Question

Is it possible to implement a function to access a private data vector foo in a class via class[i][j]? This should call the inner function of the vector foo[i][j].

Answer 1

A simple solution is to implement operator[] so it “peels off” only the first dimension:

#include <cstdlib>

class MyClass
{
    std::vector<std::vector<int> > foo;

public:

    // grant write access
    std::vector<int>& operator[](size_t index)
    {
        return foo[index];
    }

    // grant read access
    const std::vector<int>& operator[](size_t index) const
    {
        return foo[index];
    }
};

You need the two overloads because of const correctness. (If you only want to grant read access, not write access, you don’t need the non-const version, of course.)