Strange protection error

I'm trying to implement a generic N-Dimensional matrix class but I can't to get a pointer to a protected data member. Here's the code cut down considerable.

Code:

---

template <typename NumType, int dim1, int dim2>
class Matrix
{
protected:
 NumType m[dim1][dim2];
};

template <typename NumType, int dim1>
class SquareMatrix : public Matrix<NumType, dim1, dim1>
{
public:
 //Return the minor matrix
 SquareMatrix<NumType, dim1 - 1> Minor(int i, int j) {
  SquareMatrix<NumType, dim1 - 1> ret;
  int *pretm = (int*)&ret.m[0][0]; //compiler hates this
  for (int u = 0; u < dim1; u++)
  {
  for (int v = 0; v < dim1; v++)
  {
    if( (u!=i) && (v!=j) ) //as long as we are not on a row or column of i or j then put the number into next slot of matrix
    {
    *pretm = m[u][v];
    pretm++;
    }
  }
  }
  return ret;
 }
};

template<typename NumType>
class SquareMatrix<NumType, 2> : public Matrix<NumType, 2, 2>
{
public:
//specialized code for 2x2 matrices
};

int main(int argc, char *argv[])
{
 int arr3[3][3] = { {1, 2, 3},
        {0, 5, 6},
        {0, 8, 9} };

 SquareMatrix<int, 3> m(arr3);
 cout << m.Determinant() << endl; //determinant will call Minor()
 
 return EXIT_SUCCESS;
}

---

When I try to compile this I get the following errors.
/home/bobby/meta/src/meta.cpp:106: instantiated from `NumType SquareMatrix<NumType, dim1>::Cofactor(int, int) [with NumType = int, int dim1 = 3]'
/home/bobby/meta/src/meta.cpp:116: instantiated from `NumType SquareMatrix<NumType, dim1>::Determinant() [with NumType = int, int dim1 = 3]'
/home/bobby/meta/src/meta.cpp:231: instantiated from here
/home/bobby/meta/src/meta.cpp:35: error: `int Matrix<int, 2, 2>::m[2][2]' is protected
/home/bobby/meta/src/meta.cpp:90: error: within this context

Any ideas about why the compiler considers m to be protected in my Minor function? Everything is fine if I make m public. Thanks.

SquareMatrix<int, 3> and SquareMatrix<int, 2> are (almost) unrelated types. They do share the same base class but there is no other connection between them: they are not friends, they do not inherit each etc. So there is no reason for SquareMatrix<int, 3> to access the protected/private members from SquareMatrix<int, 2>.

Thanks, it makes sense now. Do you think I should provide an accessor function to a pointer of the first element of the array or somehow make them friends?

SquareMatrix<int,3,3> has access to Matrix<int,3,3> protected members (since it is derived from it), but not to protected members of Matrix<int,2,2>

Your program has flaws.
SquareMatrix should not derive from Matrix!
In fact Matrix is a useless class, since it has no public interface, and thus, can't be a useful base class.
Moreover your code uses public inheritance, while private inheritance would be better : If someone derive from your matrix, it should not be able to access implementation details.

You should rather put m as a public member of Matrix, and Matrix as a private member of SquareMatrix.

Quote:

---

int *pretm = (int*)&ret.m[0][0]; //compiler hates this

---

Incorrect! &ret.m[0][0] is a int (*)[dim2]. It can't be safely converted to int*, because there may be some padding at the end of an array.

Quote:

---

*pretm = m[u][v];

---

This should be *pretm = this->m[u][v];

Other compile-time errors are trivial...

Wow, I'm glad I posted. There are actually a lot more functions in the Matrix class than I posted. I wanted to make looking through the code easier. I was actually going to implement operator overloads for addition and multiplication in the base class, since this is a generic property between matrices. That's why I was using public inheritance. Otherwise, I would have to implement an arithmetic operators in every derived class.