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if (my_pointer==nullptr) causes segmentation fault

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  • Seb TurS Seb Tur

    with if(param==nullptr) I get the same segfault

    I added this-> just in case after being clueless for hours

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

    @Seb-Tur run your app with valgrind to check it out if you are working on Linux.

    1 Reply Last reply
    0
    • Chris KawaC Offline
      Chris KawaC Offline
      Chris Kawa
      Lifetime Qt Champion
      wrote on last edited by
      #7

      This will happen when you call a method on a deleted object. this points to garbage and any access to its members results in seg fault. Make sure your object is still alive when you call this function.

      Seb TurS 1 Reply Last reply
      1
      • Chris KawaC Chris Kawa

        This will happen when you call a method on a deleted object. this points to garbage and any access to its members results in seg fault. Make sure your object is still alive when you call this function.

        Seb TurS Offline
        Seb TurS Offline
        Seb Tur
        wrote on last edited by Seb Tur
        #8

        @Chris-Kawa

        I don't understand
        I'm calling this check in a method belonging to inicio_common class and param also belongs there.
        so when I create an object

        inicio_common *common_object;
        so then it gets its member from .h
        common_object->param =nullptr

        so
        method common_object->parameter_read()
        reffers to common_object->param

        Param is a sibling member , so how come it's parent "this" can be garbage? If this was garbage than its method read_paramater wouldnt't be called either?
        I don't delete param in any method in inicio_common class. It is a private member.

        Christian EhrlicherC 1 Reply Last reply
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        • JoeCFDJ Offline
          JoeCFDJ Offline
          JoeCFD
          wrote on last edited by JoeCFD
          #9

          @Seb-Tur said in if (my_pointer==nullptr) causes segmentation fault:

          inicio_common *common_object;

          how did you create this one: inicio_common *common_object{}; ? is it nullptr?

          Seb TurS 1 Reply Last reply
          0
          • Seb TurS Seb Tur

            @Chris-Kawa

            I don't understand
            I'm calling this check in a method belonging to inicio_common class and param also belongs there.
            so when I create an object

            inicio_common *common_object;
            so then it gets its member from .h
            common_object->param =nullptr

            so
            method common_object->parameter_read()
            reffers to common_object->param

            Param is a sibling member , so how come it's parent "this" can be garbage? If this was garbage than its method read_paramater wouldnt't be called either?
            I don't delete param in any method in inicio_common class. It is a private member.

            Christian EhrlicherC Offline
            Christian EhrlicherC Offline
            Christian Ehrlicher
            Lifetime Qt Champion
            wrote on last edited by
            #10

            @Seb-Tur said in if (my_pointer==nullptr) causes segmentation fault:

            If this was garbage than its method read_paramater wouldnt't be called either?

            Why not? Who would prevent it?

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            • JoeCFDJ JoeCFD

              @Seb-Tur said in if (my_pointer==nullptr) causes segmentation fault:

              inicio_common *common_object;

              how did you create this one: inicio_common *common_object{}; ? is it nullptr?

              Seb TurS Offline
              Seb TurS Offline
              Seb Tur
              wrote on last edited by
              #11

              @JoeCFD

              declared in mainwindow.h as
              inicio_common = *common_object =nullptr

              than contructed at the begining of mainwindow.cpp

              common_object = new inicio_common();

              JoeCFDJ 1 Reply Last reply
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              • Seb TurS Seb Tur

                @JoeCFD

                declared in mainwindow.h as
                inicio_common = *common_object =nullptr

                than contructed at the begining of mainwindow.cpp

                common_object = new inicio_common();

                JoeCFDJ Offline
                JoeCFDJ Offline
                JoeCFD
                wrote on last edited by
                #12

                @Seb-Tur I guess it is better for you to show more code.

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                • Chris KawaC Offline
                  Chris KawaC Offline
                  Chris Kawa
                  Lifetime Qt Champion
                  wrote on last edited by Chris Kawa
                  #13

                  @Seb-Tur It's not the param member that is deleted. It's the instance of class inicio_common.

                  One case where this would happen:

                  inicio_common *common_object;       // common_object is an uninitialized variable
                  common_object->parameter_read(...); // calling method with garbage "this" -> crash
                  

                  another way the same happens:

                  inicio_common *common_object = new inicio_common(); // common_object is initialized, ok
                  common_object->parameter_read(...);                 // works
                  delete common_object;                               // common_object now points to garbage
                  common_object->parameter_read(...);                 // calling method with garbage "this" -> crash
                  

                  If this was garbage than its method read_paramater wouldnt't be called either?

                  There's nothing in C++ language preventing that. You can create pointers pointing to random memory and call methods on them. For example:

                  QString* foo = (QString*)12345; // because why not
                  foo->clear();                   // garbage "this" -> crash
                  
                  1 Reply Last reply
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                  • Seb TurS Offline
                    Seb TurS Offline
                    Seb Tur
                    wrote on last edited by Seb Tur
                    #14

                    Thanks all for the support, your hints directed me to the right path to find an error I had no idea was an error.

                    Here's the scenario :

                    I used common_object in mainwindow doing various activities and all was fine;

                    at some point I was passing the common_object pointer in a constructor to another object of action_class that needed some properties of inicio_common object.,

                    action_class.h contained

                    inicio_common *ref_common = nullptr;
                    

                    but action class cunstructor was capturing the common_object

                    action_class::action_class(inicio_common *common)
                    {
                      ref_common = common;
                    }
                    

                    so in the mainwindow I had common_object used by mainwindow and action_class_object because I run:

                    action_class *action_class_object = new (common_object);
                    

                    After I was done with the actions I deleted action_class_object with

                    delete(action_class_object);

                    and this seems to have deleted the common_object by deleting ref_common in cascade .... I guess
                    Does it make sense or is it just my wicked thinking?

                    Anyway I still don't get why if(param==nullptr) would crash instead of returning true;

                    Chris KawaC 1 Reply Last reply
                    0
                    • Seb TurS Seb Tur

                      Thanks all for the support, your hints directed me to the right path to find an error I had no idea was an error.

                      Here's the scenario :

                      I used common_object in mainwindow doing various activities and all was fine;

                      at some point I was passing the common_object pointer in a constructor to another object of action_class that needed some properties of inicio_common object.,

                      action_class.h contained

                      inicio_common *ref_common = nullptr;
                      

                      but action class cunstructor was capturing the common_object

                      action_class::action_class(inicio_common *common)
                      {
                        ref_common = common;
                      }
                      

                      so in the mainwindow I had common_object used by mainwindow and action_class_object because I run:

                      action_class *action_class_object = new (common_object);
                      

                      After I was done with the actions I deleted action_class_object with

                      delete(action_class_object);

                      and this seems to have deleted the common_object by deleting ref_common in cascade .... I guess
                      Does it make sense or is it just my wicked thinking?

                      Anyway I still don't get why if(param==nullptr) would crash instead of returning true;

                      Chris KawaC Offline
                      Chris KawaC Offline
                      Chris Kawa
                      Lifetime Qt Champion
                      wrote on last edited by Chris Kawa
                      #15

                      @Seb-Tur said:

                      and this seems to have deleted the common_object by deleting ref_common in cascade .... I guess

                      delete something does not delete "something". It deletes the thing that "something" points to. "something" points to garbage after that. If you copy a pointer you copy a pointer. You're not copying the object it points to, so when you delete the object via one pointer both the original and the copied pointer point to the same garbage memory.

                      Anyway I still don't get why if(param==nullptr) would crash instead of returning true;

                      It's not the if that is crashing. It can't return anything because it's not even executed. It doesn't get to the comparison operator. this points to garbage, so this->param is trying to read a value from garbage memory. Reading from garbage memory is access violation and terminates your app.

                      Christian EhrlicherC 1 Reply Last reply
                      2
                      • Chris KawaC Chris Kawa

                        @Seb-Tur said:

                        and this seems to have deleted the common_object by deleting ref_common in cascade .... I guess

                        delete something does not delete "something". It deletes the thing that "something" points to. "something" points to garbage after that. If you copy a pointer you copy a pointer. You're not copying the object it points to, so when you delete the object via one pointer both the original and the copied pointer point to the same garbage memory.

                        Anyway I still don't get why if(param==nullptr) would crash instead of returning true;

                        It's not the if that is crashing. It can't return anything because it's not even executed. It doesn't get to the comparison operator. this points to garbage, so this->param is trying to read a value from garbage memory. Reading from garbage memory is access violation and terminates your app.

                        Christian EhrlicherC Offline
                        Christian EhrlicherC Offline
                        Christian Ehrlicher
                        Lifetime Qt Champion
                        wrote on last edited by
                        #16

                        @Chris-Kawa said in if (my_pointer==nullptr) causes segmentation fault:

                        this points to garbage, so this->param is trying to read a value from garbage memory. Reading from garbage memory is access violation and terminates your app.

                        Not that I said it in my first post... :)

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

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