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<ol class="chapter"><li class="affix"><a href="introduction.html">Introduction</a></li><li><a href="notation.html"><strong aria-hidden="true">1.</strong> Notation</a></li><li><a href="lexical-structure.html"><strong aria-hidden="true">2.</strong> Lexical structure</a></li><li><ol class="section"><li><a href="input-format.html"><strong aria-hidden="true">2.1.</strong> Input format</a></li><li><a href="keywords.html"><strong aria-hidden="true">2.2.</strong> Keywords</a></li><li><a href="identifiers.html"><strong aria-hidden="true">2.3.</strong> Identifiers</a></li><li><a href="comments.html"><strong aria-hidden="true">2.4.</strong> Comments</a></li><li><a href="whitespace.html"><strong aria-hidden="true">2.5.</strong> Whitespace</a></li><li><a href="tokens.html"><strong aria-hidden="true">2.6.</strong> Tokens</a></li><li><a href="paths.html"><strong aria-hidden="true">2.7.</strong> Paths</a></li></ol></li><li><a href="macros.html"><strong aria-hidden="true">3.</strong> Macros</a></li><li><ol class="section"><li><a href="macros-by-example.html"><strong aria-hidden="true">3.1.</strong> Macros By Example</a></li><li><a href="procedural-macros.html"><strong aria-hidden="true">3.2.</strong> Procedural Macros</a></li></ol></li><li><a href="crates-and-source-files.html"><strong aria-hidden="true">4.</strong> Crates and source files</a></li><li><a href="items-and-attributes.html"><strong aria-hidden="true">5.</strong> Items and attributes</a></li><li><ol class="section"><li><a href="items.html"><strong aria-hidden="true">5.1.</strong> Items</a></li><li><ol class="section"><li><a href="items/modules.html"><strong aria-hidden="true">5.1.1.</strong> Modules</a></li><li><a href="items/extern-crates.html"><strong aria-hidden="true">5.1.2.</strong> Extern crates</a></li><li><a href="items/use-declarations.html"><strong aria-hidden="true">5.1.3.</strong> Use declarations</a></li><li><a href="items/functions.html"><strong aria-hidden="true">5.1.4.</strong> Functions</a></li><li><a href="items/type-aliases.html"><strong aria-hidden="true">5.1.5.</strong> Type aliases</a></li><li><a href="items/structs.html"><strong aria-hidden="true">5.1.6.</strong> Structs</a></li><li><a href="items/enumerations.html" class="active"><strong aria-hidden="true">5.1.7.</strong> Enumerations</a></li><li><a href="items/unions.html"><strong aria-hidden="true">5.1.8.</strong> Unions</a></li><li><a href="items/constant-items.html"><strong aria-hidden="true">5.1.9.</strong> Constant items</a></li><li><a href="items/static-items.html"><strong aria-hidden="true">5.1.10.</strong> Static items</a></li><li><a href="items/traits.html"><strong aria-hidden="true">5.1.11.</strong> Traits</a></li><li><a href="items/implementations.html"><strong aria-hidden="true">5.1.12.</strong> Implementations</a></li><li><a href="items/external-blocks.html"><strong aria-hidden="true">5.1.13.</strong> External blocks</a></li></ol></li><li><a href="items/generics.html"><strong aria-hidden="true">5.2.</strong> Type and lifetime parameters</a></li><li><a href="items/associated-items.html"><strong aria-hidden="true">5.3.</strong> Associated Items</a></li><li><a href="visibility-and-privacy.html"><strong aria-hidden="true">5.4.</strong> Visibility and Privacy</a></li><li><a href="attributes.html"><strong aria-hidden="true">5.5.</strong> Attributes</a></li></ol></li><li><a href="statements-and-expressions.html"><strong aria-hidden="true">6.</strong> Statements and expressions</a></li><li><ol class="section"><li><a href="statements.html"><strong aria-hidden="true">6.1.</strong> Statements</a></li><li><a href="expressions.html"><strong aria-hidden="true">6.2.</strong> Expressions</a></li><li><ol class="section"><li><a href="expressions/literal-expr.html"><strong aria-hidden="true">6.2.1.</strong> Literal expressions</a></li><li><a href="expressions/path-expr.html"><strong aria-hidden="true">6.2.2.</strong> Path expressions</a></li><li><a href="expressions/block-expr.html"><strong aria-hidden="true">6.2.3.</strong> Block expressions</a></li><li><a href="expressions/operator-expr.html"><strong aria-hidden="true">6.2.4.</strong> Operator expressions</a></li><li><a href="expressions/grouped-expr.html"><strong aria-hidden="true">6.2.5.</strong> Grouped expressions</a></li><li><a href="expressions/array-expr.html"><strong aria-hidden="true">6.2.6.</strong> Array and index expressions</a></li><li><a href="expressions/tuple-expr.html"><strong aria-hidden="true">6.2.7.</strong> Tuple and index expressions</a></li><li><a href="expressions/struct-expr.html"><strong aria-hidden="true">6.2.8.</strong> Struct expressions</a></li><li><a href="expressions/enum-variant-expr.html"><strong aria-hidden="true">6.2.9.</strong> Enum variant expressions</a></li><li><a href="expressions/call-expr.html"><strong aria-hidden="true">6.2.10.</strong> Call expressions</a></li><li><a href="expressions/method-call-expr.html"><strong aria-hidden="true">6.2.11.</strong> Method call expressions</a></li><li><a href="expressions/field-expr.html"><strong aria-hidden="true">6.2.12.</strong> Field access expressions</a></li><li><a href="expressions/closure-expr.html"><strong aria-hidden="true">6.2.13.</strong> Closure expressions</a></li><li><a href="expressions/loop-expr.html"><strong aria-hidden="true">6.2.14.</strong> Loop expressions</a></li><li><a href="expressions/range-expr.html"><strong aria-hidden="true">6.2.15.</strong> Range expressions</a></li><li><a href="expressions/if-expr.html"><strong aria-hidden="true">6.2.16.</strong> If and if let expressions</a></li><li><a href="expressions/match-expr.html"><strong aria-hidden="true">6.2.17.</strong> Match expressions</a></li><li><a href="expressions/return-expr.html"><strong aria-hidden="true">6.2.18.</strong> Return expressions</a></li></ol></li></ol></li><li><a href="type-system.html"><strong aria-hidden="true">7.</strong> Type system</a></li><li><ol class="section"><li><a href="types.html"><strong aria-hidden="true">7.1.</strong> Types</a></li><li><a href="dynamically-sized-types.html"><strong aria-hidden="true">7.2.</strong> Dynamically Sized Types</a></li><li><a href="type-layout.html"><strong aria-hidden="true">7.3.</strong> Type layout</a></li><li><a href="interior-mutability.html"><strong aria-hidden="true">7.4.</strong> Interior mutability</a></li><li><a href="subtyping.html"><strong aria-hidden="true">7.5.</strong> Subtyping and Variance</a></li><li><a href="trait-bounds.html"><strong aria-hidden="true">7.6.</strong> Trait and lifetime bounds</a></li><li><a href="type-coercions.html"><strong aria-hidden="true">7.7.</strong> Type coercions</a></li><li><a href="destructors.html"><strong aria-hidden="true">7.8.</strong> Destructors</a></li><li><a href="lifetime-elision.html"><strong aria-hidden="true">7.9.</strong> Lifetime elision</a></li></ol></li><li><a href="special-types-and-traits.html"><strong aria-hidden="true">8.</strong> Special types and traits</a></li><li><a href="memory-model.html"><strong aria-hidden="true">9.</strong> Memory model</a></li><li><ol class="section"><li><a href="memory-allocation-and-lifetime.html"><strong aria-hidden="true">9.1.</strong> Memory allocation and lifetime</a></li><li><a href="memory-ownership.html"><strong aria-hidden="true">9.2.</strong> Memory ownership</a></li><li><a href="variables.html"><strong aria-hidden="true">9.3.</strong> Variables</a></li></ol></li><li><a href="linkage.html"><strong aria-hidden="true">10.</strong> Linkage</a></li><li><a href="unsafety.html"><strong aria-hidden="true">11.</strong> Unsafety</a></li><li><ol class="section"><li><a href="unsafe-functions.html"><strong aria-hidden="true">11.1.</strong> Unsafe functions</a></li><li><a href="unsafe-blocks.html"><strong aria-hidden="true">11.2.</strong> Unsafe blocks</a></li><li><a href="behavior-considered-undefined.html"><strong aria-hidden="true">11.3.</strong> Behavior considered undefined</a></li><li><a href="behavior-not-considered-unsafe.html"><strong aria-hidden="true">11.4.</strong> Behavior not considered unsafe</a></li></ol></li><li><a href="influences.html">Appendix: Influences</a></li><li class="affix"><a href="undocumented.html">Appendix: As-yet-undocumented Features</a></li><li class="affix"><a href="glossary.html">Appendix: Glossary</a></li></ol>
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<a class="header" href="items/enumerations.html#enumerations" id="enumerations"><h1>Enumerations</h1></a>
<blockquote>
<p><strong><sup>Syntax</sup></strong><br />
<em>Enumeration</em> :<br />
<code>enum</code>
<a href="identifiers.html">IDENTIFIER</a>
<a href="items/generics.html"><em>Generics</em></a><sup>?</sup>
<a href="items/generics.html#where-clauses"><em>WhereClause</em></a><sup>?</sup>
<code>{</code> <em>EnumItems</em><sup>?</sup> <code>}</code></p>
<p><em>EnumItems</em> :<br />
<em>EnumItem</em> ( <code>,</code> <em>EnumItem</em> )<sup>*</sup> <code>,</code><sup>?</sup></p>
<p><em>EnumItem</em> :<br />
<em>OuterAttribute</em><sup>*</sup><br />
<a href="identifiers.html">IDENTIFIER</a> ( <em>EnumItemTuple</em> | <em>EnumItemStruct</em>
| <em>EnumItemDiscriminant</em> )<sup>?</sup></p>
<p><em>EnumItemTuple</em> :<br />
<code>(</code> <a href="items/structs.html"><em>TupleFields</em></a><sup>?</sup> <code>)</code></p>
<p><em>EnumItemStruct</em> :<br />
<code>{</code> <a href="items/structs.html"><em>StructFields</em></a><sup>?</sup> <code>}</code></p>
<p><em>EnumItemDiscriminant</em> :<br />
<code>=</code> <a href="expressions.html"><em>Expression</em></a></p>
</blockquote>
<p>An <em>enumeration</em>, also referred to as <em>enum</em> is a simultaneous definition of a
nominal <a href="types.html#enumerated-types">enumerated type</a> as well as a set of <em>constructors</em>, that can be used
to create or pattern-match values of the corresponding enumerated type.</p>
<p>Enumerations are declared with the keyword <code>enum</code>.</p>
<p>An example of an <code>enum</code> item and its use:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
enum Animal {
Dog,
Cat,
}
let mut a: Animal = Animal::Dog;
a = Animal::Cat;
#}</code></pre></pre>
<p>Enum constructors can have either named or unnamed fields:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
enum Animal {
Dog(String, f64),
Cat { name: String, weight: f64 },
}
let mut a: Animal = Animal::Dog("Cocoa".to_string(), 37.2);
a = Animal::Cat { name: "Spotty".to_string(), weight: 2.7 };
#}</code></pre></pre>
<p>In this example, <code>Cat</code> is a <em>struct-like enum variant</em>, whereas <code>Dog</code> is simply
called an enum variant. Each enum instance has a <em>discriminant</em> which is an
integer associated to it that is used to determine which variant it holds. An
opaque reference to this discriminant can be obtained with the
<a href="../std/mem/fn.discriminant.html"><code>mem::discriminant</code></a> function.</p>
<a class="header" href="items/enumerations.html#custom-discriminant-values-for-field-less-enumerations" id="custom-discriminant-values-for-field-less-enumerations"><h2>Custom Discriminant Values for Field-Less Enumerations</h2></a>
<p>If there is no data attached to <em>any</em> of the variants of an enumeration,
then the discriminant can be directly chosen and accessed.</p>
<p>These enumerations can be cast to integer types with the <code>as</code> operator by a
<a href="expressions/operator-expr.html#semantics">numeric cast</a>. The enumeration can optionally specify which integer each
discriminant gets by following the variant name with <code>=</code> and then an integer
literal. If the first variant in the declaration is unspecified, then it is set
to zero. For every unspecified discriminant, it is set to one higher than the
previous variant in the declaration.</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
enum Foo {
Bar, // 0
Baz = 123, // 123
Quux, // 124
}
let baz_discriminant = Foo::Baz as u32;
assert_eq!(baz_discriminant, 123);
#}</code></pre></pre>
<p>Under the [default representation], the specified discriminant is interpreted as
an <code>isize</code> value although the compiler is allowed to use a smaller type in the
actual memory layout. The size and thus acceptable values can be changed by
using a [primitive representation] or the [<code>C</code> representation].</p>
<p>It is an error when two variants share the same discriminant.</p>
<pre><code class="language-rust ignore">enum SharedDiscriminantError {
SharedA = 1,
SharedB = 1
}
enum SharedDiscriminantError2 {
Zero, // 0
One, // 1
OneToo = 1 // 1 (collision with previous!)
}
</code></pre>
<p>It is also an error to have an unspecified discriminant where the previous
discriminant is the maximum value for the size of the discriminant.</p>
<pre><code class="language-rust ignore">#[repr(u8)]
enum OverflowingDiscriminantError {
Max = 255,
MaxPlusOne // Would be 256, but that overflows the enum.
}
#[repr(u8)]
enum OverflowingDiscriminantError2 {
MaxMinusOne = 254, // 254
Max, // 255
MaxPlusOne // Would be 256, but that overflows the enum.
}
</code></pre>
<a class="header" href="items/enumerations.html#zero-variant-enums" id="zero-variant-enums"><h2>Zero-variant Enums</h2></a>
<p>Enums with zero variants are known as <em>zero-variant enums</em>. As they have
no valid values, they cannot be instantiated.</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
enum ZeroVariants {}
#}</code></pre></pre>
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