On Sat, 13 Dec 2025, 11:04 Mike Reed via Std-Proposals, <
std-proposals_at_[hidden]> wrote:

> Just wrote something like this in my day to day c++ programming job:
>
>
> auto i = somemap.find(key);
>
> auto data = i->second.main_info;
>
> somemap.erase(i);
>
> undecl i; // this would be really nice
>
> // go on to use `data`
>
>
> I'm mindful that I really, really, don't want to accidentally use `i`
> later. And not just me, some other poor dev who comes along later to
> update the "go on to use data" code, during a production panic, and tries
> to access "i->second.meta_info".
>
>
> I have to say, c++ introduced the idea of being able to introduce an
> identifier anywhere in a block, so the idea of being able to remove an
> identifier anywhere after, does have a symmetry that feels right.
>

This is possible today using the _ identifier.

auto _ = somemap.find(key);
auto data = _->second.main_info;
somemap.erase(_);
int _ = 0;

Now you have two _ variables in scope and so it's an error to refer to it.



> I agree that this should be purely about the identifier, not the object
> lifetime.
>
>
> Personally, I don't think it should be allowed to re-use that identifier
> later in the block. To me that has all the same confusions and pitfalls
> that shadowing has.
>
>
> Mike.
>
>
>
> On 13/12/2025 04:02, SD SH via Std-Proposals wrote:
>
> >Moving has to keep the object in a valid state.
>
> The destructor will be called same as origin.
>
> >Calling the destructor would probably lead to double-destruction.
>
> Moving and destructing are additional operations. This feature not be used
> to do these things.
> Calling destructor leads double-destruction. I hope there is a way to end
> objects early and explicitly if we need, but using obj.~T() or
> std::destruct_at(&obj) may be incorrect.
>
> It isn't directly related to the feature we are talking, sorry that I
> talking far ahead.
> ------------------------------
> *发件人:* Std-Proposals <std-proposals-bounces_at_[hidden]>
> <std-proposals-bounces_at_[hidden]> 代表 Sebastian Wittmeier via
> Std-Proposals <std-proposals_at_[hidden]>
> <std-proposals_at_[hidden]>
> *发送时间:* 2025年12月13日 10:47
> *收件人:* std-proposals_at_[hidden] <std-proposals_at_[hidden]>
> <std-proposals_at_[hidden]>
> *抄送:* Sebastian Wittmeier <wittmeier_at_[hidden]>
> <wittmeier_at_[hidden]>
> *主题:* Re: [std-proposals] 回复: 回复: 回复: [PXXXXR0] Add a New Keyword ‘undecl’
>
>
> The cleanest approach is to relocate (trivial relocatability was delayed
> for after C++26) the object into nothing.
>
>
>
> That would probably just destruct it, but the compiler would know, not to
> destruct it a second time.
>
>
>
> Moving has to keep the object in a valid state.
>
>
>
> Calling the destructor would probably lead to double-destruction.
>
>
> -----Ursprüngliche Nachricht-----
> *Von:* SD SH <Z5515zwy_at_[hidden]> <Z5515zwy_at_[hidden]>
> *Gesendet:* Sa 13.12.2025 02:20
> *Betreff:* Re: [std-proposals] 回复: 回复: 回复: [PXXXXR0] Add a New Keyword
> ‘undecl’
> *An:* std-proposals_at_[hidden];
> *CC:* Sebastian Wittmeier <wittmeier_at_[hidden]>
> <wittmeier_at_[hidden]>;
> Thinking of more cases, we can move the object, call the destructor, use
> std::destroy_at or just do nothing until it end, so changing lifetimes is
> not necessary and it will introduce trouble in managing a object.
>
>
> --
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>