A nasty problem with moc

[Perdrix]

I have a class that's derived from a base class and also from QMainWindow.

The base class has:

class QString;

class DSSBase
{
public:
	// severity 2 is QMessageBox::Warning
	virtual void reportError(const QString& message, int severity = 2) = 0;

	inline static DSSBase* instance()
	{
		return theInstance;
	}

	inline static void setInstance(DSSBase* instance)
	{
		ZASSERT(nullptr == theInstance);
		theInstance = instance;
	}

private:
	static inline DSSBase* theInstance{ nullptr };
};

and the derived class has:

public:
inline static DeepSkyStacker* instance()
{
	return reinterpret_cast<DeepSkyStacker *>(DSSBase::instance());
}

So I have to code:

class DeepSkyStacker :
	public DSSBase,
	public QMainWindow
{

But if I do that, moc creates a moc_DeepSkyStacker file that results in the following errors:

1>C:\Users\amonra\Documents\GitHub\DSS\DeepSkyStacker\GeneratedFiles\moc_DeepSkyStacker.cpp(90,43): error C2039: 'staticMetaObject': is not a member of 'DSSBase'
1>C:\Users\amonra\Documents\GitHub\DSS\dssbase.h(44,7): message : see declaration of 'DSSBase'
1>C:\Users\amonra\Documents\GitHub\DSS\DeepSkyStacker\GeneratedFiles\moc_DeepSkyStacker.cpp(134,21): error C2039: 'qt_metacast': is not a member of 'DSSBase'
1>C:\Users\amonra\Documents\GitHub\DSS\dssbase.h(44,7): message : see declaration of 'DSSBase'
1>C:\Users\amonra\Documents\GitHub\DSS\DeepSkyStacker\GeneratedFiles\moc_DeepSkyStacker.cpp(139,20): error C2039: 'qt_metacall': is not a member of 'DSSBase'
1>C:\Users\amonra\Documents\GitHub\DSS\dssbase.h(44,7): message : see declaration of 'DSSBase'

Arrggghhhh!!!!!

[jeremy_k]

https://doc.qt.io/qt-6/moc.html#multiple-inheritance-requires-qobject-to-be-first:

If you are using multiple inheritance, moc assumes that the first inherited class is a subclass of QObject.

[Perdrix]

@jeremy_k Yes, that was immediately obvious.

Is there a clever way to work round this problem? As things stand right now I need DSSBase to be first, otherwise the pointer to base class and to derived class stuff doesn't work.

OTOH moc wants (in this case) QMainWindow first.

[jeremy_k]

My point wasn't that it wasn't obvious, but that this has been a documented requirement for decades. You can forgo moc entirely, either by writing the code manually, or through a 3rd party implementation such as Verdigris.

[Perdrix]

@jeremy_k Neither option there seems helpful

[Christian Ehrlicher]

@Perdrix said in A nasty problem with moc:

otherwise the pointer to base class and to derived class stuff doesn't work.

Then fix your code - dynamic_cast<> is what you're looking for.

[Perdrix]

@Christian-Ehrlicher This seems to work:

class DSSBase
{
public:
	// severity 2 is QMessageBox::Warning
	virtual void reportError(const QString& message, int severity = 2) = 0;
	inline static DSSBase* instance()
	{
		return theInstance;
	}
	inline static void setInstance(DSSBase* instance)
	{
		ZASSERT(nullptr == theInstance);
		theInstance = instance;
	}
private:
	static inline DSSBase* theInstance{ nullptr };
};
class DeepSkyStacker
	public QMainWindow,
	public DSSBase
{
private:
	static inline DeepSkyStacker* theInstance{ nullptr };
public:
	inline static DeepSkyStacker* instance()
	{
		return theInstance;
	}
	inline static void setInstance(DeepSkyStacker* instance)
	{
		ZASSERT(nullptr == theInstance);
		theInstance = instance;
		DSSBase::setInstance(static_cast<DSSBase*>(instance));
	}

I'm prepared to revert to the original with the reinterpret_cast replaced with dynamic_cast as suggested. But that's a task for tomorrow!

[Christian Ehrlicher]

@Perdrix I will not read or comment your code until you add proper formatting with the code tags...

[Perdrix]

@Christian-Ehrlicher Updated

[Perdrix]

@Perdrix Replacing with dynamic_cast in original code also worked.

My this crow is tasty :)

[jeremy_k]

An intermediate class can also work, so moc isn't asked to deal with not having the QObject base first.

class Intermediate : public QObject {
    Q_OBJECT
signals:
    void mySignal();
};

class OtherClass {};

class derived : public OtherClass, public Intermediate { };