macro autocxx-macro

Safe autogenerated interop between Rust and C++

22 releases (11 breaking)

Uses new Rust 2021

0.15.0 Jan 8, 2022
0.14.0 Nov 29, 2021
0.13.2 Nov 22, 2021
0.11.0 Jun 9, 2021
0.4.0 Nov 10, 2020

#8 in #cxx

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Used in 2 crates (via autocxx)


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GitHub crates.io docs.rs

This project is a tool for calling C++ from Rust in a heavily automated, but safe, fashion.

The intention is that it has all the fluent safety from cxx whilst generating interfaces automatically from existing C++ headers using a variant of bindgen. Think of autocxx as glue which plugs bindgen into cxx.


autocxx::include_cpp! {
    #include "url/origin.h"

fn main() {
    let o = ffi::url::Origin::CreateFromNormalizedTuple("https",
        "google.com", 443);
    let uri = o.Serialize();
    println!("URI is {}", uri.to_str().unwrap());

Getting started

If you're here, you want to call some C++ from Rust, right?

You will need:

  • Some C++ header files (.h files)
  • The C++ "include path". That is, the set of directories containing those headers. (That's not necessarily the directory in which each header file lives; C++ might contain #include "foo/bar.h" and so your include path would need to include the directory containing the foo directory).
  • A list of the APIs (types and functions) from those header files which you wish to make available in Rust.
  • Either a Cargo or non-Cargo build system.
  • To know how to link the C++ libraries into your Cargo project. This is beyond the scope of what autocxx helps with, but one solution is to emit a print from your build script.
  • LLVM to be installed.
  • Some patience. This is not a magic solution. C++/Rust interop is hard. Avoid it if you can!

The rest of this 'getting started' section assumes Cargo - if you're using something else, see the include_cpp documentation.

First, add autocxx to your dependencies and autocxx-build to your build-dependencies in your Cargo.toml.

autocxx = "0.13.2"

autocxx-build = "0.13.2"

Now, add a build.rs. This is where you need your include path:

fn main() {
    let path = std::path::PathBuf::from("src"); // include path
    let mut b = autocxx_build::Builder::new("src/main.rs", &[&path]).expect_build();
        // This assumes all your C++ bindings are in main.rs
    // Add instructions to link to any C++ libraries you need.

Finally, in your main.rs you can use the include_cpp macro which is the heart of autocxx:

autocxx::include_cpp! {
    #include "my_header.h" // your header file name
    safety!(unsafe) // see details of unsafety policies described in include_cpp
    generate!("MyAPIFunction") // add this line for each function or type you wish to generate

You should then find you can call the function by referring to an ffi namespace:

fn main() {
    println!("Hello, world! - answer from C++ is {}", ffi::MyAPIFunction(4));

C++ types such as std::string and std::unique_ptr are represented using the types provided by the marvellous cxx library. This provides good ergonomics and safety norms, so unlike with normal bindgen bindings, you won't normally need to write unsafe code for every function call.

Some caveats:

  • Not all C++ features are supported. You will come across APIs - possibly many APIs - where autocxx doesn't work. It should emit reasonable diagnostics explaining the problem. See the section on "dealing with failure" in the include_cpp documentation.
  • autocxx can be frustrating when you run up against its limitations. It's designed to allow importing of APIs from complex existing codebases. It's often a better choice to use cxx directly.

A full user manual can be found in the documentation for include_cpp. See demo/src/main.rs for a basic example, and the examples directory for more.

On safety

This crate mostly intends to follow the lead of the cxx crate in where and when unsafe is required. But, this crate is opinionated. It believes some unsafety requires more careful review than other bits, along the following spectrum:

  • Rust unsafe code (requires most review)
  • Rust code calling C++ with raw pointers
  • Rust code calling C++ with shared pointers, or anything else where there can be concurrent mutation
  • Rust code calling C++ with unique pointers, where the Rust single-owner model nearly always applies (but we can't prove that the C++ developer isn't doing something weird)
  • Rust safe code (requires least review)

If your project is 90% Rust code, with small bits of C++, don't use this crate. You need something where all C++ interaction is marked with big red "this is terrifying" flags. This crate is aimed at cases where there's 90% C++ and small bits of Rust, and so we want the Rust code to be pragmatically reviewable without the signal:noise ratio of unsafe in the Rust code becoming so bad that unsafe loses all value.

See [safety!] in the documentation for more details.

Building without cargo

See instructions in the documentation for include_cpp. This interop inevitably involves lots of fiddly small functions. It's likely to perform far better if you can achieve cross-language LTO. This issue may give some useful hints - see also all the build-related help in the cxx manual which all applies here too.

Directory structure

  • demo - a very simple demo example
  • examples - will gradually fill with more complex examples
  • parser - code which parses a single include_cpp! macro. Used by both the macro (which doesn't do much) and the code generator (which does much more, by means of engine below)
  • engine - all the core code for actual code generation.
  • macro - the procedural macro which expands the Rust code.
  • gen/build - a library to be used from build.rs scripts to generate .cc and .h files from an include_cxx section.
  • gen/cmd - a command-line tool which does the same.
  • src (outermost project) - a wrapper crate which imports the procedural macro and a few other things.

Where to start reading

The main algorithm is in engine/src/lib.rs, in the function generate(). This asks bindgen to generate a heap of Rust code and then passes it into engine/src/conversion to convert it to be a format suitable for input to cxx.

However, most of the actual code is in engine/src/conversion/mod.rs.

At the moment we're using a slightly branched version of bindgen called autocxx-bindgen. It's hoped this is temporary; some of our changes are sufficiently weird that it would be presumptious to try to get them accepted upstream until we're sure autocxx has roughly the right approach.

How to develop

If you're making a change, here's what you need to do to get useful diagnostics etc. First of all, cargo run in the demo directory. If it breaks, you don't get much in the way of useful diagnostics, because stdout is swallowed by cargo build scripts. So, practically speaking, you would almost always move onto running one of the tests in the test suite. With suitable options, you can get plenty of output. For instance:

RUST_BACKTRACE=1 RUST_LOG=autocxx_engine=info cargo test --all test_cycle_string_full_pipeline -- --nocapture

This is especially valuable to see the bindgen output Rust code, and then the converted Rust code which we pass into cxx. Usually, most problems are due to some mis-conversion somewhere in engine/src/conversion. See here for documentation and diagrams on how the engine works.

Reporting bugs

If you've found a problem, and you're reading this, thank you! Your diligence in reporting the bug is much appreciated and will make autocxx better. In order of preference here's how we would like to hear about your problem:

  • Raise a pull request adding a new failing integration test to engine/src/integration_tests.rs.
  • Minimize the test using tools/reduce, something like this: target/debug/autocxx-reduce -d "safety!(unsafe_ffi)" -d 'generate_pod!("A")' -I ~/my-include-dir -h my-header.h -p problem-error-message -- --remove-pass pass_line_markers This is a wrapper for the amazing creduce which will take thousands of lines of C++, preprocess it, and then identify the minimum required lines to reproduce the same problem.
  • Use the C++ preprocessor to give a single complete C++ file which demonstrates the problem, along with the include_cpp! directive you use. Alternatively, run your build using AUTOCXX_PREPROCESS=output.h which should put everything we need into output.h. If necessary, you can use the CLANG_PATH or CXX environment variables to specify the path to the Clang compiler to use.
  • Failing all else, build using cargo clean -p <your package name> && RUST_LOG=autocxx_engine=info cargo build -vvv and send the entire log to us. This will include two key bits of logging: the C++ bindings as distilled by bindgen, and then the version which we've converted and moulded to be suitable for use by cxx.


David Tolnay did much of the hard work here, by inventing the underlying cxx crate, and in fact nearly all of the parsing infrastructure on which this crate depends. bindgen is also awesome. This crate stands on the shoulders of giants!

License and usage notes

This is not an officially supported Google product.

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


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