Why do I HAVE to use consts in my code?

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The following piece of code runs fine but if I omit the const qualifier after the parenthesis overload function, I get a compile error saying: "passing 'const matrix' as 'this' argument of 'double& matrix::operator()(unsigned int, unsigned int)' discards qualifiers". Also, if I omit the const before the argument type name in the function copy(const matrix&, matrix&) or both this and the one after the parenthesis overload, I get an exception. Could someone please explain to me why these happen?

#include <string>
#include <iostream>

struct matrix
{
protected:
  double* mat; //array that stores matrix components
public:
  unsigned nrows; unsigned ncols; //ncols = number of columns, nrows = number of rows
  matrix(unsigned m, unsigned n): nrows(m), ncols(n)
  {
    mat = new double[m*n]; //creates an array of size nrows*ncols
    for (unsigned k=1; k<=m*n; k++)
      mat[k]=0.0;
  }
  double& operator()(unsigned i, unsigned j) const //parenthesis operator (to access matrix components)
  {
    if (i>nrows || i<1 || j>ncols || j<1) throw(-1); //index out of bounds
    return mat[(i-1)*ncols+(j-1)];
  }
 ~matrix() {delete[] mat;}
};

void copy(const matrix& src, matrix& dest)
{
  unsigned m=src.nrows; //sets m to number of rows
  unsigned n=src.ncols; //sets n to number of columns
  for (unsigned i=1; i<=m; i++)
    for (unsigned j=1; j<=n; j++)
      dest(i,j)=src(i,j); //copies all the components of src to dest
}

int main()
{
  matrix A(2,1);
  A(1,1)=11;
  A(2,1)=21;
  matrix B(2,1);
  copy(A,B);
  std::cout << B(1,1) << std::endl;
  return 0;
}

asked 1 min ago

Mohammad Sanei

85