dtolnay/cxx

Support passing function pointers across the FFI

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#52 opened on Mar 2, 2020

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help wanted

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Description

i.e. function parameters with the type fn(...) -> ....

#[cxx::bridge]
mod ffi {
    extern "Rust" {
        fn f(arg: u8, callback: fn(u8));
    }
}

fn f(arg: u8, callback: fn(u8)) {
    callback(256 - arg);
}

Contributor guide

Research direction
Design a way to represent function pointer types in the cxx code generation. Look at how cxx handles other types like strings and references. You'll need to modify the Rust side to accept function pointer syntax in the bridge macro, generate appropriate C++ function pointer type definitions, and implement the FFI marshalling logic. Study existing cxx source code in the 'syntax' and 'gen' modules, and test with simple examples.
Tech stack
rust
Domain
api
Issue type
Feature
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
Half day
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
RustFFI concepts
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
40