On Wednesday, 5 April 2017 at 11:59, Ole Begemann wrote: > On 05.04.2017 00:33, Pavol Vaskovic via swift-dev wrote: > > ## Abstract Base Class with Single Concrete Implementation > > > > What is the purpose of having an internal abstract base `class > > _AnyIteratorBoxBase`[3][], that is only inherited by the `internal final > > class _IteratorBox`? [4][] It is used as the type to store the underlying > > base in the `struct AnyIterator`, but the initializers in `AnyIterator` > > create only concrete instances of `_IteratorBox` and the remaining > > occurrence of `_AnyIteratorBoxBase` is as parameter for `internal init` > > that isn’t used anywhere. My search found no other subclasses of > > `_AnyIteratorBoxBase`, so I guess this is not an extension point. > This is not an answer to your performance questions, but I believe I can > explain why the `_AnyIteratorBoxBase` base class is needed. > > The purpose of `AnyIterator<Element>` is to hide the specific type of > the iterator it wraps. Hence it can't have a property whose type is > generic over the iterator type `I` (because then `AnyIterator` would > have to be generic over `I` too). Hence `AnyIterator` can't have a > property `let _box: _IteratorBox<I>` directly. > > Here's where the base class comes in. `_AnyIteratorBoxBase` is _not_ > generic over `I` and thus erases the iterator type. Only the initializer > must be generic over `I` because it's the only place that needs to > handle the concrete `_IteratorBox<I>` type.
Thanks Ole! This explanation of type erasure makes total sense. I have overlooked they are generic over different axis. But I think I have confirmed the performance implications of this design and created https://bugs.swift.org/browse/SR-4499 to track the issue. As is, all elements that get vended through this are being created using virtual dispatch on generic protocol - super slow. If I recall correctly, there is alternative approach to type erasure using closures, but its possible it has similar performance characteristics? Does this show a need for a different approach to type erasure in Swift? Some language or compiler support? Best regards Pavol Vaskovic _______________________________________________ swift-dev mailing list swift-dev@swift.org https://lists.swift.org/mailman/listinfo/swift-dev
