[libcxx] r203587 - Implement LWG 2360: 'reverse_iterator::operator*() is unimplementable'. Note that this is a (small) behavior change in the library. Reverse iterators whose base iterators' operator* return references to 'within themselves' have been sacrificed to the greater goal of avoiding data races.

[Richard Smith]

On Tue, Mar 11, 2014 at 2:00 PM, Marshall Clow <mclow.lists at gmail.com>wrote:

> On Mar 11, 2014, at 10:32 AM, Richard Smith <richard at metafoo.co.uk> wrote:
>
>  {
>> -private:
>> -    mutable _Iter __t;
>>
>
> This looks like an ABI break; is that OK?
>
>
> Sigh. Yes; yes, it is.
> This changes the size of a reverse_iterator.
>
> Thanks for catching this, Richard. Apparently I was asleep at the wheel.
>
> I'm open to suggestions here:
> a) I can put the instance variable back, and never use it. No ABI break,
> but an unused iterator in every reverse iterator.
>

Any code that passes a reverse_iterator between TUs built against different
libc++ versions would still not work.


> b) I can leave it the way it is, and say - yeah, it's an ABI break.
> Fortunately, I suspect that very few people hvea a reverse_iterator stored
> in a data file somewhere.
> c) Something else.. ?
>

You could push the choice onto the user: leave it how it was in std::__1,
 add a "correct" reverse_iterator in std::__2, and give people an option to
pick whether they want the newest-available version or the ABI-stable
version (maybe selected by a macro).

(Hopefully this will clarify what I mean:

#ifndef _LIBCPP_UNSTABLE_ABI
_LIBCPP_BEGIN_NAMESPACE_STD // std::__1
  // old reverse_iterator
_LIBCPP_END_NAMESPACE_STD
#else
_LIBCPP_BEGIN_NAMESPACE_STD_UNSTABLE // std::__2
  // new reverse_iterator
_LIBCPP_BEGIN_NAMESPACE_STD_UNSTABLE
#endif

... or even:

_LIBCPP_BEGIN_NAMESPACE_STD_MULTI_ABI // std::__1 or std::__2 depending on
UNSTABLE_ABI macro
class reverse_iterator { //...
#ifdef _LIBCPP_UNSTABLE_ABI
private:
  mutable _Iter __t;
#endif
// ...
};
_LIBCPP_END_NAMESPACE_STD_MULTI_ABI

... with the intent that the __2 ABI is subject to change for now, but when
any platform next gets to do an ABI break, we define __2 as the new stable
ABI, and use __3 as the next unstable ABI).
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