What does this C++ snippet output?

1#include <iostream>
2#include <vector>
3void push(std::vector<int>& v) { v.push_back(9); }
4int main() {
5    std::vector<int> data = {1, 2};
6    push(data);          // by ref: grows caller's vector
7    std::cout << data.size() << "\n";
8    return 0;
9}