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QML object access through model crashes

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  • mzimmersM Offline
    mzimmersM Offline
    mzimmers
    wrote on last edited by
    #1

    Hi all -

    My app is doing something rather strange - upon entering a QML component the first time, everything works, but upon entering it a second time, it crashes. I can't tell exactly where it's crashing because of an RTTI issue, but I think it has to do with the final line in this passage:

    class Outcome : public QObject
    {
        Q_OBJECT
        QML_ELEMENT
        Q_PROPERTY(bool isRunning READ isRunning WRITE setIsRunning NOTIFY isRunningChanged FINAL)
        bool isRunning() { return m_isRunning; }
        bool setIsRunning(bool running);
        ...
    }
    
    Outcome* OutcomeModel::getOutcome(const QUuid &uuid)
    {
        auto outcome {std::make_shared<Outcome>(this)};
        const auto i { getIndex(uuid) };
        if (i == NOT_IN_LIST) {
            qWarning() << __PRETTY_FUNCTION__ << "uuid not found in list.";
        } else {
            outcome = m_list.at(i);
        }
        return outcome.get();
    }
    
    // qml
    text: "running: " + outcomeModel.getOutcome(modelData).isRunning 
    

    I've used other properties instead of my isRunning, and the behavior is the same. Is there something wrong with my getOutcome() routine, or the way I'm calling it from QML?

    Thanks...

    JoeCFDJ jeremy_kJ 2 Replies Last reply
    0
    • mzimmersM mzimmers

      @GrecKo from my updateModel() function:

      std::shared_ptr<Outcome> pOutcome = nullptr;
      
      // determine whether the outcome in this message already exists in list.
      const auto listIndex { getIndex(uuid) };
      if (listIndex == NgaUI::NOT_IN_LIST) { // will append to list below.
          pOutcome = std::make_shared<Outcome>(this);
      } else {
          pOutcome = m_list.at(listIndex);
      }
      ...
      if (listIndex == NgaUI::NOT_IN_LIST) {
          beginInsertRows(QModelIndex(), m_list.size(),  m_list.size());
          m_list.append(pOutcome);
          endInsertRows();
      

      EDIT:

      Regarding who they're shared with, I do create an instance in QML, using my getOutcome() function, but this works with a raw pointer, so I don't think it's sharing the smart pointer, right?

      GrecKoG Offline
      GrecKoG Offline
      GrecKo
      Qt Champions 2018
      wrote on last edited by
      #34

      @mzimmers Does the Outcome constructor set a QObject parent? By calling the base constructor QObject(parent) or with an explicit call to QObject::setParent.

      mzimmersM 1 Reply Last reply
      2
      • mzimmersM mzimmers

        Hi all -

        My app is doing something rather strange - upon entering a QML component the first time, everything works, but upon entering it a second time, it crashes. I can't tell exactly where it's crashing because of an RTTI issue, but I think it has to do with the final line in this passage:

        class Outcome : public QObject
        {
            Q_OBJECT
            QML_ELEMENT
            Q_PROPERTY(bool isRunning READ isRunning WRITE setIsRunning NOTIFY isRunningChanged FINAL)
            bool isRunning() { return m_isRunning; }
            bool setIsRunning(bool running);
            ...
        }
        
        Outcome* OutcomeModel::getOutcome(const QUuid &uuid)
        {
            auto outcome {std::make_shared<Outcome>(this)};
            const auto i { getIndex(uuid) };
            if (i == NOT_IN_LIST) {
                qWarning() << __PRETTY_FUNCTION__ << "uuid not found in list.";
            } else {
                outcome = m_list.at(i);
            }
            return outcome.get();
        }
        
        // qml
        text: "running: " + outcomeModel.getOutcome(modelData).isRunning 
        

        I've used other properties instead of my isRunning, and the behavior is the same. Is there something wrong with my getOutcome() routine, or the way I'm calling it from QML?

        Thanks...

        JoeCFDJ Offline
        JoeCFDJ Offline
        JoeCFD
        wrote on last edited by JoeCFD
        #2

        @mzimmers you are returning the raw pointer of outcome from this func. And outcome is destroyed after this func call. I guess you are supposed to return outcome or make outcome a local variable in OutcomeModel

        mzimmersM 1 Reply Last reply
        0
        • mzimmersM mzimmers

          Hi all -

          My app is doing something rather strange - upon entering a QML component the first time, everything works, but upon entering it a second time, it crashes. I can't tell exactly where it's crashing because of an RTTI issue, but I think it has to do with the final line in this passage:

          class Outcome : public QObject
          {
              Q_OBJECT
              QML_ELEMENT
              Q_PROPERTY(bool isRunning READ isRunning WRITE setIsRunning NOTIFY isRunningChanged FINAL)
              bool isRunning() { return m_isRunning; }
              bool setIsRunning(bool running);
              ...
          }
          
          Outcome* OutcomeModel::getOutcome(const QUuid &uuid)
          {
              auto outcome {std::make_shared<Outcome>(this)};
              const auto i { getIndex(uuid) };
              if (i == NOT_IN_LIST) {
                  qWarning() << __PRETTY_FUNCTION__ << "uuid not found in list.";
              } else {
                  outcome = m_list.at(i);
              }
              return outcome.get();
          }
          
          // qml
          text: "running: " + outcomeModel.getOutcome(modelData).isRunning 
          

          I've used other properties instead of my isRunning, and the behavior is the same. Is there something wrong with my getOutcome() routine, or the way I'm calling it from QML?

          Thanks...

          jeremy_kJ Offline
          jeremy_kJ Offline
          jeremy_k
          wrote on last edited by jeremy_k
          #3

          @mzimmers said in QML object access through model crashes:

          Hi all -

          My app is doing something rather strange - upon entering a QML component

          What does entering mean?

          Outcome* OutcomeModel::getOutcome(const QUuid &uuid)
          {
              auto outcome {std::make_shared<Outcome>(this)};
              const auto i { getIndex(uuid) };
              if (i == NOT_IN_LIST) {
                  qWarning() << __PRETTY_FUNCTION__ << "uuid not found in list.";
              } else {
                  outcome = m_list.at(i);
              }
              return outcome.get();
          }
          

          This looks extremely suspicious. The QML engine doesn't know about shared_ptr, and the get() is stripping that information anyway. The engine presumes ownership of created objects unless told otherwise. That means that when the object goes out of scope, the engine will eventually invoke the Outcome destructor. Holding a shared pointer to it elsewhere won't prevent that.

          If I understand the else section, under some circumstance the OutcomeModel factory will attempt to reuse a previously created object that may have been deleted by the engine.

          Asking a question about code? http://eel.is/iso-c++/testcase/

          mzimmersM 1 Reply Last reply
          1
          • jeremy_kJ jeremy_k

            @mzimmers said in QML object access through model crashes:

            Hi all -

            My app is doing something rather strange - upon entering a QML component

            What does entering mean?

            Outcome* OutcomeModel::getOutcome(const QUuid &uuid)
            {
                auto outcome {std::make_shared<Outcome>(this)};
                const auto i { getIndex(uuid) };
                if (i == NOT_IN_LIST) {
                    qWarning() << __PRETTY_FUNCTION__ << "uuid not found in list.";
                } else {
                    outcome = m_list.at(i);
                }
                return outcome.get();
            }
            

            This looks extremely suspicious. The QML engine doesn't know about shared_ptr, and the get() is stripping that information anyway. The engine presumes ownership of created objects unless told otherwise. That means that when the object goes out of scope, the engine will eventually invoke the Outcome destructor. Holding a shared pointer to it elsewhere won't prevent that.

            If I understand the else section, under some circumstance the OutcomeModel factory will attempt to reuse a previously created object that may have been deleted by the engine.

            mzimmersM Offline
            mzimmersM Offline
            mzimmers
            wrote on last edited by
            #4

            @jeremy_k said in QML object access through model crashes:

            What does entering mean?

            I mean when a component is loaded.

            In this case, the list itself is "good" -- that is, there have been only additions to it, no deletions. My list is declared like this:

            typedef QList<std::shared_ptr<Outcome>> OutcomeList;
            OutcomeList m_list;
            

            I don't really need to maintain a list of pointers; my list could be the actual objects, but then code like this:

            outcome = m_list.at(listIndex);
            

            becomes problematic because, as a QObject-derived class, Outcome has no copy c'tor. So, I'm not sure what the best way to go is here.

            1 Reply Last reply
            0
            • JoeCFDJ JoeCFD

              @mzimmers you are returning the raw pointer of outcome from this func. And outcome is destroyed after this func call. I guess you are supposed to return outcome or make outcome a local variable in OutcomeModel

              mzimmersM Offline
              mzimmersM Offline
              mzimmers
              wrote on last edited by
              #5

              @JoeCFD said in QML object access through model crashes:

              you are returning the raw pointer of outcome from this func. And outcome is destroyed after this func call.

              But isn't the pointer itself still valid (meaning it's pointing to the correct item in the list)?

              JoeCFDJ jeremy_kJ kshegunovK 3 Replies Last reply
              0
              • mzimmersM mzimmers

                @JoeCFD said in QML object access through model crashes:

                you are returning the raw pointer of outcome from this func. And outcome is destroyed after this func call.

                But isn't the pointer itself still valid (meaning it's pointing to the correct item in the list)?

                JoeCFDJ Offline
                JoeCFDJ Offline
                JoeCFD
                wrote on last edited by
                #6

                @mzimmers It is a dangled pointer after return.

                1 Reply Last reply
                1
                • mzimmersM mzimmers

                  @JoeCFD said in QML object access through model crashes:

                  you are returning the raw pointer of outcome from this func. And outcome is destroyed after this func call.

                  But isn't the pointer itself still valid (meaning it's pointing to the correct item in the list)?

                  jeremy_kJ Offline
                  jeremy_kJ Offline
                  jeremy_k
                  wrote on last edited by jeremy_k
                  #7

                  @mzimmers said in QML object access through model crashes:

                  @JoeCFD said in QML object access through model crashes:

                  you are returning the raw pointer of outcome from this func. And outcome is destroyed after this func call.

                  But isn't the pointer itself still valid (meaning it's pointing to the correct item in the list)?

                  Add some logging to the destructor to see when the object is destroyed.

                  Asking a question about code? http://eel.is/iso-c++/testcase/

                  1 Reply Last reply
                  0
                  • mzimmersM mzimmers

                    @JoeCFD said in QML object access through model crashes:

                    you are returning the raw pointer of outcome from this func. And outcome is destroyed after this func call.

                    But isn't the pointer itself still valid (meaning it's pointing to the correct item in the list)?

                    kshegunovK Offline
                    kshegunovK Offline
                    kshegunov
                    Moderators
                    wrote on last edited by kshegunov
                    #8

                    Also, unless you're fan of pain and suffering don't hold QObjects in shared pointers. You're basically saying that the object owns itself (semantically) and have no guarantee when this object is going to be freed. This in turn may cause you to execute a slot on an object that's getting/got deleted.

                    If you want to own a QObject use std::unique_ptr/QScopedPointer, if you're going to hold a weak reference to the object (i.e. to hold a pointer without owning the actual object) then use QPointer.

                    Read and abide by the Qt Code of Conduct

                    mzimmersM 1 Reply Last reply
                    0
                    • kshegunovK kshegunov

                      Also, unless you're fan of pain and suffering don't hold QObjects in shared pointers. You're basically saying that the object owns itself (semantically) and have no guarantee when this object is going to be freed. This in turn may cause you to execute a slot on an object that's getting/got deleted.

                      If you want to own a QObject use std::unique_ptr/QScopedPointer, if you're going to hold a weak reference to the object (i.e. to hold a pointer without owning the actual object) then use QPointer.

                      mzimmersM Offline
                      mzimmersM Offline
                      mzimmers
                      wrote on last edited by
                      #9

                      I added a member to my OutcomeModel:

                          Outcome *m_outcome;
                      

                      and modified my get routine:

                      Outcome *OutcomeModel::getOutcome(const QUuid &uuid)
                      {
                          bool found = false;
                          m_outcome = nullptr;
                          for (const auto &o: m_list) {
                              if (o->uuid() == uuid) {
                                  m_outcome = o.get();
                                  found = true;
                                  break;
                              }
                          }
                          if (!found) {
                              m_outcome = m_list.at(0).get();
                          }
                          return m_outcome;
                      }
                      

                      (I realize this isn't the desired ultimate behavior, but I wanted to try to ensure that m_outcome would always point to something valid - the list will always contain at least one item).

                      I'm getting the same behavior. Obviously I'm missing something, but I don't see what it is.

                      kshegunovK 1 Reply Last reply
                      0
                      • mzimmersM mzimmers

                        I added a member to my OutcomeModel:

                            Outcome *m_outcome;
                        

                        and modified my get routine:

                        Outcome *OutcomeModel::getOutcome(const QUuid &uuid)
                        {
                            bool found = false;
                            m_outcome = nullptr;
                            for (const auto &o: m_list) {
                                if (o->uuid() == uuid) {
                                    m_outcome = o.get();
                                    found = true;
                                    break;
                                }
                            }
                            if (!found) {
                                m_outcome = m_list.at(0).get();
                            }
                            return m_outcome;
                        }
                        

                        (I realize this isn't the desired ultimate behavior, but I wanted to try to ensure that m_outcome would always point to something valid - the list will always contain at least one item).

                        I'm getting the same behavior. Obviously I'm missing something, but I don't see what it is.

                        kshegunovK Offline
                        kshegunovK Offline
                        kshegunov
                        Moderators
                        wrote on last edited by
                        #10

                        As usual, if we are tracking a segfault here, do provide a stack trace.

                        Read and abide by the Qt Code of Conduct

                        mzimmersM 1 Reply Last reply
                        0
                        • kshegunovK kshegunov

                          As usual, if we are tracking a segfault here, do provide a stack trace.

                          mzimmersM Offline
                          mzimmersM Offline
                          mzimmers
                          wrote on last edited by
                          #11

                          @kshegunov I can't - the error occurs somewhere within QObject and there's no RTTI information for it.

                          BTW: I'm not ignoring your suggestion about shared_ptr; I just want to get this figured out first.

                          JonBJ kshegunovK 2 Replies Last reply
                          0
                          • mzimmersM mzimmers

                            @kshegunov I can't - the error occurs somewhere within QObject and there's no RTTI information for it.

                            BTW: I'm not ignoring your suggestion about shared_ptr; I just want to get this figured out first.

                            JonBJ Offline
                            JonBJ Offline
                            JonB
                            wrote on last edited by
                            #12

                            @mzimmers said in QML object access through model crashes:

                            @kshegunov I can't - the error occurs somewhere within QObject and there's no RTTI information for it.

                            I know you mentioned this elsewhere, but this ought not stop you getting a stack trace from a seg fault. It might stop you seeing the internals of a QObject, but are you saying this is somehow an "error" in the debugger which prevents you accessing a stack trace? Even if it does inside a QObject, you should still see information about where it was in your or Qt code.

                            1 Reply Last reply
                            1
                            • mzimmersM mzimmers

                              @kshegunov I can't - the error occurs somewhere within QObject and there's no RTTI information for it.

                              BTW: I'm not ignoring your suggestion about shared_ptr; I just want to get this figured out first.

                              kshegunovK Offline
                              kshegunovK Offline
                              kshegunov
                              Moderators
                              wrote on last edited by kshegunov
                              #13

                              @mzimmers said in QML object access through model crashes:

                              BTW: I'm not ignoring your suggestion about shared_ptr; I just want to get this figured out first.

                              Substitute your shared_ptr with QPointer and when you fill in that list use new. At the point of crash if you are seeing dereferencing a nullptr, then something's not right with the ownership; that is to say something (probably QML) took ownership of the objects and freed them and you were left with dangling pointers all over the place.

                              PS.
                              You should almost never use shared_ptr::get, which erases the notion of the control block and who/when this object is going to be culled.

                              Read and abide by the Qt Code of Conduct

                              mzimmersM 1 Reply Last reply
                              2
                              • kshegunovK kshegunov

                                @mzimmers said in QML object access through model crashes:

                                BTW: I'm not ignoring your suggestion about shared_ptr; I just want to get this figured out first.

                                Substitute your shared_ptr with QPointer and when you fill in that list use new. At the point of crash if you are seeing dereferencing a nullptr, then something's not right with the ownership; that is to say something (probably QML) took ownership of the objects and freed them and you were left with dangling pointers all over the place.

                                PS.
                                You should almost never use shared_ptr::get, which erases the notion of the control block and who/when this object is going to be culled.

                                mzimmersM Offline
                                mzimmersM Offline
                                mzimmers
                                wrote on last edited by
                                #14

                                Thanks to your suggestions, I think I'm making progress. In my OutcomeModel, I've eliminated the Outcome element, and done the following:

                                typedef QPointer<Outcome> OutcomePtr;
                                typedef QList<OutcomePtr> OutcomeList;
                                OutcomePtr OutcomeModel::getOutcome(const QUuid &uuid)
                                {
                                   // auto outcome {std::make_shared<Outcome>(this)};
                                   OutcomePtr outcome { new Outcome };
                                   const auto i { getIndex(uuid) };
                                   if (i == NgaUI::NOT_IN_LIST) {
                                       qWarning() << __PRETTY_FUNCTION__ << "uuid not found in list."; // I never see this warning.
                                   } else {
                                       delete outcome;
                                       outcome = m_list.at(i);
                                   }
                                   return outcome.data();
                                }
                                

                                I'm no longer getting segmentation faults, so I think we may have solved that problem. So, now the remaining issue is how to use the QPointer to access the Outcome properties from my QML? I tried this:

                                ListView {
                                    model: outcomeList // a list of UUIDs
                                    delegate: RowLayout {
                                        Label {
                                            text: "running: " + outcomeModel.getOutcome(modelData).isRunning
                                

                                but the Label shows as undefined. I've verified that the getOutcome() routine seems to be returning good data. Any ideas what I'm doing wrong?

                                Thanks...

                                JoeCFDJ GrecKoG 2 Replies Last reply
                                0
                                • mzimmersM mzimmers

                                  Thanks to your suggestions, I think I'm making progress. In my OutcomeModel, I've eliminated the Outcome element, and done the following:

                                  typedef QPointer<Outcome> OutcomePtr;
                                  typedef QList<OutcomePtr> OutcomeList;
                                  OutcomePtr OutcomeModel::getOutcome(const QUuid &uuid)
                                  {
                                     // auto outcome {std::make_shared<Outcome>(this)};
                                     OutcomePtr outcome { new Outcome };
                                     const auto i { getIndex(uuid) };
                                     if (i == NgaUI::NOT_IN_LIST) {
                                         qWarning() << __PRETTY_FUNCTION__ << "uuid not found in list."; // I never see this warning.
                                     } else {
                                         delete outcome;
                                         outcome = m_list.at(i);
                                     }
                                     return outcome.data();
                                  }
                                  

                                  I'm no longer getting segmentation faults, so I think we may have solved that problem. So, now the remaining issue is how to use the QPointer to access the Outcome properties from my QML? I tried this:

                                  ListView {
                                      model: outcomeList // a list of UUIDs
                                      delegate: RowLayout {
                                          Label {
                                              text: "running: " + outcomeModel.getOutcome(modelData).isRunning
                                  

                                  but the Label shows as undefined. I've verified that the getOutcome() routine seems to be returning good data. Any ideas what I'm doing wrong?

                                  Thanks...

                                  JoeCFDJ Offline
                                  JoeCFDJ Offline
                                  JoeCFD
                                  wrote on last edited by
                                  #15

                                  @mzimmers delegate in ListView is a component from here:
                                  https://doc.qt.io/qt-6/qml-qtquick-listview.html#delegate-prop
                                  Is RowLayout a component?

                                  mzimmersM 1 Reply Last reply
                                  0
                                  • JoeCFDJ JoeCFD

                                    @mzimmers delegate in ListView is a component from here:
                                    https://doc.qt.io/qt-6/qml-qtquick-listview.html#delegate-prop
                                    Is RowLayout a component?

                                    mzimmersM Offline
                                    mzimmersM Offline
                                    mzimmers
                                    wrote on last edited by
                                    #16

                                    @JoeCFD if I understand your point, I need to do this instead:

                                    ListView {
                                        model: outcomeList
                                        delegate: rowComponent
                                        Component {
                                            id: rowComponent
                                            RowLayout {
                                                Label {
                                                    id: runningLabel
                                                    text: "running: " + outcomeModel.getOutcome(modelData).isRunning
                                    

                                    Is this what you were getting at? The behavior seems the same.

                                    Thanks...

                                    1 Reply Last reply
                                    0
                                    • mzimmersM mzimmers

                                      Thanks to your suggestions, I think I'm making progress. In my OutcomeModel, I've eliminated the Outcome element, and done the following:

                                      typedef QPointer<Outcome> OutcomePtr;
                                      typedef QList<OutcomePtr> OutcomeList;
                                      OutcomePtr OutcomeModel::getOutcome(const QUuid &uuid)
                                      {
                                         // auto outcome {std::make_shared<Outcome>(this)};
                                         OutcomePtr outcome { new Outcome };
                                         const auto i { getIndex(uuid) };
                                         if (i == NgaUI::NOT_IN_LIST) {
                                             qWarning() << __PRETTY_FUNCTION__ << "uuid not found in list."; // I never see this warning.
                                         } else {
                                             delete outcome;
                                             outcome = m_list.at(i);
                                         }
                                         return outcome.data();
                                      }
                                      

                                      I'm no longer getting segmentation faults, so I think we may have solved that problem. So, now the remaining issue is how to use the QPointer to access the Outcome properties from my QML? I tried this:

                                      ListView {
                                          model: outcomeList // a list of UUIDs
                                          delegate: RowLayout {
                                              Label {
                                                  text: "running: " + outcomeModel.getOutcome(modelData).isRunning
                                      

                                      but the Label shows as undefined. I've verified that the getOutcome() routine seems to be returning good data. Any ideas what I'm doing wrong?

                                      Thanks...

                                      GrecKoG Offline
                                      GrecKoG Offline
                                      GrecKo
                                      Qt Champions 2018
                                      wrote on last edited by
                                      #17

                                      Any ideas what I'm doing wrong?

                                      To be blunt I'd say having trouble explaining what your issue is and blindly following strangers advices.

                                      QPointer is an observing pointer. Don't use it to keep ownership.
                                      shared_ptr was fine, unique_ptr may have been better. raw pointers could also be used if using the QObject parent ownership system, it would require to manually delete the object on removal though.

                                      Your unusual code rules do not help you there (the "always init your variables at the start of a function" and "only one return").

                                      Your getOutcome function is leaking (when not finding the uuid, doing unnecessary temporary allocation when finding it).

                                      the Label shows as undefined

                                      Is the text "undefined", "running: undefined", something else?
                                      what does outcomeModel.getOutcome(modelData) returns?

                                      The source issue of your problem was most likely that your object has no QObject::parent, thus the QML engine taking ownership of it when it access it from your Q_INVOKABLE.

                                      https://doc.qt.io/qt-6/qtqml-cppintegration-data.html#data-ownership

                                      I'd go back to using shared_ptr or unique_ptr and making sure to set a parent to your contained objects so the QML engine don't take ownership of it.

                                      Keep in mind I'm also an internet stranger, so don't apply blindly what I'm claiming.

                                      mzimmersM 1 Reply Last reply
                                      1
                                      • GrecKoG GrecKo

                                        Any ideas what I'm doing wrong?

                                        To be blunt I'd say having trouble explaining what your issue is and blindly following strangers advices.

                                        QPointer is an observing pointer. Don't use it to keep ownership.
                                        shared_ptr was fine, unique_ptr may have been better. raw pointers could also be used if using the QObject parent ownership system, it would require to manually delete the object on removal though.

                                        Your unusual code rules do not help you there (the "always init your variables at the start of a function" and "only one return").

                                        Your getOutcome function is leaking (when not finding the uuid, doing unnecessary temporary allocation when finding it).

                                        the Label shows as undefined

                                        Is the text "undefined", "running: undefined", something else?
                                        what does outcomeModel.getOutcome(modelData) returns?

                                        The source issue of your problem was most likely that your object has no QObject::parent, thus the QML engine taking ownership of it when it access it from your Q_INVOKABLE.

                                        https://doc.qt.io/qt-6/qtqml-cppintegration-data.html#data-ownership

                                        I'd go back to using shared_ptr or unique_ptr and making sure to set a parent to your contained objects so the QML engine don't take ownership of it.

                                        Keep in mind I'm also an internet stranger, so don't apply blindly what I'm claiming.

                                        mzimmersM Offline
                                        mzimmersM Offline
                                        mzimmers
                                        wrote on last edited by mzimmers
                                        #18

                                        @GrecKo points taken.

                                        @GrecKo said in QML object access through model crashes:

                                        Your unusual code rules do not help you there (the "always init your variables at the start of a function" and "only one return").
                                        Your getOutcome function is leaking (when not finding the uuid, doing unnecessary temporary allocation when finding it).

                                        Is this better? EDIT: I can't do this with unique_ptr; the second return statement is invalid.

                                        OutcomePtr OutcomeModel::getOutcome(const QUuid &uuid)
                                        {
                                            const auto i { getIndex(uuid) };
                                            if (i == NgaUI::NOT_IN_LIST) {
                                                qWarning() << __PRETTY_FUNCTION__ << "uuid not found in list.";
                                                return nullptr;
                                            } else {
                                                return m_list.at(i);
                                            }
                                        }
                                        

                                        @GrecKo said in QML object access through model crashes:

                                        Is the text "undefined", "running: undefined", something else?

                                        "running: undefined"

                                        @GrecKo said in QML object access through model crashes:

                                        what does outcomeModel.getOutcome(modelData) returns?

                                        Screenshot 2024-02-12 092033.png
                                        I also notice that the address of outcome doesn't match the address of m_list[2], which I find somewhat strange (if these were old-fashioned pointers, it should, I believe).
                                        EDIT: this screenshot might be more helpful (this was using a shared_ptr):
                                        Screenshot 2024-02-12 101535.png
                                        I do notice that now the addresses of outcome and m_list[2] agree, so I guess this is progress. Still having the "undefined" issue, though.

                                        mzimmersM 1 Reply Last reply
                                        0
                                        • mzimmersM mzimmers

                                          @GrecKo points taken.

                                          @GrecKo said in QML object access through model crashes:

                                          Your unusual code rules do not help you there (the "always init your variables at the start of a function" and "only one return").
                                          Your getOutcome function is leaking (when not finding the uuid, doing unnecessary temporary allocation when finding it).

                                          Is this better? EDIT: I can't do this with unique_ptr; the second return statement is invalid.

                                          OutcomePtr OutcomeModel::getOutcome(const QUuid &uuid)
                                          {
                                              const auto i { getIndex(uuid) };
                                              if (i == NgaUI::NOT_IN_LIST) {
                                                  qWarning() << __PRETTY_FUNCTION__ << "uuid not found in list.";
                                                  return nullptr;
                                              } else {
                                                  return m_list.at(i);
                                              }
                                          }
                                          

                                          @GrecKo said in QML object access through model crashes:

                                          Is the text "undefined", "running: undefined", something else?

                                          "running: undefined"

                                          @GrecKo said in QML object access through model crashes:

                                          what does outcomeModel.getOutcome(modelData) returns?

                                          Screenshot 2024-02-12 092033.png
                                          I also notice that the address of outcome doesn't match the address of m_list[2], which I find somewhat strange (if these were old-fashioned pointers, it should, I believe).
                                          EDIT: this screenshot might be more helpful (this was using a shared_ptr):
                                          Screenshot 2024-02-12 101535.png
                                          I do notice that now the addresses of outcome and m_list[2] agree, so I guess this is progress. Still having the "undefined" issue, though.

                                          mzimmersM Offline
                                          mzimmersM Offline
                                          mzimmers
                                          wrote on last edited by
                                          #19

                                          Update: I've got something now that works (doesn't crash and properly updates the QML). Here's the C++:

                                          typedef std::shared_ptr<Outcome> OutcomePtr;
                                          typedef QList<OutcomePtr> OutcomeList;
                                          
                                          Outcome *OutcomeModel::getOutcome(const QUuid &uuid)
                                          {
                                              const auto i { getIndex(uuid) };
                                              if (i == NgaUI::NOT_IN_LIST) {
                                                  return nullptr;
                                              } else {
                                                  OutcomePtr outcome(m_list.at(i));
                                                  return outcome.get();
                                              }
                                          }
                                          

                                          And the QML:

                                          ListView {
                                              model: outcomeList
                                              delegate: rowComponent
                                              Component {
                                                  id: rowComponent
                                                  RowLayout {
                                                      property Outcome outcome: outcomeModel.getOutcome(modelData)
                                                      Label {
                                                          text: "running: " + outcome.isRunning
                                                      }
                                          

                                          So, I have 2 follow up questions:

                                          1. if I were to use unique_ptrs instead of shared_ptrs, how do I code this line? I can't understand from the docs.
                                              OutcomePtr outcome(m_list.at(i));
                                          
                                          1. any other comments/room for improvement that anyone can see?

                                          Thanks to everyone for the help on this...

                                          kshegunovK 1 Reply Last reply
                                          0
                                          • mzimmersM mzimmers

                                            Update: I've got something now that works (doesn't crash and properly updates the QML). Here's the C++:

                                            typedef std::shared_ptr<Outcome> OutcomePtr;
                                            typedef QList<OutcomePtr> OutcomeList;
                                            
                                            Outcome *OutcomeModel::getOutcome(const QUuid &uuid)
                                            {
                                                const auto i { getIndex(uuid) };
                                                if (i == NgaUI::NOT_IN_LIST) {
                                                    return nullptr;
                                                } else {
                                                    OutcomePtr outcome(m_list.at(i));
                                                    return outcome.get();
                                                }
                                            }
                                            

                                            And the QML:

                                            ListView {
                                                model: outcomeList
                                                delegate: rowComponent
                                                Component {
                                                    id: rowComponent
                                                    RowLayout {
                                                        property Outcome outcome: outcomeModel.getOutcome(modelData)
                                                        Label {
                                                            text: "running: " + outcome.isRunning
                                                        }
                                            

                                            So, I have 2 follow up questions:

                                            1. if I were to use unique_ptrs instead of shared_ptrs, how do I code this line? I can't understand from the docs.
                                                OutcomePtr outcome(m_list.at(i));
                                            
                                            1. any other comments/room for improvement that anyone can see?

                                            Thanks to everyone for the help on this...

                                            kshegunovK Offline
                                            kshegunovK Offline
                                            kshegunov
                                            Moderators
                                            wrote on last edited by
                                            #20

                                            @GrecKo said in QML object access through model crashes:

                                            To be blunt I'd say having trouble explaining what your issue is and blindly following strangers advices.

                                            Hey, I take pride in my internet stranger's random advices.

                                            QPointer is an observing pointer. Don't use it to keep ownership.

                                            As already stated ...

                                            @mzimmers said in QML object access through model crashes:

                                            if I were to use unique_ptrs instead of shared_ptrs, how do I code this line? I can't understand from the docs.

                                            You don't. unique_ptr/QScopedPointer is an owning wrapper. You'd return the underlying raw pointer instead (which you already do with shared_ptr).
                                            I believe this is what you're looking for: https://en.cppreference.com/w/cpp/memory/unique_ptr/get

                                            Read and abide by the Qt Code of Conduct

                                            mzimmersM 1 Reply Last reply
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