insert elements between reads in vector in theads

Below sample crashes while thread print is reading the vector and in between the thread insert gets turn and inserts some elements after switching back to thread print program carashes , how to overcome this problem

// Thread_vector.cpp 
//

#include "stdafx.h"
#include <vector>
#include <iostream>
#include <thread>


//thead insert
using namespace std;
void insert_elements(vector<int> & v) {
    for (int i = 1; i < 20; i++) {
        v.push_back(i);
        cout << " : inserted - " << i << "  ";
    }

}
//thead print

void print_elements(vector<int> & vec) {
    for (auto & p:vec)
    {
        std::cout << p << "  ";
    }
    cout << endl;
}

int main()
{
    std::vector<int> b;
    thread ins(insert_elements, std::ref(b));
    thread prn(print_elements, std::ref(b));
    ins.join();
    prn.join();

    return 0;
}