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How do I create a subscript operator for a Matrix class?

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How do I create a subscript operator for a Matrix class?
posted Dec 19, 2014 by Emran

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1 Answer

+1 vote
 
Best answer

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(const Matrix& m); // Copy constructor
Matrix& operator= (const Matrix& 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);
...
}

answer Dec 23, 2014 by Mohammed Hussain
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