[swift-evolution] RFC: Proposed rewrite of Unmanaged<T>

[Joe Groff]

> On Dec 17, 2015, at 5:37 PM, Dave Abrahams via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Hi Everybody,
> 
> We’ve been working on a rewrite of the Unmanaged<T> component, and are soliciting comments.  First, a little background:
> 
> Unmanaged <https://developer.apple.com/library/ios/documentation/Swift/Conceptual/BuildingCocoaApps/WorkingWithCocoaDataTypes.html#//apple_ref/doc/uid/TP40014216-CH6-ID79> is primarily used as a return type from imported CoreFoundation functions that haven’t been annotated with reference-counting semantic information
> A secondary known use-case is as a vehicle for creating a COpaquePointer containing a reference’s bits, e.g. for when you need to pass a reference through C APIs that use “void*” as a universal “give me some info and I’ll give it back to your callback” mechanism.
> 
> We saw several problems with Unmanaged that we wanted to fix:
> It was poorly-named (the reference is managed by somebody, we just aren't representing that management in the type system).
> Its interface was much broader than it needs to be to cover the use-cases
> The purpose of many of its APIs was unclear
> Its documentation was vague and hard to understand.
> It didn’t establish a maximally-safe usage pattern for handling the results of un-annotated CoreFoundation functions.
> 
> The code for the proposed replacement, called UnsafeReference, is here <https://github.com/dabrahams/swift/blob/6eb86b48d150342709da3f3be9c738df23382866/stdlib/public/core/UnsafeReference.swift>, and a commit that updates Swift to use it is here <https://github.com/dabrahams/swift/commit/6eb86b48d150342709da3f3be9c738df23382866>. 
> 
> Maximally Safe Usage
> 
> The recommended usage pattern for handling an UnsafeReference<T> returned by a function CFSomething is to always use the T instance produced by one of the forms:
> 
>     CFSomething(arguments…).release() // when the result is returned at +1
> 
> or
> 
>     CFSomething(arguments…).object    // when the result is returned at +0
> 
> In other words, turn the UnsafeReference<T> into a safe T as quickly as possible, and never store the UnsafeReference<T> in a variable so that it can’t be (mis)used thereafter.
> 
> Points of Discussion
> 
> We’re interested in any feedback you might have, but there are a few points we’d especially like to address:
> 
> The name of the release() method has been contentious.
> 👍: Documentation—or naming conventions such as the “create rule <https://developer.apple.com/library/mac/documentation/CoreFoundation/Conceptual/CFMemoryMgmt/Concepts/Ownership.html>”—normally says something like “you are responsible for releasing the result” in those cases where release() must be called, so there’s a very direct way to know which variant of the recommended usage pattern to employ.
> 👎: Some people who are very familiar with existing manual retain/release programming find the recommended usage pattern really counter-intuitive because they're “using something after calling release on it,” which one never does in Objective-C.
> The alternative names we’ve been able to think of so far are verbose, clumsy, and don’t match up with anything in the documentation of the called function, so this seems like a really hard naming problem.  Better ideas from the community would be most welcome here.
> We’re not sure about the terminology <https://github.com/dabrahams/swift/blob/6eb86b48d150342709da3f3be9c738df23382866/stdlib/public/core/UnsafeReference.swift#L27> (Unretained/Retained/Released) used to precisely describe the semantics of UnsafeReference. We’d like to know if these terms make sense to you or whether you have better ideas.
> We want to know whether the usage pattern recommended above works for you.
> We want to know if the API is sufficiently broad or if there are things you currently get—and need—from Unmanaged that we’ve left out.

`UnsafeReference` is a great name, and the proposed API definitely feels cleaner for the CF use case. Just a couple comments:

- The `bitPattern:` constructors should be between UnsafeReference and Unsafe[Mutable]Pointer<Void>, not COpaquePointer. Let COpaquePointer retire gracefully.
- `Unmanaged` has also been promoted as a solution for people who need to do manual reference counting, for performance or other reasons, so I think we might want to keep the 'retain()' method. Conveniently enough, `release()`-ing and dropping the return value would have the net effect of decrementing the refcount by one, though the admonitions about the `UnsafeReference` become invalid after that point wouldn't hold if you're using `release()` purely for that effect, so maybe `manuallyRetain()`/`manuallyRelease()` would be more appropriate for manual refcounting applications.

-Joe
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