Template class casting

[cmajor28]

I have this code here and I keep getting errors. I understand them but I don't know how to fix it.

NOTE: matrix is a pointer to the matrix elements, m = the number of rows, and n = the number of cols

template <class T>
template <class U> Matrix<T>& Matrix<T>::operator = (const Matrix<U>& mat) {
	
	if (sizeof(T) != sizeof(U)) {
		size_t i,j;
		
		// Free memory
		free(matrix);
			
		// Set new rows and columns
		m = mat.row();
		n = mat.col();
		
		// Allocate memory for matrix
		matrix = (T*)malloc(m*n*sizeof(T));
		
		for (i=0;i<m;i++) {
			for (j=0;j<n;j++) {
				*(matrix+n*i+j) = *(mat.data()+n*i+j);
			}
		}
	}
	else {
		if (this != &mat) {
			// Free memory
			free(matrix);
			
			// Set new rows and columns
			m = mat.row();
			n = mat.col();
			
			// Set matrix to copy of mat
			memcpy(matrix, mat.data(), m*n*sizeof(T));
		}
	}
	return *this;
}

In file included from Matrix.h:118:0,
                 from Main.cpp:4:
Matrix.cpp: In instantiation of ‘Matrix<T>& Matrix<T>::operator=(const Matrix<U>&) [with U = int; T = double]’:
Main.cpp:15:8:   required from here
Matrix.cpp:533:5: error: passing ‘const Matrix<int>’ as ‘this’ argument of ‘size_t Matrix<T>::row() [with T = int; size_t = long unsigned int]’ discards qualifiers [-fpermissive]
   m = mat.row();
     ^
Matrix.cpp:534:5: error: passing ‘const Matrix<int>’ as ‘this’ argument of ‘size_t Matrix<T>::col() [with T = int; size_t = long unsigned int]’ discards qualifiers [-fpermissive]
   n = mat.col();
     ^
Matrix.cpp:546:12: error: comparison between distinct pointer types ‘Matrix<double>*’ and ‘const Matrix<int>*’ lacks a cast
   if (this != &mat) {
            ^
Matrix.cpp:551:6: error: passing ‘const Matrix<int>’ as ‘this’ argument of ‘size_t Matrix<T>::row() [with T = int; size_t = long unsigned int]’ discards qualifiers [-fpermissive]
    m = mat.row();
      ^
Matrix.cpp:552:6: error: passing ‘const Matrix<int>’ as ‘this’ argument of ‘size_t Matrix<T>::col() [with T = int; size_t = long unsigned int]’ discards qualifiers [-fpermissive]
    n = mat.col();
      ^

[kbw]

You need to check methods row() and col(), they ought to be const members, but they aren't.

You can only compare mat and *this if they're the same type. I think this is the problem you can see. You can work around it by reinterpreting those addresses as some suitably large integral type, but you have a real problem, that memcpy only works if T and U are the same type and a POD type. You should try to support user-defined types with template classes/functions.

[cmajor28]

They weren't const. That was a major error. And now the only thing is it says I can't compare pointers of different types but that check only happens if T == U which means they will be of the same type. But I know the compiler can't tell that. Maybe I could rearrange my logic and do a cast of the pointer.

EDIT: if (this != reinterpret_cast<Matrix<T>*>(const_cast<Matrix<U>*>(&mat)))

That's what I have but I just feel like its bad programming practice.