cxx-async

Overview

cxx-async is a Rust crate that extends the cxx library to provide seamless 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 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.

Quick tutorial

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 {
    // Give each future type that you want to bridge a name.
    extern "Rust" {
        type RustFutureString;
    }

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

    
     // 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
     
      ;
    }
}
     
    

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

// The inner type is the Rust type that this future yields.
#[cxx_async::bridge_future]
struct RustFutureString(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 name you gave the future, and the second argument is the corresponding
// C++ type. The latter 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.
CXXASYNC_DEFINE_FUTURE(RustFutureString, rust::String);

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

rust::Box 
   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() -> Box
    {
    
   // 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 
   call_rust() {
    
   co_return 
   hello_from_rust();
}
  

That's it! You should now be able to freely await futures on either side.

Code of Conduct

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

License

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.