rust
/
book
mirror of https://github.com/rust-lang/book
1
0
Fork 0
Code Issues Releases Wiki Activity

Don’t recommend empty enums for opaque types 

Same as https://github.com/rust-lang-nursery/nomicon/pull/44
This commit is contained in:
Simon Sapin 2017-11-23 14:04:20 +01:00 committed by GitHub
parent 131859023a
commit a3d7bc224a
No known key found for this signature in database
GPG Key ID: 4AEE18F83AFDEB23
1 changed files with 7 additions and 5 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

12
first-edition/src/ffi.md
View File

@ -737,11 +737,11 @@ void foo(struct Foo *arg);
void bar(struct Bar *arg);
```
To do this in Rust, let’s create our own opaque types with `enum`:
To do this in Rust, let’s create our own opaque types:
```rust
pub enum Foo {}
pub enum Bar {}
#[repr(C)] pub struct Foo { private: [u8; 0] }
#[repr(C)] pub struct Bar { private: [u8; 0] }
extern "C" {
pub fn foo(arg: *mut Foo);
@ -750,7 +750,9 @@ extern "C" {
# fn main() {}
```
By using an `enum` with no variants, we create an opaque type that we can’t
instantiate, as it has no variants. But because our `Foo` and `Bar` types are
By including a private field and no constructor,
we create an opaque type that we can’t instantiate outside of this module.
An empty array is both zero-size and compatible with `#[repr(C)]`.
But because our `Foo` and `Bar` types are
different, we’ll get type safety between the two of them, so we cannot
accidentally pass a pointer to `Foo` to `bar()`.