c++ - Can't assign a value to object of my custom class, that shall behave like a matrix -

i have custom class, behaves matrix. works well, except assigning value other instance of same class.

so can stuff like:

matrix a(5,7); // du stuff matrix b(5,7); matrix d=a+b; d=a*5; d[3][2]=1;  //but can't this: double x=d[3][2];  //at point error: 

main.cpp:604:12: error: passing ‘const matrix’ ‘this’ argument of ‘matrix::proxy matrix::operator[](int)’ discards qualifiers

does have idea, how fix this? :(

implementation of matrix class here:

class matrix { public:  matrix(int x, int y); ~matrix(void); //overloaded operators matrix operator+(const matrix &matrix) const;   matrix operator-() const; matrix operator-(const matrix &matrix) const;  matrix operator*(const double x) const;  matrix operator*(const matrix &matrix) const;   friend istream& operator>>(istream &in, matrix& a);  class proxy {     matrix& _a;     int _i; public:      proxy(matrix& a, int i) : _a(a), _i(i) {      }      double& operator[](int j) {         return _a._arrayofarrays[_i][j];     } };  proxy operator[](int i) {     return proxy(*this, i); }  // copy constructor  matrix(const matrix& other) : _arrayofarrays() {     _arrayofarrays = new double*[other.x ];     (int = 0; != other.x; i++)         _arrayofarrays[i] = new double[other.y];      (int = 0; != other.x; i++)          (int j = 0; j != other.y; j++)             _arrayofarrays[i][j] = other._arrayofarrays[i][j];      x = other.x;     y = other.y; } int x, y; double** _arrayofarrays; }; 

you have 1 operator[] signature:

proxy operator[](matrix *this, int i) 

you're trying call this:

proxy operator[](const matrix *, int) 

the error saying in order convert const matrix * matrix *, const has discarded, bad. should provide const version inside class:

proxy operator[](int) const {...} 

when inside class, gains first parameter of this, , const after parameter list means first parameter pointer constant object of class, not non-constant one.