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Casting a custom class to QVariant (2020 but still relevant)

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  • mrjjM Offline
    mrjjM Offline
    mrjj
    Lifetime Qt Champion
    wrote on last edited by
    #5

    Hi
    To used classes based on QObject, you must store them as pointers.
    QObject itself cannot be copied and hence cannot be stored in any container
    as plain type. Only via pointers.

    1 Reply Last reply
    3
    • D Offline
      D Offline
      Doug Beck
      wrote on last edited by
      #6

      I'm making some progress. I used QMap like this QMap<QString, MyClass*> as a QProperty in my class. Then, in the ComboBox, I used this. I have a property in the QML file called "controlName".

                  ComboBox {
                      model: MainViewMgr.controls[controlName].nodeList
                      width: 132
      
                      onActivated: {
                          MainViewMgr.controls[controlName].currentIndex = currentIndex;
                      }
      
                      onModelChanged: {
                          currentIndex = Qt.binding( function(){return MainViewMgr.controls[controlName].currentIndex} )
                      }
                  }
      

      However, I get this error. Does this mean the Javascript doesn't support using maps?

      TypeError: Cannot read property 'nodeList' of undefined

      1 Reply Last reply
      0
      • D Offline
        D Offline
        Doug Beck
        wrote on last edited by
        #7

        I have seen some posts on-line that suggest I should be using QVariantMap instead of QMap but I wasn't able to get that to compile with a class based upon QObject.

        mrjjM 1 Reply Last reply
        0
        • D Doug Beck

          I have seen some posts on-line that suggest I should be using QVariantMap instead of QMap but I wasn't able to get that to compile with a class based upon QObject.

          mrjjM Offline
          mrjjM Offline
          mrjj
          Lifetime Qt Champion
          wrote on last edited by
          #8

          Hi
          Yes https://doc.qt.io/qt-5/qtqml-cppintegration-data.html mentions
          QVariantMap so that should work.

          What error did you get with QVariantMap ?
          Also you must also use pointers to "cMyClass"
          And it should be supported
          https://doc.qt.io/qt-5/qmetatype.html#Type-enum
          as it has QMetaType::QObjectStar

          D 1 Reply Last reply
          0
          • mrjjM mrjj

            Hi
            Yes https://doc.qt.io/qt-5/qtqml-cppintegration-data.html mentions
            QVariantMap so that should work.

            What error did you get with QVariantMap ?
            Also you must also use pointers to "cMyClass"
            And it should be supported
            https://doc.qt.io/qt-5/qmetatype.html#Type-enum
            as it has QMetaType::QObjectStar

            D Offline
            D Offline
            Doug Beck
            wrote on last edited by
            #9

            @mrjj The problem is getting myClass into the QVariant. Using the following required me to use Q_DECLARE_METATYPE.

            QVariant v = QVariant::fromValue<cMyClass>(MyObject);

            However, as soon as I use Q_DECLARE_METATYPE, I get errors about using a deleted function. Without this reference, it compiles just fine. But I can't see a method to insert myClass into QVariantMap without using QVariant::fromValue.

            Christian EhrlicherC 1 Reply Last reply
            0
            • Christian EhrlicherC Offline
              Christian EhrlicherC Offline
              Christian Ehrlicher
              Lifetime Qt Champion
              wrote on last edited by
              #10

              Again: you can't copy a QObject! use a pointer...

              Qt Online Installer direct download: https://download.qt.io/official_releases/online_installers/
              Visit the Qt Academy at https://academy.qt.io/catalog

              D 1 Reply Last reply
              0
              • Christian EhrlicherC Christian Ehrlicher

                Again: you can't copy a QObject! use a pointer...

                D Offline
                D Offline
                Doug Beck
                wrote on last edited by
                #11

                @Christian-Ehrlicher There's something I'm not understanding here. Here's my class. When I uncomment the part about Q_DECLARE_METATYPE, I immediately get the compiler error with the use of deleted function.

                #include <QObject>
                // #include <QMetaType>
                #include "PropertyHelper.h"

                class cMyClass: public QObject
                {
                Q_OBJECT
                AUTO_PROPERTY(QString, nodeValue)
                AUTO_PROPERTY(QString, nodeUnits)
                AUTO_PROPERTY(QStringList, nodeList)
                AUTO_PROPERTY(qint16, currentIndex)
                public:
                explicit cMyClass(QObject* parent = nullptr);
                explicit cMyClass(QObject* parent, QString controlName, QString value, std::vector<QString> nodeStringVector);
                QString controlName() const {return mControlName;}

                private:
                QString mControlName;
                };

                // Q_DECLARE_METATYPE(cMyClass);

                In my ViewMgr class, I used this as my Q_PROPERTY.

                Q_PROPERTY(QVariantMap *myObject READ myObject WRITE setMyObject NOTIFY myObjectChanged)

                I guess I'm not sure where I should be using a pointer that I'm not.

                1 Reply Last reply
                0
                • D Doug Beck

                  @mrjj The problem is getting myClass into the QVariant. Using the following required me to use Q_DECLARE_METATYPE.

                  QVariant v = QVariant::fromValue<cMyClass>(MyObject);

                  However, as soon as I use Q_DECLARE_METATYPE, I get errors about using a deleted function. Without this reference, it compiles just fine. But I can't see a method to insert myClass into QVariantMap without using QVariant::fromValue.

                  Christian EhrlicherC Offline
                  Christian EhrlicherC Offline
                  Christian Ehrlicher
                  Lifetime Qt Champion
                  wrote on last edited by
                  #12

                  @Doug-Beck said in Casting a custom class to QVariant (2020 but still relevant):

                  QVariant v = QVariant::fromValue<cMyClass>(MyObject);

                  here

                  Qt Online Installer direct download: https://download.qt.io/official_releases/online_installers/
                  Visit the Qt Academy at https://academy.qt.io/catalog

                  D 1 Reply Last reply
                  1
                  • Christian EhrlicherC Christian Ehrlicher

                    @Doug-Beck said in Casting a custom class to QVariant (2020 but still relevant):

                    QVariant v = QVariant::fromValue<cMyClass>(MyObject);

                    here

                    D Offline
                    D Offline
                    Doug Beck
                    wrote on last edited by
                    #13

                    @Christian-Ehrlicher Still get the same errors even without that line of code.

                    Start of compiler errors*
                    error: C2280: 'cMyClass::cMyClass:(const cMyClass: &)': attempting to reference a deleted function
                    compiler has generated 'cMyClass::cMyClass' here
                    while compiling class template member function 'void *QtMetaTypePrivate::QMetaTypeFunctionHelper<T,true>::Construct(void *,const void *)'
                    with
                    [
                    T=cMyClass
                    ]

                    see reference to function template instantiation 'void *QtMetaTypePrivate::QMetaTypeFunctionHelper<T,true>::Construct(void *,const void *)' being compiled
                    with
                    [
                    T=cMyClass
                    ]

                    see reference to class template instantiation 'QtMetaTypePrivate::QMetaTypeFunctionHelper<T,true>' being compiled
                    with
                    [
                    T=cFakeControl
                    ]
                    End of compiler errors*

                    I sincerely appreciate your help on this! I have spent much of the last decade using C# so it would not be surprising if I didn't use a pointer correctly. But I'm not sure where the problem is.

                    1 Reply Last reply
                    0
                    • Christian EhrlicherC Offline
                      Christian EhrlicherC Offline
                      Christian Ehrlicher
                      Lifetime Qt Champion
                      wrote on last edited by
                      #14

                      I don't know what else I can tell. You can't put a QObject into a QVariant, only a pointer to a QObject. Therefore pass a pointer ... c++ basics.

                      Qt Online Installer direct download: https://download.qt.io/official_releases/online_installers/
                      Visit the Qt Academy at https://academy.qt.io/catalog

                      D 1 Reply Last reply
                      0
                      • Christian EhrlicherC Christian Ehrlicher

                        I don't know what else I can tell. You can't put a QObject into a QVariant, only a pointer to a QObject. Therefore pass a pointer ... c++ basics.

                        D Offline
                        D Offline
                        Doug Beck
                        wrote on last edited by Doug Beck
                        #15

                        @Christian-Ehrlicher I get the part about pointers. However, when I copy cMyClass to an empty QML project and don't reference it at all and even remove all of the Q_PROPERTIES from cMyClass I still get the same error as before.

                        This new project does not use QVariant at all. Nor does it ever refer to cMyClass, which is just an empty class. But, the line Q_DECLARE_METATYPE does cause compiler errors referring to a deleted function. (Which admittedly is just like I'd have if I was trying to use QObject instead of QObject*). So, this has nothing to do QVariant or even my use of cMyClass since I'm not using it at all.

                        I hope you are saying that Q_DECLARE_METATYPE is completely unnecessary for the use of QVariant.

                        #include <QObject>
                        #include <QMetaType>

                        class cMyClass: public QObject
                        {
                        Q_OBJECT
                        public:
                        explicit cMyClass(QObject* parent = nullptr);
                        };

                        Q_DECLARE_METATYPE(cMyClass);

                        ** Still happens
                        'cMyClass::cMyClass(const cMyClass &)': function was implicitly deleted because a base class invokes a deleted or inaccessible function 'QObject::QObject(const QObject &)'

                        mrjjM 1 Reply Last reply
                        0
                        • D Doug Beck

                          @Christian-Ehrlicher I get the part about pointers. However, when I copy cMyClass to an empty QML project and don't reference it at all and even remove all of the Q_PROPERTIES from cMyClass I still get the same error as before.

                          This new project does not use QVariant at all. Nor does it ever refer to cMyClass, which is just an empty class. But, the line Q_DECLARE_METATYPE does cause compiler errors referring to a deleted function. (Which admittedly is just like I'd have if I was trying to use QObject instead of QObject*). So, this has nothing to do QVariant or even my use of cMyClass since I'm not using it at all.

                          I hope you are saying that Q_DECLARE_METATYPE is completely unnecessary for the use of QVariant.

                          #include <QObject>
                          #include <QMetaType>

                          class cMyClass: public QObject
                          {
                          Q_OBJECT
                          public:
                          explicit cMyClass(QObject* parent = nullptr);
                          };

                          Q_DECLARE_METATYPE(cMyClass);

                          ** Still happens
                          'cMyClass::cMyClass(const cMyClass &)': function was implicitly deleted because a base class invokes a deleted or inaccessible function 'QObject::QObject(const QObject &)'

                          mrjjM Offline
                          mrjjM Offline
                          mrjj
                          Lifetime Qt Champion
                          wrote on last edited by
                          #16

                          @Doug-Beck said in Casting a custom class to QVariant (2020 but still relevant):

                          Q_DECLARE_METATYPE

                          Hi
                          https://doc.qt.io/qt-5/qmetatype.html#details
                          "Any class or struct that has a public default constructor, a public copy constructor, and a public destructor can be registered."
                          Which means you cant use
                          Q_DECLARE_METATYPE(cMyClass);

                          Also I think QObject * is already registered.

                          D 1 Reply Last reply
                          1
                          • mrjjM mrjj

                            @Doug-Beck said in Casting a custom class to QVariant (2020 but still relevant):

                            Q_DECLARE_METATYPE

                            Hi
                            https://doc.qt.io/qt-5/qmetatype.html#details
                            "Any class or struct that has a public default constructor, a public copy constructor, and a public destructor can be registered."
                            Which means you cant use
                            Q_DECLARE_METATYPE(cMyClass);

                            Also I think QObject * is already registered.

                            D Offline
                            D Offline
                            Doug Beck
                            wrote on last edited by
                            #17

                            @mrjj Thanks! Clearly that was a dead end. I think I understand what Christian-Ehrlicher was telling me.

                            1 Reply Last reply
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