Question

#include <iostream>

using namespace std;

class Street;

class CrossStreet

{

        private:

             char m_chName;
             Street* m_acLoS;
             int m_nNoS;
             static int m_nCSid;
        public:
             CrossStreet()
             {
                  Init();
                  m_chName = m_nCSid;
             }
             CrossStreet(char chName)
             {
                  Init();
                  m_chName = chName;
             }

             /** Problem is here **/
             void AddStreet(Street* const cStreet)
             {
                  m_acLoS[++m_nNoS] = cStreet;
             }

             ~CrossStreet()
             {
                  delete[] m_acList;
             }

             void Init()
             {
                  m_nNoS = 0;
                  m_acLoS = 0;
                  m_nCSid++;
             }

             friend class Street;
};

class Street

{

         private:

              int m_nDistance;

         public:

              Street(CrossStreet& cHead, CrossStreet& cTail, int nDistance)
              {
                   CreateStreet(cHead,cTail,nDistance);
              }

              void CreateStreet(CrossStreet& cHead, CrossStreet& cTail, int nDistance)
              {
                   m_nDistance = nDistance;
                   cHead.AddStreet(Street* const THIS);
                   cTail.AddStreet(Street* const THIS);

              }

};

when I compile above code, this throw error:

-->>invalid use of incomplete 'struct Street'
-->>forward declaration of 'struct Street'

function AddStreet of an object of ‘CrossStreet’ was called at an object of ‘Street’, to add ‘THIS street object to member variable of CrossStreet object. So I think I can use THIS pointer in this case but it not compile.

Answer 1

There is no such things as a THIS pointer. There is a this pointer though. So your code in CreateStreet is invalid (and not only for that). The correct syntax would be:

cHead.AddStreet(this); // no cast needed

The problem in AddStreet is that you have defined m_acLoS as a pointer-to-Street. So m_acLoS[x] is of type Street, and not Street*. But you can’t use a Street object there since you don’t have a complete definition of Street at that point. You can only use pointers to Street in CrossStreet. (Additionally, you’re lacking storage allocation for m_acLoS, so even if you could use that type there, and if the assignment was legal, it would still be a bug).

Depending on what you’re after, a std::vector<Street*> or std::list<Street*> or some other container could be what you’re looking for, instead of that incorrect member array. (But beware that you’re responsible for managing the lifetime of those pointers.)