Cin in a while 循环

The goal of the current exercise is to ask for user input from the values of 0 - 15, and converting that number into it's "Hex Equivelant" (Where 0 = 1, 1 = 2, 2 = 3, ... , 10 = A, 11 = B). I tried doing this without the book and failed, but then started questioning the code in the book. Here's the code in the book:

//Note: <iostream> and <string> have been included. using namespace std is used 
const string hexdigits = "0123456789ABCDEF";
cout << "Enter a series of numbers between 0 and 15"
     << " separated by spaces. Hit ENTER when finished: "
     << endl;
string result;
string::size_type n;

while(cin >> n)
    if (n < hexdigits.size())
        result += hexdigits[n];
cout << "Your hex number is: " << result << endl;

如果我要运行这段代码，它永远不会在没有输入任何代码的情况下按 Enter 键后终止 while 循环(基本上是输入我认为的空格?).