cxx-async is a Rust crate that extends the cxx library to provide interoperability between asynchronous Rust code using async/await and C++20 coroutines using co_await. If your C++ code is asynchronous, cxx-async can provide a more convenient, and potentially more efficient, alternative to callbacks. You can freely convert between C++ coroutines and Rust futures and/or streams and await one from the other.

It's important to emphasize what cxx-async isn't: it isn't a C++ binding to Tokio or any other Rust I/O library. Nor is it a Rust binding to boost::asio or similar. Such bindings could in principle be layered on top of cxx-async if desired, but this crate doesn't provide them out of the box. (Note that this is a tricky problem even in theory, since Rust async I/O code is generally tightly coupled to a single library such as Tokio, in much the same way C++ async I/O code tends to be tightly coupled to libraries like boost::asio.) If you're writing server code, you can still use cxx-async, but you will need to ensure that both the Rust and C++ sides run separate I/O executors.

cxx-async aims for compatibility with popular C++ coroutine support libraries. Right now, both the lightweight cppcoro and the more comprehensive Folly are supported. Pull requests are welcome to support others.

To use cxx-async, first start by adding cxx to your project. Then add the following to your Cargo.toml:

[dependencies]
cxx-async = "0.1"

Now, inside your #[cxx::bridge] module, declare a future type and some methods like so:

#[cxx::bridge]
mod ffi {
    // Declare type aliases for each of the future types you wish to use here. Then declare
    // async C++ methods that you wish Rust to call. Make sure they return one of the future
    // types you declared.
    unsafe extern "C++" {
        type RustFutureString = crate::RustFutureString;

        fn hello_from_cpp() -> RustFutureString;
    }

    // Async Rust methods that you wish C++ to call go here. Again, make sure they return one of the
    // boxed future types you declared above.
    extern "Rust" {
        fn hello_from_rust() -> Box<RustFutureString>;
    }
}

After the #[cxx::bridge] block, define the future types using the #[cxx_async::bridge] attribute:

// The inner type is the Rust type that this future yields.
#[cxx_async::bridge]
unsafe impl Future for RustFutureString {
    type Output = String;
}

Now, in your C++ file, make sure to #include the right headers:

#include "rust/cxx.h"
#include "rust/cxx_async.h"
#include "rust/cxx_async_cppcoro.h"  // Or cxx_async_folly.h, as appropriate.

And add a call to the CXXASYNC_DEFINE_FUTURE macro to define the C++ side of the future:

// The first argument is the C++ type that the future yields, and the second argument is the
// fully-qualified name of the future, with `::` namespace separators replaced with commas. (For
// instance, if your future is named `mycompany::myproject::RustFutureString`, you might write
// `CXXASYNC_DEFINE_FUTURE(rust::String, mycompany, myproject, RustFutureString);`. The first
// argument is the C++ type that `cxx` maps your Rust type to: in this case, `String` maps to
// `rust::String`, so we supply `rust::String` here.
//
// This macro must be invoked at the top level, not in a namespace.
CXXASYNC_DEFINE_FUTURE(rust::String, RustFutureString);

You're all set! Now you can define asynchronous C++ code that Rust can call:

RustFutureString hello_from_cpp() {
    co_return std::string("Hello world!");
}

On the Rust side:

async fn call_cpp() -> String {
    // This returns a Result (with the error variant populated if C++ threw an exception), so you
    // need to unwrap it:
    ffi::hello_from_cpp().await.unwrap()
}

And likewise, define some asynchronous Rust code that C++ can call:

use cxx_async::CxxAsyncResult;
fn hello_from_rust() -> RustFutureString {
    // You can instead use `fallible` if your async block returns a Result.
    RustFutureString::infallible(async { "Hello world!".to_owned() })
}

Over on the C++ side:

cppcoro::task<rust::String> call_rust() {
    co_return hello_from_rust();
}

That's it! You should now be able to freely await futures on either side. An analogous procedure can be followed to wrap C++ coroutines that yield values with co_yield in Rust streams.

You will need a C++ compiler that implements the coroutines TS, which generally coincides with support for C++20. Some C++ compilers (e.g. Apple clang 13.0.0) that implement the coroutines TS crash when compiling Folly. It's also recommended to use libc++ instead of libstdc++, as the former has more complete support for coroutines.

Usage of cxx-async with Folly requires that Folly have been built with coroutine support. Many distributions of Folly (e.g. the one in Homebrew) don't have coroutine support enabled; a common symptom of this is a linker error mentioning a missing symbol folly::resumeCoroutineWithNewAsyncStackRoot.

To build Folly with coroutine support on macOS, try:

$ git clone https://github.com/facebook/folly.git
$ cd folly
$ ./build.sh --install-dir=/usr/local --extra-cmake-defines \
    '{"CXX_STD": "c++20", "CMAKE_CXX_FLAGS": "-fcoroutines-ts"}'

Note that, depending on how your permissions are set up, you might get an error like Permission denied: '/usr/local/.built-by-getdeps'; this is harmless.

cxx-async follows the same Code of Conduct as Rust itself. Reports can be made to the crate authors.

Licensed under either of Apache License, Version 2.0 or MIT license at your option.

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in this project by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.