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

[Jordan Rose via swift-users]

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: ) // this works, but 'pos' is now an Airplane
move(item: ) // 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  
> 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 
> > 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 
> >:
> 
> 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 
> > > 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: ) // 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(item: inout P) {
>   item.x += 1
> }
> 
> move(item: ) // this works, since it’s now calling the generic function.
> assert(car.x == 51) // works
> 
> Cheers,
> Guillaume Lessard
> 
> 
> 
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Re: [swift-users] Why inout protocol parameter in function work with strange

[Zhao Xin via swift-users]

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@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 <
> gless...@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@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: ) // 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(item: inout P) {
>   item.x += 1
> }
>
> move(item: ) // this works, since it’s now calling the generic
> function.
> assert(car.x == 51) // works
>
> Cheers,
> Guillaume Lessard
>
>
>
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>
>
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Re: [swift-users] Why inout protocol parameter in function work with strange

[Седых Александр via swift-users]

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 
>:
>
>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: )// 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(item: inout P) {
>  item.x += 1
>}
>
>move(item: )// this works, since it’s now calling the generic function.
>assert(car.x == 51) // works
>
>Cheers,
>Guillaume Lessard
>

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

[Guillaume Lessard via swift-users]

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 
>  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: )// 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(item: inout P) {
  item.x += 1
}

move(item: )// this works, since it’s now calling the generic function.
assert(car.x == 51) // works

Cheers,
Guillaume Lessard

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