On 3/1/13 11:51 AM, Graydon Hoare wrote:
It's also to represent enums known-to-C++ with the same values when
projected into rust types.
It'll always be a less-than-faithful translation, though, because C++
enum values don't have to correspond to any of the variants. You can
even represent bitfields with C++ enums:
enum e {
a = 0x00,
b = 0x01,
c = 0x10,
};
int main() {
e x = (e)(a | c);
return 0;
}
More importantly, the size of an enum is *undefined* in C++. So outside
of C++11 `enum class` it's always dangerous to assume that a given Rust
enum is bit-compatible with a C++ enum.
This much I think we need to address generally via explicit control over
packing. Especially on x86, _most_ of our enums should be bytes at most.
https://github.com/mozilla/rust/issues/1704
I'm a bit hesitant to commit to a particular bit representation of
enums, since there's lots of room for compiler optimization here. For
example, we might want to collapse `Option<~int>` into a nullable
pointer, we might want to collapse `Result<(),~str>` into a nullable
string, or we might want to collapse `Either<u8,~str>` into 1 word,
assuming that strings can never occupy the top 256 bytes of the address
space. I've thought for a while that perhaps it's best to just say that
the bit pattern of Rust enums is unspecified, to give us as much room as
possible to play with optimizations.
Casting _to_ is permitted currently via transmute, which is probably a
fine place to leave it. I agree it'd be nice to provide an auto_enum
syntax extension that defines a bad-value condition, and produces a
.from_int() safe function that hits that condition when given a bad integer.
Agreed.
3. There is no way to have multiple values correspond to a single enum
variant.
Is there in C++? The point of this feature has always been (to my mind)
emulation of C++ where sensible.
No. I guess this point is irrelevant. (Mostly I was thinking that it
seems clear that we will need a syntax extension to properly support
bitfields and that it is a bit odd to require a syntax extension for
bitfields but to have support for explicit enum values in the language,
given that they're very similar features.)
Patrick
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