[swift-users] returning an array of Protocol-conforming objects from a function

[Ryan Walklin]

Hi Jordan,

Thanks for your reply. I’ve since discovered the differences in memory representation that you mention, and the performance implications of the conversion. 

I’ve worked around this by just using a typed array, as the eventual destination for the array is a Metal buffer via memcpy, so I need a contiguous array.

Regards,

Ryan


> On 27 Jan 2016, at 5:56 AM, Jordan Rose <jordan_rose at apple.com> wrote:
> 
> The problem is that an array of protocols is stored using a different representation from an array of plain structs (or classes, for that matter). Converting from [Float] to [MyProtocol] is therefore an O(N) operation, while a normal upcast between class types is O(1) (by sharing the underlying storage).
> 
> This does keep tripping people up, though, so maybe an explicit "as" should be considered good enough. The typical workarounds are either to use "as [MyProtocol]" to begin with (as Trent mentioned) or to use 'map' to build a new array ("return a.map { $0 }").
> 
> (Any change in this space would need to go through the Swift Evolution Process <https://github.com/apple/swift-evolution/blob/master/process.md>.)
> 
> Best,
> Jordan
> 
> 
>> On Jan 25, 2016, at 19:58, Ryan Walklin via swift-users <swift-users at swift.org <mailto:swift-users at swift.org>> wrote:
>> 
>> Hi Austin, 
>> 
>> Thanks for your reply. Sorry you’re right, my struct types don’t work if they are in in existing array of type [MyType] as below.
>> 
>> I’ve been trying to work round it and found that with the following code: 
>> 
>> extension MyType: MyProtocol {}
>> 
>> var array1: [MyType]
>> 
>> fun foo() -> [MyProtocol] {
>>     return array1
>> }
>> 
>> func foo2 -> [MyProtocol] {
>>     return [myType1, myType2]
>> }
>> 
>> 
>> The foo pattern fails, but the foo2 pattern works. My existing code with struct type was all of type foo2, so I didn’t appreciate the error. As you stay, still seems like a problem with type inference.
>> 
>> Ryan
>> 
>>> On 26 Jan 2016, at 2:47 PM, Austin Zheng <austinzheng at gmail.com <mailto:austinzheng at gmail.com>> wrote:
>>> 
>>> Hi Ryan,
>>> 
>>> Are you sure it works with user-defined struct types? I tried the following code, and got the same error:
>>> 
>>> protocol MyProtocol {}
>>> 
>>> struct Blah : MyProtocol { }
>>> 
>>> func foo() -> [MyProtocol] {
>>>   let a = [Blah(), Blah()] as [Blah]
>>>   return a
>>> }
>>> 
>>> 'class Blah' works fine.
>>> 
>>> Even so, this is not the behavior I would have expected. Perhaps someone else can explain why [T] can't be upcasted to [P] when T is a struct and T : P.
>>> 
>>> Best,
>>> Austin
>>> 
>>> 
>>>> On Jan 25, 2016, at 7:36 PM, Ryan Walklin via swift-users <swift-users at swift.org <mailto:swift-users at swift.org>> wrote:
>>>> 
>>>> Hi All,
>>>> 
>>>> I have a function which returns a closure, which then returns an array of objects conforming to a protocol. 
>>>> 
>>>> ie. 
>>>> 
>>>> protocol MyProtocol {}
>>>> 
>>>> extension Float: MyProtocol {}
>>>> extension OtherType: MyProtocol {}
>>>> 
>>>> When I try to return an array of Floats, I get the error “Cannot convert value of type '[Float]' to closure result type ‘[MyProtocol]”.
>>>> 
>>>> Other struct-based types work fine.
>>>> 
>>>> Is this not possible with standard library types?
>>>> 
>>>> Regards,
>>>> 
>>>> Ryan
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>>> 
>> 
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