Difference between self and Self in Swift

In Swift, ‘Self’ represents the current Class, Struct, Enum and Protocol. While ‘self’ refers to the current instance itself.

Let’s take examples to get Difference between self and Self in Swift:

Self:

struct Car {
    var color: UIColor
    var wheels: Int
    
    static func createObject() -> Self {
        return Car(color: .white, wheels: 4)
    }
}

 let carObj = Car.createObject() //It will return the ‘Car’ object

The ‘Self’ point in the ‘Car’ class in the example above. Instead, we can use “Car” on Self as shown below.

struct Car {
    var color: UIColor
    var wheels: Int
    
    static func createObject() -> Car {
        return Car(color: .white, wheels: 4)
    }
}

Above both example will work same.

We take another example:

extension Int {
    static var one: Self {
        Self(1.0) 
// Here we are converting float number (1.0) to Int, Self refer to Int Struct
    }
}
print(Int.one) 

//Output - 1

Usage of Self in Protocols:

protocol InstanceCreator {
    static func create() -> Self 
    //Self return an instance of the same type as the conforming type
}

class Car: InstanceCreator {
    var carModel: String?
    var color: UIColor?

    init(carModel: String? = nil, color: UIColor? = nil) {
        self.carModel = carModel 
        //This self point to current class instance
        self.color = color
    }
    
    static func create() -> Self {
        return Car(carModel: "BMW", color: .black) as! Self
    }
}

Here, in ‘InstanceCreator’ protocol – we are using ‘Self’  that return the instance of the same type as the conforming type. 

‘self’- This is a keyword in swift point to current instance of a class or struct within scope of that instance is methods or properties.

self:

class Car {
    var carModel: String?
    var color: UIColor?

    init(carModel: String? = nil, color: UIColor? = nil) {
        self.carModel = carModel
        self.color = color
    }
    
    func displayCar() {
        print("Car Model: \(carModel)")
    }
}

In init method – carModel and colour are distinguished by ‘self’. ‘self’ point to the current class object. self.carModel means carModel object of car class. 

Conclusion:

If we define difference between self and Self in Swift in simple definition:

“The current instance of a class or structure is denoted by “self” (lowercase “s”), whereas the type of the current instance is denoted by “Self” (capital “S”).”