十进制转十六进制

 int dec = 123;
 std::string str = "0x";

 for (int i = 0; i < 8; i++)
 {
  int tmp = (dec >> (7 - i % 8) * 4) & 0x0f;

  if (tmp < 10)
   str += (tmp + '0');
  else
   str += ('A' + (tmp - 10));
 }