mask in enum [C++]

Question

Hi, I'm playing at the moment with bits and was looking at microsoft code for io states and discovered something like this:

enum _Iostate
 { // constants for stream states
 _Statmask = 0x17};//What is this mask for???

static const _Iostate goodbit = (_Iostate)0x0;
static const _Iostate eofbit = (_Iostate)0x1;
static const _Iostate failbit = (_Iostate)0x2;
static const _Iostate badbit = (_Iostate)0x4;
static const _Iostate _Hardfail = (_Iostate)0x10;

I just wonder why is this mask for because code works without this mask and values stayed the same with as whitout this mask. Thank you.

Answer 1

It makes sure that the _Iostate enum has the correct size to hold all the bit constants defined afterward, and their combinations.

Answer 2

That's a bitwise OR of all possible flags. You could perhaps use it to extract the part containing the flags from other bits in the integer.