Here is an offending example (Playground):
// Some traits
trait Behaviour {
type Sub: SubBehaviour;
}
trait SubBehaviour {}
// Some implementations of these traits
struct A;
impl Behaviour for A {
type Sub = B;
}
struct B;
impl SubBehaviour for B {}
// Struct that holds a collection of these traits.
struct Example<'a> {
behaviours: Vec<&'a dyn Behaviour>,
}
impl<'a> Example<'a> {
fn add_behaviour<T: Behaviour>(&mut self, b: &'a T) {
self.behaviours.push(b);
}
}
fn main() {
let b = A;
let mut e = Example {
behaviours: Vec::new(),
};
e.add_behaviour(&b);
}
I get:
error[E0191]: the value of the associated type `Sub` (from trait `Behaviour`) must be specified
--> src/main.rs:17:29
|
3 | type Sub: SubBehaviour;
| ----------------------- `Sub` defined here
...
17 | behaviours: Vec<&'a dyn Behaviour>,
| ^^^^^^^^^ help: specify the associated type: `Behaviour<Sub = Type>`
Why must this type must be specified, particularly in this case where we are only storing a reference to the object? How can I get this code to work?
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