How to emit signal from static callback function

[blizniak83]

Hi,

Is there a way of emitting a signal from static callback function?
Below is the example I found online which apparently should be fine.

// MyCustomClass.h
...
protected:
         static void XN_CALLBACK_TYPE OnPointUpdate(const XnVHandPointContext* pContext, void* cxt);
 
signals:
         mySignal(QVector3D v);
...
 
//=======================
//=======================
 
// MyCustomClass.cpp
...
void XN_CALLBACK_TYPE MyCustomClass::OnPointUpdate(const XnVHandPointContext* pContext, void* cxt)
{
        QVector3D v;
	v.setX(pContext->ptPosition.X);
	v.setY(pContext->ptPosition.Y);
	v.setZ(pContext->ptPosition.Z);
 
// this is where the problems start...
 
        emit static_cast<MyCustomClass*>(cxt)->mySignal(v);
 
}
...

// MyCustomClass.h
...
protected:
         static void XN_CALLBACK_TYPE OnPointUpdate(const XnVHandPointContext* pContext, void* cxt);

signals:
         mySignal(QVector3D v);
...

//=======================
//=======================

// MyCustomClass.cpp
...
void XN_CALLBACK_TYPE MyCustomClass::OnPointUpdate(const XnVHandPointContext* pContext, void* cxt)
{
        QVector3D v;
	v.setX(pContext->ptPosition.X);
	v.setY(pContext->ptPosition.Y);
	v.setZ(pContext->ptPosition.Z);

// this is where the problems start...

        emit static_cast<MyCustomClass*>(cxt)->mySignal(v);

}
...

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When I tried to debug my program to see where it fails, this is what I got:

"Unhandled exception at 0x60893f3a (QtCored4.dll) in test.exe: 0xC0000005: Access violation reading location 0x00000004."

... and that happened in QScopedPointer.h at line 135.

Currently I'm using a global variable to access my 'QVector3D v'.
Is there a better way of overcoming this problem?

Thank you.

Krzysztof

[Lykurg]

Does it work if you call a function of "static_cast<MyCustomClass*>" which emits the signal?

[ChrisW67]

The error message is because the value of "cxt" is not valid, i.e. is not a valid pointer to instance of MyCustomClass. Chances are good that it is 0 (NULL).

[blizniak83]

Thanks Chris,

Not valid pointer, that was the problem and yes it was NULL.
It was just the case of changing NULL to this, and now works fine.