> On Jan 6, 2016, at 12:46 PM, Austin Zheng <[email protected]> wrote:
>
> Thanks Joe, this is incredibly helpful, especially the note about how enums
> are passed in 'piecewise'.
To clarify, what you're seeing in EnumMirror is not the enum itself, but the
_EnumMirror structure, which is itself designed to be bitwise-compatible with a
generic (owner, type, pointer to value) triple that can reference an arbitrary
value in memory while keeping it alive if reference-counted. The 'value' field
in this structure is always a pointer to the value of the 'type' type in
memory.
>
> re. your question: I was indeed calling the C++ function from Swift and
> directly passing in the enum (in pseudo-Swift):
> let myEnum : SomeEnumType = ...
> cplusplusFunction(value: myEnum, type: SomeEnumType.self) // incorrect
>
> How would I pass a pointer to the enum's value from Swift? I don't think
> Swift exposes any mechanism to get the memory address of a Swift object.
> Perhaps I should create an UnsafeMutablePointer<SomeEnumType>, set its memory
> to (a copy of) the enum value, and pass that pointer object in instead?
If all you want is the enum tag, you could implement a more straightforward
interface. In Swift:
@_silgen_name("_swift_stdlib_getEnumTag")
func _getEnumTag<T>(value: T) -> Int32
In C++:
/// Return the tag number for an enum, or crash if the value is not of enum
type.
int32_t _swift_stdlib_getEnumTag(OpaqueValue *value, const Metadata *type) {
assert(type->getKind() == MetadataKind::Enum);
auto tag = type->vw_getEnumTag(value);
type->vw_destroy(value);
return tag;
}
-Joe
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