Hi Marcin,

On 2023-07-10 14:52, Marcin Jaczewski wrote:
> pon., 10 lip 2023 o 14:40 Alejandro Colomar via Std-Proposals
> <std-proposals_at_[hidden]> napisał(a):
[...]

>> An idea floats my mind from time to time, that having a [[flags]]
>> attribute for such an enum would be useful to mark pow-of-2 cases. It
>> could make that the enumerators are prev<<1 if the value is unspecified,
>> instead of prev++ that is default for enums.
>>
>> However, I was never convinced by such idea because removing attributes
>> should produce working code, and having the default values of an enum
>> change depending on the attribute would be problematic. An alternative
>> idea would be to use 'unsigned enum', since flags lists should be
>> unsigned, to allow bitwise operations without UB.
>>
>
> This could work if user need to do all work and attribute only to check
> if given work was done correctly. Simply:
>
> ```
> enum [[flag]] F : int
> {
> Two = 2,
> Three = 3, //compilation error, expected 4
> };
> ```
>
> Ignoring this enum only allows compiling "invalid" programs,
> it would not change behavior of "correct" ones.

Yeah, that would be a possibility. However, I'm worried that in some
cases it might be useful to have a sentinel UINT_MAX value even for
flags enums, and that attribute would prohibit that. Also, sometimes
one wants to define enumerators to be an alias to the combination of
other enumerators. So it's a bit too restrictive.

I think the more useful idea I've had regarding this is the following:

unsigned enum foo_e {
 FOO_A, // = 1,
        FOO_B, // = 2,
 FOO_AB = FOO_A | FOO_B, // = 3,
 FOO_C, // = 4,
 FOO_D, // = 8,

 FOO_ALL = FOO_A | FOO_B | FOO_C | FOO_D, // = 15,
 FOO_ERR = -1 // = UINT_MAX
};

- Default for the first field is 1
- Default for consecutive fields is stdc_bit_ceil(prev+1);
- Default underlying type is unsigned int instead of int.

Cheers,
Alex

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