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Re: How to transfer an allocated object into a function closure

From	usenet@stegropa.de (Stefan Große Pawig)
Newsgroups	comp.lang.c++
Subject	Re: How to transfer an allocated object into a function closure
Date	2025-03-22 22:44 +0100
Organization	Dark Sky
Message-ID	<87ecyoyc3i.fsf@ID-208667.user.individual.de> (permalink)
References	<vr20f7$2e5bi$3@gwaiyur.mb-net.net> <vr2487$21gi7$1@dont-email.me> <vr3fvn$2lage$1@gwaiyur.mb-net.net> <87o6y0vplb.fsf@ID-208667.user.individual.de>

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usenet@stegropa.de (Stefan Große Pawig) writes:
>   Hi Marcel!
>
> Marcel Mueller <news.5.maazl@spamgourmet.org> writes:
>> Am 14.03.25 um 21:40 schrieb Paavo Helde:
>>> In general, passing non-copyable objects to a lambda should be done via
>>> std::move() (requires C++14):
>>> auto u = std::make_unique<FOO>();
>>> bar( [ u = std::move(u) ]() { /*...*/ } );
>>
>> I know. I tried this first, but I still get the same error:
>>
>>
>> auto u(make_unique(child_error, g_error_free));
>> gIdleAdd(new function<void()>([child_dir = move(child_dir), child_error =
>> move(u)]()
>> { OnDirFailed(child_dir.get(), child_error.get()); }));
>>
>>
>> /usr/include/c++/9/bits/std_function.h:176:6: error: use of deleted function 
>> ‘ReadDirectoryWorker::DirScan(gObject<_GFileEnumerator>)::<lambda()>::<lambda>(const
>> ReadDirectoryWorker::DirScan(gObject<_GFileEnumerator>)::<lambda()>&)’
>> ...
>> src/easytag.cc:1144:87: error: use of deleted function ‘std::unique_ptr<_Tp,
>> _Dp>::unique_ptr(const std::unique_ptr<_Tp, _Dp>&) [with _Tp = _GError; _Dp =
>> void (*)(_GError*)]’
>>
>>
>> Not the lambda but std::function is the problem.
>
> The functor that is passed to std::function has to be copy-constructible
> (20.8.11.2.1/7 in C++11).
>
> If you can use C++23, there is std::move_only_function.
> <https://en.cppreference.com/w/cpp/utility/functional/move_only_function>
>
>   Regards,
>     Stefan

BTW, now that https://www.youtube.com/watch?v=clpQVn_LAiM is out: maybe
the MoveWrapper introduced around 30:30 helps in your case, too.

  Regards,
    Stefan

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Thread

How to transfer an allocated object into a function closure Marcel Mueller <news.5.maazl@spamgourmet.org> - 2025-03-14 20:36 +0100
  Re: How to transfer an allocated object into a function closure Paavo Helde <eesnimi@osa.pri.ee> - 2025-03-14 22:40 +0200
    Re: How to transfer an allocated object into a function closure Marcel Mueller <news.5.maazl@spamgourmet.org> - 2025-03-15 10:07 +0100
      Re: How to transfer an allocated object into a function closure Bonita Montero <Bonita.Montero@gmail.com> - 2025-03-15 10:34 +0100
        Re: How to transfer an allocated object into a function closure Marcel Mueller <news.5.maazl@spamgourmet.org> - 2025-03-16 10:02 +0100
          Re: How to transfer an allocated object into a function closure Bonita Montero <Bonita.Montero@gmail.com> - 2025-03-16 14:07 +0100
      Re: How to transfer an allocated object into a function closure usenet@stegropa.de (Stefan Große Pawig) - 2025-03-16 18:43 +0100
        Re: How to transfer an allocated object into a function closure Bonita Montero <Bonita.Montero@gmail.com> - 2025-03-18 13:18 +0100
        Re: How to transfer an allocated object into a function closure usenet@stegropa.de (Stefan Große Pawig) - 2025-03-22 22:44 +0100

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