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

Austin Zheng austinzheng at gmail.com
Mon Jan 25 21:47:44 CST 2016

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.


> On Jan 25, 2016, at 7:36 PM, Ryan Walklin via swift-users <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
> _______________________________________________
> swift-users mailing list
> swift-users at swift.org
> https://lists.swift.org/mailman/listinfo/swift-users

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