loop runs n times, so n objects are created.

How Execution Grows With Input

Each new object takes a small, fixed amount of time to create.

Input Size (n)	Approx. Operations
10	10 object creations
100	100 object creations
1000	1000 object creations

As n grows, the total work grows in a straight line with it.

Final Time Complexity

Time Complexity: O(n)

This means the time to create all readonly objects grows directly with the number of objects.

Common Mistake

[X] Wrong: "Readonly classes make object creation instant or free."

[OK] Correct: Readonly means properties can't change after creation, but creating each object still takes time proportional to how many you make.

Interview Connect

Understanding how object creation scales helps you write clear and efficient code, a skill valued in many coding discussions.

Self-Check

"What if we added a nested loop inside the constructor that runs m times? How would the time complexity change?"

Practice

1. What does a readonly class in PHP ensure about its properties?

easy

A. Properties are static and shared between all instances.

B. Properties can be changed anytime after creation.

C. All properties cannot be changed after the object is created.

D. Properties are private and cannot be accessed outside the class.

Solution

1. Step 1: Understand the meaning of readonly class

   A readonly class means all its properties are readonly by default, so they cannot be changed after the object is created.

2. Step 2: Compare options with this definition

   Only the option stating "All properties cannot be changed after the object is created." correctly states that properties cannot be changed after creation.

3. Final Answer:

   All properties cannot be changed after the object is created. -> Option C

4. Quick Check:

   Readonly class = immutable properties [OK]

Hint: Readonly means no property changes after creation [OK]

Common Mistakes:

• Thinking readonly means private or static
• Assuming properties can be changed anytime
• Confusing readonly with access modifiers

2. Which of the following is the correct syntax to declare a readonly class in PHP?

easy

A. readonly class User { public readonly string $name; }

B. class readonly User {}

C. class User readonly {}

D. readonly class User {}

Solution

1. Step 1: Recall PHP readonly class syntax

   In PHP, the keyword readonly comes before class to declare a readonly class.

2. Step 2: Check each option

   readonly class User {} correctly uses readonly class User {}. class readonly User {} reverses the order, class User readonly {} places readonly incorrectly, and readonly class User { public readonly string $name; } is invalid syntax.

3. Final Answer:

   readonly class User {} -> Option D

4. Quick Check:

   Keyword readonly before class [OK]

Hint: Put readonly before class keyword [OK]

Common Mistakes:

• Placing readonly after class name
• Using readonly as a property only
• Mixing readonly with access modifiers

3. What will be the output of this PHP code?

readonly class Point {
    public int $x;
    public int $y;
    public function __construct(int $x, int $y) {
        $this->x = $x;
        $this->y = $y;
    }
}

$p = new Point(5, 10);
echo $p->x . ', ' . $p->y;

medium

A. 5, 10

B. Error: Cannot modify readonly property

C. 0, 0

D. 5 10

Solution

1. Step 1: Understand readonly class property initialization

   Readonly properties can be set inside the constructor. Here, $x and $y are set to 5 and 10 respectively.

2. Step 2: Check output of echo statement

   The echo prints 5, 10 as expected because the properties were initialized correctly and not modified later.

3. Final Answer:

   5, 10 -> Option A

4. Quick Check:

   Readonly properties set in constructor output values [OK]

Hint: Readonly properties can be set in constructor only [OK]

Common Mistakes:

• Thinking readonly properties cannot be set at all
• Expecting error on property access
• Confusing output format with space instead of comma

4. Identify the error in this PHP code using a readonly class:

readonly class User {
    public string $name;
    public function __construct(string $name) {
        $this->name = $name;
    }
}

$user = new User('Alice');
$user->name = 'Bob';

medium

A. Cannot assign to readonly property after construction

B. Missing readonly keyword before property declaration

C. Constructor must be static in readonly class

D. No error, code runs fine

Solution

1. Step 1: Understand readonly property behavior

   In a readonly class, properties can only be assigned once during construction. Changing them later causes an error.

2. Step 2: Analyze the code

   The code tries to change $user->name after construction, which is not allowed and causes an error.

3. Final Answer:

   Cannot assign to readonly property after construction -> Option A

4. Quick Check:

   Readonly properties are immutable after construction [OK]

Hint: Readonly properties cannot be changed after constructor [OK]

Common Mistakes:

• Thinking readonly properties can be changed anytime
• Confusing readonly class with readonly property keyword
• Ignoring error on property reassignment

5. Given this readonly class, which code snippet correctly creates an immutable user and tries to update the name without error?

readonly class User {
    public string $name;
    public function __construct(string $name) {
        $this->name = $name;
    }
}

Choose the correct approach:

hard

A. $user = new User('Alice');
$user->name = 'Bob';

B. $user = new User('Alice');
$newUser = new User('Bob');

C. $user = new User('Alice');
$user->name = strtoupper($user->name);

D. $user = new User('Alice');
unset($user->name);

Solution

1. Step 1: Understand immutability of readonly classes

   Readonly class properties cannot be changed after construction, so to 'change' data, create a new object with new values.

2. Step 2: Evaluate each option

   $user = new User('Alice'); $user->name = 'Bob'; tries to change property directly - error. $user = new User('Alice'); $user->name = strtoupper($user->name); tries to modify property - error. $user = new User('Alice'); $newUser = new User('Bob'); creates a new User object with new name - correct. $user = new User('Alice'); unset($user->name); tries to unset property - invalid.

3. Final Answer:

   Create a new User object with updated name -> Option B

4. Quick Check:

   Immutable means create new object for changes [OK]

Hint: Change readonly data by creating new object [OK]

Common Mistakes:

• Trying to modify readonly property directly
• Using unset on readonly properties
• Assuming readonly means properties can be changed