#include <iostream>
#include <vector>
#include <string>
#include <boost/unordered_map.hpp>

int main()
{
    boost::unordered_map<int, std::vector<std::string> > map;

        std::cout << "Usage without any initialization:\n";
        std::cout << map[0].size() << "\n\n";

    map[1].push_back("qwerty");
        std::cout << "Insertion with non-existing key\n";
        std::cout << map[1].size() << " " << map[1][0] << "\n\n";

    map[1].push_back("asdfgh");
        std::cout << "Insertion with existing key\n";
        std::cout << map[1].size() << " " << map[1][0] << " " << map[1][1] << "\n\n";

    map[0] = map[1];
        std::cout << "Overwriting of existing key\n";
        std::cout << map[0].size() << " " << map[0][0] << " " << map[0][1] << "\n\n";

    return 0;
}
/*
Output:

Usage without any initialization:
0

Insertion with non-existing key
1 qwerty

Insertion with existing key
2 qwerty asdfgh

Overwriting of existing key
2 qwerty asdfgh

*/