[swift-evolution] [Pitch] `Int.init(ObjectIdentifier)` and `UInt.init(ObjectIdentifier)` should have a `bitPattern:` label

[Dave Abrahams]

on Fri Jul 15 2016, Arnold Schwaighofer <swift-evolution at swift.org> wrote:

> This is probably uncontroversial.
>
> PR: https://github.com/apple/swift-evolution/pull/434

+1

>> On Jul 13, 2016, at 4:39 PM, Arnold Schwaighofer via swift-evolution <swift-evolution at swift.org> wrote:
>> 
>> 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.
>> 
>> 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: Int.init(bitPattern: ObjectIdentifier).
>> 
>>  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))
>>    }
>>  }
>> 
>> 
>> Motivation
>> ==========
>> 
>> 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)))"
>> 
>> Impact on existing code
>> =======================
>> 
>> Existing code will have to add the bitPattern label.
>> 
>> 
>> Alternatives
>> ============
>> 
>> Leave as is.
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-- 
Dave