[swift-users] Why inout protocol parameter in function work with strange

[Jordan Rose]

I think you're missing the underlying semantic problem here:

func transform(item: inout Position) {
  item = Airplane()
}

var car = Car(x: 50)
var pos: Position = car
move(item: &pos) // this works, but 'pos' is now an Airplane
move(item: &car) // this doesn't work because 'car' has to stay a Car

Dave Abrahams likes to phrase this lesson as "generics preserve type information, while protocol values erase it". In this case your Car needs to stay a Car, so you need the type information to be preserved.

Hope that helps,
Jordan


> On May 27, 2017, at 03:07, Zhao Xin via swift-users <swift-users at swift.org> wrote:
> 
> Because generic uses `Car` instead of `Position` when running the code, so there is no casting as `car as Position` as in your original code. The `T:Position` part restricts that the type conforms `Position`, but it won't use `Position`, it uses the type.
> 
> Zhaoxin
> 
> On Sat, May 27, 2017 at 4:58 PM, Седых Александр via swift-users <swift-users at swift.org <mailto:swift-users at swift.org>> wrote:
> Thanks. Why generic function don not require memory allocate for inout variable, even if it is protocol type?
> 
> 
> Пятница, 26 мая 2017, 19:35 +03:00 от Guillaume Lessard <glessard at tffenterprises.com <mailto:glessard at tffenterprises.com>>:
> 
> In your example, the compiler needs a parameter of type Position. Car is a type of Position, but they are not interchangeable. See below:
> 
> > On May 26, 2017, at 00:33, Седых Александр via swift-users <swift-users at swift.org <mailto:swift-users at swift.org>> wrote:
> > 
> > protocol Position {
> > var x: Double { getset }
> > }
> > 
> > struct Car: Position {
> > var x: Double
> > }
> > 
> > func move(item: inout Position) {
> > item.x += 1
> > }
> > 
> > var car = Car(x: 50)
> 
> var pos: Position = car
> 
> move(item: &pos) // this works.
> assert(pos.x == 51) // works
> 
> The move function as you wrote it requires the memory representation of a Position variable, which Car does not have; when you assign it to a Position variable, the Car struct gets accessed through an indirection layer. (There was a WWDC talk about this last year or the year before.)
> 
> You may want a generic function instead:
> 
> func move<P: Position>(item: inout P) {
>   item.x += 1
> }
> 
> move(item: &car) // this works, since it’s now calling the generic function.
> assert(car.x == 51) // works
> 
> Cheers,
> Guillaume Lessard
> 
> 
> 
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