heres a quick sample. no guarantee on it working, or being good. just ideas. I hope this helps.


class Button{
public:
//give it variables
float left,right,top,bottom;//relative to window
char *name;
//and a callback function
int (*fn)(void);
}

class Window{
public:
//give it variables
float left,right,top,bottom;
char *title;
int nButtons;//number of buttons
Button *buttons;//pointer to buttons in this window
//and functions like:
void Load(char* file);
void Draw();
}

I have also been working on some GUI-Elements like Buttons and Scrollbars which are implemented with OpenGL. I can tell you how I have designed that things. First I have introduced a common Baseclass which i named GraphicObject. This class contains all attributes and methods every Graphical Object have. Then there are some specific basical Graphical-Objects Classes which are inherited from the GraphicObject-Class, like a simple Line or a Rectangle and so on. Then I introduce a Composite-Class which contains of many different GraphicObjects(Here I use the Composite-Pattern for Details see E.Gamma,... "Design-Patterns"). A button or another GUI-Element is a Composite of different GraphicObjects like lines, rectangles and so on. So I created a Button-Class which inherits the Composite-Class.
For the Message-queue thing I use the Chain of Responsibility-Pattern (also see E.Gamma, ... "Design-Patterns" for more Details). That means that every GUI-Element has a method like HandleEvent() which get the Input like Windows callback Function and inside this you can Handle every Event. Do this for every GUI-Element and even the Window-itself(The Window is a composite of other GUI-Elements). For setting-up the Messagequeue call in your Windows-Main Function the HandleEvent()-Functions of your Window(s) which delegates the Event to their Child-Objects.
This is one Idea how you can implement your GUI in OpenGL. It is not an easy things and maybe there exists easier Designs. Look at www.OpenGL.org there exists alrady some GUI-Interfaces for OpenGL perhaps to use such an Library would be easier as implementing your own GUI.
Good luck

Peter Gmeiner