OPLOutputBlock and XMLOutputBlock not copyable about libosmium

[joto]

The proposed solution at alex85k/libosmium@b397605 will not work.

Basically the problem is that a buffer needs to be moved into the *OutputBlock class and when the object is copied where does the Buffer go?

[alex85k]

I had a bad feeling about that buffer :) Currently the buffer is re-created, I guess. If we move the buffer or copy the pointer, can we guarantee that the objects will work as desired? I am completely new to move-constructors and had used only the trivial (primitive-type) cases of copy-constructors...

[alex85k]

Maybe this is the bug described in
http://stackoverflow.com/questions/11768273/why-cant-c11-move-a-noncopyable-functor-to-a-stdfunction
(only Visual Studio compiler wants to have copyable object for casting it into function)

/outdated/

[alex85k]

http://stackoverflow.com/questions/16639131/can-stdfunction-be-move-constructed-from-rvalue-reference-to-a-temporary-funct

So, it seems that all functors should be copyable to be used as std::functions?
(Too C++-ish... So that is the reason why Java abandoned type conversions and operation overloadind :) )

/outdated/

[alex85k]

New info:

That is not a copy-constructor problem, that is just the same packaged_task<void()> compiler bug-or-feature!

Close looking at the error:

                std::packaged_task<result_type()> task(std::move(f));

2>          D:\build_d\libosmium7\include\osmium/thread/pool.hpp(167) : see reference to function template instantiation 'std::packaged_task<result_type (void)>::packaged_task<osmium::io::detail::XMLOutputBlock>(_Fty2 &&)' being compiled

[joto]

Yes, as far as I can tell this is the same issue we had with XMLParser. packaged_task uses std::function internally and that needs a copyable functor on Windows. It works with a movable functor on Linux/OSX. Unfortunately the bug fix for XMLParser for Windows broke the OSX compile and I don't know why. So this is a complicated problem and needs more looking into.

[alex85k]

Here is the alternative fix:
alex85k@5318c1f
It compiles, however, I do no believe it works as needed :)

[joto]

Your code creates a dangling reference. Can you verify my commit works?

[alex85k]

Yes, it works! Thank you for another demonstration of C++ magic :)

[alex85k]

Should the solution for #32 (reference) (already added copy-constructor in the same situation) be the same or copying is really needed there?

[joto]

For #32 the solution is reasonably clean, so it should be kept that way. The fix for OPLOutputBlock and XMLOutputBlock is somewhat icky because it adds another redirection that should really not be necessary. But it was the simplest and cleanest I could come up with at the moment. There are some other problems with the threading code so I have to look at that code again anyway, maybe I can come up with a cleaner solution then.