const int D_SIZE = 16;

class Cirbuf
{  friend class CirbufIterator;
   public:
      int produce(char c);
      int consume();
      int is_empty() { return(length == 0); }
      int is_full() { return(length == size); }
      int next(char& c);                    // iteration
      Cirbuf& operator = (const Cirbuf& b); // assignment
      Cirbuf(const Cirbuf& b)               // copy constructor
      {  *this = b; }
      Cirbuf(int size = D_SIZE);            // constructor
      ~Cirbuf();                            // destructor
   private:
      int head;   // first char in buffer
      int tail;   // first empty slot in buffer
      int length; // number of characters in buffer
      int size;   // capacity of buffer
      char *cb;   // pointer to buffer
      int mod(int x)
      { return(x >= size ?  x - size : x); }
};

class CirbufIterator
{   public:
      CirbufIterator(Cirbuf& buf);
      int operator() (char&);
    private:
      Cirbuf& b;
      int ind;
};