Base class and subclass in Swift - Step-by-Step Execution

Concept Flow - Base class and subclass

Define Base Class

↓

Define Subclass inheriting Base

↓

Create Subclass Object

↓

Access Base and Subclass Properties/Methods

↓

Use Overridden Methods if any

↓

Program Ends

This flow shows how a subclass inherits from a base class, creates an object, and accesses properties and methods.

Execution Sample

Swift

class Animal {
    func sound() -> String {
        return "Some sound"
    }
}

class Dog: Animal {
    override func sound() -> String {
        return "Bark"
    }
}

let pet = Dog()
print(pet.sound())

Defines a base class Animal and subclass Dog that overrides a method; then creates a Dog and prints its sound.

Execution Table

Step	Action	Evaluation	Result
1	Define class Animal with method sound()	No output	Animal class ready
2	Define subclass Dog inheriting Animal, override sound()	No output	Dog class ready
3	Create instance pet of Dog	pet is Dog object	pet created
4	Call pet.sound()	Calls Dog's overridden sound()	"Bark" returned
5	Print pet.sound()	Output to console	Bark
6	Program ends	No further action	Execution complete

💡 Program ends after printing the overridden method result from subclass instance

Variable Tracker

Variable	Start	After Step 3	After Step 4	Final
pet	nil	Dog instance	Dog instance	Dog instance

Key Moments - 2 Insights

Why does pet.sound() print "Bark" and not "Some sound"?

Because pet is an instance of Dog, which overrides the sound() method from Animal. See execution_table step 4 where Dog's method is called.

What happens if Dog does not override sound()?

Then calling pet.sound() would use Animal's sound() method and print "Some sound". This is shown by the inheritance in execution_table steps 2 and 4.

Visual Quiz - 3 Questions

Look at the execution table, what is the output at step 5?

A "Some sound"

B "Bark"

C No output

D Error

💡 Hint

Check the 'Result' column at step 5 in the execution_table.

At which step is the Dog instance created?

A Step 1

B Step 4

C Step 3

D Step 5

💡 Hint

Look at the 'Action' column for instance creation in execution_table.

If Dog did not override sound(), what would pet.sound() return?

A "Some sound"

B "Bark"

C nil

D Error

💡 Hint

Refer to key_moments explanation about method overriding.

Concept Snapshot

Base class defines common behavior.
Subclass inherits and can override methods.
Create subclass object to use overridden methods.
Calling method on subclass uses subclass version if overridden.
If not overridden, base class method runs.

Full Transcript

This example shows how a base class Animal defines a method sound(). The subclass Dog inherits from Animal and overrides the sound() method to return "Bark". When we create an instance pet of Dog and call pet.sound(), the overridden method in Dog runs, printing "Bark". This demonstrates inheritance and method overriding in Swift.