0124-bitpattern-label-for-int-initializer-objectidentfier.md

Int.init(ObjectIdentifier) and UInt.init(ObjectIdentifier) should have a bitPattern: label

• Proposal: SE-0124
• Author: Arnold Schwaighofer
• Review Manager: Chris Lattner
• Status: Implemented (Swift 3)
• Decision Notes: Rationale

Introduction

Int.init(ObjectIdentifier) and UInt.init(ObjectIdentifier) should have a 'bitPattern:’ label to make it clear at the use site that we interpret the value as a bit pattern.

  public func <(lhs: ObjectIdentifier, rhs: ObjectIdentifier) -> Bool {
    return UInt(bitPattern: lhs) < UInt(bitPattern: rhs)
  }

• Swift-evolution thread: Pitch
• Swift Bug: SR-2064
• Branch with change to stdlib: [int_init_objectidentifier_label] (https://github.com/aschwaighofer/swift/tree/int_init_objectidentifier_label)

Motivation

In Swift we have ObjectIdentifier values which uniquely identify a class instance or metatype. They are implemented as a struct which holds the value of the reference to the instance or metatype as a raw pointer.

  /// A unique identifier for a class instance or metatype.
  public struct ObjectIdentifier : Hashable, Comparable {
    internal let _value: Builtin.RawPointer
    ...
  }

We have constructors for Int and UInt that capture this value. These constructors don’t have an argument label.

  extension UInt {
    /// Create a `UInt` that captures the full value of `objectID`.
    public init(_ objectID: ObjectIdentifier) {
      self.init(Builtin.ptrtoint_Word(objectID._value))
    }
  }

  extension Int {
    /// Create an `Int` that captures the full value of `objectID`.
    public init(_ objectID: ObjectIdentifier) {
      self.init(bitPattern: UInt(objectID))
    }
  }

This proposals suggest adding a label bitPattern: to the constructor.

  extension UInt {
    /// Create a `UInt` that captures the full value of `objectID`.
    public init(bitPattern objectID: ObjectIdentifier) {
      self.init(Builtin.ptrtoint_Word(objectID._value))
    }
  }

  extension Int {
    /// Create an `Int` that captures the full value of `objectID`.
    public init(bitPattern objectID: ObjectIdentifier) {
      self.init(bitPattern: UInt(objectID))
    }
  }

Adding a label bitPattern to the constructors makes it clear that we interpret the pointer value as a bit pattern at the use site. It is similar to what we do in other APIs, for example in UInt(bitPattern: UnsafePointer<Void>(value))).

Proposed solution

See above.

Detailed design

We will change the initializers of Int and UInt as shown above. The compiler will suggest corrections in existing code because we mark the old API unavailable.

Impact on existing code

Existing code will have to add the argument label.

Alternatives considered

Leave as is. The API will be inconsistent with other APIs such as the UInt(bitPattern: UnsafePointer<T>) API.