Overloading operator []

Question

I have a task to write a class matrix in C++ and there is a condition, to override the operator [] for matrix so if I have a matrix with name Matrix with this "Matrix[0][0]" I must take it's first element, on it's first line. I have represented the matrix with two dimensional dynamic array and templates (T **matrix). Could you help me, please?

PS: This method I'm using to create the two dimensional array:

template <class T>
T ** Matrix<T>::createMatrix(unsigned int rows, unsigned int cols)
{
    T** matrix = new T*[rows];
    for (unsigned int i = 0; i < rows; i++) {

        matrix[i] = new T[cols];

    }
    return matrix;
}

Answer 1

i assume matrix is a member variable of type T** of Matrix.

template< class T >
T* operator []( Matrix<T>& m, unsigned int row )
{
  // maybe remember rows and assert(row<rows);
  return m.matrix[ row ];
}

Now you may write something like

Matrix<T> m(50,9999);
m[42][1337]; 

to access element 1337 in row 42.

Answer 2

You can do this with two classes. The matrix class overrides [] and returns a row object. The row object overrides [] and returns a scalar.

Answer 3

What you want to do is return a T* in operator[], as it can then have operator[] applied to it natively and get the result that you want. However, I want to point out that it's pretty bad practice to leave your pointers raw.

Answer 4

You can't do that natively in C++, but you could simply instanciate your matrix this way:

Matrix<Matrix<int>> m;

This way the first [ ] returns another matrix which will return the item you want. Of course a much better name for your class then would be Vector, and a wrapper class called Matrix that creates an internal Vector<Vector<T>>.

Answer 5

The method you are using to create the matrix doesn't have anything to do with a class, as your requirements state. You are creating a simple T**, it doesn't make a difference that you are doing it in a class method.

Creating a matrix of T that can be used as in matrix[i][j] means that the expression matrix[i] must return a type on which operator [] is also defined to return a T. Therefore, the usual way to do it is break it down in steps:

• Matrix<T> operator[] (int) returns a MatrixRow<T>&
• MatrixRow<T>& operator[] (int) returns a T&

There will also be const versions of these operators to be used when reading values from the matrix (non-const versions will be used when writing to the matrix).

So, at the minimum, you should create a class with this interface:

template <typename T>
class Matrix {
    public:
        Matrix(int rows, int cols);

        const MatrixRow<T>& operator[] (int row) const;
        MatrixRow<T>& operator[] (int row);
}

PS: This reads like a homework question, is it one?