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  4. Correct syntax to register object reference using qRegisterMetaType?

Correct syntax to register object reference using qRegisterMetaType?

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  • S Offline
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    SGaist
    Lifetime Qt Champion
    wrote on 6 Jan 2021, 15:37 last edited by
    #10

    Hi,

    Don't pass references in parameters. As @JKSH wrote earlier, most of Qt datatypes are implicitly shared so there's really no need to try to go that way.

    If you are forcing the Qt::DirectConnection because of that then it means you have a design issue. It's currently pretty rare to have to do that and it also has consequences when doing multithreading.

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

    S 1 Reply Last reply 6 Jan 2021, 15:48
    1
    • S SGaist
      6 Jan 2021, 15:37

      Hi,

      Don't pass references in parameters. As @JKSH wrote earlier, most of Qt datatypes are implicitly shared so there's really no need to try to go that way.

      If you are forcing the Qt::DirectConnection because of that then it means you have a design issue. It's currently pretty rare to have to do that and it also has consequences when doing multithreading.

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      SPlatten
      wrote on 6 Jan 2021, 15:48 last edited by
      #11

      @SGaist , I want to pass by reference because my understanding of passing by reference is that it is far less costly on stack space. A QJsonObject can be very large.

      Kind Regards,
      Sy

      K 1 Reply Last reply 6 Jan 2021, 15:55
      0
      • S SPlatten
        6 Jan 2021, 15:48

        @SGaist , I want to pass by reference because my understanding of passing by reference is that it is far less costly on stack space. A QJsonObject can be very large.

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        kshegunov
        Moderators
        wrote on 6 Jan 2021, 15:55 last edited by
        #12

        @SPlatten said in Correct syntax to register object reference using qRegisterMetaType?:

        I want to pass by reference because my understanding of passing by reference is that it is far less costly on stack space. A QJsonObject can be very large.

        Your understanding is irrelevant in this case. You were told it isn't possible with a queued connection, and it still isn't, and it is not going to be, no matter how much we argue about it. The warning is there for a reason - to warn you that you have a serious bug in your code. You can either use a direct connection and suffer the inevitable consequences of that, or use const QJsonObject &/QJsonObject for your signal arguments.

        Read and abide by the Qt Code of Conduct

        S 1 Reply Last reply 6 Jan 2021, 16:36
        1
        • K kshegunov
          6 Jan 2021, 15:55

          @SPlatten said in Correct syntax to register object reference using qRegisterMetaType?:

          I want to pass by reference because my understanding of passing by reference is that it is far less costly on stack space. A QJsonObject can be very large.

          Your understanding is irrelevant in this case. You were told it isn't possible with a queued connection, and it still isn't, and it is not going to be, no matter how much we argue about it. The warning is there for a reason - to warn you that you have a serious bug in your code. You can either use a direct connection and suffer the inevitable consequences of that, or use const QJsonObject &/QJsonObject for your signal arguments.

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          SPlatten
          wrote on 6 Jan 2021, 16:36 last edited by
          #13

          @kshegunov , I am using const QJsonObject& for my signal arguments, these are working, the only thing that is displayed in the Application Output is the message I've raised. I'm not experience any other problems, crashes or bugs.

          Kind Regards,
          Sy

          J S 2 Replies Last reply 6 Jan 2021, 16:49
          0
          • S SPlatten
            6 Jan 2021, 16:36

            @kshegunov , I am using const QJsonObject& for my signal arguments, these are working, the only thing that is displayed in the Application Output is the message I've raised. I'm not experience any other problems, crashes or bugs.

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            JonB
            wrote on 6 Jan 2021, 16:49 last edited by
            #14

            @SPlatten
            The signal argument type is "OK", insofar as it would work OK with non-cross-thread direct connection connect()s. But the problem is that you are trying to use it when you need a queued connection. That is why you get the error message on the QObject::connect statement.

            S 1 Reply Last reply 6 Jan 2021, 17:51
            1
            • J JonB
              6 Jan 2021, 16:49

              @SPlatten
              The signal argument type is "OK", insofar as it would work OK with non-cross-thread direct connection connect()s. But the problem is that you are trying to use it when you need a queued connection. That is why you get the error message on the QObject::connect statement.

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              SPlatten
              wrote on 6 Jan 2021, 17:51 last edited by
              #15

              @JonB , thank you, I'll take a look.

              Kind Regards,
              Sy

              1 Reply Last reply
              0
              • S SPlatten
                6 Jan 2021, 16:36

                @kshegunov , I am using const QJsonObject& for my signal arguments, these are working, the only thing that is displayed in the Application Output is the message I've raised. I'm not experience any other problems, crashes or bugs.

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                SGaist
                Lifetime Qt Champion
                wrote on 6 Jan 2021, 18:01 last edited by
                #16

                @SPlatten said in Correct syntax to register object reference using qRegisterMetaType?:

                @kshegunov , I am using const QJsonObject& for my signal arguments, these are working, the only thing that is displayed in the Application Output is the message I've raised. I'm not experience any other problems, crashes or bugs.

                For the slots as well ?

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

                S 1 Reply Last reply 6 Jan 2021, 18:20
                2
                • S SGaist
                  6 Jan 2021, 18:01

                  @SPlatten said in Correct syntax to register object reference using qRegisterMetaType?:

                  @kshegunov , I am using const QJsonObject& for my signal arguments, these are working, the only thing that is displayed in the Application Output is the message I've raised. I'm not experience any other problems, crashes or bugs.

                  For the slots as well ?

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                  SPlatten
                  wrote on 6 Jan 2021, 18:20 last edited by
                  #17

                  @SGaist Of course.

                  Kind Regards,
                  Sy

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                    SPlatten
                    wrote on 6 Jan 2021, 18:23 last edited by
                    #18

                    Ok, I've now got rid of the warning, by going through all the QObject::connect calls and only using Qt::DirectionConnection where the signal and slot pointers are the same.

                    Kind Regards,
                    Sy

                    J 1 Reply Last reply 6 Jan 2021, 18:29
                    0
                    • S SPlatten
                      6 Jan 2021, 18:23

                      Ok, I've now got rid of the warning, by going through all the QObject::connect calls and only using Qt::DirectionConnection where the signal and slot pointers are the same.

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                      JonB
                      wrote on 6 Jan 2021, 18:29 last edited by
                      #19

                      @SPlatten
                      A lot of your questions are about threads. Are your connect()s' senders/receivers never in separate threads?

                      S 1 Reply Last reply 6 Jan 2021, 18:30
                      0
                      • J JonB
                        6 Jan 2021, 18:29

                        @SPlatten
                        A lot of your questions are about threads. Are your connect()s' senders/receivers never in separate threads?

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                        SPlatten
                        wrote on 6 Jan 2021, 18:30 last edited by
                        #20

                        @JonB , not for the ones where I've used Qt::DirectConnection

                        Kind Regards,
                        Sy

                        1 Reply Last reply
                        1
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                          Christian Ehrlicher
                          Lifetime Qt Champion
                          wrote on 6 Jan 2021, 18:38 last edited by
                          #21

                          @SPlatten said in Correct syntax to register object reference using qRegisterMetaType?:

                          not for the ones where I've used Qt::DirectConnection

                          Then it's useless since then Qt::AutoConnection evaluates to Qt::DirectConnection. Specifying a connection type is not needed in most of the cases.

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

                          S 1 Reply Last reply 6 Jan 2021, 18:45
                          0
                          • C Christian Ehrlicher
                            6 Jan 2021, 18:38

                            @SPlatten said in Correct syntax to register object reference using qRegisterMetaType?:

                            not for the ones where I've used Qt::DirectConnection

                            Then it's useless since then Qt::AutoConnection evaluates to Qt::DirectConnection. Specifying a connection type is not needed in most of the cases.

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                            SPlatten
                            wrote on 6 Jan 2021, 18:45 last edited by
                            #22

                            @Christian-Ehrlicher , Well that's the only change I'm implemented and now there is no message, so perhaps this is something that needs looking at?

                            Kind Regards,
                            Sy

                            S C K 3 Replies Last reply 6 Jan 2021, 18:46
                            0
                            • S SPlatten
                              6 Jan 2021, 18:45

                              @Christian-Ehrlicher , Well that's the only change I'm implemented and now there is no message, so perhaps this is something that needs looking at?

                              S Offline
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                              SGaist
                              Lifetime Qt Champion
                              wrote on 6 Jan 2021, 18:46 last edited by
                              #23

                              @SPlatten said in Correct syntax to register object reference using qRegisterMetaType?:

                              @Christian-Ehrlicher , Well that's the only change I'm implemented and now there is no message, so perhaps this is something that needs looking at?

                              For that a minimal compilable example would be needed.

                              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
                              0
                              • S SPlatten
                                6 Jan 2021, 18:45

                                @Christian-Ehrlicher , Well that's the only change I'm implemented and now there is no message, so perhaps this is something that needs looking at?

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                                Christian Ehrlicher
                                Lifetime Qt Champion
                                wrote on 6 Jan 2021, 18:50 last edited by
                                #24

                                @SPlatten said in Correct syntax to register object reference using qRegisterMetaType?:

                                so perhaps this is something that needs looking at?

                                This means that there is a thread context switch somewhere even you tell us it isn't.

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

                                1 Reply Last reply
                                2
                                • S SPlatten
                                  6 Jan 2021, 18:45

                                  @Christian-Ehrlicher , Well that's the only change I'm implemented and now there is no message, so perhaps this is something that needs looking at?

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                                  kshegunov
                                  Moderators
                                  wrote on 6 Jan 2021, 19:34 last edited by
                                  #25

                                  @SPlatten said in Correct syntax to register object reference using qRegisterMetaType?:

                                  @Christian-Ehrlicher , Well that's the only change I'm implemented and now there is no message, so perhaps this is something that needs looking at?

                                  Run with QT_FATAL_WARNINGS=1 as an environment variable and extract the stack trace, then you will have leveraged your debugger to show you exactly where you're passing a reference between threads.

                                  Read and abide by the Qt Code of Conduct

                                  S 1 Reply Last reply 6 Jan 2021, 20:11
                                  2
                                  • K kshegunov
                                    6 Jan 2021, 19:34

                                    @SPlatten said in Correct syntax to register object reference using qRegisterMetaType?:

                                    @Christian-Ehrlicher , Well that's the only change I'm implemented and now there is no message, so perhaps this is something that needs looking at?

                                    Run with QT_FATAL_WARNINGS=1 as an environment variable and extract the stack trace, then you will have leveraged your debugger to show you exactly where you're passing a reference between threads.

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                                    SPlatten
                                    wrote on 6 Jan 2021, 20:11 last edited by
                                    #26

                                    @kshegunov said in Correct syntax to register object reference using qRegisterMetaType?:

                                    QT_FATAL_WARNINGS=1

                                    I've opened both .pro files and added that line, then cleaned and rebuilt, the applications have been running for several minutes, nothing to report, no issues.

                                    Kind Regards,
                                    Sy

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                                      SGaist
                                      Lifetime Qt Champion
                                      wrote on 6 Jan 2021, 20:15 last edited by
                                      #27

                                      It's an environnement variable, not a define.

                                      You have to set it in the Run tab of the Project panel.

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

                                      S 1 Reply Last reply 6 Jan 2021, 20:16
                                      1
                                      • S SGaist
                                        6 Jan 2021, 20:15

                                        It's an environnement variable, not a define.

                                        You have to set it in the Run tab of the Project panel.

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                                        SPlatten
                                        wrote on 6 Jan 2021, 20:16 last edited by
                                        #28

                                        @SGaist , Yes, just googled it and then added as environment variable:

                                        export QT_FATAL_WARNINGS=1
                                        

                                        Then restarted, same, not seeing anything additional.

                                        Kind Regards,
                                        Sy

                                        1 Reply Last reply
                                        0
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                                          SGaist
                                          Lifetime Qt Champion
                                          wrote on 6 Jan 2021, 20:18 last edited by
                                          #29

                                          Restarted what ?
                                          How are you currently running your application ?

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

                                          S 1 Reply Last reply 7 Jan 2021, 07:50
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