在学习C++过程中写了如下一串代码，调试过程中只有输入英文字母才会结束程序（跳出循环），按照我的理解只要输入的参数n不是string::size _ type就能跳出while循环了啊。
#include
using namespace std;
int main() {

  const string hexdigits = "0123456789ABCDEFF";
  cout << "please enter a series of number between 0 and 15\n"
    << "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;
   return 0;

}