`init(capacity:)` on RangeReplaceableCollection

benrimmington · October 29, 2018, 1:59am

I filed SR-1431 (with a confusing title) a few months before the Swift 3.0 release, to suggest making the following APIs standard:

var capacity: Int { get }
init(minimumCapacity: Int)
mutating func removeAll(keepingCapacity: Bool = false)
mutating func reserveCapacity(_ minimumCapacity: Int)

Since then, SE-0165 added the missing APIs to Dictionary and Set.

Using init(capacity:) rather than init(minimumCapacity:) could lead to false assumptions, if I understand the pitch for SE-0223 correctly.

RangeReplaceableCollection has a default implementation of reserveCapacity(_:) which does nothing.

github.com

apple/swift/blob/a05dd3ed26214a66ce11a84d08d20168b65c5c98/stdlib/public/core/RangeReplaceableCollection.swift#L645-L656


      
          /// Prepares the collection to store the specified number of elements, when
          /// doing so is appropriate for the underlying type.
          ///
          /// If you will be adding a known number of elements to a collection, use
          /// this method to avoid multiple reallocations. A type that conforms to
          /// `RangeReplaceableCollection` can choose how to respond when this method
          /// is called. Depending on the type, it may make sense to allocate more or
          /// less storage than requested or to take no action at all.
          ///
          /// - Parameter n: The requested number of elements to store.
          @inlinable
          public mutating func reserveCapacity(_ n: Int) {}

Foundation.Data only has this no-op reserveCapacity(_:) AFAICT. There's an existing Data.init(capacity:) which could be deprecated and renamed.

Should a get-only capacity property also be added, to RangeReplaceableCollection and/or concrete collection types? A default implementation could return the count, if that makes sense.