On Fri, Jul 23, 2021 at 7:40 AM Petr Viktorin <encu...@gmail.com> wrote: [snip] > On 21. 07. 21 14:18, Nick Coghlan wrote:
> [snip] > > Please don't put the enum in the stable ABI. If we would add another > > value and then an older extension would receive it, we'd get > undefined > > behavior. > > > > > > After researching a bit more, I see that casting unknown values to enum > is only undefined/unspecified behavior in C++. But we do support C++ > extensions, and so I'll try to get enums out of the stable ABI. > > (In both C & C++, the size of an `enum` is implementation-defined. > That's unlikely to be a problem in practice, but one more point against > enum.) > I can't speak much for C, but for C++ this is fine I think? enums have the same range of values as their "underlying type" (see e.g. [basic.fundamental] <https://eel.is/c++draft/basic.fundamental> and [dcl.enum] <https://eel.is/c++draft/dcl.enum>). Aside from representation, [expr.static.cast] <https://eel.is/c++draft/expr.static.cast> explicitly allows conversion when in-range: "the value is unchanged if the original value is within the range of the enumeration values". (Note if you click the link -- "fixed underlying type" refers to a form of enum which doesn't exist in C; the C-like enums are the ones without a fixed underlying type.) And as Larry points out, you can force the range of values to be as large as you want by specifying a large enumerator. However, the standard is not meant for reading by us mere mortals, so I usually use cppreference, which has this to say at https://en.cppreference.com/w/cpp/language/enum : Values of integer, floating-point, and enumeration types can be converted > by static_cast <https://en.cppreference.com/w/cpp/language/static_cast> > or explicit cast > <https://en.cppreference.com/w/cpp/language/explicit_cast>, to any > enumeration type. If the underlying type is not fixed and the source value > is out of range, [the result is unspecified (until C++17) | the behavior > is undefined (since C++17)]. [...] Note that the value after such > conversion may not necessarily equal any of the named enumerators defined > for the enumeration. -- Devin
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