Overriding methods and properties in Swift - Step-by-Step Execution

Concept Flow - Overriding methods and properties

Define base class

↓

Define method/property

↓

Define subclass

↓

Override method/property

↓

Create subclass instance

↓

Call overridden method/property

↓

Execute subclass version

↓

End

This flow shows how a subclass replaces a base class method or property with its own version when called.

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())

This code defines a base class Animal with a method sound, then a subclass Dog overrides sound to return "Bark". It creates a Dog instance and prints its sound.

Execution Table

Step	Action	Evaluation	Result
1	Define class Animal with method sound()	Method sound returns "Some sound"	Animal.sound() ready
2	Define subclass Dog overriding sound()	Dog.sound() returns "Bark"	Dog.sound() ready
3	Create instance pet of Dog	pet is Dog instance	pet created
4	Call pet.sound()	Calls overridden Dog.sound()	Returns "Bark"
5	Print result	Output printed	Bark
6	End	No more code	Execution stops

💡 Execution stops after printing overridden method result

Variable Tracker

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

Key Moments - 2 Insights

Why does pet.sound() print "Bark" instead of "Some sound"?

Because pet is an instance of Dog, and Dog overrides the sound() method. See execution_table step 4 where the overridden Dog.sound() is called.

What does the override keyword do?

It tells Swift that the subclass method replaces the base class method. Without override, the compiler gives an error. See execution_table step 2 where Dog.sound() is defined with override.

Visual Quiz - 3 Questions

Look at the execution_table, what is the output printed at step 5?

A "Some sound"

B "Bark"

C "Woof"

D No output

💡 Hint

Check the Result column at step 5 in execution_table.

At which step is the Dog instance created?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the Action column for instance creation in execution_table.

If we remove the override keyword from Dog.sound(), what happens?

A Compiler error occurs

B Code runs normally, prints "Bark"

C Base class method is called instead

D Dog.sound() is ignored

💡 Hint

Recall key_moments explanation about the override keyword necessity.

Concept Snapshot

class BaseClass {
    func method() -> ReturnType { ... }
}

class SubClass: BaseClass {
    override func method() -> ReturnType { ... }
}

- Use override to replace base class methods or properties.
- Calls on subclass instances use overridden versions.
- Compiler enforces override keyword for safety.

Full Transcript

This example shows how a subclass in Swift can replace a method from its base class by overriding it. The base class Animal has a method sound() that returns "Some sound". The subclass Dog overrides sound() to return "Bark". When we create an instance of Dog and call sound(), the overridden Dog version runs, printing "Bark". The override keyword is required to tell Swift this is intentional. Without it, the code will not compile. This behavior lets subclasses customize or extend base class behavior safely.