explain the empty type better, and explain to NOT use enums

pull/82/head
Ralf Jung 6 years ago
parent 5b45a89c37
commit b3d532f55b

@ -751,8 +751,17 @@ extern "C" {
```
By including a private field and no constructor,
we create an opaque type that we cant instantiate outside of this module.
An empty array is both zero-size and compatible with `#[repr(C)]`.
we create an opaque type that we can't instantiate outside of this module.
(A struct with no field could be instantiated by anyone.)
We also want to use this type in FFI, so we have to add `#[repr(C)]`.
And to avoid warning around using `()` in FFI, we instead use an empty array,
which works just as well as an empty type but is FFI-compatible.
But because our `Foo` and `Bar` types are
different, well get type safety between the two of them, so we cannot
accidentally pass a pointer to `Foo` to `bar()`.
Notice that it is a really bad idea to use an empty enum as FFI type.
The compiler relies on empty enums being uninhabited, so handling values of type
`&Empty` is a huge footgun and can lead to buggy program behavior (by triggering
undefined behavior).

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