[swift-evolution] [Idea] Set variables to "_" in two-stage init; remove IUO

[Charlie Monroe]

Is there a particular reason for not using lazy var here?

class MySuperClass {
	init() {}
}

class MyClass : MySuperClass {

	let x: Int
	lazy var y: String = self.someInstanceMethod()

	init(x: Int) {
		self.x = x
		super.init()
	}

	func someInstanceMethod() -> String {
		return "Kaboom"
	}
}



> On Jun 15, 2016, at 2:22 PM, Karl via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Say you have the following code. There is a property on MyClass (‘y’) which is derived from other data via an instance method; Two-stage initialisation.
> 
> class MySuperClass {
> 	init() {}
> }
> 
> class MyClass : MySuperClass {
> 
> 	let x : Int
> 	var y : String
> 
> 	init(x: Int) {
> 
> 		self.x = x
> 		super.init()
> 		y = someInstanceMethod()
> 	}
> }
> 
> The code won’t compile because you call super.init before initialising all properties. The way to work-around this so far is to make the type of ‘y’ an implicitly-unwrapped optional. I don’t think it’s very elegant to have to change the type of ‘y’ in this case - it exposes implementation details and implies that the value may sometimes be nil, which is not the case.
> 
> What about if we allowed you to explicitly declare that it’s okay for ‘y’ not to be initialised before calling super.init? Perhaps by assigning it to the underscore:
> 
> self.x = x
> y = _
> super.init()
> y = someInstanceMethod()
> 
> 'y' would still effectively work as an implicitly-unwrapped optional - the value would be initialised to nil, and any attempt to use it before it is initialised would be a fatal runtime error as with an IUO. This also means that it couldn’t be a “let” value.
> 
> This change wouldn’t give you any additional safety when using two-stage initialisation; it would simply not require you to change the type of the property when doing so.
> 
> Thoughts?
> 
> Karl
> _______________________________________________
> swift-evolution mailing list
> swift-evolution at swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution