Προγραμματισμός Υπολογιστών με C++ (3321)

#include <random>
#include <iostream>

template <typename PREV, typename NEXT>
class Link
{
protected:
   PREV* m_prev;
   NEXT* m_next;

public:
   Link(PREV* p=0, NEXT* n=0) : m_prev(p), m_next(n) {}
   void setPrev(PREV* p) { m_prev = p; }
   void setNext(NEXT* n) { m_next = n; }
};

#define ADD 0
#define SUBTRACT 1
#define MULTIPLY 2

class Operator : public Link<long, long>
{
   int m_type = ADD;

public:
   Operator(int t) : m_type(t) {}
   long operator ()() const
   {
      if (!m_prev)
         return 0;
      if (!m_next)
         return *m_prev;
      switch (m_type)
      {
         case ADD  : return *m_prev + *m_next; break;
         case SUBTRACT: return *m_prev - *m_next; break;
         case MULTIPLY: return *m_prev * *m_next; break;
         default: return *m_prev;
      }
   }

  operator std::string() const
  {
     switch (m_type)
     {
        case ADD: return " + "; break;
        case SUBTRACT: return " - "; break;
        case MULTIPLY: return " * "; break;
     }
  }
};

int main()
{
   long student_id = 1000000;
   srand(student_id);
   int num_operands = 10 + rand() % 10;
   long left_operand = rand() % 20 + 1;
  
   for (int i = 0; i < num_operands; i++)
   {
      long right_operand = rand() % 20 + 1;
     
      Operator* op = new Operator(rand() % 3);
      op->setPrev(&left_operand);
      op->setNext(&right_operand);
     
      long res = (*op)();
      std::cout << left_operand << (std::string)*op << right_operand << " = " << res << "\n";
      left_operand = res;

      delete op;
   }
   getchar();
}