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Re: [std-proposals] Laundering arrays of unknown bound

From: Jason Cobb <jason.e.cobb_at_[hidden]>
Date: Sun, 3 Apr 2022 20:07:09 -0400
On 4/3/22 19:33, Jason McKesson via Std-Proposals wrote:
> On Sun, Apr 3, 2022 at 6:55 PM Jason Cobb via Std-Proposals
> <std-proposals_at_[hidden]> wrote:
>> On 4/3/22 09:21, Jason McKesson via Std-Proposals wrote:
>>> I don't believe this is necessary. While `T` may not be implicit
>>> lifetime, arrays of `T` are. So if your allocator returns a `T*`, if
>>> the caller uses that `T*` in a way that would require that `T*` to be
>>> a pointer into an array of `T`, the system will create that array
>>> around the pointer and declare that the pointer was always a pointer
>>> to an array of `T`.
>>> It does this without creating the `T` elements of the array.
>> The text doesn't explicitly say this, but I read the requirements on
>> allocate() as requiring it to return a pointer to storage for the first
>> array element (otherwise why would it be required to create an array
>> object?). If the intent was for it to return a pointer to the array
>> itself, the return type should be T(*)[].
>> But in either case, the question isn't whether implicit object creation
>> can create a T[n] (I think we all agree that it can), it's whether you
>> can get a pointer to that T[n] so that you can return it.
> The pointer will be such a pointer once it gets used as such. Or
> rather, the pointer will have always been such a pointer when you use
> it as such. That's kind of the point of IOC; so long as the memory is
> set to implicitly create objects, you can use any pointer as a pointer
> to an array of `T`.
> All your allocator has to do is make sure that the memory is such that
> objects will be implicitly created within it.

Having the objects be implicitly created and actually getting a pointer
to the implicitly created objects are distinct operations, as shown in
the standard by the distinction between implicit object creation and
returning a pointer to a suitable created object. If all that was
necessary was for the objects to be implicitly created, the "suitable
created object" wording wouldn't need to exist.

Jason Cobb

Received on 2022-04-04 00:07:12