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Appending QStringList crashes

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  • H Hubbard

    In fact it seems to crash using 'mac' for anything at all outside of using it to initialize a class and output it through qDebug() once, Its hard for me to show the code as it is on a different platform.

    jsulmJ Offline
    jsulmJ Offline
    jsulm
    Lifetime Qt Champion
    wrote on last edited by
    #6

    @Hubbard Well, then I have no idea.
    You should at least try to debug your app. Most probably the pointer is invalid at the time the app is crashing. What type of crash is it actually (SIGSEGV, ...)?

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

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    • O ollarch

      @Hubbard Use the debugger. Maybe you are using an invalid pointer.

      H Offline
      H Offline
      Hubbard
      wrote on last edited by
      #7

      @ollarch You're right...

      none of it is the problem apart from trying to use ownedMacAddresses
      just

      qDebug() << ownedMacAddresses[0];

      in the slot

      crashes the program

      jsulmJ 1 Reply Last reply
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      • H Hubbard

        I have a large program which does many things at once (including multiple threads)

        and a small addition to a particular part of my code crashes it, in the mainWindow class a pointer QStringList exists that stores a set of mac addresses, this QStringList is initialised initially with ownedMacAddresses = &tempStringList, and later on in the code prompted by the user a mac address can be found and passed by way of signal to a slot which processes it, which is fine (QString mac is passed through the connection), however, when I try to ownedMacAddresses->append(mac), it crashes when the user clicks the button at this point, why?

        JonBJ Online
        JonBJ Online
        JonB
        wrote on last edited by
        #8

        @Hubbard said in Appending QStringList crashes:

        &tempStringList

        Has tempStringList gone out of scope?

        H 1 Reply Last reply
        1
        • JonBJ JonB

          @Hubbard said in Appending QStringList crashes:

          &tempStringList

          Has tempStringList gone out of scope?

          H Offline
          H Offline
          Hubbard
          wrote on last edited by
          #9

          @JonB No but when I qDebug() << it from somewhere else it doesn't crash but outputs the element in brackets

          ("2C:3A:E8:37:D8:6A") as opposed to "2C:3A:E8:37:D8:6A"

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          • H Offline
            H Offline
            Hubbard
            wrote on last edited by
            #10

            Never ->at(0) instead fixes that

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            • H Offline
              H Offline
              Hubbard
              wrote on last edited by
              #11

              Still crashes inside the slot

              qDebug() << ownedMacAddresses->at(0);

              anywhere else in mainWindow is fine, in this slot, nah

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              • H Hubbard

                @ollarch You're right...

                none of it is the problem apart from trying to use ownedMacAddresses
                just

                qDebug() << ownedMacAddresses[0];

                in the slot

                crashes the program

                jsulmJ Offline
                jsulmJ Offline
                jsulm
                Lifetime Qt Champion
                wrote on last edited by jsulm
                #12

                @Hubbard said in Appending QStringList crashes:

                qDebug() << ownedMacAddresses[0];

                This can't work the way you expect it to work, because ownedMacAddresses is a pointer. If you want to do it this way then so:

                qDebug() << (*ownedMacAddresses)[0];
                

                "Still crashes inside the slot
                qDebug() << ownedMacAddresses->at(0);" - did you try to place a breakpoint at that line and see whether ownedMacAddresses is a valid pointer? Also - what kind of crash is it (SIGSEGV?)?

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

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                • H Offline
                  H Offline
                  Hubbard
                  wrote on last edited by
                  #13

                  What do you mean by 'valid pointer'?

                  jsulmJ 1 Reply Last reply
                  0
                  • H Offline
                    H Offline
                    Hubbard
                    wrote on last edited by
                    #14

                    If its related I have begun to pass the pointer through the signal

                    now the crash error gives me an 'index out of range' error at element 0, implying something is definitely not right with the pointer to the array

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                    • H Offline
                      H Offline
                      Hubbard
                      wrote on last edited by
                      #15

                      Like 'emit signal(..., QStringList* currentMacs);'

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                      • H Hubbard

                        What do you mean by 'valid pointer'?

                        jsulmJ Offline
                        jsulmJ Offline
                        jsulm
                        Lifetime Qt Champion
                        wrote on last edited by jsulm
                        #16

                        @Hubbard Mit "valid pointer" I mean a pointer pointing to reserved memory and not to nirvana.

                        QStringList* p1; // Invalid pointer
                        QStringList* p2 = new QStringList(); // Valid pointer
                        

                        "Index out of range at element 0" means that you do not have any elements in the list. Check that with:

                        qDebug() << currentMacs->size();
                        

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

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                        1
                        • H Offline
                          H Offline
                          Hubbard
                          wrote on last edited by
                          #17

                          I see now

                          QStringList declared in the header file makes a tonne more sense and works better than QStringList*.

                          Thinking about it QStringList and arrays and things come with their own addresses anyway.

                          Thank you guys

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