Default Parameters and Type Annotation

Swift support default Default Parameter for functions

You can define a default value for any parameter as part of a function’s definition. If a default value is defined, you can omit that parameter when calling the function.

Quick simple example

class Car {
    func drive(nitro: Bool = false) {
        println("Drive. Use nitro: \(nitro)")
    }
}

class RaisingCar : Car {
    override func drive(nitro: Bool = true) {
        println("Drive. Use nitro: \(nitro)")
    }
}

let car = Car()
car.drive()
//Drive. Use nitro: false

let raising = RaisingCar()
raising.drive()
//Drive fast. Use nitro: true

This is pretty easy example. We don't provide any argument to the function so default value is used.
Let's try this one

let rasing2: Car = RaisingCar()
rasing2.drive()
//Drive fast. Use nitro: false

The result is Drive fast. Use nitro: false.
What happens here is:

1. Because of polymorphism RaisingCar drive function is being called
   
2. But it's called with false default parameter. This happens because swift binds default parameters statically. The static type of rasing2 variable is Car, because we have specified it.
   

Thanks to Airspeed Velocity about Great post