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.
It's intended that eventually this exposes a single procedural macro, something like this:
cpp
class Bob {
public:
Bob(std::string name);
...
void do_a_thing();
}
```rust use autocxx::include_cxx;
include_cxx!( Header("base/bob.h"), Allow("Bob"), )
let a = ffi::base::Bob::makeunique("hello".into()); a.doa_thing(); ```
See demo/src/main.rs for a real example.
The existing cxx facilities are used to allow safe ownership of C++ types from Rust; specifically things like std::unique_ptr and std::string - so the Rust code should not typically require use of unsafe code, unlike with normal bindgen bindings.
The macro and code generator will both need to know the include path to be passed to bindgen. At the moment, this is passed in via an
environment variable, AUTOCXX_INC. See the demo/build.rs file for details.
It is effectively a three-stage procedural macro, which:
bindgen to generate some bindings (with all the usual unsafe, #[repr(C)] etc.)cxx::bridge.cxx::bridge to convert them to Rust code.The same code can be passed through tools that generate .cc and .h bindings too:
bindgen to generate some bindings (with all the usual unsafe, #[repr(C)] etc.) - in exactly the same way as above.cxx::bridge - in the same way as above.cxx to generate .cc and .h filesThere is an example of this macro working within the demo directory.
The project also contains test code which does this end-to-end, for all sorts of C++ types and constructs which we eventually would like to support.
| Type | Status |
| ---- | ------ |
| Primitives (u8, etc.) | Works |
| Plain-old-data structs | Works |
| std::uniqueptr of POD | Works |
| std::uniqueptr of std::string | Works |
| std::uniqueptr of opaque types | - |
| Reference to POD | Works |
| Reference to std::string | Works |
| Classes | Works, but with warnings |
| Methods | Works (classes give warnings) |
| Int #defines | Works |
| String #defines | Works |
| Primitive constants | Works |
| Enums | Works, though more thought needed |
| #ifdef, #if etc. | - |
| Typedefs | - |
| Structs containing UniquePtr | Works |
| Structs containing strings | Works (opaque only) |
| Passing opaque structs (owned by UniquePtr) into C++ APIs which take them by value | Works, not yet for methods |
| Constructors/makeunique | Works, though probably many problems |
| Field access to opaque objects via UniquePtr | - |
| Reference counting | - |
| std::optional | - |
| Function pointers | - |
| Unique ptrs to primitives | - |
| Inheritance from pure virtual classes | - |
| Destructors | Works via cxx UniquePtr already |
| Namespaces | - |
The plan is (roughly) to work through the above list of features. Some are going to be very hard, and it's not at all clear that a plan will present itself. In particular, some will require that C++ structs are owned by UniquePtr yet passed to C++ by value. It's not clear how ergonomic the results will be. Until we are much further, I don't advise using this for anything in production.
This crate mostly intends to follow the lead of the cxx crate in where and when unsafe is required (it doesn't currently do that, because cxx has recently changed policy). But, this crate is opinionated. It believes some unsafety requires more careful review than other bits, along the following spectrum:
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.
It's not clear yet how this opinion will play out in practice. Perhaps we will get something like unsafe(ffi) or might_be_a_bit_unsafe (joking!) All that is yet to be determined.
Because this uses bindgen, and bindgen may depend on the state of your system C++ headers, it is somewhat sensitive. The following known build problems exist:
type-parameter-0-0 which is not a valid identifier.This runs bindgen within a procedural macro. There are limited opportunities to pass information into procedural macros, yet bindgen needs to know a lot about the build environment.
The plan is: * The Rust code itself will specify the include file(s) and allowlist by passing them into the macro. This is the sort of thing that developers within an existing C++ codebase would specify in C++ (give or take) so it makes sense for it to be specific in the Rust code. * However, all build settings (e.g. bindgen compiler configuration, include path etc.) will be passed into the macro by means of environment variables. The build environment should set these before building the code. (An alternative means will be provided to pass these into the C++ code generator tools.)
demo - a demo exampleengine - all the core code. Currently a Rust library, but we wouldn't want to support
these APIs for external users, so maybe it needs to be a directory of code symlinked
into all the other sub-crates. All the following three sub-crates are thin wrappers
for part of this engine. This also contains the test 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. Except this isn't written yet.src (outermost project)- the procedural macro include_cxx as described above.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!
This is not an officially supported Google product.
Licensed under either of Apache License, Version 2.0 or MIT license at your option.