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error: static assertion failed: result type must be constructible from input type when building with -DQT6=ON #11167

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Holzhaus opened this issue Dec 30, 2022 · 11 comments · Fixed by #11180
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error: static assertion failed: result type must be constructible from input type when building with -DQT6=ON #11167

Holzhaus opened this issue Dec 30, 2022 · 11 comments · Fixed by #11180
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bug confirmed

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@Holzhaus
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Bug Description

I cannot build mixxx with -DQT6=ON due to this error:

In file included from /usr/include/c++/12.2.0/memory:66,
                 from /usr/include/qt6/QtCore/qcontainertools_impl.h:19,
                 from /usr/include/qt6/QtCore/qarraydataops.h:9,
                 from /usr/include/qt6/QtCore/qarraydatapointer.h:7,
                 from /usr/include/qt6/QtCore/qlist.h:8,
                 from /usr/include/qt6/QtCore/qmap.h:9,
                 from /usr/include/qt6/QtCore/QMap:1,
                 from /home/jan/Projects/mixxx/src/effects/backends/builtin/autopaneffect.h:3,
                 from /home/jan/Projects/mixxx/src/effects/backends/builtin/autopaneffect.cpp:1:
/usr/include/c++/12.2.0/bits/stl_uninitialized.h: In instantiation of ‘constexpr bool std::__check_constructible() [with _ValueType = unique_ptr<AutoPanGroupState, default_delete<AutoPanGroupState> >; _Tp = const unique_ptr<AutoPanGroupState, default_delete<AutoPanGroupState> >&]’:
/usr/include/c++/12.2.0/bits/stl_uninitialized.h:182:4:   required from ‘_ForwardIterator std::uninitialized_copy(_InputIterator, _InputIterator, _ForwardIterator) [with _InputIterator = __gnu_cxx::__normal_iterator<const unique_ptr<AutoPanGroupState, default_delete<AutoPanGroupState> >*, vector<unique_ptr<AutoPanGroupState, default_delete<AutoPanGroupState> >, allocator<unique_ptr<AutoPanGroupState, default_delete<AutoPanGroupState> > > > >; _ForwardIterator = unique_ptr<AutoPanGroupState, default_delete<AutoPanGroupState> >*]’
/usr/include/c++/12.2.0/bits/stl_uninitialized.h:372:37:   required from ‘constexpr _ForwardIterator std::__uninitialized_copy_a(_InputIterator, _InputIterator, _ForwardIterator, allocator<_Tp>&) [with _InputIterator = __gnu_cxx::__normal_iterator<const unique_ptr<AutoPanGroupState, default_delete<AutoPanGroupState> >*, vector<unique_ptr<AutoPanGroupState, default_delete<AutoPanGroupState> >, allocator<unique_ptr<AutoPanGroupState, default_delete<AutoPanGroupState> > > > >; _ForwardIterator = unique_ptr<AutoPanGroupState, default_delete<AutoPanGroupState> >*; _Tp = unique_ptr<AutoPanGroupState, default_delete<AutoPanGroupState> >]’
/usr/include/c++/12.2.0/bits/stl_vector.h:601:31:   required from ‘constexpr std::vector<_Tp, _Alloc>::vector(const std::vector<_Tp, _Alloc>&) [with _Tp = std::unique_ptr<AutoPanGroupState, std::default_delete<AutoPanGroupState> >; _Alloc = std::allocator<std::unique_ptr<AutoPanGroupState, std::default_delete<AutoPanGroupState> > >]’
/usr/include/qt6/QtCore/qvarlengtharray.h:754:17:   required from ‘void QVLABase<T>::reallocate_impl(qsizetype, void*, qsizetype, qsizetype, const T*) [with T = std::vector<std::unique_ptr<AutoPanGroupState, std::default_delete<AutoPanGroupState> >, std::allocator<std::unique_ptr<AutoPanGroupState, std::default_delete<AutoPanGroupState> > > >; qsizetype = long long int]’
/usr/include/qt6/QtCore/qvarlengtharray.h:211:7:   required from ‘void QVLABase<T>::resize_impl(qsizetype, void*, qsizetype, const T*) [with T = std::vector<std::unique_ptr<AutoPanGroupState, std::default_delete<AutoPanGroupState> >, std::allocator<std::unique_ptr<AutoPanGroupState, std::default_delete<AutoPanGroupState> > > >; qsizetype = long long int]’
/usr/include/qt6/QtCore/qvarlengtharray.h:376:50:   required from ‘void QVarLengthArray<T, Prealloc>::resize(qsizetype) [with T = std::vector<std::unique_ptr<AutoPanGroupState, std::default_delete<AutoPanGroupState> >, std::allocator<std::unique_ptr<AutoPanGroupState, std::default_delete<AutoPanGroupState> > > >; long long int Prealloc = 256; qsizetype = long long int]’
/home/jan/Projects/mixxx/src/engine/channelhandle.h:245:30:   required from ‘void ChannelHandleMap<T>::maybeExpand(int) [with T = std::vector<std::unique_ptr<AutoPanGroupState, std::default_delete<AutoPanGroupState> >, std::allocator<std::unique_ptr<AutoPanGroupState, std::default_delete<AutoPanGroupState> > > >]’
/home/jan/Projects/mixxx/src/engine/channelhandle.h:208:9:   required from ‘T& ChannelHandleMap<T>::operator[](const ChannelHandle&) [with T = std::vector<std::unique_ptr<AutoPanGroupState, std::default_delete<AutoPanGroupState> >, std::allocator<std::unique_ptr<AutoPanGroupState, std::default_delete<AutoPanGroupState> > > >]’
/home/jan/Projects/mixxx/src/effects/backends/effectprocessor.h:203:57:   required from ‘void EffectProcessorImpl<EffectSpecificState>::initializeInputChannel(ChannelHandle, const mixxx::EngineParameters&) [with EffectSpecificState = AutoPanGroupState]’
/home/jan/Projects/mixxx/src/effects/backends/effectprocessor.h:183:10:   required from here
/usr/include/c++/12.2.0/bits/stl_uninitialized.h:90:56: error: static assertion failed: result type must be constructible from input type
   90 |       static_assert(is_constructible<_ValueType, _Tp>::value,
      |                                                        ^~~~~
/usr/include/c++/12.2.0/bits/stl_uninitialized.h:90:56: note: ‘std::integral_constant<bool, false>::value’ evaluates to false
In file included from /usr/include/c++/12.2.0/variant:45,
                 from /usr/include/qt6/QtCore/qtypeinfo.h:7,
                 from /usr/include/qt6/QtCore/qglobal.h:1397,
                 from /usr/include/qt6/QtCore/qiterator.h:7,
                 from /usr/include/qt6/QtCore/qmap.h:8:
/usr/include/c++/12.2.0/bits/stl_construct.h: In instantiation of ‘constexpr void std::_Construct(_Tp*, _Args&& ...) [with _Tp = unique_ptr<AutoPanGroupState, default_delete<AutoPanGroupState> >; _Args = {const unique_ptr<AutoPanGroupState, default_delete<AutoPanGroupState> >&}]’:
/usr/include/c++/12.2.0/bits/stl_uninitialized.h:120:21:   required from ‘constexpr _ForwardIterator std::__do_uninit_copy(_InputIterator, _InputIterator, _ForwardIterator) [with _InputIterator = __gnu_cxx::__normal_iterator<const unique_ptr<AutoPanGroupState, default_delete<AutoPanGroupState> >*, vector<unique_ptr<AutoPanGroupState, default_delete<AutoPanGroupState> >, allocator<unique_ptr<AutoPanGroupState, default_delete<AutoPanGroupState> > > > >; _ForwardIterator = unique_ptr<AutoPanGroupState, default_delete<AutoPanGroupState> >*]’
/usr/include/c++/12.2.0/bits/stl_uninitialized.h:370:30:   required from ‘constexpr _ForwardIterator std::__uninitialized_copy_a(_InputIterator, _InputIterator, _ForwardIterator, allocator<_Tp>&) [with _InputIterator = __gnu_cxx::__normal_iterator<const unique_ptr<AutoPanGroupState, default_delete<AutoPanGroupState> >*, vector<unique_ptr<AutoPanGroupState, default_delete<AutoPanGroupState> >, allocator<unique_ptr<AutoPanGroupState, default_delete<AutoPanGroupState> > > > >; _ForwardIterator = unique_ptr<AutoPanGroupState, default_delete<AutoPanGroupState> >*; _Tp = unique_ptr<AutoPanGroupState, default_delete<AutoPanGroupState> >]’
/usr/include/c++/12.2.0/bits/stl_vector.h:601:31:   required from ‘constexpr std::vector<_Tp, _Alloc>::vector(const std::vector<_Tp, _Alloc>&) [with _Tp = std::unique_ptr<AutoPanGroupState, std::default_delete<AutoPanGroupState> >; _Alloc = std::allocator<std::unique_ptr<AutoPanGroupState, std::default_delete<AutoPanGroupState> > >]’
/usr/include/qt6/QtCore/qvarlengtharray.h:754:17:   required from ‘void QVLABase<T>::reallocate_impl(qsizetype, void*, qsizetype, qsizetype, const T*) [with T = std::vector<std::unique_ptr<AutoPanGroupState, std::default_delete<AutoPanGroupState> >, std::allocator<std::unique_ptr<AutoPanGroupState, std::default_delete<AutoPanGroupState> > > >; qsizetype = long long int]’
/usr/include/qt6/QtCore/qvarlengtharray.h:211:7:   required from ‘void QVLABase<T>::resize_impl(qsizetype, void*, qsizetype, const T*) [with T = std::vector<std::unique_ptr<AutoPanGroupState, std::default_delete<AutoPanGroupState> >, std::allocator<std::unique_ptr<AutoPanGroupState, std::default_delete<AutoPanGroupState> > > >; qsizetype = long long int]’
/usr/include/qt6/QtCore/qvarlengtharray.h:376:50:   required from ‘void QVarLengthArray<T, Prealloc>::resize(qsizetype) [with T = std::vector<std::unique_ptr<AutoPanGroupState, std::default_delete<AutoPanGroupState> >, std::allocator<std::unique_ptr<AutoPanGroupState, std::default_delete<AutoPanGroupState> > > >; long long int Prealloc = 256; qsizetype = long long int]’
/home/jan/Projects/mixxx/src/engine/channelhandle.h:245:30:   required from ‘void ChannelHandleMap<T>::maybeExpand(int) [with T = std::vector<std::unique_ptr<AutoPanGroupState, std::default_delete<AutoPanGroupState> >, std::allocator<std::unique_ptr<AutoPanGroupState, std::default_delete<AutoPanGroupState> > > >]’
/home/jan/Projects/mixxx/src/engine/channelhandle.h:208:9:   required from ‘T& ChannelHandleMap<T>::operator[](const ChannelHandle&) [with T = std::vector<std::unique_ptr<AutoPanGroupState, std::default_delete<AutoPanGroupState> >, std::allocator<std::unique_ptr<AutoPanGroupState, std::default_delete<AutoPanGroupState> > > >]’
/home/jan/Projects/mixxx/src/effects/backends/effectprocessor.h:203:57:   required from ‘void EffectProcessorImpl<EffectSpecificState>::initializeInputChannel(ChannelHandle, const mixxx::EngineParameters&) [with EffectSpecificState = AutoPanGroupState]’
/home/jan/Projects/mixxx/src/effects/backends/effectprocessor.h:183:10:   required from here
/usr/include/c++/12.2.0/bits/stl_construct.h:115:28: error: no matching function for call to ‘construct_at(std::unique_ptr<AutoPanGroupState, std::default_delete<AutoPanGroupState> >*&, const std::unique_ptr<AutoPanGroupState, std::default_delete<AutoPanGroupState> >&)’
  115 |           std::construct_at(__p, std::forward<_Args>(__args)...);
      |           ~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/12.2.0/bits/stl_construct.h:94:5: note: candidate: ‘template<class _Tp, class ... _Args> constexpr decltype (::new(void*(0)) _Tp) std::construct_at(_Tp*, _Args&& ...)’
   94 |     construct_at(_Tp* __location, _Args&&... __args)
      |     ^~~~~~~~~~~~
/usr/include/c++/12.2.0/bits/stl_construct.h:94:5: note:   template argument deduction/substitution failed:
/usr/include/c++/12.2.0/bits/stl_construct.h: In substitution of ‘template<class _Tp, class ... _Args> constexpr decltype (::new(void*(0)) _Tp) std::construct_at(_Tp*, _Args&& ...) [with _Tp = std::unique_ptr<AutoPanGroupState, std::default_delete<AutoPanGroupState> >; _Args = {const std::unique_ptr<AutoPanGroupState, std::default_delete<AutoPanGroupState> >&}]’:
/usr/include/c++/12.2.0/bits/stl_construct.h:115:21:   required from ‘constexpr void std::_Construct(_Tp*, _Args&& ...) [with _Tp = unique_ptr<AutoPanGroupState, default_delete<AutoPanGroupState> >; _Args = {const unique_ptr<AutoPanGroupState, default_delete<AutoPanGroupState> >&}]’
/usr/include/c++/12.2.0/bits/stl_uninitialized.h:120:21:   required from ‘constexpr _ForwardIterator std::__do_uninit_copy(_InputIterator, _InputIterator, _ForwardIterator) [with _InputIterator = __gnu_cxx::__normal_iterator<const unique_ptr<AutoPanGroupState, default_delete<AutoPanGroupState> >*, vector<unique_ptr<AutoPanGroupState, default_delete<AutoPanGroupState> >, allocator<unique_ptr<AutoPanGroupState, default_delete<AutoPanGroupState> > > > >; _ForwardIterator = unique_ptr<AutoPanGroupState, default_delete<AutoPanGroupState> >*]’
/usr/include/c++/12.2.0/bits/stl_uninitialized.h:370:30:   required from ‘constexpr _ForwardIterator std::__uninitialized_copy_a(_InputIterator, _InputIterator, _ForwardIterator, allocator<_Tp>&) [with _InputIterator = __gnu_cxx::__normal_iterator<const unique_ptr<AutoPanGroupState, default_delete<AutoPanGroupState> >*, vector<unique_ptr<AutoPanGroupState, default_delete<AutoPanGroupState> >, allocator<unique_ptr<AutoPanGroupState, default_delete<AutoPanGroupState> > > > >; _ForwardIterator = unique_ptr<AutoPanGroupState, default_delete<AutoPanGroupState> >*; _Tp = unique_ptr<AutoPanGroupState, default_delete<AutoPanGroupState> >]’
/usr/include/c++/12.2.0/bits/stl_vector.h:601:31:   required from ‘constexpr std::vector<_Tp, _Alloc>::vector(const std::vector<_Tp, _Alloc>&) [with _Tp = std::unique_ptr<AutoPanGroupState, std::default_delete<AutoPanGroupState> >; _Alloc = std::allocator<std::unique_ptr<AutoPanGroupState, std::default_delete<AutoPanGroupState> > >]’
/usr/include/qt6/QtCore/qvarlengtharray.h:754:17:   required from ‘void QVLABase<T>::reallocate_impl(qsizetype, void*, qsizetype, qsizetype, const T*) [with T = std::vector<std::unique_ptr<AutoPanGroupState, std::default_delete<AutoPanGroupState> >, std::allocator<std::unique_ptr<AutoPanGroupState, std::default_delete<AutoPanGroupState> > > >; qsizetype = long long int]’
/usr/include/qt6/QtCore/qvarlengtharray.h:211:7:   required from ‘void QVLABase<T>::resize_impl(qsizetype, void*, qsizetype, const T*) [with T = std::vector<std::unique_ptr<AutoPanGroupState, std::default_delete<AutoPanGroupState> >, std::allocator<std::unique_ptr<AutoPanGroupState, std::default_delete<AutoPanGroupState> > > >; qsizetype = long long int]’
/usr/include/qt6/QtCore/qvarlengtharray.h:376:50:   required from ‘void QVarLengthArray<T, Prealloc>::resize(qsizetype) [with T = std::vector<std::unique_ptr<AutoPanGroupState, std::default_delete<AutoPanGroupState> >, std::allocator<std::unique_ptr<AutoPanGroupState, std::default_delete<AutoPanGroupState> > > >; long long int Prealloc = 256; qsizetype = long long int]’
/home/jan/Projects/mixxx/src/engine/channelhandle.h:245:30:   required from ‘void ChannelHandleMap<T>::maybeExpand(int) [with T = std::vector<std::unique_ptr<AutoPanGroupState, std::default_delete<AutoPanGroupState> >, std::allocator<std::unique_ptr<AutoPanGroupState, std::default_delete<AutoPanGroupState> > > >]’
/home/jan/Projects/mixxx/src/engine/channelhandle.h:208:9:   required from ‘T& ChannelHandleMap<T>::operator[](const ChannelHandle&) [with T = std::vector<std::unique_ptr<AutoPanGroupState, std::default_delete<AutoPanGroupState> >, std::allocator<std::unique_ptr<AutoPanGroupState, std::default_delete<AutoPanGroupState> > > >]’
/home/jan/Projects/mixxx/src/effects/backends/effectprocessor.h:203:57:   required from ‘void EffectProcessorImpl<EffectSpecificState>::initializeInputChannel(ChannelHandle, const mixxx::EngineParameters&) [with EffectSpecificState = AutoPanGroupState]’
/home/jan/Projects/mixxx/src/effects/backends/effectprocessor.h:183:10:   required from here
/usr/include/c++/12.2.0/bits/stl_construct.h:96:17: error: use of deleted function ‘std::unique_ptr<_Tp, _Dp>::unique_ptr(const std::unique_ptr<_Tp, _Dp>&) [with _Tp = AutoPanGroupState; _Dp = std::default_delete<AutoPanGroupState>]’
   96 |     -> decltype(::new((void*)0) _Tp(std::declval<_Args>()...))
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/c++/12.2.0/memory:76:
/usr/include/c++/12.2.0/bits/unique_ptr.h:514:7: note: declared here
  514 |       unique_ptr(const unique_ptr&) = delete;
      |       ^~~~~~~~~~
/usr/include/c++/12.2.0/bits/stl_construct.h: In instantiation of ‘constexpr void std::_Construct(_Tp*, _Args&& ...) [with _Tp = unique_ptr<AutoPanGroupState, default_delete<AutoPanGroupState> >; _Args = {const unique_ptr<AutoPanGroupState, default_delete<AutoPanGroupState> >&}]’:
/usr/include/c++/12.2.0/bits/stl_uninitialized.h:120:21:   required from ‘constexpr _ForwardIterator std::__do_uninit_copy(_InputIterator, _InputIterator, _ForwardIterator) [with _InputIterator = __gnu_cxx::__normal_iterator<const unique_ptr<AutoPanGroupState, default_delete<AutoPanGroupState> >*, vector<unique_ptr<AutoPanGroupState, default_delete<AutoPanGroupState> >, allocator<unique_ptr<AutoPanGroupState, default_delete<AutoPanGroupState> > > > >; _ForwardIterator = unique_ptr<AutoPanGroupState, default_delete<AutoPanGroupState> >*]’
/usr/include/c++/12.2.0/bits/stl_uninitialized.h:370:30:   required from ‘constexpr _ForwardIterator std::__uninitialized_copy_a(_InputIterator, _InputIterator, _ForwardIterator, allocator<_Tp>&) [with _InputIterator = __gnu_cxx::__normal_iterator<const unique_ptr<AutoPanGroupState, default_delete<AutoPanGroupState> >*, vector<unique_ptr<AutoPanGroupState, default_delete<AutoPanGroupState> >, allocator<unique_ptr<AutoPanGroupState, default_delete<AutoPanGroupState> > > > >; _ForwardIterator = unique_ptr<AutoPanGroupState, default_delete<AutoPanGroupState> >*; _Tp = unique_ptr<AutoPanGroupState, default_delete<AutoPanGroupState> >]’
/usr/include/c++/12.2.0/bits/stl_vector.h:601:31:   required from ‘constexpr std::vector<_Tp, _Alloc>::vector(const std::vector<_Tp, _Alloc>&) [with _Tp = std::unique_ptr<AutoPanGroupState, std::default_delete<AutoPanGroupState> >; _Alloc = std::allocator<std::unique_ptr<AutoPanGroupState, std::default_delete<AutoPanGroupState> > >]’
/usr/include/qt6/QtCore/qvarlengtharray.h:754:17:   required from ‘void QVLABase<T>::reallocate_impl(qsizetype, void*, qsizetype, qsizetype, const T*) [with T = std::vector<std::unique_ptr<AutoPanGroupState, std::default_delete<AutoPanGroupState> >, std::allocator<std::unique_ptr<AutoPanGroupState, std::default_delete<AutoPanGroupState> > > >; qsizetype = long long int]’
/usr/include/qt6/QtCore/qvarlengtharray.h:211:7:   required from ‘void QVLABase<T>::resize_impl(qsizetype, void*, qsizetype, const T*) [with T = std::vector<std::unique_ptr<AutoPanGroupState, std::default_delete<AutoPanGroupState> >, std::allocator<std::unique_ptr<AutoPanGroupState, std::default_delete<AutoPanGroupState> > > >; qsizetype = long long int]’
/usr/include/qt6/QtCore/qvarlengtharray.h:376:50:   required from ‘void QVarLengthArray<T, Prealloc>::resize(qsizetype) [with T = std::vector<std::unique_ptr<AutoPanGroupState, std::default_delete<AutoPanGroupState> >, std::allocator<std::unique_ptr<AutoPanGroupState, std::default_delete<AutoPanGroupState> > > >; long long int Prealloc = 256; qsizetype = long long int]’
/home/jan/Projects/mixxx/src/engine/channelhandle.h:245:30:   required from ‘void ChannelHandleMap<T>::maybeExpand(int) [with T = std::vector<std::unique_ptr<AutoPanGroupState, std::default_delete<AutoPanGroupState> >, std::allocator<std::unique_ptr<AutoPanGroupState, std::default_delete<AutoPanGroupState> > > >]’
/home/jan/Projects/mixxx/src/engine/channelhandle.h:208:9:   required from ‘T& ChannelHandleMap<T>::operator[](const ChannelHandle&) [with T = std::vector<std::unique_ptr<AutoPanGroupState, std::default_delete<AutoPanGroupState> >, std::allocator<std::unique_ptr<AutoPanGroupState, std::default_delete<AutoPanGroupState> > > >]’
/home/jan/Projects/mixxx/src/effects/backends/effectprocessor.h:203:57:   required from ‘void EffectProcessorImpl<EffectSpecificState>::initializeInputChannel(ChannelHandle, const mixxx::EngineParameters&) [with EffectSpecificState = AutoPanGroupState]’
/home/jan/Projects/mixxx/src/effects/backends/effectprocessor.h:183:10:   required from here
/usr/include/c++/12.2.0/bits/stl_construct.h:119:7: error: use of deleted function ‘std::unique_ptr<_Tp, _Dp>::unique_ptr(const std::unique_ptr<_Tp, _Dp>&) [with _Tp = AutoPanGroupState; _Dp = std::default_delete<AutoPanGroupState>]’
  119 |       ::new((void*)__p) _Tp(std::forward<_Args>(__args)...);
      |       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/12.2.0/bits/unique_ptr.h:514:7: note: declared here
  514 |       unique_ptr(const unique_ptr&) = delete;
      |       ^~~~~~~~~~
make[2]: *** [CMakeFiles/mixxx-lib.dir/build.make:1462: CMakeFiles/mixxx-lib.dir/src/effects/backends/builtin/autopaneffect.cpp.o] Error 1
make[1]: *** [CMakeFiles/Makefile2:298: CMakeFiles/mixxx-lib.dir/all] Error 2
make: *** [Makefile:166: all] Error 2

IIUC the issue is that it tries to make a copy of the unique_ptr when accessing the ChannelHandleMap<std::vector<std::unique_ptr<EffectSpecificState>>>:

auto& outputChannelStates = m_channelStateMatrix[inputChannel];

Version

Latest main (86a280b)

OS

Arch Linux with gcc 12.2.0-1 on AMD64
@Holzhaus Holzhaus added the bug label Dec 30, 2022
@Swiftb0y
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Swiftb0y commented Dec 31, 2022 •
edited
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I think the problem in particular is that the ChannelHandleMap<T>::operator[] might resize and for some reason it resizes by copying the contained vector<unique_ptr> instead of moving. IMO this is a Qt bug (I don't see why it shouldn't be possible to move instead of copy).
Also std::map requires that the map value is CopyConstructible, so the QMap requirements are already laxer than the ones from the standard.
Why are we using a std::vector<unique_ptr<EffectSpecificState>> anyways? Is there any benefit gained from the indirection? Can't we just make it std::vector<EffectSpecificState>?

@daschuer
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daschuer commented Jan 2, 2023

I think we hit a bug introduced in Qt 6.4.0:

It fails here:
https://github.com/qt/qtbase/blob/067b53864112c084587fa9a507eb4bde3d50a6e1/src/corelib/tools/qvarlengtharray.h#L754
The code has been introduced here:
qt/qtbase@a00a1d8#diff-3f4287973382b16af82c02e80740e5efbb0e5753771a19d6309eb2f7f6f6cd6d

Even though the code of question is not used, it is compiled and fails:
if constexpr (std::is_copy_constructible_v<T>) {

becomes
if constexpr (std::is_copy_constructible_v<std::vector<std::unique_ptr<EffectSpecificState>>>>) {
which is true
but the subsequent
static_assert(is_constructible<std::unique_ptr<EffectSpecificState>...
is false.

See also:
https://godbolt.org/z/948xd73xT

I will report a QT bug.

@daschuer
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daschuer commented Jan 2, 2023

https://bugreports.qt.io/browse/QTBUG-109745

@Swiftb0y
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Swiftb0y commented Jan 2, 2023

Thanks for your further research and filing the Qt bug, seems like they're taking it seriously. Conversely, I'm wondering why static_assert(std::is_copy_constructible_v<std::vector<std::unique_ptr<int>>>); compiles even though copying such a vector (fortunately) doesn't work: https://godbolt.org/z/fWPKsh7f8

@daschuer daschuer mentioned this issue Jan 2, 2023
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@daschuer
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daschuer commented Jan 2, 2023

This is a workaround.
#11180

I also had a look into the idea to hold a copy by value but that is difficult because we don't know the effect setup at compile time.

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Swiftb0y commented Jan 3, 2023

I also had a look into the idea to hold a copy by value but that is difficult because we don't know the effect setup at compile time.

Can you elaborate? EffectSpecificState is a template parameter for a concrete type, there is no runtime type-erasure that would require indirection.

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daschuer commented Jan 3, 2023

The effect specific state is created dynamically according to the registered channels. That's why I think we need a container class.
It is cheaper to fill the container with pointers instead of copy or move the whole object.

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Swiftb0y commented Jan 3, 2023

Ah I see, good point... making the entire thing a std::vector<std::optional<EffectSpecificState>> ain't great either because that probably results in a bunch of wasted space.

@marcmutz
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marcmutz commented Jan 3, 2023

An instance of the problem described here: https://quuxplusone.github.io/blog/2020/02/05/vector-is-copyable-except-when-its-not/ I'll see what I can do. As a work-around,wrap the vector:

 template <typename T, typename .... Args>
 struct moveonly_vector : std::vector<T, Args...>
 {
     using base = std::vector<T, Args...>;
     using base::base;
     moveonly_vector(moveonly_vector &&) = default;
     moveonly_vector &operator=(moveonly_vector &&) = default; 
     moveonly_vector(const moveonly_vector &&) = delete;
     moveonly_vector &operator=(moveonly_vector &&) = delete;
};

QVarLengthArray<moveonly_vector<std::unique_ptr<~~~>>> fixed;

@daschuer
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daschuer commented Jan 3, 2023

Thank you for coming back to us. This is almost the same we have implemented in #11180

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The upstream bug has been fixed here:
qt/qtbase@89672ef

@Swiftb0y Swiftb0y added this to Qt6 May 28, 2024
@Swiftb0y Swiftb0y moved this to Done in Qt6 May 28, 2024
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Add class unique_ptr_vector to workaround a QT 6.4 Bug daschuer/mixxx
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@daschuer @Holzhaus @Swiftb0y @marcmutz