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@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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