On 9/24/22 23:17, roger tunnicliffe wrote:

Thanks guys, I have found the difference but do not understand it..

I had coded:-

       Fl_Tabs *cards = new Fl_Tabs(40,40,300,200);

while your demonstration has coded:-

      cards = new Fl_Tabs(40,40,300,200);

Would be very grateful if you could explain the difference

    You should probably start by learning the C++ language first
    using a tutorial or book so you can get down the basics of the language.
    Then segue into FLTK once you're comfortable with C++.

    With that said, the C++ programming topics you're running into here are:

        "variable scope"        -- once a variable is declared, how long it remains visible
        "variable shadowing"    -- how variables of the same name can 'shadow' each other, depending on scope
        "pointers"              -- accessing objects and memory via indirection

    When you added the global variable 'cards', but also declared a local variable 'cards'
    and initialized that, the global variable remains unset, since the local variable shadows it.
    This causes the global to remain uninitialized.

    And since the variable is a pointer, the machine segfaults your app (crashes it) the moment
    you try to indirect through the uninitialized pointer (via cards->xxx), causing a memory violation.

    You can read up on C++ variable scoping rules here: https://codescracker.com/cpp/cpp-scope-rules.htm
    ..and variable shadowing here: https://www.learncpp.com/cpp-tutorial/variable-shadowing-name-hiding/

    But this is kinda sliding off topic for this forum, as these are questions not so much
    about FLTK, and more about the basics of the C++ language itself. I know it's hard to
    know that when you're new to the language, which is why I think it'll help if you learn
    the language first.