C++ vector constructors

#include <iostream>
#include <vector>
using namespace std;
int main()/*from   w ww . j a  va 2s. co  m*/
{                              // an array of doubles
    double arr[] = { 1.1, 2.2, 3.3, 4.4 };
    vector<double> v1(arr, arr+4); // initialize vector to array
    vector<double> v2(4);          // empty vector of size 4
    v1.swap(v2);                   // swap contents of v1 and v2
    while( !v2.empty() )           // until vector is empty,
    {
       cout << v2.back() << ' ';   // display the last element
       v2.pop_back();              // remove the last element
    }                           // output: 4.4 3.3 2.2 1.1
    cout << endl;
    return 0;
}