How to call c++ methods from rust?

Question

I would like to call the c++ methods from rust. I heard I need to create vtables(VMTs), but how can I do that? How is that different from what I did?

C++:

struct numbers {
    int addnums(int a, int b) {
        return a + b;
    }
};

struct v_numbers {
    virtual int v_addnums(int a, int b) {
        return a + b;
    }
};

Rust(wrong, not linking):

extern "system" {
    fn addnums(a: i32, b: i32) -> i32;
    fn v_addnums(a: i32, b: i32) -> i32;
}

fn main() {
    unsafe {
        println!("{}", addnums(1, 2));
        println!("{}", v_addnums(1, 2));
    }
}

In Rust, what represents the `numbers` instance object and the `v_numbers` instance object? — Eljay, Oct 02 '22 at 17:43
Actually, you need the exact *opposite* of vtables. You want to remove all of the fancy name-mangling template-expansion shiny C++ magic and get down to a classic C-style function. That is, write an `extern "C"` function that delegates to your structure function. — Silvio Mayolo, Oct 02 '22 at 18:21
Can you actually modify the C++ code? If yes, then what @SilvioMayolo says. — Finomnis, Oct 02 '22 at 18:52

score 0 · Accepted Answer · answered Oct 03 '22 at 23:08

To the people who commented: The example I gave to you, was a simplified version of what I was trying to do. I was thinking this example is so simple, that people would recognize that it is a simplified version of something. I am sorry for assuming something like that. You know, not all C++ libraries have "classic C-style functions".

But on the other hand, you had no intentions to help me, right? Yes, obviously I can modify the C++ code in this example, but what would be the point in doing that? I came up with a problem, I needed an answer for that. I could not change the problem, because it would be a different problem then.

I solved it. It is working only for pure virtual method structs, but I only need those so, I'm fine with it.

C++:

struct numbers {
    virtual int addnums(int a, int b) {
        return a + b;
    }
};

extern "C" numbers* get_numbers() {
    numbers* num = new numbers();
    return num;
}

Rust:

#![allow(non_snake_case)]

extern "system" {
    fn get_numbers() -> *mut usize;
}

#[repr(C)]
pub struct numbers_vtbl {
    pub addnums: unsafe extern "system" fn(This: *mut numbers, a: i32, b: i32) -> i32,
}

#[repr(C)]
pub struct numbers {
    pub lpVtbl: *const numbers_vtbl,
}

impl numbers {
    #[inline]
    pub unsafe fn addnums(&self, a: i32, b: i32) -> i32 {
        ((*self.lpVtbl).addnums)(self as *const _ as *mut _, a, b)
    }
}

fn main() {
    unsafe {
        let a = get_numbers() as *mut numbers;
        let b = (*a).addnums(14, 53);
        println!("{}", b);
    }
}

How to call c++ methods from rust?

1 Answers1