Important: Please read the Qt Code of Conduct -

[SOLVED]How do I use a global varaible in qt

  • I have a structure that is a linked list, and I need to use it in multiple windows. My question is how I can make the variable global or other method to used the variable in all the windows.

  • Lifetime Qt Champion

    Hi and welcome
    Global vars are not the best design. It would be better to
    give the list as parameter to the other windows.
    mywindow.setlist( thelist );
    so each would have its own pointer to the list versus just having a global one.
    Anyway, to create a global list:
    create a mylist.h file and a mylist.cpp to contain your list.
    In the .h file
    extern list mylist;
    and in the cpp file
    mylist list;
    then include mylist.h in the other windows.
    and it will know mylist;

  • Moderators

    Depending on what the list contains using a global for this might range from moderately bad idea to a horrible bug.

    Remember that you can't make global QObjects, as they should be born and die during the lifespan of QApplication object. So if the list contains QObjects it's a no-no.
    Design-wise there should be no state that belongs to no one, which is the case for global variables. The data should always belong to some object that will be responsible for cleaning it up when its time is due, otherwise you've got a curious case of questionable responsibility.

    So my advice is to make it a member of the class that feels best for this and expose it to others via an access method. Passing it to all the classes via parameter like @mrjj suggested is also an option (a good one in some cases), although from my experience the responsibility for the data tends to blur this way the deeper into class structure it gets passed. Qt has a nice tree-like dependency structure provided by its parent-child relations. If you can't find a suitable object to host the data in that tree, tie it to the root - the application object, but don't make it hang around rootless - it usually causes a bug or a maintenance headache down the line.

  • @mrjj Well I tried give the list as parameter to the windows but when I do, it appears a lot of errors, and when I include the mylist.h dont recognize mylist.

  • Lifetime Qt Champion

    well, if you post your code we can try to help.
    Also the errors if possible.

    If you are new to c++ , maybe try some simple stuff before trying with Qt ?

  • Well now it's working, I only put "extern list mylist" below the structure of list

  • Lifetime Qt Champion

    Yes, the "extern list mylist" must be under the definition of "list" or the
    type (in this case list) is unknown for compiler and it gives errors.
    Should have mentioned that.
    Happy coding.

  • @TLoZ_KZP You can take a look at the singleton design pattern if you want a more OO approach to having a global variable.

    The singleton is a highly debated pattern but sometimes it is the correct solution, in most situations it is better than global variables.

    Although the best and recommended approach is as @mrjj pointed out: Pass the list to the objects that needs it through a function.

Log in to reply