Section 13:
[13.10] How do I create a subscript operator for a Matrix class?

Use operator() rather than operator[].

When you have multiple subscripts, the cleanest way to do it is with operator() rather than with operator[]. The reason is that operator[] always takes exactly one parameter, but operator() can take any number of parameters (in the case of a rectangular matrix, two parameters are needed).

For example:

```class Matrix {
public:
Matrix(unsigned rows, unsigned cols);
double& operator() (unsigned row, unsigned col);        ← subscript operators often come in pairs
double  operator() (unsigned row, unsigned col) const;  ← subscript operators often come in pairs
...
~Matrix();                              // Destructor
Matrix(Matrix const& m);               // Copy constructor
Matrix& operator= (Matrix const& m);   // Assignment operator
...
private:
unsigned rows_, cols_;
double* data_;
};

inline
Matrix::Matrix(unsigned rows, unsigned cols)
: rows_ (rows)
, cols_ (cols)
//data_ <--initialized below (after the 'if/throw' statement)
{
if (rows == 0 || cols == 0)
throw BadIndex("Matrix constructor has 0 size");
data_ = new double[rows * cols];
}

inline
Matrix::~Matrix()
{
delete[] data_;
}

inline
double& Matrix::operator() (unsigned row, unsigned col)
{
if (row >= rows_ || col >= cols_)
throw BadIndex("Matrix subscript out of bounds");
return data_[cols_*row + col];
}

inline
double Matrix::operator() (unsigned row, unsigned col) const
{
if (row >= rows_ || col >= cols_)
throw BadIndex("const Matrix subscript out of bounds");
return data_[cols_*row + col];
}
```
Then you can access an element of Matrix m using m(i,j) rather than m[i][j]:
```int main()
{
Matrix m(10,10);
m(5,8) = 106.15;
std::cout << m(5,8);
...
}
```
See the next FAQ for more detail on the reasons to use m(i,j) vs. m[i][j].