运行不输出，是哪里出问题了啊for循环吗

Question

#include<iostream>
#include<vector>
using namespace std;

void test01() {
    vector<vector<int>> v;
    vector<int> v1;
    vector<int> v2;
    vector<int> v3;
    vector<int> v4;
    for (int i = 0; i < 4; i++) {
        v1.push_back(i + 1);
        v2.push_back(i + 2);
        v3.push_back(i + 3);
        v4.push_back(i + 4);
    }
    for (vector<vector<int>>::iterator it = v.begin(); it !=v.end(); it++) {
        for (vector<int>::iterator vit = (*it).begin(); vit != (*it).end(); vit++) {
            cout << *vit << " ";
        }
        cout << endl;
    }
}
int main() {
    test01();
    system("pause");
    return 0;
}

Answer 1

v什么都没做，能输出个啥呢，第一个for后面加上

v.push_back(v1);
v.push_back(v2);
v.push_back(v3);
v.push_back(v4);