dtolnay/cxx

Box clone() member function to call Clone on opaque Rust types

Open

#105 opened on Apr 11, 2020

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 (3 comments) (0 reactions) (0 assignees)Rust (4,472 stars) (253 forks)batch import
help wanted

Description

We should provide a way to constrain that some opaque Rust type implements Clone and then expose the Clone impl to C++ via a clone() member function on rust::Box.

Something like:

mod ffi {
    extern "Rust" {
        type ClonableR: Clone;
    }

    extern "C++" {
        fn f(b: Box<ClonableR>); // C++ will be able to clone b
    }
}

  template <typename T>
  class Box {
+   Box clone() const noexcept;
  };

Contributor guide

Tech stack
rustcpp
Domain
backendapi
Issue type
feature
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
4
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
3-5 days
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
RustC++cxx librarytrait system
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
30
Research direction
Investigate how cxx currently handles opaque Rust types and the Box class template. The goal is to add a clone() member function to Box that calls Clone on the underlying Rust type. Examine existing Box member functions (e.g., from Rust, into raw) to follow the pattern. The implementation will require modifying the Rust side to generate C++ bindings that expose Clone, and updating the C++ header for Box. Check issue comments for any prior discussion; there are 3 comments that may contain additional constraints. No linked PRs or assignees, so the issue is available for contribution.