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<ol class="chapter"><li class="affix"><a href="README.html">Introduction</a></li><li><a href="getting-started.html"><strong aria-hidden="true">1.</strong> Getting Started</a></li><li><a href="guessing-game.html"><strong aria-hidden="true">2.</strong> Tutorial: Guessing Game</a></li><li><a href="syntax-and-semantics.html"><strong aria-hidden="true">3.</strong> Syntax and Semantics</a></li><li><ol class="section"><li><a href="variable-bindings.html"><strong aria-hidden="true">3.1.</strong> Variable Bindings</a></li><li><a href="functions.html"><strong aria-hidden="true">3.2.</strong> Functions</a></li><li><a href="primitive-types.html"><strong aria-hidden="true">3.3.</strong> Primitive Types</a></li><li><a href="comments.html"><strong aria-hidden="true">3.4.</strong> Comments</a></li><li><a href="if.html"><strong aria-hidden="true">3.5.</strong> if</a></li><li><a href="loops.html"><strong aria-hidden="true">3.6.</strong> Loops</a></li><li><a href="vectors.html"><strong aria-hidden="true">3.7.</strong> Vectors</a></li><li><a href="ownership.html"><strong aria-hidden="true">3.8.</strong> Ownership</a></li><li><a href="references-and-borrowing.html"><strong aria-hidden="true">3.9.</strong> References and Borrowing</a></li><li><a href="lifetimes.html"><strong aria-hidden="true">3.10.</strong> Lifetimes</a></li><li><a href="mutability.html"><strong aria-hidden="true">3.11.</strong> Mutability</a></li><li><a href="structs.html"><strong aria-hidden="true">3.12.</strong> Structs</a></li><li><a href="enums.html"><strong aria-hidden="true">3.13.</strong> Enums</a></li><li><a href="match.html"><strong aria-hidden="true">3.14.</strong> Match</a></li><li><a href="patterns.html"><strong aria-hidden="true">3.15.</strong> Patterns</a></li><li><a href="method-syntax.html"><strong aria-hidden="true">3.16.</strong> Method Syntax</a></li><li><a href="strings.html"><strong aria-hidden="true">3.17.</strong> Strings</a></li><li><a href="generics.html"><strong aria-hidden="true">3.18.</strong> Generics</a></li><li><a href="traits.html"><strong aria-hidden="true">3.19.</strong> Traits</a></li><li><a href="drop.html"><strong aria-hidden="true">3.20.</strong> Drop</a></li><li><a href="if-let.html"><strong aria-hidden="true">3.21.</strong> if let</a></li><li><a href="trait-objects.html"><strong aria-hidden="true">3.22.</strong> Trait Objects</a></li><li><a href="closures.html"><strong aria-hidden="true">3.23.</strong> Closures</a></li><li><a href="ufcs.html"><strong aria-hidden="true">3.24.</strong> Universal Function Call Syntax</a></li><li><a href="crates-and-modules.html"><strong aria-hidden="true">3.25.</strong> Crates and Modules</a></li><li><a href="const-and-static.html"><strong aria-hidden="true">3.26.</strong> const and static</a></li><li><a href="attributes.html"><strong aria-hidden="true">3.27.</strong> Attributes</a></li><li><a href="type-aliases.html"><strong aria-hidden="true">3.28.</strong> type aliases</a></li><li><a href="casting-between-types.html"><strong aria-hidden="true">3.29.</strong> Casting between types</a></li><li><a href="associated-types.html" class="active"><strong aria-hidden="true">3.30.</strong> Associated Types</a></li><li><a href="unsized-types.html"><strong aria-hidden="true">3.31.</strong> Unsized Types</a></li><li><a href="operators-and-overloading.html"><strong aria-hidden="true">3.32.</strong> Operators and Overloading</a></li><li><a href="deref-coercions.html"><strong aria-hidden="true">3.33.</strong> Deref coercions</a></li><li><a href="macros.html"><strong aria-hidden="true">3.34.</strong> Macros</a></li><li><a href="raw-pointers.html"><strong aria-hidden="true">3.35.</strong> Raw Pointers</a></li><li><a href="unsafe.html"><strong aria-hidden="true">3.36.</strong> unsafe</a></li></ol></li><li><a href="effective-rust.html"><strong aria-hidden="true">4.</strong> Effective Rust</a></li><li><ol class="section"><li><a href="the-stack-and-the-heap.html"><strong aria-hidden="true">4.1.</strong> The Stack and the Heap</a></li><li><a href="testing.html"><strong aria-hidden="true">4.2.</strong> Testing</a></li><li><a href="conditional-compilation.html"><strong aria-hidden="true">4.3.</strong> Conditional Compilation</a></li><li><a href="documentation.html"><strong aria-hidden="true">4.4.</strong> Documentation</a></li><li><a href="iterators.html"><strong aria-hidden="true">4.5.</strong> Iterators</a></li><li><a href="concurrency.html"><strong aria-hidden="true">4.6.</strong> Concurrency</a></li><li><a href="error-handling.html"><strong aria-hidden="true">4.7.</strong> Error Handling</a></li><li><a href="choosing-your-guarantees.html"><strong aria-hidden="true">4.8.</strong> Choosing your Guarantees</a></li><li><a href="ffi.html"><strong aria-hidden="true">4.9.</strong> FFI</a></li><li><a href="borrow-and-asref.html"><strong aria-hidden="true">4.10.</strong> Borrow and AsRef</a></li><li><a href="release-channels.html"><strong aria-hidden="true">4.11.</strong> Release Channels</a></li><li><a href="using-rust-without-the-standard-library.html"><strong aria-hidden="true">4.12.</strong> Using Rust without the standard library</a></li><li><a href="procedural-macros.html"><strong aria-hidden="true">4.13.</strong> Procedural Macros (and custom derive)</a></li></ol></li><li><a href="glossary.html"><strong aria-hidden="true">5.</strong> Glossary</a></li><li><a href="syntax-index.html"><strong aria-hidden="true">6.</strong> Syntax Index</a></li><li><a href="bibliography.html"><strong aria-hidden="true">7.</strong> Bibliography</a></li></ol>
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<a class="header" href="associated-types.html#associated-types" id="associated-types"><h1>Associated Types</h1></a>
<p>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 <code>Graph</code> trait, you have two types to be generic over: the node type
and the edge type. So you might write a trait, <code>Graph<N, E></code>, that looks like
this:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
trait Graph<N, E> {
fn has_edge(&self, &N, &N) -> bool;
fn edges(&self, &N) -> Vec<E>;
// Etc.
}
#}</code></pre></pre>
<p>While this sort of works, it ends up being awkward. For example, any function
that wants to take a <code>Graph</code> as a parameter now <em>also</em> needs to be generic over
the <code>N</code>ode and <code>E</code>dge types too:</p>
<pre><code class="language-rust ignore">fn distance<N, E, G: Graph<N, E>>(graph: &G, start: &N, end: &N) -> u32 { ... }
</code></pre>
<p>Our distance calculation works regardless of our <code>Edge</code> type, so the <code>E</code> stuff in
this signature is a distraction.</p>
<p>What we really want to say is that a certain <code>E</code>dge and <code>N</code>ode type come together
to form each kind of <code>Graph</code>. We can do that with associated types:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#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>;
// Etc.
}
#}</code></pre></pre>
<p>Now, our clients can be abstract over a given <code>Graph</code>:</p>
<pre><code class="language-rust ignore">fn distance<G: Graph>(graph: &G, start: &G::N, end: &G::N) -> u32 { ... }
</code></pre>
<p>No need to deal with the <code>E</code>dge type here!</p>
<p>Let’s go over all this in more detail.</p>
<a class="header" href="associated-types.html#defining-associated-types" id="defining-associated-types"><h2>Defining associated types</h2></a>
<p>Let’s build that <code>Graph</code> trait. Here’s the definition:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#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>;
}
#}</code></pre></pre>
<p>Simple enough. Associated types use the <code>type</code> keyword, and go inside the body
of the trait, with the functions.</p>
<p>These type declarations work the same way as those for functions. For example,
if we wanted our <code>N</code> type to implement <code>Display</code>, so we can print the nodes out,
we could do this:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
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>;
}
#}</code></pre></pre>
<a class="header" href="associated-types.html#implementing-associated-types" id="implementing-associated-types"><h2>Implementing associated types</h2></a>
<p>Just like any trait, traits that use associated types use the <code>impl</code> keyword to
provide implementations. Here’s a simple implementation of Graph:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#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()
}
}
#}</code></pre></pre>
<p>This silly implementation always returns <code>true</code> and an empty <code>Vec<Edge></code>, but it
gives you an idea of how to implement this kind of thing. We first need three
<code>struct</code>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 going to
use <code>struct</code>s for all three here.</p>
<p>Next is the <code>impl</code> line, which is an implementation like any other trait.</p>
<p>From here, we use <code>=</code> to define our associated types. The name the trait uses
goes on the left of the <code>=</code>, and the concrete type we’re <code>impl</code>ementing this
for goes on the right. Finally, we use the concrete types in our function
declarations.</p>
<a class="header" href="associated-types.html#trait-objects-with-associated-types" id="trait-objects-with-associated-types"><h2>Trait objects with associated types</h2></a>
<p>There’s one more bit of syntax we should talk about: trait objects. If you
try to create a trait object from a trait with an associated type, like this:</p>
<pre><code class="language-rust ignore"># 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>;
</code></pre>
<p>You’ll get two errors:</p>
<pre><code class="language-text">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>;
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
</code></pre>
<p>We can’t create a trait object like this, because we don’t know the associated
types. Instead, we can write this:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#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>>;
#}</code></pre></pre>
<p>The <code>N=Node</code> syntax allows us to provide a concrete type, <code>Node</code>, for the <code>N</code>
type parameter. Same with <code>E=Edge</code>. If we didn’t provide this constraint, we
couldn’t be sure which <code>impl</code> to match this trait object to.</p>
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