Correct way to return stl map by reference / Is this behavior well defined?

I have 2 classes and one of them has map<string, vector<string> >I want to be able to use this in other class. Here is my code:

class a
{
    map<string, vector<string> > m;
    public:
    const map<string, vector<string> > & get()
    {
        return m;
    }
};

class b
{
    a obj;
    public:
    void test()
    {
        map<string, vector<string> > m= obj.get();
        // and then print
    }
};

int main(int argc, char ** argv)
{
    b bobj;
    bobj.test();
    return 0;
}

Is the way I returned reference to map in class a correct? It worked,but I just want to confirm if it was done properly / I got lucky / any other comments about the code.

Thank you for your help.