Poking a Rust vtable from C++

doathing.rs

#![feature(box_syntax)]

pub struct MyThing {
    stuff: Vec<i32>,
}

pub trait DoAThing {
    fn add(&mut self, item: i32);
    fn remove(&mut self) -> i32;
}

impl DoAThing for MyThing {
    fn add(&mut self, item: i32) {
        self.stuff.push(item);
    }

    fn remove(&mut self) -> i32 {
        match self.stuff.pop() {
            Some(value) => value,
            None => 0,
        }
    }
}

#[no_mangle]
pub extern "C" fn new_thing() -> Box<DoAThing> {
    box MyThing { stuff: vec![] }
}

#[no_mangle]
pub extern "C" fn free_thing(_: Box<DoAThing>) {

}

main.cpp

#include <Windows.h>
#include <stdint.h>
#include <assert.h>

struct DoAThingVtable
{
    void(*destructor)(void *self);
    size_t size;
    size_t alignment;

    void(*add)(void *self, int32_t item);
    int32_t(*remove)(void *self);
};

struct BoxDoAThing
{
    void *data;
    DoAThingVtable *vtable;
};

int main(void)
{
    auto lib = LoadLibrary(TEXT("doathing.dll"));
    auto new_thing = (BoxDoAThing(*)()) GetProcAddress(lib, "new_thing");
    auto free_thing = (void(*)(BoxDoAThing)) GetProcAddress(lib, "free_thing");

    auto thing = new_thing();

    thing.vtable->add(thing.data, 5);
    thing.vtable->add(thing.data, 6);
    assert(thing.vtable->remove(thing.data) == 6);
    assert(thing.vtable->remove(thing.data) == 5);
    assert(thing.vtable->remove(thing.data) == 0);

    free_thing(thing);
}