How do i use it in C++ ?
when is it useful to use ?
Please give me an example of a problem where bitmask is used , how it actually works .
Thanks!
Answer
Briefly bitmask helps to manipulate position of multiple values. There is a good example here ;
Bitflags are a method of storing multiple values, which are not mutually exclusive, in one variable. You've probably seen them before. Each flag is a bit position which can be set on or off. You then have a bunch of bitmasks #defined for each bit position so you can easily manipulate it:
#define LOG_ERRORS 1 // 2^0, bit 0
#define LOG_WARNINGS 2 // 2^1, bit 1
#define LOG_NOTICES 4 // 2^2, bit 2
#define LOG_INCOMING 8 // 2^3, bit 3
#define LOG_OUTGOING 16 // 2^4, bit 4
#define LOG_LOOPBACK 32 // and so on...
// Only 6 flags/bits used, so a char is fine
unsigned char flags;
// initialising the flags
// note that assigning a value will clobber any other flags, so you
// should generally only use the = operator when initialising vars.
flags = LOG_ERRORS;
// sets to 1 i.e. bit 0
//initialising to multiple values with OR (|)
flags = LOG_ERRORS | LOG_WARNINGS | LOG_INCOMING;
// sets to 1 + 2 + 8 i.e. bits 0, 1 and 3
// setting one flag on, leaving the rest untouched
// OR bitmask with the current value
flags |= LOG_INCOMING;
// testing for a flag
// AND with the bitmask before testing with ==
if ((flags & LOG_WARNINGS) == LOG_WARNINGS)
...
// testing for multiple flags
// as above, OR the bitmasks
if ((flags & (LOG_INCOMING | LOG_OUTGOING))
== (LOG_INCOMING | LOG_OUTGOING))
...
// removing a flag, leaving the rest untouched
// AND with the inverse (NOT) of the bitmask
flags &= ~LOG_OUTGOING;
// toggling a flag, leaving the rest untouched
flags ^= LOG_LOOPBACK;
**
WARNING: DO NOT use the equality operator (i.e. bitflags == bitmask) for testing if a flag is set - that expression will only be true if that flag is set and all others are unset. To test for a single flag you need to use & and == :
**
if (flags == LOG_WARNINGS) //DON'T DO THIS
...
if ((flags & LOG_WARNINGS) == LOG_WARNINGS) // The right way
...
if ((flags & (LOG_INCOMING | LOG_OUTGOING)) // Test for multiple flags set
== (LOG_INCOMING | LOG_OUTGOING))
...
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