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QML object access through model crashes

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  • 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
                          0
                          • kshegunovK kshegunov

                            @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

                            mzimmersM Offline
                            mzimmersM Offline
                            mzimmers
                            wrote on last edited by
                            #21

                            @kshegunov said in QML object access through model crashes:

                            You'd return the underlying raw pointer instead

                            Got it. Now, how about something like this:

                            auto outcome { std::make_unique<Outcome>(this) };
                            const auto listIndex { getIndex(uuid) };
                            
                            if (listIndex == NgaUI::NOT_IN_LIST) { // will append to list below.
                                outcome->setUuid(uuid);
                            } else {
                                outcome = m_list.at(listIndex);
                            }
                            

                            This is used in a function that updates the model, either by adding items to the model list, or updating the existing items. The last line won't compile. I could delete and re-create the pointer here, but I suspect there's a better way to go about this.

                            kshegunovK 1 Reply Last reply
                            0
                            • mzimmersM mzimmers

                              @kshegunov said in QML object access through model crashes:

                              You'd return the underlying raw pointer instead

                              Got it. Now, how about something like this:

                              auto outcome { std::make_unique<Outcome>(this) };
                              const auto listIndex { getIndex(uuid) };
                              
                              if (listIndex == NgaUI::NOT_IN_LIST) { // will append to list below.
                                  outcome->setUuid(uuid);
                              } else {
                                  outcome = m_list.at(listIndex);
                              }
                              

                              This is used in a function that updates the model, either by adding items to the model list, or updating the existing items. The last line won't compile. I could delete and re-create the pointer here, but I suspect there's a better way to go about this.

                              kshegunovK Offline
                              kshegunovK Offline
                              kshegunov
                              Moderators
                              wrote on last edited by kshegunov
                              #22

                              @mzimmers said in QML object access through model crashes:
                              Well assuming this is similar to the code we saw above:

                              if (listIndex != NgaUI::NOT_IN_LIST)
                                  return m_list.at(listIndex).get();
                              
                              auto outcome = std::make_unique<Outcome>(this);
                              outcome->setUuid(uuid);
                              return m_list.emplaceBack(std::move(outcome)).get();
                              

                              Or something of this sort, I imagine.

                              Read and abide by the Qt Code of Conduct

                              JoeCFDJ 1 Reply Last reply
                              0
                              • kshegunovK kshegunov

                                @mzimmers said in QML object access through model crashes:
                                Well assuming this is similar to the code we saw above:

                                if (listIndex != NgaUI::NOT_IN_LIST)
                                    return m_list.at(listIndex).get();
                                
                                auto outcome = std::make_unique<Outcome>(this);
                                outcome->setUuid(uuid);
                                return m_list.emplaceBack(std::move(outcome)).get();
                                

                                Or something of this sort, I imagine.

                                JoeCFDJ Offline
                                JoeCFDJ Offline
                                JoeCFD
                                wrote on last edited by JoeCFD
                                #23

                                @kshegunov why is the unique pointer even needed in the first place?
                                to @mzimmers: is OutcomeModel a subclass of Qt class? I can not see it.

                                kshegunovK mzimmersM GrecKoG 3 Replies Last reply
                                0
                                • JoeCFDJ JoeCFD

                                  @kshegunov why is the unique pointer even needed in the first place?
                                  to @mzimmers: is OutcomeModel a subclass of Qt class? I can not see it.

                                  kshegunovK Offline
                                  kshegunovK Offline
                                  kshegunov
                                  Moderators
                                  wrote on last edited by kshegunov
                                  #24

                                  @JoeCFD said in QML object access through model crashes:

                                  @kshegunov why is the unique pointer even needed in the first place?

                                  It isn't, strictly speaking. If it were me, I'd've implemented it with regular ol' raw pointers. The difference is only in the semantics - std::unique_ptr means "I own the stuff" (also implying you can't have two different std::unique_ptr instances pointing to the same object, hence the copy constructor being deleted).

                                  to @mzimmers: is OutcomeModel a subclass of Qt class? I can not see it.

                                  Both the model and the Outcome class are derived from Qt classes. I imagine the OutcomeModel is derived from QAbstractTableModel, where as the Outcome is derived from QObject (or something akin).

                                  Read and abide by the Qt Code of Conduct

                                  JoeCFDJ 1 Reply Last reply
                                  0
                                  • JoeCFDJ JoeCFD

                                    @kshegunov why is the unique pointer even needed in the first place?
                                    to @mzimmers: is OutcomeModel a subclass of Qt class? I can not see it.

                                    mzimmersM Offline
                                    mzimmersM Offline
                                    mzimmers
                                    wrote on last edited by
                                    #25

                                    @JoeCFD said in QML object access through model crashes:

                                    is OutcomeModel a subclass of Qt class?

                                    Yes, a subclass of QAbstractListModel. And Outcome is a subclass of QObject.

                                    1 Reply Last reply
                                    0
                                    • kshegunovK kshegunov

                                      @JoeCFD said in QML object access through model crashes:

                                      @kshegunov why is the unique pointer even needed in the first place?

                                      It isn't, strictly speaking. If it were me, I'd've implemented it with regular ol' raw pointers. The difference is only in the semantics - std::unique_ptr means "I own the stuff" (also implying you can't have two different std::unique_ptr instances pointing to the same object, hence the copy constructor being deleted).

                                      to @mzimmers: is OutcomeModel a subclass of Qt class? I can not see it.

                                      Both the model and the Outcome class are derived from Qt classes. I imagine the OutcomeModel is derived from QAbstractTableModel, where as the Outcome is derived from QObject (or something akin).

                                      JoeCFDJ Offline
                                      JoeCFDJ Offline
                                      JoeCFD
                                      wrote on last edited by JoeCFD
                                      #26

                                      @kshegunov Ok. Qt has its own garbage collection mechanism. Therefore, any derived classes from Qt classes may not need any shared or unique pointers unless they are needed to be deleted immediately for example inside one func call(sometimes I do it). I guess in most cases, raw pointers are good enough while Qt will take care of memory release.
                                      In his case, the pointer is used(returned). I guess a raw pointer is good enough.

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                                      • JoeCFDJ JoeCFD

                                        @kshegunov why is the unique pointer even needed in the first place?
                                        to @mzimmers: is OutcomeModel a subclass of Qt class? I can not see it.

                                        GrecKoG Offline
                                        GrecKoG Offline
                                        GrecKo
                                        Qt Champions 2018
                                        wrote on last edited by
                                        #27

                                        @JoeCFD the advantage of using an owning smart pointer in a container is that when removing an element from a container, the pointed-to object will be automatically deleted.

                                        @mzimmers the question is now why are you using this getOutcome() function at all in your model?

                                        How do you expose the uuid in the first place? Do you get it from your OutcomeList? Can't you directly add a role returning an Outcome*?

                                        JoeCFDJ mzimmersM 2 Replies Last reply
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                                        • GrecKoG GrecKo

                                          @JoeCFD the advantage of using an owning smart pointer in a container is that when removing an element from a container, the pointed-to object will be automatically deleted.

                                          @mzimmers the question is now why are you using this getOutcome() function at all in your model?

                                          How do you expose the uuid in the first place? Do you get it from your OutcomeList? Can't you directly add a role returning an Outcome*?

                                          JoeCFDJ Offline
                                          JoeCFDJ Offline
                                          JoeCFD
                                          wrote on last edited by JoeCFD
                                          #28

                                          @GrecKo That is ok. This case is same as I do in a func call. But, I do not think his usage of shared or unique pointer in this func is correct.

                                          raw pointers will work for mzimmers as well. He got confused with shared or unique pointers.

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