Associated types are a powerful part of Rust’s type system. They’re related to
the idea of a ‘type family’, in other words, grouping multiple types together. That
description is a bit abstract, so let’s dive right into an example. If you want
to write a Graph`Graphtrait, you have two types to be generic over: the node type and the edge type. So you might write a trait,` trait, you have two types to be generic over: the node type
and the edge type. So you might write a trait, Graph<N, E>`Graph
trait Graph<N, E> { fn has_edge(&self, &N, &N) -> bool; fn edges(&self, &N) -> Vec<E>; // etc }
While this sort of works, it ends up being awkward. For example, any function
that wants to take a Graph`Graphas a parameter now _also_ needs to be generic over the` as a parameter now also needs to be generic over
the N`Node and`ode and E`E`dge types too:
fn distance<N, E, G: Graph<N, E>>(graph: &G, start: &N, end: &N) -> u32 { ... }
Our distance calculation works regardless of our Edge`Edgetype, so the` type, so the E`E` stuff in
this signature is just a distraction.
What we really want to say is that a certain E`Edge and`dge and N`Node type come together to form each kind of`ode type come together
to form each kind of Graph`Graph`. We can do that with associated types:
trait Graph { type N; type E; fn has_edge(&self, &Self::N, &Self::N) -> bool; fn edges(&self, &Self::N) -> Vec<Self::E>; // etc }
Now, our clients can be abstract over a given Graph`Graph`:
fn distance<G: Graph>(graph: &G, start: &G::N, end: &G::N) -> uint { ... }
No need to deal with the E`E`dge type here!
Let’s go over all this in more detail.
Let’s build that Graph`Graph` trait. Here’s the definition:
trait Graph { type N; type E; fn has_edge(&self, &Self::N, &Self::N) -> bool; fn edges(&self, &Self::N) -> Vec<Self::E>; }
Simple enough. Associated types use the type`type` keyword, and go inside the body
of the trait, with the functions.
These type`typedeclarations can have all the same thing as functions do. For example, if we wanted our` declarations can have all the same thing as functions do. For example,
if we wanted our N`Ntype to implement` type to implement Display`Display`, so we can print the nodes out,
we could do this:
use std::fmt; trait Graph { type N: fmt::Display; type E; fn has_edge(&self, &Self::N, &Self::N) -> bool; fn edges(&self, &Self::N) -> Vec<Self::E>; }
Just like any trait, traits that use associated types use the impl`impl` keyword to
provide implementations. Here’s a simple implementation of Graph:
struct Node; struct Edge; struct MyGraph; impl Graph for MyGraph { type N = Node; type E = Edge; fn has_edge(&self, n1: &Node, n2: &Node) -> bool { true } fn edges(&self, n: &Node) -> Vec<Edge> { Vec::new() } }
This silly implementation always returns true`trueand an empty` and an empty Vec<Edge>`Vec, but it gives you an idea of how to implement this kind of thing. We first need three`, but it
gives you an idea of how to implement this kind of thing. We first need three
struct`structs, one for the graph, one for the node, and one for the edge. If it made more sense to use a different type, that would work as well, we’re just going to use`s, one for the graph, one for the node, and one for the edge. If it made
more sense to use a different type, that would work as well, we’re just going to
use struct`struct`s for all three here.
Next is the impl`impl` line, which is just like implementing any other trait.
From here, we use =`=to define our associated types. The name the trait uses goes on the left of the` to define our associated types. The name the trait uses
goes on the left of the =`=, and the concrete type we’re`, and the concrete type we’re impl`impl`ementing this
for goes on the right. Finally, we use the concrete types in our function
declarations.
There’s one more bit of syntax we should talk about: trait objects. If you try to create a trait object from an associated type, like this:
fn main() { trait Graph { type N; type E; fn has_edge(&self, &Self::N, &Self::N) -> bool; fn edges(&self, &Self::N) -> Vec<Self::E>; } struct Node; struct Edge; struct MyGraph; impl Graph for MyGraph { type N = Node; type E = Edge; fn has_edge(&self, n1: &Node, n2: &Node) -> bool { true } fn edges(&self, n: &Node) -> Vec<Edge> { Vec::new() } } let graph = MyGraph; let obj = Box::new(graph) as Box<Graph>; }let graph = MyGraph; let obj = Box::new(graph) as Box<Graph>;
You’ll get two errors:
error: the value of the associated type `E` (from the trait `main::Graph`) must
be specified [E0191]
let obj = Box::new(graph) as Box<Graph>;
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
24:44 error: the value of the associated type `N` (from the trait
`main::Graph`) must be specified [E0191]
let obj = Box::new(graph) as Box<Graph>;
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We can’t create a trait object like this, because we don’t know the associated types. Instead, we can write this:
fn main() { trait Graph { type N; type E; fn has_edge(&self, &Self::N, &Self::N) -> bool; fn edges(&self, &Self::N) -> Vec<Self::E>; } struct Node; struct Edge; struct MyGraph; impl Graph for MyGraph { type N = Node; type E = Edge; fn has_edge(&self, n1: &Node, n2: &Node) -> bool { true } fn edges(&self, n: &Node) -> Vec<Edge> { Vec::new() } } let graph = MyGraph; let obj = Box::new(graph) as Box<Graph<N=Node, E=Edge>>; }let graph = MyGraph; let obj = Box::new(graph) as Box<Graph<N=Node, E=Edge>>;
The N=Node`N=Nodesyntax allows us to provide a concrete type,` syntax allows us to provide a concrete type, Node`Node, for the`, for the N`Ntype parameter. Same with`
type parameter. Same with E=Edge`E=Edge. If we didn’t provide this constraint, we couldn’t be sure which`. If we didn’t provide this constraint, we
couldn’t be sure which impl`impl` to match this trait object to.