[C++] Hex string convert to integer with stringstream

Sometimes we need to convert hex string to integer.
Usually we will use this alrorithm.

char xtod(char c) {
 if (c>='0' && c<='9') return c-'0';
 if (c>='A' && c<='F') return c-'A'+10;
 if (c>='a' && c<='f') return c-'a'+10;
 return c=0;        // not Hex digit
}

This is a conventional usage.
Tonight I found a question in CSDN, then I try to found a better solution.
In stackoverflow, I found this answer.
It is very nice.

#include <sstream>
#include <iostream>

int main() {
  unsigned int x; 
  std::stringstream ss;
  ss << std::hex << "FF";
  ss >> x;
  // output it as a signed type
  std::cout << static_cast<int>(x) << std::endl;
}