Const operator overloading problems in C++

Question

I'm having trouble with overloading operator() with a const version:

#include <iostream>
#include <vector>
using namespace std;

class Matrix {
public:
    Matrix(int m, int n) { 
        vector<double> tmp(m, 0.0);
        data.resize(n, tmp);
    }
    ~Matrix() { }


    const double & operator()(int ii, int jj) const {
        cout << " - const-version was called - ";
        return data[ii][jj];
    }

    double & operator()(int ii, int jj) {
        cout << " - NONconst-version was called - ";
        if (ii!=1) {
            throw "Error: you may only alter the first row of the matrix.";
        }
        return data[ii][jj];
     }


protected:  
    vector< vector<double> > data;
};

int main() {
try {
    Matrix A(10,10);
    A(1,1) = 8.8;
    cout << "A(1,1)=" << A(1,1) << endl;
    cout << "A(2,2)=" << A(2,2) << endl;
    double tmp = A(3,3);
} catch (const char* c) { cout << c << endl; }
}

This gives me the following output:

• NONconst-version was called - - NONconst-version was called - A(1,1)=8.8
• NONconst-version was called - Error: you may only alter the first row of the matrix.

How can I achieve that C++ call the const-version of operator()? I am using GCC 4.4.0.

Answer 1

The overloading looks fine but you never call it on a const object. You can try this:

void foo(const Matrix& A) {
  cout << "A(1,1)=" << A(1,1) << endl;
}

Matrix A(10,10);
foo(A);

This gives you:

 - const-version was called - A(1,1)=0