[cfe-dev] [libc++] use of allocator_traits

Richard Smith via cfe-dev cfe-dev at lists.llvm.org
Fri Apr 13 14:39:11 PDT 2018


This appears to make allocator_traits not really fit for purpose, since
extending the allocator interface is still a breaking change. :-(

On Fri, 13 Apr 2018, 22:34 Marshall Clow via cfe-dev, <
cfe-dev at lists.llvm.org> wrote:

> On Fri, Apr 13, 2018 at 1:23 PM, Eric Fiselier via cfe-dev <
> cfe-dev at lists.llvm.org> wrote:
>
>>
>> On Thu, Apr 12, 2018 at 3:20 AM, Axel Naumann via cfe-dev <
>> cfe-dev at lists.llvm.org> wrote:
>>
>>> Hi,
>>>
>>> I'm confused by libc++'s use of allocator_traits, for instance in
>>> vector:898:
>>>     __alloc_traits::__construct_backward(this->__alloc(),
>>> this->__begin_, this->__end_, __v.__begin_);
>>>
>>> The standard seems to not require the existence of a member called
>>> __construct_backward. Seemingly standard-conforming, user provided
>>> specializations of this trait (for a user provided allocator) thus fail
>>> to compile.
>>>
>>> Am I misinterpreting allocator_trait as a customization point?
>>>
>>
>> I would have thought so, because my understanding was that the allocator
>> was the customization point, where as allocator_traits was the interface
>> used
>> to deal with the custom allocator.
>>
>> However, the standard seems to disagree with me, noting that users
>> may specialize `allocator_traits`.
>>
>>
> interesting. :-(
>
> Please open a bug at bugs.llvm.org, and I'll look into it when I get back
> from the LLVM devcon.
>
> -- Marshall
>
>
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