class ui_system
{	
	int base_window(ui_system::window win, int xb, int yb, int zb);
	int window(int xw, int yw, int zw);

} ui_system;

int ui_system::window(int xw, int yw, int zw)
{
	//draw window and set it's default position
}

int ui_system::base_window(ui_system::window win, int xb, int yb, int zb){
    win.xw = xb/2 //however you would do that... this would center the window inside the base window on the x axis
	win.yw = yb/2 //same thing but for y axis
}


int main(int argc, char *Argv[])
{
    ui_system uis;
	int windowhandle uis.window(10,10,1);
    uis.base_window(windowhandle, 25, 25, 0);
};