C++ overload operator [ ][ ]

Question

I have class CMatrix, where is "double pointer" to array of values.

class CMatrix {
public:
    int rows, cols;
    int **arr;
};

I simply need to access the values of matrix by typing:

CMatrix x;
x[0][0] = 23;

I know how to do that using:

x(0,0) = 23;

But I really need to do that the other way. Can anyone help me with that? Please?

Thank you guys for help at the end I did it this way...

class CMatrix {
public:
   int rows, cols;
   int **arr;

public:
   int const* operator[]( int const y ) const
   {
      return &arr[0][y];
   }

   int* operator[]( int const y )
   {
      return &arr[0][y];
   }

   ....

Thank you for your help I really appreciate it!

Answer 1

You cannot overload operator [][], but the common idiom here is using a proxy class, i.e. overload operator [] to return an instance of different class which has operator [] overloaded. For example:

class CMatrix {
public:
    class CRow {
        friend class CMatrix;
    public:
        int& operator[](int col)
        {
            return parent.arr[row][col];
        }
    private:
        CRow(CMatrix &parent_, int row_) : 
            parent(parent_),
            row(row_)
        {}

        CMatrix& parent;
        int row;
    };

    CRow operator[](int row)
    {
        return CRow(*this, row);
    }
private:
    int rows, cols;
    int **arr;
};

Answer 2

There is no operator[][] in C++. However, you can overload operator[] to return another structure, and in that overload operator[] too to get the effect you want.

Answer 3

You can do it by overloading operator[] to return an int*, which is then indexed by the second application of []. Instead of int* you could also return another class representing a row, whose operator[] gives access to individual elements of the row.

Essentially, subsequent applications of operator[] work on the result of the previous application.

Answer 4

If you create a matrix using Standard Library containers, it's trivial:

class Matrix {
    vector<vector<int>> data;

public:
    vector<int>& operator[] (size_t i) { return data[i]; }
};

Answer 5

You could operator[] and make it return a pointer to the respective row or column of the matrix. Because pointers support subscripting by [ ], access by the 'double-square' notation [][] is possible then.