Qt emit base class signal from derived class

Question

I basically just want to use multiple derived classes to change a member variable of a base class and to forward that value to qml using qproperty, but emitting the signal isn't working and I am not able to make the signal static

car.h

    #include <QObject>

    class Car : public QObject{
        Q_OBJECT
        Q_PROPERTY(int seats MEMBER m_seats NOTIFY updateSeats)

    public:
        explicit Car(QObject *parent = 0);
        ~Car();
        static int m_seats;

    signals:
        void updateSeats();
    };

car.cpp

    #include "car.h"

    Car::Car(QObject *parent) :
        QObject(parent)
    {

    }

    int Car::m_seats = 0;

    Car::~Car(){}

toyota.h

    #include "car.h"

    class Toyota : public Car{
        Q_OBJECT


    public:
        explicit Toyota(QObject *parent = 0);
        ~Toyota();
        Q_INVOKABLE void foundCar();
    };

toyota.cpp

    #include "toyota.h"

    Toyota::Toyota(QObject *parent) 
    {

    }

    Toyota::foundCar(){
        m_seats = 4;
        emit updateSeats();  // This isn't working
    }

    Toyota::~Toyota(){}

main.cpp

    #include <QGuiApplication>
    #include "car.h"
    #include "toyota.h"

    int main(int argc, char *argv[])
    {

        QGuiApplication app(argc, argv);
        Car car = new Car();
        Toyota ty = new Toyota();
        QQmlApplicationEngine engine;
        QQmlContext *ctxt = engine.rootContext();
        ctxt->setContextProperty("car", car);
        ctxt->setContextProperty("toyota", ty);
        engine.load(QUrl(QLatin1String("qrc:/main.qml")));

        app.exec();
        engine.quit();
    }

In qml when I print car.seats after invoking the foundCar function

main.qml

    toyota.foundCar()
    console.log(car.seats)

the output is still 0. The reason the signal being emitted is not updating is probably because it is from a different object (Toyota). So how can I emit the signal of the base class from the derived class?

Amfasis Amfasis · Accepted Answer · 2019-10-02T18:27:40

Your toyota and car properties are two separate ones.

You have to read the seats property from toyota:

main.qml

toyota.foundCar()
console.log(toyota.seats) //<--- here

Update after comment:

Ok, that's a different approach, but in that case I would set the car property to the toyota pointer:

main.cpp

    Car car = new Toyota();

    ctxt->setContextProperty("car", ty);
    ctxt->setContextProperty("toyota", ty);

This probably can be fitted in a overarching class (something like car_manager or car_store) in which you have a list of available cars and a function to select one car as the current, then you update the generic car or current property of that overarching class.

I say this because you will get nasty code when you want to work from the root context and furthermore, root properties don't signal that they are changed

Qt emit base class signal from derived class

1 Answers