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<ul class="chapter"><li class="affix"><a href="README.html">Introduction</a></li><li><a href="getting-started.html"><strong>1.</strong> Getting Started</a></li><li><a href="guessing-game.html"><strong>2.</strong> Tutorial: Guessing Game</a></li><li><a href="syntax-and-semantics.html"><strong>3.</strong> Syntax and Semantics</a></li><li><ul class="section"><li><a href="variable-bindings.html"><strong>3.1.</strong> Variable Bindings</a></li><li><a href="functions.html"><strong>3.2.</strong> Functions</a></li><li><a href="primitive-types.html"><strong>3.3.</strong> Primitive Types</a></li><li><a href="comments.html"><strong>3.4.</strong> Comments</a></li><li><a href="if.html"><strong>3.5.</strong> if</a></li><li><a href="loops.html"><strong>3.6.</strong> Loops</a></li><li><a href="vectors.html"><strong>3.7.</strong> Vectors</a></li><li><a href="ownership.html"><strong>3.8.</strong> Ownership</a></li><li><a href="references-and-borrowing.html"><strong>3.9.</strong> References and Borrowing</a></li><li><a href="lifetimes.html"><strong>3.10.</strong> Lifetimes</a></li><li><a href="mutability.html"><strong>3.11.</strong> Mutability</a></li><li><a href="structs.html"><strong>3.12.</strong> Structs</a></li><li><a href="enums.html" class="active"><strong>3.13.</strong> Enums</a></li><li><a href="match.html"><strong>3.14.</strong> Match</a></li><li><a href="patterns.html"><strong>3.15.</strong> Patterns</a></li><li><a href="method-syntax.html"><strong>3.16.</strong> Method Syntax</a></li><li><a href="strings.html"><strong>3.17.</strong> Strings</a></li><li><a href="generics.html"><strong>3.18.</strong> Generics</a></li><li><a href="traits.html"><strong>3.19.</strong> Traits</a></li><li><a href="drop.html"><strong>3.20.</strong> Drop</a></li><li><a href="if-let.html"><strong>3.21.</strong> if let</a></li><li><a href="trait-objects.html"><strong>3.22.</strong> Trait Objects</a></li><li><a href="closures.html"><strong>3.23.</strong> Closures</a></li><li><a href="ufcs.html"><strong>3.24.</strong> Universal Function Call Syntax</a></li><li><a href="crates-and-modules.html"><strong>3.25.</strong> Crates and Modules</a></li><li><a href="const-and-static.html"><strong>3.26.</strong> <code>const</code> and <code>static</code></a></li><li><a href="attributes.html"><strong>3.27.</strong> Attributes</a></li><li><a href="type-aliases.html"><strong>3.28.</strong> <code>type</code> aliases</a></li><li><a href="casting-between-types.html"><strong>3.29.</strong> Casting between types</a></li><li><a href="associated-types.html"><strong>3.30.</strong> Associated Types</a></li><li><a href="unsized-types.html"><strong>3.31.</strong> Unsized Types</a></li><li><a href="operators-and-overloading.html"><strong>3.32.</strong> Operators and Overloading</a></li><li><a href="deref-coercions.html"><strong>3.33.</strong> Deref coercions</a></li><li><a href="macros.html"><strong>3.34.</strong> Macros</a></li><li><a href="raw-pointers.html"><strong>3.35.</strong> Raw Pointers</a></li><li><a href="unsafe.html"><strong>3.36.</strong> <code>unsafe</code></a></li></ul></li><li><a href="effective-rust.html"><strong>4.</strong> Effective Rust</a></li><li><ul class="section"><li><a href="the-stack-and-the-heap.html"><strong>4.1.</strong> The Stack and the Heap</a></li><li><a href="testing.html"><strong>4.2.</strong> Testing</a></li><li><a href="conditional-compilation.html"><strong>4.3.</strong> Conditional Compilation</a></li><li><a href="documentation.html"><strong>4.4.</strong> Documentation</a></li><li><a href="iterators.html"><strong>4.5.</strong> Iterators</a></li><li><a href="concurrency.html"><strong>4.6.</strong> Concurrency</a></li><li><a href="error-handling.html"><strong>4.7.</strong> Error Handling</a></li><li><a href="choosing-your-guarantees.html"><strong>4.8.</strong> Choosing your Guarantees</a></li><li><a href="ffi.html"><strong>4.9.</strong> FFI</a></li><li><a href="borrow-and-asref.html"><strong>4.10.</strong> Borrow and AsRef</a></li><li><a href="release-channels.html"><strong>4.11.</strong> Release Channels</a></li><li><a href="using-rust-without-the-standard-library.html"><strong>4.12.</strong> Using Rust without the standard library</a></li><li><a href="procedural-macros.html"><strong>4.13.</strong> Procedural Macros (and custom derive)</a></li></ul></li><li><a href="glossary.html"><strong>5.</strong> Glossary</a></li><li><a href="syntax-index.html"><strong>6.</strong> Syntax Index</a></li><li><a href="bibliography.html"><strong>7.</strong> Bibliography</a></li></ul>
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<h1 class="menu-title">The Rust Programming Language</h1>
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<a class="header" href="enums.html#enums" id="enums"><h1>Enums</h1></a>
<p>An <code>enum</code> in Rust is a type that represents data that is one of
several possible variants. Each variant in the <code>enum</code> can optionally
have data associated with it:</p>
<pre><pre class="playpen"><code class="language-rust"># #![allow(unused_variables)]
#fn main() {
enum Message {
Quit,
ChangeColor(i32, i32, i32),
Move { x: i32, y: i32 },
Write(String),
}
#}</code></pre></pre>
<p>The syntax for defining variants resembles the syntaxes used to define structs:
you can have variants with no data (like unit-like structs), variants with named
data, and variants with unnamed data (like tuple structs). Unlike
separate struct definitions, however, an <code>enum</code> is a single type. A
value of the enum can match any of the variants. For this reason, an
enum is sometimes called a ‘sum type’: the set of possible values of the
enum is the sum of the sets of possible values for each variant.</p>
<p>We use the <code>::</code> syntax to use the name of each variant: they’re scoped by the name
of the <code>enum</code> itself. This allows both of these to work:</p>
<pre><pre class="playpen"><code class="language-rust"># #![allow(unused_variables)]
#fn main() {
# enum Message {
# Move { x: i32, y: i32 },
# }
let x: Message = Message::Move { x: 3, y: 4 };
enum BoardGameTurn {
Move { squares: i32 },
Pass,
}
let y: BoardGameTurn = BoardGameTurn::Move { squares: 1 };
#}</code></pre></pre>
<p>Both variants are named <code>Move</code>, but since they’re scoped to the name of
the enum, they can both be used without conflict.</p>
<p>A value of an <code>enum</code> type contains information about which variant it is,
in addition to any data associated with that variant. This is sometimes
referred to as a ‘tagged union’, since the data includes a ‘tag’
indicating what type it is. The compiler uses this information to
enforce that you’re accessing the data in the enum safely. For instance,
you can’t simply try to destructure a value as if it were one of the
possible variants:</p>
<pre><code class="language-rust ignore">fn process_color_change(msg: Message) {
let Message::ChangeColor(r, g, b) = msg; // This causes a compile-time error.
}
</code></pre>
<p>Not supporting these operations may seem rather limiting, but it’s a limitation
which we can overcome. There are two ways: by implementing equality ourselves,
or by pattern matching variants with <a href="match.html"><code>match</code></a> expressions, which you’ll
learn in the next section. We don’t know enough about Rust to implement
equality yet, but we’ll find out in the <a href="traits.html"><code>traits</code></a> section.</p>
<a class="header" href="enums.html#constructors-as-functions" id="constructors-as-functions"><h1>Constructors as functions</h1></a>
<p>An <code>enum</code> constructor can also be used like a function. For example:</p>
<pre><pre class="playpen"><code class="language-rust"># #![allow(unused_variables)]
#fn main() {
# enum Message {
# Write(String),
# }
let m = Message::Write("Hello, world".to_string());
#}</code></pre></pre>
<p>is the same as</p>
<pre><pre class="playpen"><code class="language-rust"># #![allow(unused_variables)]
#fn main() {
# enum Message {
# Write(String),
# }
fn foo(x: String) -> Message {
Message::Write(x)
}
let x = foo("Hello, world".to_string());
#}</code></pre></pre>
<p>This is not immediately useful to us, but when we get to
<a href="closures.html"><code>closures</code></a>, we’ll talk about passing functions as arguments to
other functions. For example, with <a href="iterators.html"><code>iterators</code></a>, we can do this
to convert a vector of <code>String</code>s into a vector of <code>Message::Write</code>s:</p>
<pre><pre class="playpen"><code class="language-rust"># #![allow(unused_variables)]
#fn main() {
# enum Message {
# Write(String),
# }
let v = vec!["Hello".to_string(), "World".to_string()];
let v1: Vec<Message> = v.into_iter().map(Message::Write).collect();
#}</code></pre></pre>
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