CXX Bridge Question (problem with dependency cycle)

[felixguendling]

Hey! 👋 C++/Rust interop newbie here.

I am experimenting to make functionality from a C++ library available in Rust (e.g. to write a HTTP API for the C++ code in Rust). This works fine so far. However, I guess I made a mistake since I needed to manually copy the cxx.h file into the C++ project to break a dependency cycle.

• So there is an existing C++ library (here: nigiri) that I rather not want to modify.
• Then I have some wrapper code (also in C++): here nigiri-rust. This already requires the cxx.h include since it uses rust::* types.
• CXX / corosion then generates another library (here nigiri-cxx), which is then linked into the Rust executable (srv) via: corrosion_link_libraries(srv nigiri-cxx nigiri-rust)

The full change set is here:
motis-project/nigiri@master...rust

Is this basically the right way?

I would have liked to pull in the cxx.h file in nigiri-rust from the generated nigiri-cxx. However, this is not possible because nigiri-cxx also requires nigiri-rust for includes. So CMake rightfully complains about a cycle.

How can I get writ of this cycle and have a codebase that's free of cxx.h. As I understand it, this should only be generated (probably to stay up-to-date with the CXX version defined in Cargo.toml).

[felixguendling]

Figured it out based on this test:
https://github.com/corrosion-rs/corrosion/blob/master/test/cxxbridge/cxxbridge_cpp2rust/CMakeLists.txt

I think it would be helpful to add a link in the book.

[jschwe]

a pull-request improving the documentation or linking the test would be very welcome.