How does one write the hex values of a char in ASCII to a text file?

Here is what I currently have so far:

void WriteHexToFile( std::ofstream &stream, void *ptr, int buflen, char *prefix )
{
    unsigned char *buf = (unsigned char*)ptr;

    for( int i = 0; i < buflen; ++i ) {
     if( i % 16 == 0 ) {
      stream << prefix;
     }

     stream << buf[i] << ' ';
    }
}

I've tried doing stream.hex, stream.setf( std::ios::hex ), as well as searching Google for a bit. I've also tried:

stream << stream.hex << (int)buf[i] << ' ';

But that doesn't seem to work either.

Here is an example of some output that it currently produces:

Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í 
Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í 
Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í 
Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í 
Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í 
Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í Í

I would like the output to look like the following:

FF EE DD CC BB AA 99 88 77 66 55 44 33 22 11 00
FF EE DD CC BB AA 99 88 77 66 55 44 33 22 11 00
FF EE DD CC BB AA 99 88 77 66 55 44 33 22 11 00
FF EE DD CC BB AA 99 88 77 66 55 44 33 22 11 00
FF EE DD CC BB AA 99 88 77 66 55 44 33 22 11 00
FF EE DD CC BB AA 99 88 77 66 55 44 33 22 11 00

From stackoverflow

• char upperToHex(int byteVal)
  {
      int i = (byteVal & 0xF0) >> 4;
      return nibbleToHex(i);
  }
  
  char lowerToHex(int byteVal)
  {
      int i = (byteVal & 0x0F);
      return nibbleToHex(i);
  }
  
  char nibbleToHex(int nibble)
  {
      const int ascii_zero = 48;
      const int ascii_a = 65;
  
      if((nibble >= 0) && (nibble <= 9))
      {
          return (char) (nibble + ascii_zero);
      }
      if((nibble >= 10) && (nibble <= 15))
      {
          return (char) (nibble - 10 + ascii_a);
      }
      return '?';
  }
  

  More code here.

• I usually make a function which returns the digits and just use it:

  void CharToHex(char c, char *Hex)
  {
     Hex[0]=HexDigit(c>>4);
     Hex[1]=HexDigit(c&0xF);
  }
  
  
  char HexDigit(char c)
  {
     if(c<10)
       return c;
     else
       return c-10+'A';
  }

• #include <iostream>
  using namespace std;
  int main() {
      char c = 123;
      cout << hex << int(c) << endl;
  }
  

  Edit: with zero padding:

  #include <iostream>
  #include <iomanip>
  using namespace std;
  int main() {
      char c = 13;
      cout << hex << setw(2) << setfill('0') << int(c) << endl;
  }
  

  kitchen : Won't that just output it to the console?

  Rob Kennedy : Could you explain why that should work any differently from using stream.hex, like the asker already tried?

  Arnold Spence : I find the 'hex' stream modifier a bit of a pain. If you are outputting other types you have to keep switching the stream back (using 'dec' for example) and as far as I know, it doesn't do leading zeros.

  anon : @kitchen - replace cout with your own stream.

  anon : @rob because the OP's code is wrong?

  kitchen : @Neil: It worked, but what's the difference between using ofstream.hex and std::hex?

  Rob Kennedy : OK, I'll explain it myself. Using stream.hex prints the value of the member-function pointer ostream::hex. (For me the value is 8.) Then it prints the integer value of the character in decimal.

  anon : @rob the hex manipul;ator is not a member of the stream class. The hex you are printing is acually an enumeration value

  Mr.Ree : You might want to use unsigned(c) instead of int(c) if c<0.

  Johannes Schaub - litb : mrree better (int)(unsigned char)c :)

  RBerteig : And this is why I *love* C++ streams: cout << hex << setw(2) << setfill('0') << int(c) << endl is just *so* much clearer than printf("%02x", c) ;-) I know its typesafe(er) but couldn't they have dreamed up something less grotesque?

  Edouard A. : Clarity is a question of point of view... I don't think printf("%02x", c) can be considered as "clear"

  Mr.Ree : litb: You're right. Or perhaps int(c)&0xff

• You simply need to configure your stream once:

  stream << std::hex << std::setfill('0') << std::setw(2)
  

• Try:

  #include <iomanip>
  ....
  stream << std::hex << static_cast<int>(buf[i]);
  

• You can also do it using something a bit more old-fashioned:

  char buffer[3];//room for 2 hex digits and \0
  sprintf(buffer,"%02X ",onebyte);