FAQ 23.09 How can a Matrix -like class have a subscript operator that takes more than one subscript?

FAQ 23.09 How can a Matrix-like class have a subscript operator that takes more than one subscript?

It should use operator() rather than operator[].

When multiple subscripts are needed, the cleanest approach is to use operator() rather than 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-like class, an element can be accessed using an (i,j) pair of subscripts. For example,

 #include <stdexcept> using namespace std; class Matrix { public:   double& operator() (unsigned row, unsigned col)     throw(out_of_range);   double  operator() (unsigned row, unsigned col) const     throw(out_of_range); private:   enum { rows_ = 100, cols_ = 100 };   double data_[rows_ * cols_]; }; inline double& Matrix::operator() (unsigned row, unsigned col) throw(out_of_range) {   if (row >= rows_ || col >= cols_)     throw out_of_range("Matrix::operator()");   return data_[cols_*row + col]; } inline double Matrix::operator() (unsigned row, unsigned col) const throw(out_of_range) {   if (row >= rows_ || col >= cols_)     throw out_of_range("Matrix::operator()");   return data_[cols_*row + col]; } 

To access an element of a Matrix object, users use m(i,j) rather than m[i][j] or m[i,j]:

 int main() {   Matrix m;   m(5,8) = 106.15;                                   <-- 1   cout << m(5,8);                                    <-- 2 } 

(1) Set element (5,8)

(2) Get element (5,8)