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

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On 14.03.2025 21:36, Marcel Mueller wrote:
> I have allocated objects like GFile* or whatever that are not copyable, 
> at least not cheap.
> This objects should be passed to a std::function object that takes the 
> ownership.
> 
> Normally I would use unique_ptr with move semantics. But for some reason 
> the function objects wants to create a copy of the unique_ptr.
> 
> 
> In fact I allocate the function object on the heap and pass it to 
> another thread that takes the ownership.
> It is a GTK application with C++14.
> 
> guint gIdleAdd(std::function<void()>* func, gint priority)
> { return g_idle_add_full(
>      G_PRIORITY_DEFAULT_IDLE,
>      [](void* user_data)
>      { (*static_cast<std::function<void()>*>(user_data))();
>        return G_SOURCE_REMOVE;
>      },
>      func,
>      [](void* user_data)
>      { delete static_cast<std::function<void()>*>(user_data);
>      });
> }
> 
> 
> This works fine:
> (xPtr is a intrusive reference counted smart pointer.)
> 
> gIdleAdd(new function<void()>([that = xPtr<ReplayGainWorker>(this), 
> error]()
>    { that->OnFinished(error); }));
> 
> 
> But this does not work:
> 
> template <typename T, typename D>
> std::unique_ptr<T, D> make_unique(T* ptr, D deleter)
> { return std::unique_ptr<T, D>(ptr, deleter);
> }
> 
> GError* error;
> ...
> 
> gIdleAdd(new std::function<void()>([error = make_unique(error, 
> g_error_free)]()
>    { OnDirFailed(nullptr, error.get()); }));
> 
> 
> g_error_free needs to be invoked exactly once after the function has 
> been called or the event is cancelled, i.e. when the function object is 
> deleted.
> 

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) ]() { /*...*/ } );

HTH
Paavo

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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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