//main.h:
#if !defined(MAIN_H)
#define MAIN_H

class BASE {
   public:
      BASE() { }
      BASE(int val) { this->m_val = val; }
      virtual ~BASE() { }

      virtual int func(int val);
      int m_val;
};

class DERIVED : public BASE {
   public:
      DERIVED() : BASE() {}
      DERIVED(int val) : BASE(val) {}
      virtual ~DERIVED() {}

      int func(int val);
};

#endif //MAIN_H

//main.cpp:
#include "main.h"

int DERIVED::func(int val) {
   this->m_val = val;
}

int main()
{
   int tmp;

   BASE b;
   DERIVED d(tmp);

   return 0;
}