Overloading operator << for containers

Today, I saw a line of code in one of our cpp files, it looks like this:
marshaller << obj_list;
what interesting is that obj_list is an instance of type std::vector, and one of my colleagues asked me if he can do similar things with an instance of type std::map<>.

The answer is "I don't know", because it depends on if the operator << is overloaded for the marshaller type for std::map<>.

The secret why "marshaller << obj_list;" works is that, the marshaller type has an overloaded operator << for std::vector. Something looks like this, assuming marshaller has type T:
T  &operator << (T &t, const vector avector){...}

Here is an example with cout:
ostream &operator << (ostream &out, const vector avector)
{
    vector ::const_iterator itr = avector.begin();
    for (; itr != avector.end(); ++itr)
    {
        out << (*itr);
    }
}

int main(void)
{
    vector str_vector;
    for (int i = 0; i < 10; i++)
    {
        stringstream str_stream;
        str_stream << i;
        str_vector.push_back(str_stream.str());
    }
    cout << str_vector;
}
The output is:
0123456789

So for this works with std::map<>, we need to overload << for std::map<>. 

Finally, it turned out that there is no such an overload, unfortunately. So we have to do it for each pare in the map.