{
InfoType data;
{
auto i = somemap.find(key);
data = i->second.main_info;
somemap.erase(i);
}
// go on to use `data`
}
Just create an inner block.
------------------------------------------
InfoType fetchNext(StoreType store, Key key)
{
auto i = store.find(key);
auto data = i->second.main_info;
somemap.erase(i);
return data;
}
auto data = fetchNext(someMap, key);
// go on to use `data`
Or extract into a function.
-----Ursprüngliche Nachricht-----
Von: Mike Reed via Std-Proposals <std-proposals@lists.isocpp.org>
Gesendet: Sa 13.12.2025 12:04
Betreff: Re: [std-proposals] 回复: 回复: 回复: 回复: [PXXXXR0] Add a New Keyword ‘undecl’
An: std-proposals@lists.isocpp.org;
CC: Mike Reed <mike.reed10@gmail.com>;
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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