| /// A type for which the layout is determined by its C++ definition. |
| /// |
| /// This trait serves the following two related purposes. |
| /// |
| /// <br> |
| /// |
| /// ## Safely unifying occurrences of the same extern type |
| /// |
| /// `ExternType` makes it possible for CXX to safely share a consistent Rust |
| /// type across multiple #\[cxx::bridge\] invocations that refer to a common |
| /// extern C++ type. |
| /// |
| /// In the following snippet, two #\[cxx::bridge\] invocations in different |
| /// files (possibly different crates) both contain function signatures involving |
| /// the same C++ type `example::Demo`. If both were written just containing |
| /// `type Demo;`, then both macro expansions would produce their own separate |
| /// Rust type called `Demo` and thus the compiler wouldn't allow us to take the |
| /// `Demo` returned by `file1::ffi::create_demo` and pass it as the `Demo` |
| /// argument accepted by `file2::ffi::take_ref_demo`. Instead, one of the two |
| /// `Demo`s has been defined as an extern type alias of the other, making them |
| /// the same type in Rust. The CXX code generator will use an automatically |
| /// generated `ExternType` impl emitted in file1 to statically verify that in |
| /// file2 `crate::file1::ffi::Demo` really does refer to the C++ type |
| /// `example::Demo` as expected in file2. |
| /// |
| /// ```no_run |
| /// // file1.rs |
| /// # mod file1 { |
| /// #[cxx::bridge(namespace = example)] |
| /// pub mod ffi { |
| /// extern "C" { |
| /// type Demo; |
| /// |
| /// fn create_demo() -> UniquePtr<Demo>; |
| /// } |
| /// } |
| /// # } |
| /// |
| /// // file2.rs |
| /// #[cxx::bridge(namespace = example)] |
| /// pub mod ffi { |
| /// extern "C" { |
| /// type Demo = crate::file1::ffi::Demo; |
| /// |
| /// fn take_ref_demo(demo: &Demo); |
| /// } |
| /// } |
| /// # |
| /// # fn main() {} |
| /// ``` |
| /// |
| /// <br><br> |
| /// |
| /// ## Integrating with bindgen-generated types |
| /// |
| /// Handwritten `ExternType` impls make it possible to plug in a data structure |
| /// emitted by bindgen as the definition of an opaque C++ type emitted by CXX. |
| /// |
| /// By writing the unsafe `ExternType` impl, the programmer asserts that the C++ |
| /// namespace and type name given in the type id refers to a C++ type that is |
| /// equivalent to Rust type that is the `Self` type of the impl. |
| /// |
| /// ```no_run |
| /// # const _: &str = stringify! { |
| /// mod folly_sys; // the bindgen-generated bindings |
| /// # }; |
| /// # mod folly_sys { |
| /// # #[repr(transparent)] |
| /// # pub struct StringPiece([usize; 2]); |
| /// # } |
| /// |
| /// use cxx::{type_id, ExternType}; |
| /// |
| /// unsafe impl ExternType for folly_sys::StringPiece { |
| /// type Id = type_id!("folly::StringPiece"); |
| /// } |
| /// |
| /// #[cxx::bridge(namespace = folly)] |
| /// pub mod ffi { |
| /// extern "C" { |
| /// include!("rust_cxx_bindings.h"); |
| /// |
| /// type StringPiece = crate::folly_sys::StringPiece; |
| /// |
| /// fn print_string_piece(s: &StringPiece); |
| /// } |
| /// } |
| /// |
| /// // Now if we construct a StringPiece or obtain one through one |
| /// // of the bindgen-generated signatures, we are able to pass it |
| /// // along to ffi::print_string_piece. |
| /// # |
| /// # fn main() {} |
| /// ``` |
| pub unsafe trait ExternType { |
| /// A type-level representation of the type's C++ namespace and type name. |
| /// |
| /// This will always be defined using `type_id!` in the following form: |
| /// |
| /// ``` |
| /// # struct TypeName; |
| /// # unsafe impl cxx::ExternType for TypeName { |
| /// type Id = cxx::type_id!("name::space::of::TypeName"); |
| /// # } |
| /// ``` |
| type Id; |
| } |
| |
| #[doc(hidden)] |
| pub fn verify_extern_type<T: ExternType<Id = Id>, Id>() {} |