| /// 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 a 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"); |
| /// type Kind = cxx::kind::Opaque; |
| /// } |
| /// |
| /// #[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 Kind = cxx::kind::Opaque; |
| /// # } |
| /// ``` |
| type Id; |
| |
| /// Either `cxx::kind::Opaque` or `cxx::kind::Trivial`. If in doubt, use |
| /// `cxx::kind::Opaque`. |
| /// |
| /// Some C++ types are safe to hold and pass around in Rust, by value. |
| /// Those C++ types must have a trivial move constructor, and must |
| /// have no destructor. |
| /// |
| /// If you believe your C++ type is indeed trivial, you can specify |
| /// ``` |
| /// # struct TypeName; |
| /// # unsafe impl cxx::ExternType for TypeName { |
| /// # type Id = cxx::type_id!("name::space::of::TypeName"); |
| /// type Kind = cxx::kind::Trivial; |
| /// # } |
| /// ``` |
| /// which will enable you to pass it into C++ functions by value, |
| /// return it by value from such functions, and include it in |
| /// `struct`s that you have declared to `cxx::bridge`. Your promises |
| /// about the triviality of the C++ type will be checked using |
| /// `static_assert`s in the generated C++. |
| /// |
| /// Opaque types can't be passed by value, but can still be held |
| /// in `UniquePtr`. |
| type Kind; |
| } |
| |
| /// Marker types identifying Rust's knowledge about an extern C++ type. |
| /// |
| /// These markers are used in the [`Kind`][ExternType::Kind] associated type in |
| /// impls of the `ExternType` trait. Refer to the documentation of `Kind` for an |
| /// overview of their purpose. |
| pub mod kind { |
| /// An opaque type which cannot be passed or held by value within Rust. |
| /// |
| /// Rust's move semantics are such that every move is equivalent to a |
| /// memcpy. This is incompatible in general with C++'s constructor-based |
| /// move semantics, so a C++ type which has a destructor or nontrivial move |
| /// constructor must never exist by value in Rust. In CXX, such types are |
| /// called opaque C++ types. |
| /// |
| /// When passed across an FFI boundary, an opaque C++ type must be behind an |
| /// indirection such as a reference or UniquePtr. |
| pub struct Opaque; |
| |
| /// A type with trivial move constructor and no destructor, which can |
| /// therefore be owned and moved around in Rust code without requiring |
| /// indirection. |
| pub struct Trivial; |
| } |
| |
| #[doc(hidden)] |
| pub fn verify_extern_type<T: ExternType<Id = Id>, Id>() {} |
| |
| #[doc(hidden)] |
| pub fn verify_extern_kind<T: ExternType<Kind = Kind>, Kind>() {} |