How to convert a number from little endian to big endian

Converting from little endian to big endian means actually reversing the bits in a number, like in a mirror. This can be done iteratively, by removing one bit at a time and building another number with the bits reversed. I don't know any other version, but I don't think there's a better solution.

   1:      //little endian to big endian

   2:      unsigned char littleEndianN = 5;

   3:      cout << (unsigned int)littleEndianN << "(size=" << sizeof(littleEndianN) * 8 << ") little endian is ";

   4:      unsigned char bigEndianN = 0;

   5:      for (int i = 0; i < sizeof(littleEndianN) * 8; i++) {

   6:          cout << (littleEndianN & 1);

   7:          bigEndianN |= (littleEndianN & 1);

   8:          littleEndianN >>= 1;

   9:          if (i < (sizeof(littleEndianN) * 8 - 1))

  10:              bigEndianN <<= 1;

  11:      }

  12:      cout << " meaning " << (unsigned int)bigEndianN << " in big endian" << endl;

Here's what this prints:

   1:  5(size=8) little endian is 10100000 meaning 160 in big endian

What happens if the number is negative? Well, if we change all the unsigned char's to char's, the program prints this:

   1:  -5(size=8) little endian is 11011111 meaning -33 in big endian

Do you know why?
Also since we're on the subject, how do you tell if the computer representation is little endian or big endian? Spoiler below :)

   1:  bool isLittleEndian = ((1 << 1) == 2);