How to use enums with bit flags

Question

I have an enum declaration using bit flags and I cant exactly figure out on how to use this.

enum 
{
  kWhite   = 0,
  kBlue    = 1 << 0,
  kRed     = 1 << 1,
  kYellow  = 1 << 2,
  kBrown   = 1 << 3,
};
typedef char ColorType;

I suppose to store multiple colors in one colorType I should OR the bits together?

ColorType pinkColor = kWhite | kRed;

But suppose I would want to check if pinkColor contains kRed, how would I do this?

Anyone care to give me an example using the provided ColorType example ?

Solution

Yes, use bitwise OR (|) to set multiple flags:

ColorType pinkColor = kWhite | kRed;

Then use bitwise AND (&) to test if a flag is set:

if ( pinkColor & kRed )
{
   // do something
}

The result of & has any bit set only if the same bit is set in both operands. Since the only bit in kRed is bit 1, the result will be 0 if the other operand doesn't have this bit set too.

If you need to get whether a particular flag is set as a BOOL rather than just testing it in an if condition immediately, compare the result of the bitwise AND to the tested bit:

BOOL hasRed = ((pinkColor & kRed) == kRed);

Comments

[Nico]

Note: This means that if pinkColor is kRed, (pinkColor & kRed) will evaluate to kRed, not 1 or YES! This can be a pitfall when assigning to a small type such as BOOL: If the value is 1 << (number of bits in a BOOL) or greater, it will be out of range. A common fix is to compare the result to the tested bit: BOOL isPink = ((pinkColor & kRed) == kRed); An alternative is to cast the result to C99's bool type: isPink = (bool)(pinkColor & kRed); And a(n admittedly uncommon) way that doesn't assume 1 is in range would use ?:: isPink = (pinkColor & kRed) ? YES : NO;

[Mark Amery]

@PeterHosey Relevant further reading for anyone interested in your comment: bignerdranch.com/blog/bools-sharp-corners. By the way, if anyone is confused (as I was) about why on earth casting to a C99 bool instead of BOOL fixes the problem, the answer is that casts to bool are magic, as discussed in stackoverflow.com/questions/16934876/…. The short version is: bool is an alias of _Bool and the C99 standard says "When any scalar value is converted to _Bool, the result is 0 if the value compares equal to 0; otherwise, the result is 1."