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From: Gavin Lambert (boost_at_[hidden])
Date: 2019-06-27 07:27:19

On 27/06/2019 18:11, JeanHeyd Meneide wrote:
> On Thu, Jun 27, 2019 at 12:23 AM Gavin Lambert wrote:
>> And "UB-based optimization" does not inspire confidence.
> There is nothing I can do for standard pointers other than to give up the
> optimization or to use the UB. (This is one of the reasons it was proposed
> to the standard, asides from being an existing practice with wide
> implementation experience.)

Correct me if I'm wrong, but the clever implementation appears to depend
on several #defines which are entirely undocumented.

Furthermore, one of them selects between whether the private members of
std::unique_ptr have the pointer as the "first" or "second" member (with
assumption that the first member is then also pointer-sized), with no
other options. This does not seem like the sort of thing that users
should be required to define or otherwise know at all -- especially in
code that might be compiled against multiple compilers or multiple
standard library implementations. (And while there is a sanity check,
it is disabled by default, and also merely asserts "oops, I corrupted
your memory" after the fact.)

And the "clever" version of inout_ptr is enabled by default, with all of
these undocumented choices.

I sincerely pity the codebase that includes this code.

Avoidance of a few picoseconds copying a raw pointer twice is simply not
worth this sort of thing.

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