set a pointer to a Rust heap-allocated value on your Ruby object and get it back out again
Clarify some terms
Use rustfmt to clean up example code


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ruby_wrap_data is a module that provides a fairly low-level means of doing what Ruby's Data_Wrap_Struct macro does. That is to say, you can store a pointer to a Rust Box<T> inside a Ruby object and get it back out again.

Any heap-allocated struct, enum, or whatever should work.


extern crate ruby_sys;
extern crate ruby_wrap_data;

use ruby_sys::{class::{rb_class_new_instance, rb_define_class}, rb_cObject, types::Value,
               value::RubySpecialConsts::Nil, vm::ruby_init};

use std::ffi::CString;
use std::mem;

const RB_NIL: Value = Value {
    value: Nil as usize,

struct MyValue {
    pub val: u16,

fn alloc(klass: Value) -> Value {
    // put your value in a Box
    let data = Box::new(MyValue { val: 1 });
    // call `wrap()`, passing your class and value
    ruby_wrap_data::wrap(klass, Some(data))

fn main() {
    // you may need to start the ruby vm
    unsafe { ruby_init() };

    // create a ruby class and attach your alloc function
    let name = CString::new("Thing").unwrap().into_raw();
    let klass = unsafe { rb_define_class(name, rb_cObject) };
    ruby_wrap_data::define_alloc_func(klass, alloc);

    // create a new instance of the class
    let thing = unsafe { rb_class_new_instance(0, &RB_NIL, klass) };

    // get your value from the ruby object
    // note: once remove() is called, the pointer in your ruby object is NULL
    let data: Option<Box<MyValue>> = ruby_wrap_data::remove(thing);

    // if you try to remove it again, you get None
    let data: Option<Box<MyValue>> = ruby_wrap_data::remove(thing);

    // set a new value on the object
    let new_data = Box::new(MyValue { val: 2 });
    ruby_wrap_data::set(thing, new_data);


Assuming you're using rbenv (if not, sorry, you're on your own):

CONFIGURE_OPTS=--enable-shared rbenv install
RUBY=$(rbenv which ruby) cargo test

You may need to help Rust find the libruby.so file, like this:

export LD_LIBRARY_PATH=$HOME/.rbenv/versions/2.5.1/lib
RUBY=$(rbenv which ruby) cargo test

License: MIT