"segmentation fault : 11" in generic class

Question

I'm trying to create a simple type erasure struct in Swift 4:

protocol DataProvider
{
  associatedtype ItemType

  subscript(index: Int) -> ItemType { get }
}

struct AnyDataProvider<providerType: DataProvider> : DataProvider
{
  private var _subscript: (_ index: Int) -> providerType.ItemType

  init<P : DataProvider>(_ base: P) where P.ItemType == providerType.ItemType
  {
    _subscript = base.subscript
  }

  subscript(index: Int) -> providerType.ItemType
  {
    return _subscript(index)
  }
}

But I am getting a segmentation fault : 11 on the line that declares the initialiser.

Any ideas, apart from reporting it as a bug?

Answer 1

Yesss!

The problem is that you cannot assign a subscript "method" to a closure reference.

To achieve this, Slava Pestov from Apple showed me the trick of assigning an anonymous closure, which calls the subscript.

Here is the finished code :

protocol DataProvider
{
  associatedtype ItemType

  subscript(index: Int) -> ItemType { get }
}

struct AnyDataProvider<itemType> : DataProvider
{
  private let _subscript: (Int) -> itemType

  subscript(index: Int) -> itemType
  {
    return _subscript(index)
  }

  init<providerType : DataProvider>(_ base: providerType) where providerType.ItemType == itemType
  {
    _subscript = { base[$0] }
  }
}