[swift-evolution] [Review] SE-0018 Flexible Memberwise Initialization

Kevin Ballard kevin at sb.org
Thu Jan 7 15:31:59 CST 2016

On Thu, Jan 7, 2016, at 07:12 AM, Matthew Johnson wrote:
>> As for my concern, it's with the following rule:
>>> If the initializer body assigns to a var property that received
>>> memberwise initialization synthesis report a warning. It is unlikely
>>> that overwriting the value provided by the caller is the desired
>>> behavior.
>> I understand why you put this in there, but this is a warning that
>> cannot be suppressed and will make it impossible to use a memberwise
>> initializer for perfectly legitimate cases where you do in fact want
>> to mutate the property after it's been assigned to.
> For normal initializers I agree with you.  However, I think it’s a
> reasonable for callers to assume that if you expose a property via
> memberwise initialization the post-initialization value will match the
> value they provide.  This warning is intended to alert you to the fact
> that you are violating that reasonable assumption.

I think that's a reasonable assumption in many cases, but I don't like
the fact that the feature cannot be used at all in the rare case where
it actually makes sense to mutate the value.

> Do you have an example of where you would want a caller to initialize
> a property, but then overwrite the value they provide *during
> initialization*?

Sure, how about something like a Rect type that always guarantees it's
in "standard" form (e.g. no negative sizes):

struct StandardRect {    var origin: CGPoint    var size: CGSize {
didSet {            // ensure standardized form here        }    }

memberwise init(...) {        if size.width < 0 {            origin.x +=
size.width            size.width = -size.width        }        if
size.height < 0 {            origin.y += size.height
size.height = -size.height        }    } }

Or how about a struct that represents a URL request complete with
headers, and forces the inclusion of Content-Type:

struct URLRequest {    var headers: [String: String] = [:]    // ...
other properties here ...    memberwise init(contentType: String, ...) {
headers["Content-Type"] = contentType    } }

-Kevin Ballard
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