Swift Academy · Lesson

Existentials with any and Type Erasure

Using any Protocol, the performance cost and when to prefer generics.

What is an Existential?

An existential type boxes any value conforming to a protocol behind a uniform interface, resolved at runtime.

protocol Drawable { func draw() }
struct Line: Drawable { func draw() { print("Line") } }
let d: any Drawable = Line()
d.draw()

Swift 5.7 requires the any keyword before protocol names used as existential types to make the boxing explicit.

func render(_ shape: any Drawable) {
  shape.draw()
}
render(Line())