Throwing exception when array is out of bounds

Question

I have a matrix class, that uses [][] to access elements. When one (or both) indexes are out of bounds, I have to throw CIndexException. It's a class that stores text in this format "Invalid index [a][b]", where a and b are numbers.

This is my current implementation of CIndexException class

class CIndexException
{
    string text;
public:
    CIndexException (int a, int b)
    {
        ostringstream oss;
        oss << "Invalid index [";
        oss << a;
        oss << "][";
        oss << b;
        oss < "]";

        text = oss.str();
     }

     string get() const
     {
         return text;
     }
};

Matrix is represented as a 2D array of doubles, it is initialized in constructor:

CMatrix(int r, int c)
{
    colls = c;
    rows = r;
    mat = new double * [rows];

    for (int i = 0; i < rows; i++)
    {
        mat[i] = new double [colls];
        for (int j = 0; j < colls; j++)
            mat[i][j] = 0;
    }
}

To get single element, I overloaded [] operator like this:

double * operator[] (int x) const
{
    return mat[x];
}

When I type a[2][3], this function resolves the first [], returns pointer to an array and the second [] gets resolved as usual.

I can easily check if the first index is out of bounds, but I have trouble checking for the second one. I thought of creating second class, MatrixRow, that represents one row of matrix. Then I would have array of MatrixRows. To get [][] working, both of those classes would have overloaded operator[]. That way I would be able to check both indexes, but I have no idea how to "join" them into one exception. How can I report both indices in the exception object while using this design?

Answer 1

Go with your MatrixRow solution, but pass the index of the row to each MatrixRow as a constructor argument. The MatrixRow can save that number, and use it in any exception messages it generates.

Answer 2

Use operator()() instead of operator[](). See this link. All what you need this link considers.