Function lift_callback_opt_param 

pub unsafe fn lift_callback_opt_param<C: CallbackInterface + ?Sized>(
    ctx: *mut c_void,
    vtable: *const C::Vtable,
    name: &str,
) -> Result<Option<Arc<C>>, FfiError>

Adopt an optional callback-interface parameter (Cb?); a null vtable is None.

Errors

Returns a marshalling error for a too-small vtable.

Safety

Same contract as lift_callback_param.