#include <vector>
#include <iostream>
using namespace std;

void func(vector<int>& vec)
{
    // modify the vector
    vec.push_back(2);
    vec.push_back(3);
}

int main()
{
    vector<int> a;  // our vector
    a.push_back(1);  // add 1 element to it

    func(a);  // pass it to our function (by reference) to add 2 more

    // how many items?
    cout << a.size();  // will output 3
}