I am having trouble figuring out what lifetime parameter will work for this, so my current workarounds include transmutes or raw pointers. I have a structure holding a function pointer with a generic as a parameter:
struct CB<Data> {
cb: fn(Data) -> usize
}
I would like to store an instance of that, parameterized by some type containing a reference, in some other structure that implements a trait with one method, and use that trait method to call the function pointer in CB.
struct Holder<'a> {
c: CB<Option<&'a usize>>
}
trait Exec {
fn exec(&self, v: &usize) -> usize;
}
impl<'a> Holder<'a> {
fn exec_aux(&self, v: &'a usize) -> usize {
(self.c.cb)(Some(v))
}
}
impl<'a> Exec for Holder<'a> {
fn exec(&self, v: &usize) -> usize
{
self.exec_aux(v)
}
}
This gives me a lifetime error for the 'Exec' impl of Holder:
error[E0495]: cannot infer an appropriate lifetime for lifetime parameter `'a` due to conflicting requirements
Simply calling exec_aux
works fine as long as I don't define that Exec
impl:
fn main() {
let h = Holder { c: CB{cb:cbf}};
let v = 12;
println!("{}", h.exec_aux(&v));
}
Also, making CB not generic also makes this work:
struct CB {
cb: fn(Option<&usize>) -> usize
}
The parameter in my actual code is not a usize
but something big that I would rather not copy.