Calling super for parent behavior in Swift - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the super keyword do in Swift?

<p>The <code>super</code> keyword calls a method or property from the parent class. It lets a child class use or extend the behavior of its parent.</p>

beginner

Why should you call super when overriding a method?

Calling super ensures the parent class's original behavior runs. This keeps important setup or logic intact while adding new behavior.

beginner

Example: How to call super.viewDidLoad() in a UIViewController subclass?

override func viewDidLoad() {
    super.viewDidLoad()
    // Your code here
}

This calls the parent viewDidLoad() to keep default setup.

intermediate

What happens if you forget to call super in an overridden initializer?

Not calling super.init() can cause your object to be incomplete or crash because the parent class's setup is skipped.

intermediate

Can you call super in a method that is not overridden?

No. You can only call super inside a method that overrides a parent method. Otherwise, the compiler will give an error.

What is the purpose of calling super.method() in Swift?

A To create a new method in the child class

B To run the parent class's version of the method

C To delete the parent class's method

D To call a method from an unrelated class

Calling super.method() runs the parent class's method, preserving its behavior.

What happens if you override viewDidLoad() in a UIViewController subclass but do NOT call super.viewDidLoad()?

A The app will crash immediately

B The method will call itself recursively

C Nothing, it works fine without calling super

D The parent setup code will not run, possibly causing issues

Not calling super.viewDidLoad() skips important setup done by the parent, which can cause unexpected behavior.

Can you call super in a method that does not override a parent method?

A No, only in overridden methods

B Yes, anytime

C Only in initializers

D Only in static methods

You can only call super inside methods that override a parent method.

When overriding an initializer in Swift, what must you do to ensure proper setup?

A Call <code>super.init()</code> to run the parent initializer

B Call <code>self.init()</code> instead

C Do nothing special

D Call <code>super.deinit()</code>

Calling super.init() runs the parent class's initializer to set up inherited properties.

What keyword do you use to access the parent class's method in Swift?

A parent

B base

C super

D self

The keyword super accesses the parent class's methods and properties.

Explain why and how you use super when overriding methods in Swift.

Think about keeping the parent's work while adding your own.

Describe what can go wrong if you forget to call super.init() in a subclass initializer.

Consider what the parent class normally does in its initializer.

Practice

1. What does calling super.methodName() inside an overridden method in Swift do?

easy

A. It runs the parent class's version of the method.

B. It creates a new method unrelated to the parent.

C. It deletes the parent class's method.

D. It calls a method from a sibling class.

Solution

1. Step 1: Understand method overriding

   When a subclass overrides a method, it replaces the parent's method behavior.

2. Step 2: Role of super

   Calling super.methodName() runs the original method from the parent class, preserving its behavior.

3. Final Answer:

   It runs the parent class's version of the method. -> Option A

4. Quick Check:

   Calling super runs parent method [OK]

Hint: Remember: super calls parent method inside override [OK]

Common Mistakes:

• Thinking super creates a new method
• Believing super deletes parent methods
• Confusing super with sibling class calls

2. Which of the following is the correct syntax to call a parent class's method named display() inside an overridden method in Swift?

easy

A. super.display()

B. parent.display()

C. self.super.display()

D. super::display()

Solution

1. Step 1: Identify Swift syntax for super calls

   Swift uses the keyword super followed by dot and method name to call parent methods.

2. Step 2: Check each option

   Only super.display() matches Swift syntax. Others are invalid or from other languages.

3. Final Answer:

   super.display() -> Option A

4. Quick Check:

   super.methodName() is correct syntax [OK]

Hint: Use super.methodName() to call parent method [OK]

Common Mistakes:

• Using parent instead of super
• Adding self before super
• Using :: like in C++

3. Consider this Swift code:

class Parent {
    func greet() {
        print("Hello from Parent")
    }
}

class Child: Parent {
    override func greet() {
        print("Child starts")
        super.greet()
        print("Child ends")
    }
}

let c = Child()
c.greet()

What is the output when c.greet() is called?

medium

A. Child starts\nChild ends\nHello from Parent

B. Child starts\nHello from Parent\nChild ends

C. Hello from Parent

D. Hello from Parent\nChild starts\nChild ends

Solution

1. Step 1: Trace the overridden method

   The greet() method in Child prints "Child starts", then calls super.greet() which prints "Hello from Parent", then prints "Child ends".

2. Step 2: Combine outputs in order

   Output lines are: "Child starts", "Hello from Parent", "Child ends" in that order.

3. Final Answer:

   Child starts Hello from Parent Child ends -> Option B

4. Quick Check:

   super call runs parent method in middle [OK]

Hint: Follow print order: before, super call, after [OK]

Common Mistakes:

• Ignoring order of prints
• Assuming super call is last
• Mixing output lines

4. This Swift code tries to override a method but causes an error:

class Parent {
    func show() {
        print("Parent show")
    }
}

class Child: Parent {
    override func show() {
        print("Child show")
        show()  // Line X
    }
}

What is the problem and how to fix it?

medium

A. Missing super keyword before override.

B. Method name mismatch; rename show() in Child.

C. Cannot override non-virtual method; remove override keyword.

D. Line X causes infinite recursion; fix by calling super.show() instead.

Solution

1. Step 1: Identify recursion cause

   Inside Child.show(), calling show() calls itself again, causing infinite recursion.

2. Step 2: Correct call to parent method

   Use super.show() to call the parent class's method and avoid recursion.

3. Final Answer:

   Line X causes infinite recursion; fix by calling super.show() instead. -> Option D

4. Quick Check:

   Call super.method() to avoid recursion [OK]

Hint: Use super.method() inside override to avoid recursion [OK]

Common Mistakes:

• Calling method without super inside override
• Removing override keyword incorrectly
• Changing method name unnecessarily

5. You have a class hierarchy where Grandparent has a method log(), Parent overrides it and calls super.log(), and Child overrides it too. You want Child.log() to add its own message, then call Parent.log(), which calls Grandparent.log(). How should you write Child.log() to achieve this?

hard

A. override func log() { print("Child log") }

B. override func log() { super.log() ; print("Child log") }

C. override func log() { print("Child log") ; super.log() }

D. override func log() { Parent.log() ; print("Child log") }

Solution

1. Step 1: Understand call chain

   Calling super.log() in Child calls Parent.log(), which calls Grandparent.log().

2. Step 2: Add Child's message before calling super

   To add Child's message first, print it, then call super.log().

3. Final Answer:

   override func log() { print("Child log") ; super.log() } -> Option C

4. Quick Check:

   Print then super.log() for correct order [OK]

Hint: Print first, then call super to keep parent behavior [OK]

Common Mistakes:

• Calling super before print changes order
• Not calling super loses parent logs
• Trying to call Parent.log() directly (not allowed)