I did think of a linked list but secondary pointers can go out of scope too which would leave the list dangling.
________________________________
From: Std-Proposals <std-proposals-bounces_at_[hidden]> on behalf of Ell via Std-Proposals <std-proposals_at_[hidden]>
Sent: Monday, February 24, 2025 9:21 PM
To: std-proposals_at_[hidden] <std-proposals_at_[hidden]>
Cc: Ell <ell_se_at_[hidden]>
Subject: Re: [std-proposals] A non-owning but self zeroing smart pointer for single ownership

On 2/24/25 20:39, Thiago Macieira via Std-Proposals wrote:
> On Monday, 24 February 2025 12:16:16 Brasilia Standard Time Jason McKesson via
> Std-Proposals wrote:
>> As such, the non-aesthetic reason to specifically create a
>> `ptr_to_unique` instead of just using `shared/weak_ptr` is
>> performance: the latter involves book-keeping that (presumably) a
>> `ptr_to_unique` implementation could avoid.
>
> It can't.
>
> If you remove the book-keeping, then ptr_to_unique really *is* a pointer to
> std::unique_ptr. We don't need a new class for that; just use the pointer and
> make sure the std::unique_ptr itself doesn't get destroyed or replaced while
> something is pointing to it.
>
> If you want to be able to have the ptr_to_unique be able to survive the
> destruction or replacement of the unique_ptr they referred to, you need the
> ref-counted block containing something that says the pointed object is still
> alive. And THAT is what shared_ptr + weak_ptr do.

Since unique_ptr doesn't use a control block to begin with, there might
be other implementation options. If we don't care about thread safety,
the ptrs_to_unique could form an (intrusive) linked list, and get
individually cleared when the object gets destroyed. This does make
destruction more expensive, but it has the benefit of not requiring a
separate dynamic allocation, which makes the construction of
ptr_to_unique noexcept. It also makes the actual use of the pointer
comparable to using a raw pointer, instead of having to validate the
pointer against the control block first.

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