Hi,
As others have pointed out, weak pointers are the tool to use here. I think
it's very hard to make an argument for some sort of weak pointer to unique
(which implies a system that only works with some unique_pointers or an
entirely new unique_ptr type that always has a control block) when you can
just use shared pointers.

Cheers,
Jeremy

On Thu, Feb 13, 2025 at 1:37 PM JOHN MORRISON via Std-Proposals <
std-proposals_at_[hidden]> wrote:

> *A non-owning but self zeroing smart pointer for single ownership *
> We need this when we want hold a persistent reference to an object already
> owned by a unique_ptr to use later if it is still valid. Not having it
> leaves us using raw pointers and we don't have a good art of preventing
> them from dangling (knowing if they are still valid). Our mitigations tend
> to be fragile and often break. It is scary enough to deter us from doing
> such a thing but sometimes we have to and it is here that dangling pointers
> remain a huge hazard.
> It doesn't have to be like that, We can have a smart pointer for the role
> but it will be involve intrusion on the owning unique_ptr declaration so it
> can notify deletes and some reference counting overhead but with that it is
> rock solid (valid or null). Here is a working example:
> https://github.com/make-cpp-nice/ptr_to_unique A smart pointer to an
> object already owned by a unique_ptr. It doesn't own the object but it self
> zeroes when the object is deleted so that it can never dangle.
> The addition of something like this to the Standard Library would complete
> safe smart pointer coverage for heap allocated objects. It is the missing
> piece needed to do this. Its safe encapsulation of an idiom that was
> unreasonably hazardous will also open up new design opportunities.
>
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