C ++ istringstream tokenizing

Question

I want to read a line from stdinand mark it with a space using the class istringstream. But it does not work as expected:

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

int main()
{
    string input_line;
    istringstream input_stream;
    string word;

    cout << "> ";
    getline(cin, input_line);
    input_stream.str(input_line);
    input_stream >> word;
    cout << "  line:   " << input_line << endl;
    cout << "  stream: " << input_stream.str() << endl;
    cout << "  word:   " << word << endl;

    cout << "> ";
    getline(cin, input_line);
    input_stream.str(input_line);
    input_stream >> word;
    cout << "  line:   " << input_line << endl;
    cout << "  stream: " << input_stream.str() << endl;
    cout << "  word:   " << word << endl;
}

If I introduce separate separable lines, everything is fine:

> aa bb
  line:   aa bb
  stream: aa bb
  word:   aa
> xx yy
  line:   xx yy
  stream: xx yy
  word:   xx

However, if I enter lines without spaces, the strange thing is that the operator >>correctly reads from the stream the first time, but not the second time:

> aa
  line:   aa
  stream: aa
  word:   aa
> xx
  line:   xx
  stream: xx
  word:   aa

What am I doing wrong?