<html><body><div id="edo-message"><div>Maybe there are several initialisers, with different ways of calculating 'y' depending on what you give it.</div><div><br></div><div>Karl<br></div></div><div id="edo-original"><div><br><br><blockquote type="cite" style="margin:1ex 0 0 0;border-left:1px #ccc solid;padding-left:0.5ex;"><div>On Jun 15, 2016 at 2:26 PM, <<a href="mailto:charlie@charliemonroe.net">Charlie Monroe</a>> wrote:<br><br></div><div><pre>Is there a particular reason for not using lazy var here?
<br>
<br>class MySuperClass {
<br> init() {}
<br>}
<br>
<br>class MyClass : MySuperClass {
<br>
<br> let x: Int
<br> lazy var y: String = self.someInstanceMethod()
<br>
<br> init(x: Int) {
<br> self.x = x
<br> super.init()
<br> }
<br>
<br> func someInstanceMethod() -> String {
<br> return "Kaboom"
<br> }
<br>}
<br>
<br>
<br>
<br>> On Jun 15, 2016, at 2:22 PM, Karl via swift-evolution <<a dir="ltr" href="mailto:swift-evolution@swift.org" x-apple-data-detectors="true" x-apple-data-detectors-type="link" x-apple-data-detectors-result="2">swift-evolution@swift.org</a>> wrote:
<br>>
<br>> 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.
<br>>
<br>> class MySuperClass {
<br>> init() {}
<br>> }
<br>>
<br>> class MyClass : MySuperClass {
<br>>
<br>> let x : Int
<br>> var y : String
<br>>
<br>> init(x: Int) {
<br>>
<br>> self.x = x
<br>> super.init()
<br>> y = someInstanceMethod()
<br>> }
<br>> }
<br>>
<br>> 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.
<br>>
<br>> 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:
<br>>
<br>> self.x = x
<br>> y = _
<br>> super.init()
<br>> y = someInstanceMethod()
<br>>
<br>> '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.
<br>>
<br>> 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.
<br>>
<br>> Thoughts?
<br>>
<br>> Karl
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<br>
<br></pre></div></blockquote></div></div></body></html>