[swift-evolution] [Proposal] Scoped resources (like C# using statement)
clattner at apple.com
Thu Dec 31 13:09:39 CST 2015
> On Dec 30, 2015, at 1:22 PM, Kevin Ballard <kevin at sb.org> wrote:
> A uniquely-owned class that guarantees stack allocation is pretty much the same thing as a move-only value type, isn't it? The only real difference I can think of is classes allow for subclassing.
At this point, we’re talking about two unspecified and hypothetical models, so of course they’re both equivalent and completely different :-)
We should talk about this in more detail later (perhaps next year, perhaps the year after), but I am pretty concerned with saying that unique ownership of classes replaces move-only types. From a programming model perspective (how the programmer thinks about & designs their code) both capabilities are important. You want move-only struct types in various cases and unique ownership of class instances.
For example, IMO, a uniquely-owned class instance has to be on the heap, because it would have to convert to a multiply owned reference in many cases, and “moving” a class from the stack to the heap is, uh, complicated.
> On Wed, Dec 30, 2015, at 01:18 PM, Chris Lattner wrote:
>>> On Dec 30, 2015, at 10:31 AM, Joe Groff <jgroff at apple.com <mailto:jgroff at apple.com>> wrote:
>>>> On Dec 30, 2015, at 10:26 AM, Chris Lattner <clattner at apple.com <mailto:clattner at apple.com>> wrote:
>>>>> On Dec 30, 2015, at 9:53 AM, Joe Groff via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>>>> On Dec 29, 2015, at 8:55 PM, Kevin Ballard via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>>>> An alternative solution is to do what Rust and C++ do, which is to use RAII. Which is to say, instead of introducing a new language construct that's explicitly tied to a scope, you just use a struct to represent the resource that you hold (e.g. a File that represents an open file). Of course, this does require some changes to structs, notably the addition of a deinit. And if structs have a deinit, then they also need to have a way to restrict copies. This is precisely what Rust does; any struct in Rust that implements Drop (the equivalent to deinit) loses the ability to be implicitly copied (a second trait called Clone provides a .clone() method that is the normal way to copy such non-implicitly-copyable structs).
>>>>> deinit doesn't make sense for value types.
>>>> It would if we extended the model for value types to be richer, e.g. to introduce the notion of "move only” structs.
>>> Perhaps, but I feel like it's a more natural extension of our existing model to support uniquely-owned classes though, which would give you all the same benefits.
>> So long as it guarantees no heap allocation for the class instance, ok.
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