[swift-evolution] [swift-evolution-announce] [Review] SE-0017: Change Unmanaged to use UnsafePointer

[Andrew Trick]

> On Apr 29, 2016, at 5:10 PM, Jordan Rose <jordan_rose at apple.com> wrote:
> 
>> 
>> On Apr 28, 2016, at 17:22, Andrew Trick via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> 
>> 
>>> On Apr 28, 2016, at 11:10 AM, Chris Lattner <clattner at apple.com <mailto:clattner at apple.com>> wrote:
>>> 
>>> Hello Swift community,
>>> 
>>> The review of "SE-0017: Change Unmanaged to use UnsafePointer" begins now and runs through May 3. The proposal is available here:
>>> 
>>> 	https://github.com/apple/swift-evolution/blob/master/proposals/0017-convert-unmanaged-to-use-unsafepointer.md <https://github.com/apple/swift-evolution/blob/master/proposals/0017-convert-unmanaged-to-use-unsafepointer.md>
>> 
>> 
>> I have some concerns, but let me just suggest a simple alternative and see what people think...
>> 
>> - Leave the existing from/toOpaque API until we can come up with a better plan for moving away from OpaquePointer.
>> 
>> - Add initializers to avoid boilerplate, but only for "safe" variants of the cast:
>> 
>> extension Unmanaged {
>>  @_transparent
>>  public init(_ from : UnsafePointer<Instance>)
>> 
>>  @_transparent
>>  public init?(_ from : UnsafePointer<Instance>?)
>> }
>> 
>> extension UnsafeMutablePointer where Pointee : AnyObject {
>>  @_transparent
>>  public init(_ from : Unmanaged<Pointee>)
>> 
>>  @_transparent
>>  public init?(_ from : Unmanaged<Pointee>?)
>> }
> 
> This isn’t correct; an UnsafeMutablePointer<Foo> is a pointer to a reference to Foo. Unmanaged<Foo> is a wrapper around ‘unowned Foo’, i.e. it’s just the reference.
> 
> 
>> 
>> - This doesn't solve the stated problem of passing unmanaged pointers to 'void*' imports. Is that really an issue? I believe the correct fix is to stop importing 'void*' as UnsafePointer<Void>. We should have a nominally distinct "opaque" pointer type, 'void*' should be imported as that type, and casting from any UnsafePointer to the opaque pointer type should be inferred and implicit for function arguments. I can send a proposal for eliminating UnsafePointer<Void> next week, but the scope of that proposal will be much broader.
> 
> This is one of the few major use cases for Unmanaged: passing objects through C context pointers. If the type of a ‘void *’ pointer changes, then this proposal should use that type.
> 
> (The other supported uses of Unmanaged are interacting with existing CF APIs that haven’t been audited, and dealing with fields of structs with class type, neither of which use fromOpaque/toOpaque. That last actually isn’t implemented correctly at the moment; we’re assuming those are all strong references, which they aren’t.)

Thanks. Now I understand the purpose of this proposal. My concern was that UnsafePointer<Void> is probably not the right type for opaque pointers. I didn’t want users to begin rewriting COpaquePointer APIs with UnsafePointer<Void>, then force them to rewrite the same calls again back to some other opaque pointer type once we decide what that should be. I was hoping to sidestep my concerns and meet the goal of reduced syntax, but my suggestion was nonsense.

I can start a separate thread next week on replacing UnsafePointer<Void> and see where that goes. This proposal is probably fine as-is, but there might be less impact for users if we change the imported void* type first. 

-Andy

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