You could even write [&] for the one-off lamba.   -----Ursprüngliche Nachricht----- Von:Mike Reed via Std-Proposals <std-proposals_at_[hidden]> Gesendet:Sa 13.12.2025 14:53 Betreff:Re: [std-proposals] 回复: 回复: 回复: 回复: [PXXXXR0] Add a New Keyword ‘undecl’ An:std-proposals_at_[hidden]; CC:Mike Reed <mike.reed10_at_[hidden]>; Just thought     const auto data =     [&somemap, &key]()     {         auto i = somemap.find(key);         auto data = i->second.main_info;         somemap.erase(i);         return data;     }();     Slightly clunkier, but good enough not to need a language change to add undecl.       On 13/12/2025 12:32, Sebastian Wittmeier via Std-Proposals wrote: {    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_at_[hidden]> <mailto:std-proposals_at_[hidden]> 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_at_[hidden] <mailto:std-proposals_at_[hidden]> ; CC: Mike Reed <mike.reed10_at_[hidden]> <mailto:mike.reed10_at_[hidden]> ; 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_at_[hidden]> <mailto:std-proposals-bounces_at_[hidden]> 代表 Sebastian Wittmeier via Std-Proposals <std-proposals_at_[hidden]> <mailto:std-proposals_at_[hidden]> 发送时间: 2025年12月13日 10:47 收件人: std-proposals_at_[hidden] <mailto:std-proposals_at_[hidden]> <std-proposals_at_[hidden]> <mailto:std-proposals_at_[hidden]> 抄送: Sebastian Wittmeier <wittmeier_at_[hidden]> <mailto: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]> <mailto: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] <mailto:std-proposals_at_[hidden]> ; CC: Sebastian Wittmeier <wittmeier_at_[hidden]> <mailto: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. -- Std-Proposals mailing list Std-Proposals_at_[hidden] <mailto:Std-Proposals_at_[hidden]> https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals -- Std-Proposals mailing list Std-Proposals_at_[hidden] https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals