On Thursday, September 08, 2011 15:04:56 Andrei Alexandrescu wrote:
> On 9/8/11 2:02 PM, Jonathan M Davis wrote:
> > I think that it makes perfect sense to use enums for flags. What I don't
> > think makes sense is making the type of the variable which holds the
> > flags to be that enum type unless _every_ possible combination of flags
> > has its own flag so that &ing or |ing enums always results in a valid
> > enum.
>
> This ain't going to work because it would require the human user to
> write by hand a combinatorial number of symbols.
>
> A ligthweight fixed-sized set with named members is a worthy abstraction
> for the standard library.
I agree. I'm not arguing that the user _should_ create such a combination of
flags. That would be horrible. I'm just arguing that having a set of flags with
enums, e.g.
enum Flag { a = 1, b = 2, c = 4, d = 8 };
and then having Flag.a | Flag.b or Flag.a & Flag.b result in a value of type
Flag is not a good idea, because the result isn't a valid Flag. It should
result in whatever the base type is (int in this case), and functions which
take such flags &ed or |ed should take them using the base type, not the enum
type.
- Jonathan M Davis
