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.

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@lists.isocpp.org> 代表 Sebastian Wittmeier via Std-Proposals <std-proposals@lists.isocpp.org>
发送时间: 2025年12月13日 10:47
收件人: std-proposals@lists.isocpp.org <std-proposals@lists.isocpp.org>
抄送: Sebastian Wittmeier <wittmeier@projectalpha.org>
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@outlook.com>
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@lists.isocpp.org;
CC: Sebastian Wittmeier <wittmeier@projectalpha.org>;

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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