for stupids like me: overloading brackets [] (subscript) operator in C+

subscript_operator_overloading.cpp

/*
# for stupids like me: overloading brackets [] (subscript) operator in C++
# run:
# g++ subscript_operator_overloading.cpp && ./a.out

this example works because
map (or, i.e. array) supports [] as an assignment operator, so
it works transparently, implicitly - 
you don't need translate and implement assignment
*/
#include <iostream>
#include <map>
class data
{
private:
    std::map<int, int> numbers; // integer array

public:
   int& operator[] (int index)  { return numbers[index]; } // numbers[index] ASSIGNS before return;
    // returns reference to an integer (not the value of integer)
};

int main()
{
    data dat;
    dat[3] = 23;
    std::cout << dat[3] << "\n";
    std::cout << dat[4] << "\n";
    std::cout << "dat[4] = 17: " << "\n";
    dat[4] = 17;
    std::cout << dat[4] << "\n";
}