Rust bindings for C++ functions

Rust code can call (non-member) functions defined in C++, provided that the parameter and return types are supported by Crubit:

  • If a parameter or return type is a primitive type, then the bindings for the function use the corresponding Rust type.
  • Similarly, if a parameter or return type is a pointer type, then the bindings for the function use the corresponding Rust pointer type.
  • If a parameter or return type is a C++ reference type, then the bindings for the function use a Rust reference (if the lifetime was either explicitly specified or elided and inferred) or a Rust pointer (otherwise). See References and lifetimes.
  • If the type is a user-defined type, such as a class type or enum, then the bindings for the function use the bindings for that type.

Additionally, code can call member functions defined in C++ if the parameter and return types are supported by Crubit (see above). Non-static member functions become Rust methods, and static member functions become associated functions.

Examples

Functions

Given the following C++ header:

cs/file:examples/cpp/function/example.h function:add_two_integers

Crubit will generate the following bindings, with a safe public function that calls into the corresponding FFI glue:

cs/file:examples/cpp/function/example_generated.rs function:add_two_integers

Which can be used like this:

cs/file:examples/cpp/function/main.rs function:main

Methods

Given the following C++ header:

cs/file:examples/cpp/method/example.h class:Bar

Crubit will generate the following bindings:

cs/file:examples/cpp/method/example_generated.rs class:Bar

cs/file:examples/cpp/method/example_generated.rs content:"(?m)impl Bar.*?(^$|^    \})"

Which can be used like this:

cs/file:examples/cpp/method/main.rs function:main

The this parameter of a method becomes &self or &mut self. See References and lifetimes for more details.

unsafe functions

Which C++ functions are marked unsafe in Rust?

By default, the Rust binding to a C++ function is marked as safe or unsafe based on the types of its parameters. If a C++ function accepts only simple types like integers, the resulting Rust binding will be marked as safe. Functions which accept a raw pointer are automatically marked as unsafe.

This behavior can be overridden using the CRUBIT_UNSAFE, CRUBIT_UNSAFE_MARK_SAFE and CRUBIT_OVERRIDE_UNSAFE(is_unsafe) macros.

For example, given the following C++ header:

cs/file:examples/cpp/unsafe_attributes/example.h content:^([^/#\n])[^\n]*

Crubit will generate the following bindings:

cs/file:examples/cpp/unsafe_attributes/example_generated.rs content:^([^/\n])([^!\n]|$)[^\n]*

Which can be used like this:

cs/file:examples/cpp/unsafe_attributes/main.rs function:main

Correct usage of unsafe

Functions marked unsafe cannot be called outside of an unsafe block. In order to avoid undefined behavior when using unsafe, callers must:

  • Ensure that the pointer being passed to C++ is a valid C++ pointer. In particular, it must not be dangling (e.g. Nonnull::dangling()).

  • Ensure that the safety conditions documented in C++ are upheld. For example, if the C++ function accepts a reference or non-null pointer, then do not pass in 0 as *const _.

Soundness

Note that many “safe” C++ functions may still trigger undefined behavior if used incorrectly. Regardless of whether a C++ function is marked as unsafe, calls into C++ will only be memory-safe if the caller verifies that all function preconditions are met.

Function Visibility: pub(crate)

By default, bindings for C++ functions and methods are generated with pub visibility in Rust.

You can restrict the visibility of a function or method to the generated crate using CRUBIT_PUB_CRATE from "support/annotations.h":

#include "support/annotations.h"

CRUBIT_PUB_CRATE void HelperFunction();

class MyClass {
 public:
  CRUBIT_PUB_CRATE void InternalMethod();
};

This is especially useful when providing handwritten Rust APIs via the srcs attribute of rust_api_from_cpp (see Best Practices). The custom Rust code in srcs can call the pub(crate) functions or methods, while keeping them hidden from downstream crates.

CRUBIT_PUB_CRATE is only valid on functions and methods; applying it to types will produce an error.

Function Attributes

Most function attributes are not supported. For example, functions marked [[noreturn]] don't get bindings. [[nodiscard]] is supported and becomes #[must_use].