Using std::vector in 2 levels, but the inner one's member value is not printed out

I push_back an element into a vector by for-loop in 2 levels. After finishing, I use for-loop to print it again but can not see the value.

I tried the code on a CentOS-7.

#include<iostream>
#include<vector>

using namespace std;
class Y{
    public:
        int a;
};
class X{
    public:
        int a;
        vector <Y> b;
};
int main(){
    vector<X> root;
    X x;
    x.a = 55;
    root.push_back(x);
    for(auto e : root)
    {
        cout << e.a << ",";
        Y y;
        y.a = 66;
        e.b.push_back(y);
        for(auto e2 : e.b)
        {
            cout << e2.a << ",";
        }
    }
    // Print it again
    for(auto e : root)
    {
        cout << e.a << ",";
        for(auto e2 : e.b)
        {
            cout << e2.a << ",";
        }
    }
    cout << endl;
    return 0;
}

I expect the output to be 55,66,55,66, but the actual output is 55,66,55,