std :: hex和std :: setw无法使用某些字符

Question

What I'm trying to do is converting a string's bytes into hexadecimal format.
Based on this answer (and many others consistent) I've tried the code:

#include <sstream>
#include <iomanip>
#include <iostream>

int main ()
{
   std::string inputText = u8"A7°";

   std::stringstream ss;
   // print every char of the string as hex on 2 values
   for (unsigned int i = 0; i < inputText.size (); ++i)
   {
      ss << std::hex << std::setfill ('0') << std::setw (2) << (int) inputText[i];
   }

   std::cout << ss.str() << std::endl;
}

but with some characters coded in UTF 8 it does't work.
For Instance, in strings containing the degrees symbol ( ° ) coded in UTF8, the result is: ffffffc2ffffffb0 instead of c2b0 .
Now I would expect the algorithm to work on individual bytes regardless of their contents and furthermore the result seems to ignore the setw(2) parameter.
Why does I get such a result?

(run test program here )

Answer 1

As Pete Becker already hinted in a comment, converting a negative value to a larger integer fills the higher bits with '1'. The solution is to first cast the char to unsigned char before casting it to int :

#include <string>
#include <iostream>
#include <iomanip>

int main()
{
    std::string inputText = "-12°C";
    // print every char of the string as hex on 2 values
    for (unsigned int i = 0; i < inputText.size(); ++i)
    {
       std::cout << std::hex << std::setfill('0')  
                 << std::setw(2) << (int)(unsigned char)inputText[i];
    }
}

setw sets the minimal width, it does not truncate longer values.