Calling super for parent behavior in Swift - Step-by-Step Execution

Concept Flow - Calling super for parent behavior

Child method called

↓

Child method runs custom code

↓

Call super.method()

↓

Parent method runs

↓

Return to child method

↓

Child method finishes

When a child class method runs, it can do its own work and then call the parent class method using super to keep the parent's behavior.

Execution Sample

Swift

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

class Child: Parent {
    override func greet() {
        print("Hello from Child")
        super.greet()
    }
}

let c = Child()
c.greet()

This code shows a child class method calling super to run the parent class method after its own code.

Execution Table

Step	Action	Output	Notes
1	Create Child instance c		Child object created
2	Call c.greet()		Child's greet() method starts
3	Print "Hello from Child"	Hello from Child	Child prints its message
4	Call super.greet()		Parent's greet() method called
5	Print "Hello from Parent"	Hello from Parent	Parent prints its message
6	Return from super.greet()		Back to Child's greet()
7	Child's greet() finishes		Method ends

💡 Child's greet() method finishes after calling parent's greet() via super

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 7
c	nil	Child instance created	Child instance exists	Child instance exists

Key Moments - 2 Insights

Why do we call super.greet() inside the child's greet() method?

Calling super.greet() runs the parent class's greet() method so the child keeps the parent's behavior in addition to its own, as shown in steps 4 and 5.

What happens if we don't call super.greet() in the child's greet()?

If super.greet() is not called, the parent's greet() method won't run, so only the child's message prints. This is seen by comparing step 5's absence.

Visual Quiz - 3 Questions

Look at the execution table, what is printed at step 3?

A Nothing is printed

B "Hello from Parent"

C "Hello from Child"

D "Hello from Child" and "Hello from Parent"

💡 Hint

Check the Output column at step 3 in the execution_table

At which step does the parent's greet() method run?

A Step 4

B Step 5

C Step 2

D Step 7

💡 Hint

Look for the step where "Hello from Parent" is printed in the execution_table

If we remove super.greet() from the child's greet(), what changes in the output?

A Only "Hello from Child" prints

B Only "Hello from Parent" prints

C Both messages print as before

D No messages print

💡 Hint

Refer to key_moments explanation about skipping super.greet()

Concept Snapshot

Calling super.method() lets a child class run the parent class's method.
Use it inside an overridden method to keep parent behavior.
Without super, parent's code is skipped.
Example: override func greet() { print("Child"); super.greet() }

Full Transcript

This example shows how a child class method can run its own code and then call the parent class method using super. When c.greet() is called, the child prints "Hello from Child", then calls super.greet() which prints "Hello from Parent". This keeps both behaviors. If super.greet() is omitted, only the child's message prints. The execution table traces each step, showing when each print happens and how the method calls flow.

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)