SE-0252: Key Path Member Lookup

Evolution Proposal Reviews
28

The standard library could provide this:

@dynamicMemberLookup
public struct UnsafePointer<Pointee> {
  // ...as it is currently...
  public subscript<NewPointee>(dynamicMember: ReferenceWritableKeyPath<Pointee, NewPointee>) -> UnsafeMutablePointer<NewPointee>? {
    // standard library magic goes here--key path internals contain
    // enough data to make this work for stored properties in structs and classes.
  }
  public subscript <NewPointee>(dynamicMember: KeyPath<Pointee, NewPointee>) -> UnsafePointer<NewPointee>? {
    // The same, but immutable for immutable stored properties.
  }
}

So that you could say this:

let personPtr: UnsafePointer<Person> = ...
let postalCodePtr: UnsafePointer<PostalCode>? = personPtr.address.postalCode

(The types in that example could be inferred—I'm just writing them out to clarify the semantics.)

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