#include <iostream>
#include <vector>

/// Beware! Undefined behavior lurks within.
void double_repeat(std::vector<int>& v)
{
    for (int& each: v) {
        each *= 2;
        v.push_back(each);
    }
}

void print_ints(const std::vector<int>& v)
{
    for (int each: v)
        std::cout << each << " ";
    std::cout << "\n";
}

int main ()
{
    std::vector<int> v{1, 2, 3, 4, 5};
    double_repeat(v);
    print_ints(v);
}

// Program output is:
//
//   2 2 3 4 5 2 4 6 8 10 
//
// Why?