works

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

class A
{
public:

    A() : _bla(4)
    {

    }

    const int& operator()(int num1, int num2) const
    {
        return _bla[num1 + num2];
    }

    int & operator()(int num1, int num2)
    {
        return _bla[num1 + num2];
    }
private:
    vector<int> _bla;
};

int main(int argc, char** argv)
{
    A a;
    cout << a(1, 1) << endl;
    return 0;
}