Qt signals & inheritance question

Question

I am relatively new to programming with Qt and had a question. Short version:

How do I inherit signals defined in superclasses?

I am trying to subclass someone else's nicely made QTWidgets to change some of the default behavior:

//Plot3D is a QWidget that defines a signal "rotationChanged"
class matLinePlot : public QObject, public Plot3D {

    Q_OBJECT;
        //etc...
public:
       //etc...

        //Catch Plot3D's signal "rotationChanged" and do some magic with it:
    void initPlot(){
              QObject::connect(this, SIGNAL(rotationChanged( double , double , double )),
            this, SLOT(myRotationChanged(double, double, double)));
    }
};

The problem is in the QObject::connect line. What I would like to do is connect the rotationChanged SIGNAL (from qwt3D_plot.h) to a local function/SLOT - "myRotationChanged". However whenever I do this, at run time I get:

Object::connect: No such signal matLinePlot::rotationChanged(double, double, double)

in C:...\matrixVisualization.h. Of course, I know that rotationChanged isn't in matrixVisualization.h - it's in qwt_plot3D.h, but I thought that since I inherit from Plot3D everything should be fine. But, now that I think about it, since SIGNAL and SLOT are macros, I assume MOC doesn't know/care about inheritance.

Which leads me to my question - since MOC and SIGNALS / SLOTS don't seem to know about inheritance etc: how can I subclass a widget defined somewhere else and get access to the widget's signals?

I have a lot of examples of how to use encapsulation to accomplish something like this, but I'm afraid I don't understand how to do this with inheritance.

Sorry if this is a ridiculous question - I feel like I'm missing something obvious.

Answer 1

I believe that will work if the Plot3D::rotationChanged signal is public or protected. Are you sure the signal is not private?

Edit:

Although I could not find a specific reference, I'll have to conclude that signals are always public. At least a test I did here seemed to indicate that I could connect to a signal even if it was declared in the private section of a class.

I also verified that a signal declared in QObject could be connected using a subclass of QObject in the connect statement so signals are definitely inheritable. As I see in other answers and comments here, the issue must be elsewhere.

Answer 2

Incorrect -> see comments.

I'm using Qtopia at Uni and I believe I recall someone saying something about spacing in the SIGNAL and SLOT parameters for connect.

Try using

QObject::connect(this, SIGNAL(rotationChanged(double,double,double)),
         this, SLOT(myRotationChanged(double,double,double)));

I know it doesn't seem intuitive, as C++ isn't sensitive to whitespace, however I believe it has something to do with some of the magic that Qtopia/QT uses when connecting signals and slots. This may only apply to Qtopia, or I may have heard wrong, but give it a try. Additionally are the signals public or protected and have you included the appropriate header files?

Answer 3

SIGNAL(x) and SLOT(x) are macros that generate string literals. At runtime, slots and signals are matched up using string compares of those generated literals.

(I would have added a comment to mdec's comment, but I don't have enough rep)

Answer 4

I guess the problem is the multiple inheritance:

class matLinePlot : public QObject, public Plot3D
...

I assume that Plot3D is a subclass of QObject? In this case, you should do

class matLinePlot : public Plot3D
...

instead.