Example Bindings

CXX requires that the whole C++/Rust boundary is declared in cxx::bridge modules inside .rs source code.

mod ffi {
    extern "Rust" {
        type MultiBuf;

        fn next_chunk(buf: &mut MultiBuf) -> &[u8];

    unsafe extern "C++" {

        type BlobstoreClient;

        fn new_blobstore_client() -> UniquePtr<BlobstoreClient>;
        fn put(self: &BlobstoreClient, buf: &mut MultiBuf) -> Result<u64>;

// Definitions of Rust types and functions go here

Point out:

  • Although this looks like a regular Rust mod, the #[cxx::bridge] procedural macro does complex things to it. The generated code is quite a bit more sophisticated - though this does still result in a mod called ffi in your code.
  • Native support for C++’s std::unique_ptr in Rust
  • Native support for Rust slices in C++
  • Calls from C++ to Rust, and Rust types (in the top part)
  • Calls from Rust to C++, and C++ types (in the bottom part)

Common misconception: It looks like a C++ header is being parsed by Rust, but this is misleading. This header is never interpreted by Rust, but simply #included in the generated C++ code for the benefit of C++ compilers.