C++ Bridge Declarations

mod ffi {
    // C++ types and signatures exposed to Rust.
    unsafe extern "C++" {

        type BlobstoreClient;

        fn new_blobstore_client() -> UniquePtr<BlobstoreClient>;
        fn put(self: Pin<&mut BlobstoreClient>, parts: &mut MultiBuf) -> u64;
        fn tag(self: Pin<&mut BlobstoreClient>, blobid: u64, tag: &str);
        fn metadata(&self, blobid: u64) -> BlobMetadata;

Results in (roughly) the following Rust:

pub struct BlobstoreClient {
    _private: ::cxx::private::Opaque,

pub fn new_blobstore_client() -> ::cxx::UniquePtr<BlobstoreClient> {
    extern "C" {
        #[link_name = "org$blobstore$cxxbridge1$new_blobstore_client"]
        fn __new_blobstore_client() -> *mut BlobstoreClient;
    unsafe { ::cxx::UniquePtr::from_raw(__new_blobstore_client()) }

impl BlobstoreClient {
    pub fn put(&self, parts: &mut MultiBuf) -> u64 {
        extern "C" {
            #[link_name = "org$blobstore$cxxbridge1$BlobstoreClient$put"]
            fn __put(
                _: &BlobstoreClient,
                parts: *mut ::cxx::core::ffi::c_void,
            ) -> u64;
        unsafe {
            __put(self, parts as *mut MultiBuf as *mut ::cxx::core::ffi::c_void)

// ...
  • The programmer does not need to promise that the signatures they have typed in are accurate. CXX performs static assertions that the signatures exactly correspond with what is declared in C++.
  • unsafe extern blocks allow you to declare C++ functions that are safe to call from Rust.