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Appending QStringList crashes

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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

          1 Reply Last reply
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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

            1 Reply Last reply
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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
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                • 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

                  1 Reply Last reply
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                  • H Offline
                    H Offline
                    Hubbard
                    wrote on last edited by
                    #15

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

                    1 Reply Last reply
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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

                      1 Reply Last reply
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                      • 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

                        1 Reply Last reply
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