[swift-evolution] [Proposal Draft] Flexible memberwise initialization

Matthew Johnson matthew at anandabits.com
Mon Jan 4 20:17:16 CST 2016

> On Jan 4, 2016, at 5:48 PM, Howard Lovatt <howard.lovatt at gmail.com> wrote:
> Yes you can get close, but:
> 1. Its weird that you can only do this in an extension.

This is the way the current implicit initializer works.  It is not synthesized if you define any initializers in the body of the type.  There are good reasons it works this way and the current proposal does not change those rules.

> 2. Its not quite the same as the proposal the current member-wise initialiser does not make the init arguments optional (the arguments themselves do not have defaults), i.e. with your example `let defaultOriginRect = Rect(size: Size(width: 5.0, height: 5.0))` fails whereas it would work for the proposal (this could also be true if the existing struct memberwise init and the new `memberwise init(..)` where changed to provide init argument defaults).

The implicit memberwise initializer currently in the language does not provide defaults for parameters.  This proposal changes that behavior and provides defaults if the the member is a `var` and has an initial value.  

Unfortunately I was not able to find a solution to allow synthesized parameters for `let` members to have default values.  This is because the current semantics for `let` members do not allow the member to be initialized to anything other than the initial value if one is provided.  I am hoping a solution to this will be identified in the future and have suggested one possible mechanism `@default` in the future enhancements section.

> 3. Only ‘really' works for structs, the compiler doesn’t write a member-wise initialiser for classes (just a default initializer).

That is true about the current behavior of the language but is not true with regards to the current proposal.

> 4. Still need the compiler to provide both default and member-wise initialisers, whereas this proposal would allow the existing default and member-wise initialisers to be deprecated and just the new member-wise initialiser would remain which would simplify the language and make it clear what was happening (this could also be true if a `memberwise init(..)` where added and existing compiler written inits removed).

This proposal does not change anything with regard to the default initializer.

>> On 5 Jan 2016, at 10:16 AM, Matthew Johnson <matthew at anandabits.com <mailto:matthew at anandabits.com>> wrote:
>> struct Rect { var origin: Point = Point(), size: Size = Size() }
>> extension Rect {
>>        init(center: Point, size: Size) {
>>            let originX = center.x - (size.width / 2)
>>            let originY = center.y - (size.height / 2)
>>            self.init(origin: Point(x: originX, y: originY), size: size)
>>        }
>> }

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