On Mon, Mar 11, 2019 at 10:27 AM Edward Cree <ecree@xxxxxxxxxxxxxx> wrote: > > On 11/03/19 16:34, Andrii Nakryiko wrote: > > The only thing we should consider is that enums can have different > > sizes. And enum size is part of enum's forward declaration. So unlike > > struct/union fwd, enum's fwd has extra info. I don't think it's > > possible to specify in C (enum is always 4 bytes), > Not always, see e.g. N1458 §6.7.2.2.4: Yeah, you are right, sorry. I was mostly trying to point out that C itself doesn't allow programmer to specify size, while C++ does. > > Each enumerated type shall be compatible with char, a signed integer > > type, or an unsigned integer type. The choice of type is implementation- > > defined,¹²⁸⁾ but shall be capable of representing the values of all the > > members of the enumeration. [...] > The enumeration _constants_ have type int (§6.7.2.2.3), but the enum type > itself has implementation-defined size, per the above. > C doesn't allow to forward-declare enumerators (§6.7.2.3.3), nor even to There is a GCC extension that allows to forward-declare enums: https://gcc.gnu.org/onlinedocs/gcc/Incomplete-Enums.html#Incomplete-Enums Kernel actually has examples of using this (see patch description regrading irqchip_irq_state). > reference them from the enumerator-list in the declaration, since the > enumerator type is incomplete until the closing brace (§6.7.2.2.4). > Footnote 128 adds that "An implementation may delay the choice of which > integer type until all enumeration constants have been seen." > It appears, in any case, that no forward-declaration could be required in > BTF, since an enum type's BTF record does not reference other types. With > something like > enum foo { > BAR = sizeof(enum foo *), > }; > which is not valid C thanks to §6.7.2.3.3 (but many compilers will accept > it, e.g. gcc without -pedantic), the BTF record would read > kind=enum name_off=&"foo" vlen=1 > name_off=&"BAR" val=8 > (assuming 64-bit) which does not require any forward-declaring. > > I don't know about the C++ situation, though. C++ allows to forward-declare enums only if as part of declaration you also specify underlying integer type. > > -Ed > > PS: It's not really clear to me what the rationale for §6.7.2.3.3 is, > since all the problems with incomplete enum types also arise with other > incomplete types; why 'enum foo *' can't be treated like 'struct quux *' Seems like GCC's extension allows exactly this. > I don't know. But this isn't comp.std.c...
