#include <print>
#include <vector>

// a non-const reference modifies the caller's vector in place
void push_zero(std::vector<int>& v) {
    v.push_back(0);
}

int main() {
    std::vector<int> nums{1, 2};
    push_zero(nums);
    std::println("{}", nums.size());
}

& lets the callee change your value