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  4. error: "'QObject::QObject(const QObject&)' is private" and "use of deleted function 'QObject::QObject(const QObject&)'"
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error: "'QObject::QObject(const QObject&)' is private" and "use of deleted function 'QObject::QObject(const QObject&)'"

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  • Lucas_1603L Lucas_1603

    @jsulm I think I'm getting trouble with Song class but actually I'm not sure about that because my project has other classes too. But I just see the errors in Song class now

    0_1568630374154_5f945232-7a62-4a16-8251-6caa978974c4-image.png

    jsulmJ Offline
    jsulmJ Offline
    jsulm
    Lifetime Qt Champion
    wrote on last edited by
    #10

    @lucas_1603 Your Song class is QObject based and thus not copyable. You're apparently trying to make a copy of it.
    Double click at the first error line and see what is there.

    https://forum.qt.io/topic/113070/qt-code-of-conduct

    Lucas_1603L 1 Reply Last reply
    2
    • jsulmJ jsulm

      @lucas_1603 Your Song class is QObject based and thus not copyable. You're apparently trying to make a copy of it.
      Double click at the first error line and see what is there.

      Lucas_1603L Offline
      Lucas_1603L Offline
      Lucas_1603
      wrote on last edited by
      #11

      @jsulm I don't know what you mean by said that I'm trying to make a copy of it. I actually didn't assign any ones to others!

      1 Reply Last reply
      0
      • SGaistS Offline
        SGaistS Offline
        SGaist
        Lifetime Qt Champion
        wrote on last edited by
        #12

        Hi,

        Are you maybe using the copy constructor somewhere ?

        Interested in AI ? www.idiap.ch
        Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

        Lucas_1603L 1 Reply Last reply
        0
        • SGaistS SGaist

          Hi,

          Are you maybe using the copy constructor somewhere ?

          Lucas_1603L Offline
          Lucas_1603L Offline
          Lucas_1603
          wrote on last edited by Lucas_1603
          #13

          @sgaist Nope, I don't use it in my project and I'm quite sure about that ^^.
          I really want to know what does the error use of deleted function 'Song::Song(const Song&)' mean.

          JKSHJ 1 Reply Last reply
          0
          • SGaistS Offline
            SGaistS Offline
            SGaist
            Lifetime Qt Champion
            wrote on last edited by
            #14

            It means that somewhere in your code you are using the copy constructor which is disabled since QObject objects cannot be copied.

            Interested in AI ? www.idiap.ch
            Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

            1 Reply Last reply
            1
            • Lucas_1603L Lucas_1603

              @sgaist Nope, I don't use it in my project and I'm quite sure about that ^^.
              I really want to know what does the error use of deleted function 'Song::Song(const Song&)' mean.

              JKSHJ Offline
              JKSHJ Offline
              JKSH
              Moderators
              wrote on last edited by JKSH
              #15

              @lucas_1603 said in error: "'QObject::QObject(const QObject&)' is private" and "use of deleted function 'QObject::QObject(const QObject&)'":

              Nope, I don't use it in my project and I'm quite sure about that ^^.

              The copy constructor is invoked when you try to assign one Song object to another, for example:

              Song s1("Happy Birthday to You", "Me", "/path/to/song", "/path/to/art");
              
              Song s2(s1);  // copy-constructor invoked here
              Song s3 = s1; // operator=() and copy-constructor invoked here
              

              I really want to know what does the error use of deleted function 'Song::Song(const Song&)' mean.

              It means your code contains something like s2 or s3 above.

              • Song::Song(const Song&) is the copy-constructor
              • A "deleted function" is a function which the creator has removed/deleted from the class -- QObjects are not allowed to have copy-constructors. See https://thispointer.com/c11-c14-delete-keyword-and-deleted-functions-with-use-cases-examples/ for more info

              Qt Doc Search for browsers: forum.qt.io/topic/35616/web-browser-extension-for-improved-doc-searches

              Lucas_1603L 1 Reply Last reply
              4
              • JKSHJ JKSH

                @lucas_1603 said in error: "'QObject::QObject(const QObject&)' is private" and "use of deleted function 'QObject::QObject(const QObject&)'":

                Nope, I don't use it in my project and I'm quite sure about that ^^.

                The copy constructor is invoked when you try to assign one Song object to another, for example:

                Song s1("Happy Birthday to You", "Me", "/path/to/song", "/path/to/art");
                
                Song s2(s1);  // copy-constructor invoked here
                Song s3 = s1; // operator=() and copy-constructor invoked here
                

                I really want to know what does the error use of deleted function 'Song::Song(const Song&)' mean.

                It means your code contains something like s2 or s3 above.

                • Song::Song(const Song&) is the copy-constructor
                • A "deleted function" is a function which the creator has removed/deleted from the class -- QObjects are not allowed to have copy-constructors. See https://thispointer.com/c11-c14-delete-keyword-and-deleted-functions-with-use-cases-examples/ for more info
                Lucas_1603L Offline
                Lucas_1603L Offline
                Lucas_1603
                wrote on last edited by
                #16

                @jksh Yep, I got it. I realized what all you guys said.

                So I change the approach to my project now, assume that I have a PlaylistModel class:

                #include <QAbstractListModel>
                class PlaylistModel : public QAbstractListModel
                {
                    Q_OBJECT
                
                public:
                    enum Roles {
                        TitleRole = Qt::UserRole + 1,
                        SingerRole,
                        SourceRole,
                        AlbumArtRole
                    };
                
                    explicit PlaylistModel(QObject *parent = nullptr);
                
                    int rowCount(const QModelIndex &parent = QModelIndex()) const override;
                
                    QVariant data(const QModelIndex &index, int role = Qt::DisplayRole) const override;
                    void addSong(Song &song);
                
                protected:
                    QHash<int, QByteArray> roleNames() const override;
                
                private:
                    QList<Song> m_data;
                };
                

                Now I want to access m_data (is the private attribute) from QML file, how can I do that?

                Thanks a lot for your help.

                JonBJ 1 Reply Last reply
                0
                • Lucas_1603L Lucas_1603

                  @jksh Yep, I got it. I realized what all you guys said.

                  So I change the approach to my project now, assume that I have a PlaylistModel class:

                  #include <QAbstractListModel>
                  class PlaylistModel : public QAbstractListModel
                  {
                      Q_OBJECT
                  
                  public:
                      enum Roles {
                          TitleRole = Qt::UserRole + 1,
                          SingerRole,
                          SourceRole,
                          AlbumArtRole
                      };
                  
                      explicit PlaylistModel(QObject *parent = nullptr);
                  
                      int rowCount(const QModelIndex &parent = QModelIndex()) const override;
                  
                      QVariant data(const QModelIndex &index, int role = Qt::DisplayRole) const override;
                      void addSong(Song &song);
                  
                  protected:
                      QHash<int, QByteArray> roleNames() const override;
                  
                  private:
                      QList<Song> m_data;
                  };
                  

                  Now I want to access m_data (is the private attribute) from QML file, how can I do that?

                  Thanks a lot for your help.

                  JonBJ Offline
                  JonBJ Offline
                  JonB
                  wrote on last edited by JonB
                  #17

                  @lucas_1603
                  You can only access private variables in the class which defines them. So expose a getter/setter if you really need to. But since QAbstractListModel already has data() & setData() methods you would normally go via those.

                  1 Reply Last reply
                  2
                  • JKSHJ Offline
                    JKSHJ Offline
                    JKSH
                    Moderators
                    wrote on last edited by
                    #18

                    @lucas_1603 said in error: "'QObject::QObject(const QObject&)' is private" and "use of deleted function 'QObject::QObject(const QObject&)'":

                    QList<Song> m_data

                    Does that compile?

                    Store pointers-to-QObjects inside containers, rather than QObjects themselves.

                    Qt Doc Search for browsers: forum.qt.io/topic/35616/web-browser-extension-for-improved-doc-searches

                    Lucas_1603L 1 Reply Last reply
                    3
                    • JKSHJ JKSH

                      @lucas_1603 said in error: "'QObject::QObject(const QObject&)' is private" and "use of deleted function 'QObject::QObject(const QObject&)'":

                      QList<Song> m_data

                      Does that compile?

                      Store pointers-to-QObjects inside containers, rather than QObjects themselves.

                      Lucas_1603L Offline
                      Lucas_1603L Offline
                      Lucas_1603
                      wrote on last edited by
                      #19

                      @jksh Yes, it compiled.
                      @JonB As you said, I can access m_data via data() and setData() method. In data(), I have to assign QModelIndex &index and int role to it .
                      How I can use data() method with enum TitleRole?

                      JKSHJ 1 Reply Last reply
                      0
                      • IntruderExcluderI Offline
                        IntruderExcluderI Offline
                        IntruderExcluder
                        wrote on last edited by
                        #20

                        You should override roleNames method in your model:

                        QHash<int, QByteArray> PlaylistModel::roleNames() const
                        {
                            static QHash<int, QByteArray>* ret = nullptr;
                            if (ret)
                                return *ret;
                        
                            ret = new QHash<int, QByteArray>();
                            (*ret)[TitleRole] = "title";
                            ...
                        
                            return *ret;
                        }
                        

                        Then in QML you can use them as model property: model.title.

                        Lucas_1603L 1 Reply Last reply
                        0
                        • IntruderExcluderI IntruderExcluder

                          You should override roleNames method in your model:

                          QHash<int, QByteArray> PlaylistModel::roleNames() const
                          {
                              static QHash<int, QByteArray>* ret = nullptr;
                              if (ret)
                                  return *ret;
                          
                              ret = new QHash<int, QByteArray>();
                              (*ret)[TitleRole] = "title";
                              ...
                          
                              return *ret;
                          }
                          

                          Then in QML you can use them as model property: model.title.

                          Lucas_1603L Offline
                          Lucas_1603L Offline
                          Lucas_1603
                          wrote on last edited by Lucas_1603
                          #21

                          @intruderexcluder yes, I overrided it. When I use PlaylistModel as a model in QML, I actually just want to access m_data variable, not all of PlaylistModel attributes.
                          Any suggestions to do that? Thanks a lot

                          1 Reply Last reply
                          0
                          • IntruderExcluderI Offline
                            IntruderExcluderI Offline
                            IntruderExcluder
                            wrote on last edited by
                            #22

                            You are probably doing something wrong, but you can at any time simply add slot or invokable method to access any data.

                            1 Reply Last reply
                            0
                            • Lucas_1603L Lucas_1603

                              @jksh Yes, it compiled.
                              @JonB As you said, I can access m_data via data() and setData() method. In data(), I have to assign QModelIndex &index and int role to it .
                              How I can use data() method with enum TitleRole?

                              JKSHJ Offline
                              JKSHJ Offline
                              JKSH
                              Moderators
                              wrote on last edited by
                              #23

                              @lucas_1603 said in error: "'QObject::QObject(const QObject&)' is private" and "use of deleted function 'QObject::QObject(const QObject&)'":

                              Yes, it compiled
                              ...
                              I actually just want to access m_data variable, not all of PlaylistModel attributes.

                              Is Song still a subclass of QObject?

                              Qt Doc Search for browsers: forum.qt.io/topic/35616/web-browser-extension-for-improved-doc-searches

                              Lucas_1603L 1 Reply Last reply
                              0
                              • JKSHJ JKSH

                                @lucas_1603 said in error: "'QObject::QObject(const QObject&)' is private" and "use of deleted function 'QObject::QObject(const QObject&)'":

                                Yes, it compiled
                                ...
                                I actually just want to access m_data variable, not all of PlaylistModel attributes.

                                Is Song still a subclass of QObject?

                                Lucas_1603L Offline
                                Lucas_1603L Offline
                                Lucas_1603
                                wrote on last edited by
                                #24

                                @jksh No, it's not now. I think I'm doing right way in this time ^^
                                The challenge now is accessing to m_data to get title, singer, source, album_art.

                                JKSHJ 1 Reply Last reply
                                0
                                • Lucas_1603L Lucas_1603

                                  @jksh No, it's not now. I think I'm doing right way in this time ^^
                                  The challenge now is accessing to m_data to get title, singer, source, album_art.

                                  JKSHJ Offline
                                  JKSHJ Offline
                                  JKSH
                                  Moderators
                                  wrote on last edited by
                                  #25

                                  @lucas_1603 said in error: "'QObject::QObject(const QObject&)' is private" and "use of deleted function 'QObject::QObject(const QObject&)'":

                                  I think I'm doing right way in this time ^^

                                  There's often more than 1 way to achieve something; some are just easier or more efficient than others :)

                                  The challenge now is accessing to m_data to get title, singer, source, album_art.

                                  I see that you've implemented a Model. The idea behind a model is that you shouldn't need to access the internal data structure directly. Instead, you use the model's standard interface (rows, columns, and maybe roles) to access individual fields.

                                  In your example, you can treat each Row as an individual song. You can also treat each Column as a song field (e.g. Col 1 = Title, Col 2 = Singer, etc.). Older examples use Roles instead of Columns

                                  In QML, you normally don't call data() and setData() directly; you let the View and Delegate call them for you.

                                  Spend some time getting to know the model-view framework first:

                                  • https://doc.qt.io/qt-5/qtquick-modelviewsdata-cppmodels.html
                                  • https://doc.qt.io/qt-5/qtquick-models-objectlistmodel-example.html
                                    • https://code.qt.io/cgit/qt/qtdeclarative.git/tree/examples/quick/models/objectlistmodel?h=5.13

                                  This is quite a lengthy topic, so it's not something that we can teach you over a few forum posts. You need to spend time going through examples and modifying them to see how they behave. trying things out.

                                  Qt Doc Search for browsers: forum.qt.io/topic/35616/web-browser-extension-for-improved-doc-searches

                                  Lucas_1603L 1 Reply Last reply
                                  2
                                  • JKSHJ JKSH

                                    @lucas_1603 said in error: "'QObject::QObject(const QObject&)' is private" and "use of deleted function 'QObject::QObject(const QObject&)'":

                                    I think I'm doing right way in this time ^^

                                    There's often more than 1 way to achieve something; some are just easier or more efficient than others :)

                                    The challenge now is accessing to m_data to get title, singer, source, album_art.

                                    I see that you've implemented a Model. The idea behind a model is that you shouldn't need to access the internal data structure directly. Instead, you use the model's standard interface (rows, columns, and maybe roles) to access individual fields.

                                    In your example, you can treat each Row as an individual song. You can also treat each Column as a song field (e.g. Col 1 = Title, Col 2 = Singer, etc.). Older examples use Roles instead of Columns

                                    In QML, you normally don't call data() and setData() directly; you let the View and Delegate call them for you.

                                    Spend some time getting to know the model-view framework first:

                                    • https://doc.qt.io/qt-5/qtquick-modelviewsdata-cppmodels.html
                                    • https://doc.qt.io/qt-5/qtquick-models-objectlistmodel-example.html
                                      • https://code.qt.io/cgit/qt/qtdeclarative.git/tree/examples/quick/models/objectlistmodel?h=5.13

                                    This is quite a lengthy topic, so it's not something that we can teach you over a few forum posts. You need to spend time going through examples and modifying them to see how they behave. trying things out.

                                    Lucas_1603L Offline
                                    Lucas_1603L Offline
                                    Lucas_1603
                                    wrote on last edited by
                                    #26

                                    @jksh Thank you so much
                                    All you guys helped me a lot and I really appreciate it.
                                    I'll spend more time to learn more about model-view framework to see how really they work.

                                    1 Reply Last reply
                                    2

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