[Interest] Qt5: Problem connecting QTimer::timeout() to lambda

[Stephen Chu]

I find out what QPrivateSignal is for but it's not clear if lambda can 
still be used to connect to such private signals.

If I connect QTimer::timeout() to a slot that takes no argument, it 
compiles fine. But when I try to connect to a lambda function without 
any argument, the results in the mentioned error.

Is this designed behavior? It makes using lambda function as slot really 
hard since there's no mention in documentation which signals are limited 
this way.

FYI. I am using clang 3.1.

On 11/1/12 11:18 AM, Stephen Chu wrote:
> I just updated from Qt5 git and the following code results in compile error:
>
> 	QTimer timer;
> 	connect(&timer, &QTimer::timeout, [=](){});
>
> Error resulted from clang 3.1:
>
> /Qt/5.0/qtbase/include/QtCore/../../src/corelib/kernel/qobjectdefs_impl.h:395:13:
> error: no matching function for call to object of type
> 'PageLogPanel::<lambda at ../../Dropbox/Client2012/PageLogPanel.cpp:47:36>'
>               f((*reinterpret_cast<typename RemoveRef<SignalArgs>::Type
> *>(arg[I+1]))...), ApplyReturnValue<R>(arg[0]);
>               ^
> /Qt/5.0/qtbase/include/QtCore/../../src/corelib/kernel/qobjectdefs_impl.h:434:79:
> note: in instantiation of member function
> 'QtPrivate::FunctorCall<QtPrivate::IndexesList<0>,
> QtPrivate::List<QTimer::QPrivateSignal>, void, PageLogPanel::<lambda at
> ../../Dropbox/Client2012/PageLogPanel.cpp:47:36> >::call' requested here
>               FunctorCall<typename Indexes<N>::Value, SignalArgs, R,
> Function>::call(f, arg);
>
>          ^
> /Qt/5.0/qtbase/include/QtCore/../../src/corelib/kernel/qobject_impl.h:193:17:
> note: in instantiation of function template specialization
> 'QtPrivate::Functor<PageLogPanel::<lambda at
> ../../Dropbox/Client2012/PageLogPanel.cpp:47:36>,
> 1>::call<QtPrivate::List<QTimer::QPrivateSignal>, void>' requested here
>                   FuncType::template call<Args,
> R>(static_cast<QFunctorSlotObject*>(this_)->function, r, a);
>                   ^
> /Qt/5.0/qtbase/include/QtCore/../../src/corelib/kernel/qobject_impl.h:202:71:
> note: in instantiation of member function
> 'QtPrivate::QFunctorSlotObject<PageLogPanel::<lambda at
> ../../Dropbox/Client2012/PageLogPanel.cpp:47:36>, 1,
> QtPrivate::List<QTimer::QPrivateSignal>, void>::impl' requested here
>           explicit QFunctorSlotObject(const Func &f) :
> QSlotObjectBase(&impl), function(f) {}
>                                                                         ^
> /Qt/5.0/qtbase/include/QtCore/../../src/corelib/kernel/qobject.h:285:32:
> note: in instantiation of member function
> 'QtPrivate::QFunctorSlotObject<PageLogPanel::<lambda at
> ../../Dropbox/Client2012/PageLogPanel.cpp:47:36>, 1,
> QtPrivate::List<QTimer::QPrivateSignal>, void>::QFunctorSlotObject'
> requested here
>                              new QtPrivate::QFunctorSlotObject<Func2,
> SignalType::ArgumentCount, typename SignalType::Arguments, typename
> SignalType::ReturnType>(slot),
>                                  ^
> ../../Dropbox/Client2012/PageLogPanel.cpp:47:2: note: in instantiation
> of function template specialization 'QObject::connect<void
> (QTimer::*)(), PageLogPanel::<lambda at
> ../../Dropbox/Client2012/PageLogPanel.cpp:47:36> >' requested here
>           connect(&timer, &QTimer::timeout, [=](){});
>           ^
> ../../Dropbox/Client2012/PageLogPanel.cpp:47:36: note: candidate
> function not viable: requires 0 arguments, but 1 was provided
>           connect(&timer, &QTimer::timeout, [=](){});
>                                             ^
>
> It seems
> http://qt.gitorious.org/qt/qtbase/commit/dee57bc91080740201a0bf0b8c42eb374ee696f3
> added QPrivateSignal to the signal signature?
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