Multiple inheritance and signals/slots

AndyBrice

I understood multiple inheritance is ok, as long as only one of the base classes is derived from QObject and this class is inherited first. But I can't get it to work.

My code looks like this:

class DelayedEditSpinBox : public QSpinBox
{
Q_OBJECT

protected slots:

    void onValueChanged();
   ...
};

DelayedEditSpinBox::DelayedEditSpinBox( QWidget* parent )
: QSpinBox( parent )
{
    connect( this, SIGNAL( valueChanged( int ) ), this, SLOT( onValueChanged() ) );
}

class ParameterWidget
{
public:

    ParameterWidget( Parameter* parameter );

    virtual ~ParameterWidget();
    ...
};

class IntParameterWidget : public DelayedEditSpinBox, public ParameterWidget
{
Q_OBJECT

public:

    explicit IntParameterWidget( QWidget* parent, Parameter* parameter );
    ...
};

The issue is that in the DelayedEditSpinBox::onValueChanged() slot never receives the DelayedEditSpinBox::valueChanged( int ) signal when DelayedEditSpinBox is derived from IntParameterWidget . But it does if DelayedEditSpinBox is 'standalone'.

Everything works fine if IntParameterWidget is derived from QSpinBox instead of DelayedEditSpinBox and I connect to the SpinBox::valueChanged( int ) signal.

Any ideas?

Pl45m4

@AndyBrice said in Multiple inheritance and signals/slots:

connect( this, SIGNAL( valueChanged( int ) ), this, SLOT( onValueChanged() ) );

~~Idk if just a typo, but the slot has to accept intvalues, if signal passes an integer.~~

Nevermind. Was confused because of parameter differences :)
(e.g. Int + String wont work)

Is your class DelayedEditSpinBox pure virtual or do you actually create instances?

AndyBrice

@Pl45m4 A slot can ignore the parameters in a signal. I do it all the time.

J.Hilk

@AndyBrice said in Multiple inheritance and signals/slots:

@Pl45m4 A slot can ignore the parameters in a signal. I do it all the time.

That's indeed the case, however, I would suggest using the "new" Qt5 Signal Slot Syntax

You can even specify the signal/slot to use, if to use it from the parent class or not

DelayedEditSpinBox::DelayedEditSpinBox( QWidget* parent )
: QSpinBox( parent )
{
    connect( this, QOverLoad<int>::of(&QSpinBox::valueChanged), this, &DelayedEditSpinBox::onValueChanged);
}

in case you shadow signals or slots

VRonin

What's the implementation of IntParameterWidget::IntParameterWidget?
Does DelayedEditSpinBox have multiple constructors?

AndyBrice

@VRonin

What's the implementation of IntParameterWidget::IntParameterWidget?

IntParameterWidget::IntParameterWidget( QWidget* parent, Parameter* parameter )
: DelayedEditSpinBox( parent ), ParameterWidget( parameter )
{
...
}

It connects to a signal from DelayedEditSpinBox. But that signal is never emitted.

Does DelayedEditSpinBox have multiple constructors?

No,

AndyBrice

@J-Hilk I will try that. Thanks!

AndyBrice

@J-Hilk This syntax doesn't seem to work on MSVC. :0(

Pl45m4

@AndyBrice

Qt Version? Compiler Version?
In general it should work with MSVC
https://wiki.qt.io/New_Signal_Slot_Syntax

AndyBrice

@Pl45m4 MSVC 2015 on Windows 7.

VRonin

I tested the below with Qt 5.12.5, MinGW 7.3.0 64bit on Win 10 and it's working

#include <QSpinBox>
#include <QApplication>
class Parameter;
class DelayedEditSpinBox : public QSpinBox
{
public:
    explicit DelayedEditSpinBox( QWidget* parent =nullptr )
        : QSpinBox( parent )
        {
            connect( this,static_cast<void (QSpinBox::*)(int)>(&QSpinBox::valueChanged), this, &DelayedEditSpinBox::onValueChanged );
        }
    void onValueChanged(){
        qDebug("onValueChanged");
    }
};

class ParameterWidget
{
public:
    ParameterWidget( Parameter* parameter =nullptr ){}
    virtual ~ParameterWidget()=default;
};

class IntParameterWidget : public DelayedEditSpinBox, public ParameterWidget
{

public:

    explicit IntParameterWidget( QWidget* parent=nullptr, Parameter* parameter=nullptr )
        :DelayedEditSpinBox(parent),ParameterWidget(parameter)
    {}

};

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    IntParameterWidget w;
    w.show();
    return a.exec();
}

AndyBrice

@VRonin I will try it, thanks. It might be a while, because I am going to be away from my dev PC.

AndyBrice

It turns out that I was later calling QObject::disconnect() on IntParameterWidget, which also disconnected signals in the base classes, including the QComboBox valueChanged( int ) signal. So it isn't a Qt bug/issue. Just my stupidity. Oops.