Why use the Bitwise-Shift operator for values in a C enum definition?

Question

Apple sometimes uses the Bitwise-Shift operator in their enum definitions. For example, in the CGDirectDisplay.h file which is part of Core

Answer 1

Let's me give you a more practice example. In c++ when you want to open a file (Open for output, and in binary mode opposed to text mode), you can do it by:

const char *filename = "/home/xy/test.bin";
fstream output(filename, ios::out | ios::binary);

You can see, ios::out | ios::binary can set two mode(Open for output, and in binary mode).

How does this work ? It's by enum(bitwise-shift values):

enum _Ios_Openmode 
{ 
  _S_app        = 1L << 0,
  _S_ate        = 1L << 1,
  _S_bin        = 1L << 2,  /* 0b0000000000000100 */
  _S_in         = 1L << 3,
  _S_out        = 1L << 4,  /* 0b0000000000010000 */
  _S_trunc      = 1L << 5
  //.....
};

/// Perform input and output in binary mode (as opposed to text mode).
static const openmode binary =  _S_bin;

/// Open for input.  Default for @c ifstream and fstream.
static const openmode in =      _S_in;

/// Open for output.  Default for @c ofstream and fstream.
static const openmode out =     _S_out;

If you use value increment by 1 in enum _Ios_Openmode, you have to set(ios::out) and set(ios::binary) do two times. It may not so convenient to check and set value by one time.