visitor.swift

protocol ShapeVisitor {
    func visit(_ rectangle: Rectangle)
    func visit(_ circle: Circle)
}

protocol Shape {
    func accept(_ visitor: ShapeVisitor)
}

struct Rectangle: Shape {
    let width: Float
    let height: Float
    
    func accept(_ visitor: ShapeVisitor) {
        visitor.visit(self)
    }
}

struct Circle: Shape {
    let radius: Float
    
    func accept(_ visitor: ShapeVisitor) {
        visitor.visit(self)
    }
}

final class SquareShapeVisitor: ShapeVisitor {
    var square: Float = 0
    
    func visit(_ rectangle: Rectangle) {
        square = rectangle.width * rectangle.height
    }
    
    func visit(_ circle: Circle) {
        square = circle.radius * circle.radius * Float.pi
    }
}