How to implement a countdown clock in C++ and FLTK?

Question

I create a small game using FLTK and Gui library from Programming with C++ and i want to use a countdown clock-timer. FLTK has the the Fl::add_timeout(double t,Callback) which is very useful. The thing is that i want to use that function inside my class so i can change anything inside the window when it's called. The function has to be static so i can't access the window and make the changes i want. The Gui library includes only the useful things for amatuer programmers so i can't use the function reference_to<>(). Is there any idea how can i use that function or any other way to implement this? Thanks for your time.

MY CODE:

#include"GUI.h"
#include<FL/Fl.h>
#include"Simple_window.h"

class Game : public Window {
   Button *b;
   //variables i need for the window
public:
   Game(Point xy,int w,int h, const string& name) : Window(xy,w,h,name) {         
      b=new Button(Point(100,100),40,20,"Button"cb_button);
      Fl::add_timeout(1.0,TIME);
   }  
   ~Game(){
      delete b;
   }
   static void cb_button(Address,Address addr){
      reference_to<Game>(addr).B();
   }
   void B(){}
   static void TIME(void *d){
      //access to the variables like this->...
      Fl::repeat_timeout(1.0,TIME); 
   }
};

int main(){
  Game win(Point(300,200),400,430,"Game");
  return Fl::run();
}

Answer 1

The main points here are:

1. You want to use a function (add_timeout)

2. It takes a c style callback so you are giving it a static member function.

3. You aren't sure how to access instance variables from the static method.

From the documentation here: http://www.fltk.org/doc-2.0/html/index.html, you can see the add_timeout function takes a void* as its third argument which is passed to your callback.The quick fix here would be to pass the this pointer to the add_timeout function and then cast it to a Game* to access your member variables like so:

#include"GUI.h"
#include<FL/Fl.h>
#include"Simple_window.h"

class Game : public Window 
{    
public:
   Game(Point xy,int w,int h, const string& name) 
          : Window(xy,w,h,name), b(nullptr)
   {         
      b = new Button(Point(100,100),40,20,"Button", cb_button);
      Fl::add_timeout(1.0, callback, (void*)this);
   }

   ~Game()
   {
       delete b;
   }

   static void cb_button(Address, Address addr)
   {
       reference_to<Game>(addr).B();
   }

   void B(){}

   static void callback(void *d)
   {
       Game* instance = static_cast<Game*>(d);
       instance->b; // access variables like this->
       Fl::repeat_timeout(1.0,TIME); 
   }

private:
    //variables you need for the window
    Button *b;
};

int main()
{
    Game win(Point(300,200),400,430,"Game");
    return Fl::run();
}