Simple file writing with ostream_iterator creates file, but doesn't write

I have a really simple code to create a text file called "Input.txt", and write to it using ostream_iterator:

using namespace std;


int main()
{
    ofstream os{ "Input.txt" }; 
    ostream_iterator<int> oo{ os,"," };

    vector<int> ints;
    for (int i = 0; i < 1000; i++)
    {
        ints.push_back(i);
    }

    unique_copy(ints.begin(), ints.end(), oo);

    system("PAUSE");
    return 0;
}

The code above creates a "Input.txt", but there is nothing written to it. Am I missing something really obvious and fundamental?