What does this C++ snippet output?

1#include <iostream>
2#include <vector>
3void zero(std::vector<int> v) {
4    for (auto& e : v) e = 0;   // mutates the copy
5}
6int main() {
7    std::vector<int> d = {7, 8, 9};
8    zero(d);
9    std::cout << d[0] << d[1] << d[2] << "\n";
10    return 0;
11}