lazy var fatalError is a solution that instead of ImplicitlyUnwrappedOptional

Undefined.swift

func undefined<T>(_ funcName: String=#function) -> T {
    fatalError("\(funcName) is undefined.")
}

class A {
    lazy var parameter: Int = undefined()
}

let a = A()
a.parameter = 100
a.parameter

let b = A()
b.parameter // fatalError

let c = A()
c.parameter = 100
// c.parameter = nil // cannot insert nil. If you use Int!, you can set nil value again.