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using C++ classes and structs in QML

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  • L lemons

    @mzimmers
    I thought the Q_GADGET macro is used for meta-types only, which can't be instantiated from QML.
    To be used e.g. if the struct is a property of an QObject derived class, which is somehow accessible in QML (either passed through C++ or created in QML).
    Allows you to use e.g. Q_PROPERTY macros, without the QObject overhead.

    To be creatable from QML you need a derived class from QObject, so the Q_GADGET macro is not sufficient, as it is missing e.g. the signals and slots of the QObject class.

    Note: I can be totally wrong / outdated, but this is how I was thinking and using it all the time :D

    JoeCFDJ Offline
    JoeCFDJ Offline
    JoeCFD
    wrote on last edited by JoeCFD
    #18

    @lemons said in using C++ classes and structs in QML:

    Q_GADGET macro

    Lemons seems right.

    In QML (Qt Meta-Object Language), the Q_GADGET macro is typically used with C++ classes to create non-instantiable classes that can be registered with the Qt meta-object system. These classes are similar to Q_OBJECT classes but cannot have signals, slots, or properties. They are often used for data-only structures that need to be exposed to QML.

    I use upper case for my class registration in qmlRegisterType without issues. I guess lower case is needed in app_engine->rootContext()->setContextProperty( ... );

    1 Reply Last reply
    0
    • GrecKoG Online
      GrecKoG Online
      GrecKo
      Qt Champions 2018
      wrote on last edited by GrecKo
      #19

      You can now instantiate Gadgets from QML with undocumented macros (so maybe don't use it).

      Use QML_VALUE_TYPE(typeName) with a lower case name like you currently do.
      Then you can add QML_CONSTRUCTIBLE_VALUE or QML_STRUCTURED_VALUE (even both).

      QML_CONSTRUCTIBLE_VALUE will call a user defined constructor

      if you have Q_INVOKABLE MyStruct(int foo) : m_myInt{foo} {}
      then doing property myStruct myStruct1: 42 will call the constructor with 42 as the foo param

      QML_STRUCTURED_VALUE will assign properties depending on the js object you passed in QML.

      property myStruct myStruct1: ({myInt: 42}) will create a default constructed MyStruct and assign its myInt property to 42.

      Note that the type should always be default constructible.

      EDIT: Note that you don't have to use QML_ELEMENT, Q_DECLARE_METATYPE or qmlRegisterType if you use Q_GADGET and QML_VALUE_TYPE.

      mzimmersM L 2 Replies Last reply
      3
      • GrecKoG GrecKo

        You can now instantiate Gadgets from QML with undocumented macros (so maybe don't use it).

        Use QML_VALUE_TYPE(typeName) with a lower case name like you currently do.
        Then you can add QML_CONSTRUCTIBLE_VALUE or QML_STRUCTURED_VALUE (even both).

        QML_CONSTRUCTIBLE_VALUE will call a user defined constructor

        if you have Q_INVOKABLE MyStruct(int foo) : m_myInt{foo} {}
        then doing property myStruct myStruct1: 42 will call the constructor with 42 as the foo param

        QML_STRUCTURED_VALUE will assign properties depending on the js object you passed in QML.

        property myStruct myStruct1: ({myInt: 42}) will create a default constructed MyStruct and assign its myInt property to 42.

        Note that the type should always be default constructible.

        EDIT: Note that you don't have to use QML_ELEMENT, Q_DECLARE_METATYPE or qmlRegisterType if you use Q_GADGET and QML_VALUE_TYPE.

        mzimmersM Offline
        mzimmersM Offline
        mzimmers
        wrote on last edited by
        #20

        @GrecKo curiouser and curiouser.

        My modified struct:

        struct MyStruct {
            QML_STRUCTURED_VALUE
            QML_VALUE_TYPE(myStruct)
            Q_PROPERTY(int myInt MEMBER m_myInt)
            int m_myInt = 55;
            MyStruct() {}
        };
        

        my registration (in main.cpp):

            qmlRegisterType<MyStruct>("myStruct", 1, 0, "MyStruct");
        

        and my QML:

        property myStruct myStruct1: myStruct ({myInt: 100})
        

        produces a runtime error: "myStruct is not a type."

        Isn't using QML_VALUE_TYPE intended to allow me to use "myStruct" in the QML?

        GrecKoG 1 Reply Last reply
        0
        • mzimmersM mzimmers

          @GrecKo curiouser and curiouser.

          My modified struct:

          struct MyStruct {
              QML_STRUCTURED_VALUE
              QML_VALUE_TYPE(myStruct)
              Q_PROPERTY(int myInt MEMBER m_myInt)
              int m_myInt = 55;
              MyStruct() {}
          };
          

          my registration (in main.cpp):

              qmlRegisterType<MyStruct>("myStruct", 1, 0, "MyStruct");
          

          and my QML:

          property myStruct myStruct1: myStruct ({myInt: 100})
          

          produces a runtime error: "myStruct is not a type."

          Isn't using QML_VALUE_TYPE intended to allow me to use "myStruct" in the QML?

          GrecKoG Online
          GrecKoG Online
          GrecKo
          Qt Champions 2018
          wrote on last edited by
          #21

          As I said : you don't have to use qmlRegisterType. You do have to use Q_GADGET though.

          And Q_GADGET adds a private: so you need to add back public:

          struct MyStruct {
            Q_GADGET
            QML_VALUE_TYPE(myStruct)
            QML_STRUCTURED_VALUE
          
            Q_PROPERTY(int myInt MEMBER m_myInt)
          
          public:
            int m_myInt = 55;
            MyStruct() {};
          };
          

          and my QML:
          property myStruct myStruct1: myStruct ({myInt: 100})

          Did I write that? nope. The syntax for structured values is :

          property myStruct myStruct1: ({myInt: 100})
          

          Ditch the myStruct on the right-hand side.

          mzimmersM 1 Reply Last reply
          1
          • GrecKoG GrecKo

            As I said : you don't have to use qmlRegisterType. You do have to use Q_GADGET though.

            And Q_GADGET adds a private: so you need to add back public:

            struct MyStruct {
              Q_GADGET
              QML_VALUE_TYPE(myStruct)
              QML_STRUCTURED_VALUE
            
              Q_PROPERTY(int myInt MEMBER m_myInt)
            
            public:
              int m_myInt = 55;
              MyStruct() {};
            };
            

            and my QML:
            property myStruct myStruct1: myStruct ({myInt: 100})

            Did I write that? nope. The syntax for structured values is :

            property myStruct myStruct1: ({myInt: 100})
            

            Ditch the myStruct on the right-hand side.

            mzimmersM Offline
            mzimmersM Offline
            mzimmers
            wrote on last edited by mzimmers
            #22

            @GrecKo oh, that is a thing of beauty.

            Now...about QML_STRUCTURED_VALUE being an undocumented macro, as you said...is it safe to use? I did notice mention of it in a bug report, so maybe it's OK.

            Thanks for the help.

            EDIT: everything above works, but if possible, I'd like to directly access enums defined in the struct (actually, this was the original point behind this entire thread).

            struct MyStruct {
                Q_GADGET
                QML_STRUCTURED_VALUE
                QML_VALUE_TYPE(myStruct)
                Q_PROPERTY(int myInt MEMBER m_myInt)
            public:
                enum MyEnums {
                    Enum0,
                    Enum1,
                    Enum2,
                    Enum3
                } m_myEnums;
                Q_ENUM(MyEnums)
                int m_myInt = 55;
                MyStruct() {}
            };
            

            This attempt doesn't work:

            Label { text: "myStruct.Enum3: " + myStruct.Enum3 }
            

            (it doesn't work with "MyStruct" either.)

            Can this be made to work? Thanks...

            EDIT 2:

            I have a workaround, which is to define my enums in a separate class and expose them to QML using the guidelines here, but if possible I'd prefer to avoid this level of indirection, and keep the enums in the struct.

            GrecKoG 1 Reply Last reply
            0
            • mzimmersM mzimmers

              @GrecKo oh, that is a thing of beauty.

              Now...about QML_STRUCTURED_VALUE being an undocumented macro, as you said...is it safe to use? I did notice mention of it in a bug report, so maybe it's OK.

              Thanks for the help.

              EDIT: everything above works, but if possible, I'd like to directly access enums defined in the struct (actually, this was the original point behind this entire thread).

              struct MyStruct {
                  Q_GADGET
                  QML_STRUCTURED_VALUE
                  QML_VALUE_TYPE(myStruct)
                  Q_PROPERTY(int myInt MEMBER m_myInt)
              public:
                  enum MyEnums {
                      Enum0,
                      Enum1,
                      Enum2,
                      Enum3
                  } m_myEnums;
                  Q_ENUM(MyEnums)
                  int m_myInt = 55;
                  MyStruct() {}
              };
              

              This attempt doesn't work:

              Label { text: "myStruct.Enum3: " + myStruct.Enum3 }
              

              (it doesn't work with "MyStruct" either.)

              Can this be made to work? Thanks...

              EDIT 2:

              I have a workaround, which is to define my enums in a separate class and expose them to QML using the guidelines here, but if possible I'd prefer to avoid this level of indirection, and keep the enums in the struct.

              GrecKoG Online
              GrecKoG Online
              GrecKo
              Qt Champions 2018
              wrote on last edited by
              #23

              @mzimmers I don't believe it is possible no. Instead of defining your enums in a separate class I would do it in a namespace with Q_ENUM_NS instead.

              GrecKoG mzimmersM 2 Replies Last reply
              0
              • GrecKoG GrecKo

                You can now instantiate Gadgets from QML with undocumented macros (so maybe don't use it).

                Use QML_VALUE_TYPE(typeName) with a lower case name like you currently do.
                Then you can add QML_CONSTRUCTIBLE_VALUE or QML_STRUCTURED_VALUE (even both).

                QML_CONSTRUCTIBLE_VALUE will call a user defined constructor

                if you have Q_INVOKABLE MyStruct(int foo) : m_myInt{foo} {}
                then doing property myStruct myStruct1: 42 will call the constructor with 42 as the foo param

                QML_STRUCTURED_VALUE will assign properties depending on the js object you passed in QML.

                property myStruct myStruct1: ({myInt: 42}) will create a default constructed MyStruct and assign its myInt property to 42.

                Note that the type should always be default constructible.

                EDIT: Note that you don't have to use QML_ELEMENT, Q_DECLARE_METATYPE or qmlRegisterType if you use Q_GADGET and QML_VALUE_TYPE.

                L Offline
                L Offline
                lemons
                wrote on last edited by lemons
                #24

                @GrecKo said in using C++ classes and structs in QML:

                You can now instantiate Gadgets from QML with undocumented macros (so maybe don't use it).

                Use QML_VALUE_TYPE(typeName) with a lower case name like you currently do.
                Then you can add QML_CONSTRUCTIBLE_VALUE or QML_STRUCTURED_VALUE (even both).

                QML_CONSTRUCTIBLE_VALUE will call a user defined constructor

                if you have Q_INVOKABLE MyStruct(int foo) : m_myInt{foo} {}
                then doing property myStruct myStruct1: 42 will call the constructor with 42 as the foo param

                QML_STRUCTURED_VALUE will assign properties depending on the js object you passed in QML.

                property myStruct myStruct1: ({myInt: 42}) will create a default constructed MyStruct and assign its myInt property to 42.

                Note that the type should always be default constructible.

                EDIT: Note that you don't have to use QML_ELEMENT, Q_DECLARE_METATYPE or qmlRegisterType if you use Q_GADGET and QML_VALUE_TYPE.

                This is amazing !!
                Gonna have to explore the possibilities...

                1 Reply Last reply
                0
                • GrecKoG GrecKo

                  @mzimmers I don't believe it is possible no. Instead of defining your enums in a separate class I would do it in a namespace with Q_ENUM_NS instead.

                  GrecKoG Online
                  GrecKoG Online
                  GrecKo
                  Qt Champions 2018
                  wrote on last edited by
                  #25

                  Coming back on what I said:

                  It is possible to expose an enum from a Q_GADGET.

                  If you add pragma ValueTypeBehavior: Addressable at the top of your QML file you can use the lower case gadget type name in QML and then do property int enumValue: myStruct.TestEnum.D

                  Alternatively you could do it very verbosely with QML_FOREIGN_NAMESPACE like explained here : https://codereview.qt-project.org/c/qt/qtdeclarative/+/510832 (fresh out of the oven)

                  1 Reply Last reply
                  0
                  • GrecKoG GrecKo

                    @mzimmers I don't believe it is possible no. Instead of defining your enums in a separate class I would do it in a namespace with Q_ENUM_NS instead.

                    mzimmersM Offline
                    mzimmersM Offline
                    mzimmers
                    wrote on last edited by
                    #26

                    @GrecKo said in using C++ classes and structs in QML:

                    Instead of defining your enums in a separate class I would do it in a namespace with Q_ENUM_NS instead.

                    namespace scheduleNS {
                    Q_NAMESPACE
                    enum StartAction {
                        START_ACTION_TURN_ON,
                        START_ACTION_TURN_OFF,
                        START_ACTION_BE_READY,
                        START_ACTION_SUSPEND
                    };
                    Q_ENUM_NS(StartAction)
                    } // namespace
                    

                    but "scheduleNS" isn't recognized in my QML. How do I export a C++ namespace to QML?

                    mzimmersM kshegunovK GrecKoG 3 Replies Last reply
                    0
                    • mzimmersM mzimmers

                      @GrecKo said in using C++ classes and structs in QML:

                      Instead of defining your enums in a separate class I would do it in a namespace with Q_ENUM_NS instead.

                      namespace scheduleNS {
                      Q_NAMESPACE
                      enum StartAction {
                          START_ACTION_TURN_ON,
                          START_ACTION_TURN_OFF,
                          START_ACTION_BE_READY,
                          START_ACTION_SUSPEND
                      };
                      Q_ENUM_NS(StartAction)
                      } // namespace
                      

                      but "scheduleNS" isn't recognized in my QML. How do I export a C++ namespace to QML?

                      mzimmersM Offline
                      mzimmersM Offline
                      mzimmers
                      wrote on last edited by
                      #27

                      @mzimmers said in using C++ classes and structs in QML:

                      How do I export a C++ namespace to QML?

                      I should have asked, is there a newer alternative to the old method of manually registering it like this:

                         qmlRegisterUncreatableMetaObject(scheduleNS::staticMetaObject, // static meta object
                                                          "schedule.enums",          // import statement
                                                          1, 0,                         // major and minor version of the import
                                                          "ScheduleNS",                 // name in QML
                                                          "Error: only enums");          // error in case someone tries to create a MyNamespace object
                      

                      This works fine, but given that we no longer need to use qmlRegisterType(), I was wondering whether there was a more modern way of doing the qmlRegisterUncreatableMetaObject() call.

                      1 Reply Last reply
                      0
                      • mzimmersM mzimmers

                        @GrecKo said in using C++ classes and structs in QML:

                        Instead of defining your enums in a separate class I would do it in a namespace with Q_ENUM_NS instead.

                        namespace scheduleNS {
                        Q_NAMESPACE
                        enum StartAction {
                            START_ACTION_TURN_ON,
                            START_ACTION_TURN_OFF,
                            START_ACTION_BE_READY,
                            START_ACTION_SUSPEND
                        };
                        Q_ENUM_NS(StartAction)
                        } // namespace
                        

                        but "scheduleNS" isn't recognized in my QML. How do I export a C++ namespace to QML?

                        kshegunovK Offline
                        kshegunovK Offline
                        kshegunov
                        Moderators
                        wrote on last edited by
                        #28

                        @mzimmers said in using C++ classes and structs in QML:

                        but "scheduleNS" isn't recognized in my QML. How do I export a C++ namespace to QML?

                        Have you tried naming the namespace with a capital letter?
                        As far as I recall only value types are supposed to be lower case. QML is somewhat picky on the namings.

                        Read and abide by the Qt Code of Conduct

                        mzimmersM 1 Reply Last reply
                        0
                        • kshegunovK kshegunov

                          @mzimmers said in using C++ classes and structs in QML:

                          but "scheduleNS" isn't recognized in my QML. How do I export a C++ namespace to QML?

                          Have you tried naming the namespace with a capital letter?
                          As far as I recall only value types are supposed to be lower case. QML is somewhat picky on the namings.

                          mzimmersM Offline
                          mzimmersM Offline
                          mzimmers
                          wrote on last edited by
                          #29

                          @kshegunov I did try that - I still have to register the uncreatable, and add an import statement to any QML wishing to use it.

                          Still a small price to pay...

                          1 Reply Last reply
                          0
                          • mzimmersM mzimmers

                            @GrecKo said in using C++ classes and structs in QML:

                            Instead of defining your enums in a separate class I would do it in a namespace with Q_ENUM_NS instead.

                            namespace scheduleNS {
                            Q_NAMESPACE
                            enum StartAction {
                                START_ACTION_TURN_ON,
                                START_ACTION_TURN_OFF,
                                START_ACTION_BE_READY,
                                START_ACTION_SUSPEND
                            };
                            Q_ENUM_NS(StartAction)
                            } // namespace
                            

                            but "scheduleNS" isn't recognized in my QML. How do I export a C++ namespace to QML?

                            GrecKoG Online
                            GrecKoG Online
                            GrecKo
                            Qt Champions 2018
                            wrote on last edited by
                            #30

                            @mzimmers said in using C++ classes and structs in QML:

                            but "scheduleNS" isn't recognized in my QML. How do I export a C++ namespace to QML?

                            With QML_ELEMENT or QML_NAMED_ELEMENT.

                            1 Reply Last reply
                            1
                            • mzimmersM mzimmers has marked this topic as solved on

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