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Nested class in Qt5



  • Hello everybody ,

    I am using Qt5.14.1 with the MinGW 32-bit compiler. I am trying to nest one classe brightness in the same outer class rgbLed and call variables from brightness class and manage those variable inside a function from the class rgbLed .

    rgbLed and brightness, both are QObject type class. I would like to call variables from brightness into rgbLed methods.

    I was trying to use nested classes but I had problems with Meta Object Compiler, Is there any way to fix this issue?

    So, I write the code as follow:

    #include <QObject>
    #include <QDebug>
    #include <QtQml>
    #include <iostream>
    
    using namespace std;
    
    class brightness : public QObject
    {
        Q_OBJECT
    public:
        explicit brightness(QObject *parent = 0);
    
    public slots:
        void brightnessText(const QString& in);
    private:
        bool flag = 0;
    };
    
    class rgbLed : public QObject
    {
        Q_OBJECT
        Q_PROPERTY(int bright READ bright WRITE setBright NOTIFY brightChanged)
    public:
        explicit rgbLed(QObject *parent = nullptr);
        int regresion_red(int red);
        int regresion_green(int green);
        int regresion_blue(int blue);
    
        Q_INVOKABLE int bright();
        Q_INVOKABLE int setBright(int x);
        Q_INVOKABLE void add(rgbLed *x);
    
    
        int r;
        int g;
        int b;
        int r2;
        int g2;
        int b2;
        int brightValue;
        int myBrightness;
        QStringList list;
        QString my_in;
    
        int red = 0;
        int green = 0;
        int blue = 0;
        int pos = 0 ;
        bool flag = 0;
        int x;
    
    signals:
        void brightChanged(int x);
    
    public slots:
        void rgbRecive(const QString &in);
    };
    
    #endif // RGBLED_H
    

  • Moderators

    @Juancinho_cardecan said in Nested class in Qt5:

    I was trying to use nested classes but I had problems with Meta Object Compiler, Is there any way to fix this issue?

    What problems?

    Generally what you describe should work but I'm half-guessing. I don't know how are you calling the variables, managing them etc.

    If you get moc parsing issues you can split your classes into 2 separate files. But most of the time MOC is clever enough to handle it without such need.



  • This post is deleted!


  • @sierdzio Thanks for the reply.
    In my cpp, I want to display the variable brightValue inside the method rgbRecive that belongs to rgbLed class, brightValue belongs to brightness class and my final porpouse is to call this variable from the function rgbRecive .
    This is the following code:

    void rgbLed::rgbRecive(const QString &in)
    {
     
        QStringList listNew = in.split(",");
        QStringList a_list = listNew[0].split("(");
        QStringList b_list = listNew[2].split(")");
        /
     
        red = a_list[1].toInt(&flag);
        green = listNew[1].toInt(&flag);
        blue = b_list[0].toInt(&flag);
     
        r2 = regresion_red(red);
        g2 = regresion_green(green);
        b2 = regresion_blue(blue);
     
        r = (brightValue * r2) / 10 ;
        g = (brightValue * g2) / 10 ;
        b = (brightValue * b2) / 10 ;
     
        qDebug() << "BrightValue from other class:" << brightValue;
     
    }
     
     
    void rgbLed::brightness::brightnessText(const QString &in)
    {
    ;
        QString Bright = in;
        int brightValue;
        brightValue = Bright.toInt(&flag);
    }
    

    If I nest in my .h the class brightness inside the rgbLed class the code looks like this:

    class rgbLed : public QObject
    {
        Q_OBJECT
        class brightness : public QObject
        {
            Q_OBJECT
        public:
            explicit brightness(QObject *parent = 0);
     
        public slots:
            void brightnessText(const QString& in);
        private:
            bool flag = 0;
        };
     
        Q_PROPERTY(int bright READ bright WRITE setBright NOTIFY brightChanged)
    public:
        explicit rgbLed(QObject *parent = nullptr);
        int regresion_red(int red);
        int regresion_green(int green);
        int regresion_blue(int blue);
     
        Q_INVOKABLE int bright();
        Q_INVOKABLE int setBright(int x);
        Q_INVOKABLE void add(rgbLed *x);
     
     
        int r;
        int g;
        int b;
        int r2;
        int g2;
        int b2;
        int brightValue;
        int myBrightness;
        QStringList list;
        QString my_in;
     
        int red = 0;
        int green = 0;
        int blue = 0;
        int pos = 0 ;
        bool flag = 0;
        int x;
     
    signals:
        void brightChanged(int x);
     
    public slots:
        void rgbRecive(const QString &in);
    };
    

    After I implemented the code, I tried to compiled and got the following error
    Error: Meta object features not supported for nested classes
    [Makefile:821: moc_rgbLed.cpp] Error 1


  • Qt Champions 2019

    @Juancinho_cardecan Why do you want to use nested classes? Especially if both are QObject?


  • Moderators

    Oh that kind of nesting. Your original post has different code.

    MOC does not support that. You need to use another solution, maybe some some namespaces etc. if you really need it.


  • Lifetime Qt Champion

    Hi
    Move the class brightness : public QObject to its own .h and .cpp files
    instead and just include it in class rgbLed.
    If you doing the nested class for easy access, you can just make the classes friends or
    simply provide access functions. Adding access function with Creator is fast. Just right click it and go to the refacor menu.



  • @jsulm I'm doing nested class for easy access to the variable brightValue inside a different class (In this case brightness class)


  • Lifetime Qt Champion

    @Juancinho_cardecan
    Since nested classes will never fly with MOC then you have to use other design.
    Use Friends classes
    https://www.geeksforgeeks.org/friend-class-function-cpp/
    For same level of easy access even its bad for encapsulation
    Using access functions is also a possibility as that is just more clean.



  • @Juancinho_cardecan Tanks for your support.
    In mi case the variables that I would want to manage are in a public slot.
    Could i change my function brightnessText as a friend in order to take values inside the function belonging to a slot?

    Something such as:

    #ifndef RGBLED_H
    #define RGBLED_H
    
    class brightness : public QObject
    {
        Q_OBJECT
    
    public slots:
        friend void brightnessText(const QString& in);
    

    and then accesss to the variables inside that function in my .cpp calling those variables from the other method rgbRecive:

    void rgbLed::brightness::brightnessText(const QString &in)
    {
        QString Bright = in;
        int brightValue;
        brightValue = Bright.toInt(&flag);
    }
    void rgbLed::rgbRecive(const QString &in)
    {
     
        QStringList listNew = in.split(",");
        QStringList a_list = listNew[0].split("(");
        QStringList b_list = listNew[2].split(")");
        /
     
        red = a_list[1].toInt(&flag);
        green = listNew[1].toInt(&flag);
        blue = b_list[0].toInt(&flag);
     
        r2 = regresion_red(red);
        g2 = regresion_green(green);
        b2 = regresion_blue(blue);
     
        r = (brightValue * r2) / 10 ;
        g = (brightValue * g2) / 10 ;
        b = (brightValue * b2) / 10 ;
     
        qDebug() << "BrightValue from other class:" << brightValue;
     
    }
     
     
    void brightness::brightnessText(const QString &in)
    {
    ;
        QString Bright = in;
        int brightValue;
        brightValue = Bright.toInt(&flag);
    }
    

  • Lifetime Qt Champion

    Hi
    Actually i want you to make the outer class a friend of the inner class so when inner class
    is moved to own files, the outer class can access all private variables also in the (was) nested class
    so that the effect is the same as a nested class.

    Doing
    friend void brightnessText(const QString& in);
    allows that function to access private members (in brightness class) when defined outside but im not sure that is what you want/need
    since a normal slot can do that already.

    I though you wanted something like

    class B;
    
    class A
    {
        int val; // this is private
        friend B;
    };
    
    class B
    {
        A obj; // like the nested class.
        void func()
        {
            obj.val = 100; // this is not possible without friend
        }
    };
    

  • Qt Champions 2019

    @Juancinho_cardecan said in Nested class in Qt5:

    I'm doing nested class for easy access to the variable brightValue inside a different class

    This is really not a reason to use nested classes. You should think about clean design instead. Why not use getter/setter to access private fields?



  • @mrjj Hi

    I've been tried to implement your recommendation and also with setters/getters but at time that I want to pass the brightValue from the function void brightness::brightnessText(const QString &in) into the function void rgbLed::rgbRecive(const QString &in) unfortunately the variable b takes a random value inside the method rgbRecive:

    This is the following code:

    class rgbLed;
    
    class brightness : public QObject
    {
        Q_OBJECT
    public:
        explicit brightness(QObject *parent = 0);
        friend rgbLed;
        void setBrightValue(int newbrightValueMember)
        {
            brightValueMember = newbrightValueMember;
        };
        int getBrightValue()
        {
            return brightValueMember;
        }
    
        bool flag = 0;
        int brightValueMember;
    
    public slots:
        void brightnessText(const QString& in);
    };
    
    class rgbLed : public QObject
    {
        Q_OBJECT
        Q_PROPERTY(int bright READ brigfht WRITE setBright NOTIFY brightChanged)
    public:
        explicit rgbLed(QObject *parent = nullptr);
        brightness obj;
        int regresion_red(int red);
        int regresion_green(int green);
        int regresion_blue(int blue);
    
        Q_INVOKABLE int bright();
        Q_INVOKABLE int setBright(int x);
        Q_INVOKABLE void add(rgbLed *x);
    
    
        int r;
        int g;
        int b;
        int r2;
        int g2;
        int b2;
        int brightValue;
        int myBrightness;
        QStringList list;
        QString my_in;
    
        int red = 0;
        int green = 0;
        int blue = 0;
        int pos = 0 ;
        bool flag = 0;
        int x;
    
    signals:
        void brightChanged(int x);
    
    public slots:
        void rgbRecive(const QString &in);
    };
    
    

    and in my .cpp:

    void brightness::brightnessText(const QString &in)
    {
    
        QString Bright = in;
        int brightValue;
        brightValue = Bright.toInt(&flag);
        setBrightValue(brightValue);
    
        qDebug() << "bright: " << brightValue;
    }
    
    void rgbLed::rgbRecive(const QString &in)
    {
        qDebug() << "ColorPicked:" << in;
    
        QStringList listNew = in.split(",");
        QStringList a_list = listNew[0].split("(");
        QStringList b_list = listNew[2].split(")");
    
        int b = obj.getBrightValue();
        qDebug() << "Value: " << b;
    }
    

    Could anyone see why b is taken a random value?


  • Qt Champions 2019

    @Juancinho_cardecan Where do you call brightness::brightnessText?
    Do you call it on rgbLed::obj or maybe on another brightness instance?



  • @jsulm I called here after your words but the value doesn't change:

    void rgbLed::rgbRecive(const QString &in)
    {
    qDebug() << "ColorPicked:" << in;

    QStringList listNew = in.split(",");
    QStringList a_list = listNew[0].split("(");
    QStringList b_list = listNew[2].split(")");
    
    obj.brightnessText(in);
    int b = obj.getBrightValue();
    qDebug() << "Value: " << b;
    

    }


  • Qt Champions 2019

    @Juancinho_cardecan Called where? And did you call it on rgbLed::obj?



  • This post is deleted!


  • @jsulm No I din't call it on rgbLed::obj, I called it inside the function void rgbLed::rgbRecive


  • Lifetime Qt Champion

    Hi
    So here

    void rgbLed::rgbRecive(const QString &in)
    {
    obj.brightnessText(in);
    int b = obj.getBrightValue();
    

    b is not what you expect ?



  • @mrjj yes it's not what I'm expecting, it might be changing all the time but the value is zero always.


  • Lifetime Qt Champion

    @Juancinho_cardecan
    I would check

    brightValue = Bright.toInt(&flag);
    (in void brightness::brightnessText(const QString &in))
    and see if flag is false as that mean text to Int failed.



  • @mrjj the flag is true and the value of brightValue is passing correctly, then the text to Int didn't failed, i dont know what is happening because if the values from QML are passing correctly, I'm calling in the wrong way the function brightnessText inside:

    cpp file:

    void rgbLed::rgbRecive(const QString &in)
    {
         my_brightness.brightnessText(in);
        int b = my_brightness.getBrightValue();
        qDebug() << "Value: " << b;
    }
    
    void brightness::brightnessText(const QString &in)
    {
        QString Bright = in;
        int brightValue;
        brightValue = Bright.toInt(&flag);
        qDebug() << "brightValue: " << brightValue;
        qDebug() << "flag: " << flag;
        setBrightValue(brightValue);
    
        qDebug() << "bright: " << brightValue;
    }
    
    
    
    

    main.cpp:

    int main(int argc, char *argv[])
    {
    
        QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
        QGuiApplication app(argc, argv);
       
    
        QQmlApplicationEngine engine;
        engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
        if (engine.rootObjects().isEmpty())
                return -1;
    
        //Root Object to find Items in QML
        QObject* root = engine.rootObjects()[0];
        assert(root != nullptr);
    
        // The QML Item to which we want to inject our QML-Item-loaded-from-file
        QQuickItem* colorSelector = qobject_cast<QQuickItem*>(root->findChild<QObject*>("colorSelector"));
        assert(colorSelector != nullptr);
    
    
        // Load the QML file to a component
        QString qml_path = "ColorSelector.qml";
        QQmlComponent comp(&engine, QUrl::fromLocalFile(qml_path));
    
        QScopedPointer<rgbLed> slider (new rgbLed);
        engine.rootContext()->setContextProperty("slider", slider.data());
        QObject *topLevel = engine.rootObjects().value(0);
        QQuickWindow *window = qobject_cast<QQuickWindow *>(topLevel);
    
        // Create an instance of the component
        rgbLed my_rgbLed;//Creation an Object instance
        brightness my_brightness;
    
        //Code to conecct a signal with the slot
        QObject::connect(colorSelector, SIGNAL(colorChanged(QString)), &my_rgbLed, SLOT(rgbRecive(QString)));
    
        // connect our QML signal to our C++ slot
          QObject::connect(window, SIGNAL(submitTextField(QString)), &my_brightness, SLOT(brightnessText(QString)));
    
        return app.exec();
    }
    
    
    

    header file:

    class rgbLed;
    
    class brightness : public QObject
    {
        Q_OBJECT
    public:
        explicit brightness(QObject *parent = 0);
        friend rgbLed;
        void setBrightValue(int newbrightValueMember)
        {
            brightValueMember = newbrightValueMember;
        };
        int getBrightValue()
        {
            return brightValueMember;
        }
    
        bool flag = 0;
        int brightValueMember;
    
    public slots:
        void brightnessText(const QString& in);
    };
    
    
    class rgbLed : public QObject
    {
        Q_OBJECT
        Q_PROPERTY(int bright READ brigfht WRITE setBright NOTIFY brightChanged)
    public:
        explicit rgbLed(QObject *parent = nullptr);
        brightness my_brightness;
        int regresion_red(int red);
        int regresion_green(int green);
        int regresion_blue(int blue);
    
        Q_INVOKABLE int bright();
        Q_INVOKABLE int setBright(int x);
        Q_INVOKABLE void add(rgbLed *x);
    
    
        int r;
        int g;
        int b;
        int r2;
        int g2;
        int b2;
        int brightValue;
        int myBrightness;
        QStringList list;
        QString my_in;
    
        int red = 0;
        int green = 0;
        int blue = 0;
        int pos = 0 ;
        bool flag = 0;
        int x;
    
    signals:
        void brightChanged(int x);
    
    public slots:
        void rgbRecive(const QString &in);
    }
    

  • Lifetime Qt Champion

    Hi
    I think you have 2 instances of brightness called my_brightness

    void rgbLed::rgbRecive(const QString &in)
    {
    my_brightness.brightnessText(in); <<< is the SAME my_brightness as you have in main.cpp?
    int b = my_brightness.getBrightValue();
    qDebug() << "Value: " << b;
    }

    so
    rgbLed my_rgbLed;//Creation an Object instance
    brightness my_brightness;

    how does the my_brightness from outside come into

    void rgbLed::rgbRecive(const QString &in) ?
    It seems it has its own ?
    So one is being updated but other is not ?

    In the original code, it was called

    class rgbLed : public QObject
    {
        Q_OBJECT
        Q_PROPERTY(int bright READ brigfht WRITE setBright NOTIFY brightChanged)
    public:
        explicit rgbLed(QObject *parent = nullptr);
        brightness obj; <<< the old internal name of the instance
    

    So it works from QML
    as you do
    // connect our QML signal to our C++ slot
    QObject::connect(window, SIGNAL(submitTextField(QString)), &my_brightness, SLOT(brightnessText(QString)));

    and that changes the one in Mian.cpp
    but the rgbLed my_rgbLed; and its "obj" is not.

    I assume you want the brigness instance inside my_rgbLed to change ?


  • Qt Champions 2019

    @mrjj said in Nested class in Qt5:

    I think you have 2 instances of brightness called my_brightness

    I already tryied to explain that...



  • @Juancinho_cardecan
    Assuming @jsulm & @mrjj are correct, each time you create a brightness object anywhere why don't you set its https://doc.qt.io/qt-5/qobject.html#objectName-prop (setObjectName()) to a unique string, then you can see whether one instance is different from another. Printing out &brightnessObjectInstance may also tell you the same, I think.


  • Lifetime Qt Champion

    Hi

    Could you try to add

    brightness & rgbLed: :getBrightness() {
    return obj; // or what you called it
    }

    to
    class rgbLed : public QObject

    then in main.cpp

    QObject::connect(window, SIGNAL(submitTextField(QString)), my_rgbLed.getBrightness(), SLOT(brightnessText(QString)));

    and remove all
    brightness my_brightness;
    as we dont want to use brightness alone ever, only via rgbLed.


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