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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"><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" class="active"><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/traits.html#traits" id="traits"><h1>Traits</h1></a>
<p>A <em>trait</em> describes an abstract interface that types can implement. This
interface consists of <a href="items/associated-items.html">associated items</a>, which come in three varieties:</p>
<ul>
<li><a href="items/associated-items.html#associated-functions-and-methods">functions</a></li>
<li><a href="items/associated-items.html#associated-types">types</a></li>
<li><a href="items/associated-items.html#associated-constants">constants</a></li>
</ul>
<p>All traits define an implicit type parameter <code>Self</code> that refers to "the type
that is implementing this interface". Traits may also contain additional type
parameters. These type parameters, including <code>Self</code>, may be constrained by
other traits and so forth <a href="items/generics.html">as usual</a>.</p>
<p>Traits are implemented for specific types through separate <a href="items/implementations.html">implementations</a>.</p>
<p>Items associated with a trait do not need to be defined in the trait, but they
may be. If the trait provides a definition, then this definition acts as a
default for any implementation which does not override it. If it does not, then
any implementation must provide a definition.</p>
<a class="header" href="items/traits.html#trait-bounds" id="trait-bounds"><h2>Trait bounds</h2></a>
<p>Generic items may use traits as <a href="trait-bounds.html">bounds</a> on their type parameters.</p>
<a class="header" href="items/traits.html#generic-traits" id="generic-traits"><h2>Generic Traits</h2></a>
<p>Type parameters can be specified for a trait to make it generic. These appear
after the trait name, using the same syntax used in <a href="items/functions.html#generic-functions">generic functions</a>.</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
trait Seq<T> {
fn len(&self) -> u32;
fn elt_at(&self, n: u32) -> T;
fn iter<F>(&self, F) where F: Fn(T);
}
#}</code></pre></pre>
<a class="header" href="items/traits.html#object-safety" id="object-safety"><h2>Object Safety</h2></a>
<p>Object safe traits can be the base trait of a <a href="types.html#trait-objects">trait object</a>. A trait is
<em>object safe</em> if it has the following qualities (defined in <a href="https://github.com/rust-lang/rfcs/blob/master/text/0255-object-safety.md">RFC 255</a>):</p>
<ul>
<li>It must not require <code>Self: Sized</code></li>
<li>All associated functions must either have a <code>where Self: Sized</code> bound, or
<ul>
<li>Not have any type parameters (although lifetime parameters are allowed),
and</li>
<li>Be a <a href="items/associated-items.html#methods">method</a> that does not use <code>Self</code> except in the type of the receiver.</li>
</ul>
</li>
<li>It must not have any associated constants.</li>
<li>All supertraits must also be object safe.</li>
</ul>
<a class="header" href="items/traits.html#supertraits" id="supertraits"><h2>Supertraits</h2></a>
<p><strong>Supertraits</strong> are traits that are required to be implemented for a type to
implement a specific trait. Furthermore, anywhere a [generic] or <a href="types.html#trait-objects">trait object</a>
is bounded by a trait, it has access to the associated items of its supertraits.</p>
<p>Supertraits are declared by trait bounds on the <code>Self</code> type of a trait and
transitively the supertraits of the traits declared in those trait bounds. It is
an error for a trait to be its own supertrait.</p>
<p>The trait with a supertrait is called a <strong>subtrait</strong> of its supertrait.</p>
<p>The following is an example of declaring <code>Shape</code> to be a supertrait of <code>Circle</code>.</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
trait Shape { fn area(&self) -> f64; }
trait Circle : Shape { fn radius(&self) -> f64; }
#}</code></pre></pre>
<p>And the following is the same example, except using <a href="items/generics.html#where-clauses">where clauses</a>.</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
trait Shape { fn area(&self) -> f64; }
trait Circle where Self: Shape { fn radius(&self) -> f64; }
#}</code></pre></pre>
<p>This next example gives <code>radius</code> a default implementation using the <code>area</code>
function from <code>Shape</code>.</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
# trait Shape { fn area(&self) -> f64; }
trait Circle where Self: Shape {
fn radius(&self) -> f64 {
// A = pi * r^2
// so algebraically,
// r = sqrt(A / pi)
(self.area() /std::f64::consts::PI).sqrt()
}
}
#}</code></pre></pre>
<p>This next example calls a supertrait method on a generic parameter.</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
# trait Shape { fn area(&self) -> f64; }
# trait Circle : Shape { fn radius(&self) -> f64; }
fn print_area_and_radius<C: Circle>(c: C) {
// Here we call the area method from the supertrait `Shape` of `Circle`.
println!("Area: {}", c.area());
println!("Radius: {}", c.radius());
}
#}</code></pre></pre>
<p>Similarly, here is an example of calling supertrait methods on trait objects.</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
# trait Shape { fn area(&self) -> f64; }
# trait Circle : Shape { fn radius(&self) -> f64; }
# struct UnitCircle;
# impl Shape for UnitCircle { fn area(&self) -> f64 { std::f64::consts::PI } }
# impl Circle for UnitCircle { fn radius(&self) -> f64 { 1.0 } }
# let circle = UnitCircle;
let circle = Box::new(circle) as Box<dyn Circle>;
let nonsense = circle.radius() * circle.area();
#}</code></pre></pre>
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