where to declare object to use it (fltk)

Question

I am having a problem accessing an object or its methods in fltk. I have a Class named MyWindow which is a child class of Fl_Window. so basically i would like to use an object which is either declared int the main or Mywindow in private section. My problem is that I can't use it that way. It only lets me to use the object if it is declared global. Can i somehow put it on the heap like this: Classname *pointer = new Classname(); ? IF i can where do I do that? How would the callback function work if i need that object or its functions in the callback? Should i use a pointer to it in the callback arguements? lets say i want to click on the button and I need it to do something with the object and change a value. lots of questions I know, I am really lost. Can someone just point me to the right direction? Thank you! :)

Answer 1

Simple case for passing data to the GUI

class MyData
{
    // The data model
};

class MyWindow: public FL_Window
{
    MyData* m_data;
public:
    void Data(MyData* data) { m_data = data; }
    ...
};

int main()
{
    MyWindow gui;
    MyData data;

    // Tell the gui about the data
    gui.Data(data);
    ...
    // Setup the dialog
    gui.begin();
    ...
    gui.end();
    gui.show();
    return FL::run();
}

For the callbacks, do it in two stages

class NeedingACallback
{
public:
    void Setup()
    {
        ...
        FL_xxx* w = new FL_xxx(xpos, ypos, wid, hgt, name);
        ...
        //                    v Pass the instance to the static
        w->callback(_EventCB, this);
    }

    // The callback
    static void _EventCB(FL_Widget* w, void* client)
    {
        // Convert the void* back to the instance
        NeedingACallback* self = reinterpret_cast<NeedingACallback*>(client);
        self->EventCB();
    }

    // Make life simple so you don't have to put self-> in front of
    // all the instance data accessed
    void EventCB()
    {
        // Callback for this instance of the class
    }
};

EDIT It sounds like you're having multiple instances of data. An alternative technique is to have data as a reference. This must be done in the constructor. This way, m_data and the data in main both refer to the same area of memory.

class MyWindow: public FL_Window
{
    MyData& m_data;
public:
    MyWindow(int wid, int hgt, MyData& data, const char* title=0)
   : FL_Window(wid,hgt,title)
   , m_data(data)
    {
    ...
    }
};

int main()
{
    MyData data;
    MyWindow gui(100, 100, data, "Call me Mr");

    ...
    // Setup the dialog
    gui.begin();
    ...
    gui.end();
    gui.show();
    return FL::run();
}