where to declare object to use it (fltk) - c++

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();
}