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nesarkvechnep • yesterday at 10:05 AM • 2 replies • view on HN

I'm thinking of using C++ for a personal project specifically for the lambdas and RAII.

I have a case where I need to create a static templated lambda to be passed to C as a pointer. Such thing is impossible in Rust, which I considered at first.

Replies

pornel • yesterday at 12:43 PM

Yeah, Rust closures that capture data are fat pointers { fn*, data* }, so you need an awkward dance to make them thin pointers for C.

    let mut state = 1;
    let mut fat_closure = || state += 1;
    let (fnptr, userdata) = make_trampoline(&mut &mut fat_closure);

    unsafe {
        fnptr(userdata);
    }

    assert_eq!(state, 2);

    use std::ffi::c_void;
    fn make_trampoline<C: FnMut()>(closure: &mut &mut C) -> (unsafe fn(*mut c_void), *mut c_void) {
        let fnptr = |userdata: *mut c_void| {
            let closure: *mut &mut C = userdata.cast();
            (unsafe { &mut *closure })()
        };
        (fnptr, closure as *mut _ as *mut c_void)
    }
It requires a userdata arg for the C function, since there's no allocation or executable-stack magic to give a unique function pointer to each data instance. OTOH it's zero-cost. The generic make_trampoline inlines code of the closure, so there's no extra indirection.
➕ show 4 replies
queuebert • yesterday at 4:31 PM

In Rust, could you instead use a templated struct wrapping a function pointer along with #[repr(C)]?