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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="conditional-compilation.html"><strong aria-hidden="true">5.</strong> Conditional compilation</a></li><li><a href="items.html"><strong aria-hidden="true">6.</strong> Items</a></li><li><ol class="section"><li><a href="items/modules.html"><strong aria-hidden="true">6.1.</strong> Modules</a></li><li><a href="items/extern-crates.html"><strong aria-hidden="true">6.2.</strong> Extern crates</a></li><li><a href="items/use-declarations.html"><strong aria-hidden="true">6.3.</strong> Use declarations</a></li><li><a href="items/functions.html"><strong aria-hidden="true">6.4.</strong> Functions</a></li><li><a href="items/type-aliases.html"><strong aria-hidden="true">6.5.</strong> Type aliases</a></li><li><a href="items/structs.html"><strong aria-hidden="true">6.6.</strong> Structs</a></li><li><a href="items/enumerations.html"><strong aria-hidden="true">6.7.</strong> Enumerations</a></li><li><a href="items/unions.html"><strong aria-hidden="true">6.8.</strong> Unions</a></li><li><a href="items/constant-items.html"><strong aria-hidden="true">6.9.</strong> Constant items</a></li><li><a href="items/static-items.html"><strong aria-hidden="true">6.10.</strong> Static items</a></li><li><a href="items/traits.html"><strong aria-hidden="true">6.11.</strong> Traits</a></li><li><a href="items/implementations.html"><strong aria-hidden="true">6.12.</strong> Implementations</a></li><li><a href="items/external-blocks.html"><strong aria-hidden="true">6.13.</strong> External blocks</a></li><li><a href="items/generics.html"><strong aria-hidden="true">6.14.</strong> Type and lifetime parameters</a></li><li><a href="items/associated-items.html"><strong aria-hidden="true">6.15.</strong> Associated Items</a></li><li><a href="visibility-and-privacy.html"><strong aria-hidden="true">6.16.</strong> Visibility and Privacy</a></li></ol></li><li><a href="attributes.html"><strong aria-hidden="true">7.</strong> Attributes</a></li><li><ol class="section"><li><a href="attributes/testing.html"><strong aria-hidden="true">7.1.</strong> Testing</a></li><li><a href="attributes/derive.html"><strong aria-hidden="true">7.2.</strong> Derive</a></li><li><a href="attributes/diagnostics.html"><strong aria-hidden="true">7.3.</strong> Diagnostics</a></li><li><a href="attributes/codegen.html"><strong aria-hidden="true">7.4.</strong> Code generation</a></li><li><a href="attributes/limits.html"><strong aria-hidden="true">7.5.</strong> Limits</a></li></ol></li><li><a href="statements-and-expressions.html"><strong aria-hidden="true">8.</strong> Statements and expressions</a></li><li><ol class="section"><li><a href="statements.html"><strong aria-hidden="true">8.1.</strong> Statements</a></li><li><a href="expressions.html"><strong aria-hidden="true">8.2.</strong> Expressions</a></li><li><ol class="section"><li><a href="expressions/literal-expr.html"><strong aria-hidden="true">8.2.1.</strong> Literal expressions</a></li><li><a href="expressions/path-expr.html"><strong aria-hidden="true">8.2.2.</strong> Path expressions</a></li><li><a href="expressions/block-expr.html"><strong aria-hidden="true">8.2.3.</strong> Block expressions</a></li><li><a href="expressions/operator-expr.html"><strong aria-hidden="true">8.2.4.</strong> Operator expressions</a></li><li><a href="expressions/grouped-expr.html"><strong aria-hidden="true">8.2.5.</strong> Grouped expressions</a></li><li><a href="expressions/array-expr.html"><strong aria-hidden="true">8.2.6.</strong> Array and index expressions</a></li><li><a href="expressions/tuple-expr.html"><strong aria-hidden="true">8.2.7.</strong> Tuple and index expressions</a></li><li><a href="expressions/struct-expr.html"><strong aria-hidden="true">8.2.8.</strong> Struct expressions</a></li><li><a href="expressions/enum-variant-expr.html"><strong aria-hidden="true">8.2.9.</strong> Enum variant expressions</a></li><li><a href="expressions/call-expr.html"><strong aria-hidden="true">8.2.10.</strong> Call expressions</a></li><li><a href="expressions/method-call-expr.html"><strong aria-hidden="true">8.2.11.</strong> Method call expressions</a></li><li><a href="expressions/field-expr.html"><strong aria-hidden="true">8.2.12.</strong> Field access expressions</a></li><li><a href="expressions/closure-expr.html"><strong aria-hidden="true">8.2.13.</strong> Closure expressions</a></li><li><a href="expressions/loop-expr.html"><strong aria-hidden="true">8.2.14.</strong> Loop expressions</a></li><li><a href="expressions/range-expr.html"><strong aria-hidden="true">8.2.15.</strong> Range expressions</a></li><li><a href="expressions/if-expr.html"><strong aria-hidden="true">8.2.16.</strong> If and if let expressions</a></li><li><a href="expressions/match-expr.html"><strong aria-hidden="true">8.2.17.</strong> Match expressions</a></li><li><a href="expressions/return-expr.html"><strong aria-hidden="true">8.2.18.</strong> Return expressions</a></li></ol></li></ol></li><li><a href="patterns.html"><strong aria-hidden="true">9.</strong> Patterns</a></li><li><a href="type-system.html"><strong aria-hidden="true">10.</strong> Type system</a></li><li><ol class="section"><li><a href="types.html"><strong aria-hidden="true">10.1.</strong> Types</a></li><li><ol class="section"><li><a href="types/boolean.html"><strong aria-hidden="true">10.1.1.</strong> Boolean type</a></li><li><a href="types/numeric.html"><strong aria-hidden="true">10.1.2.</strong> Numeric types</a></li><li><a href="types/textual.html"><strong aria-hidden="true">10.1.3.</strong> Textual types</a></li><li><a href="types/never.html"><strong aria-hidden="true">10.1.4.</strong> Never type</a></li><li><a href="types/tuple.html"><strong aria-hidden="true">10.1.5.</strong> Tuple types</a></li><li><a href="types/array.html"><strong aria-hidden="true">10.1.6.</strong> Array types</a></li><li><a href="types/slice.html"><strong aria-hidden="true">10.1.7.</strong> Slice types</a></li><li><a href="types/struct.html"><strong aria-hidden="true">10.1.8.</strong> Struct types</a></li><li><a href="types/enum.html"><strong aria-hidden="true">10.1.9.</strong> Enumerated types</a></li><li><a href="types/union.html"><strong aria-hidden="true">10.1.10.</strong> Union types</a></li><li><a href="types/function-item.html"><strong aria-hidden="true">10.1.11.</strong> Function item types</a></li><li><a href="types/closure.html"><strong aria-hidden="true">10.1.12.</strong> Closure types</a></li><li><a href="types/pointer.html"><strong aria-hidden="true">10.1.13.</strong> Pointer types</a></li><li><a href="types/function-pointer.html"><strong aria-hidden="true">10.1.14.</strong> Function pointer types</a></li><li><a href="types/trait-object.html" class="active"><strong aria-hidden="true">10.1.15.</strong> Trait object types</a></li><li><a href="types/impl-trait.html"><strong aria-hidden="true">10.1.16.</strong> Impl trait type</a></li><li><a href="types/parameters.html"><strong aria-hidden="true">10.1.17.</strong> Type parameters</a></li><li><a href="types/inferred.html"><strong aria-hidden="true">10.1.18.</strong> Inferred type</a></li></ol></li><li><a href="dynamically-sized-types.html"><strong aria-hidden="true">10.2.</strong> Dynamically Sized Types</a></li><li><a href="type-layout.html"><strong aria-hidden="true">10.3.</strong> Type layout</a></li><li><a href="interior-mutability.html"><strong aria-hidden="true">10.4.</strong> Interior mutability</a></li><li><a href="subtyping.html"><strong aria-hidden="true">10.5.</strong> Subtyping and Variance</a></li><li><a href="trait-bounds.html"><strong aria-hidden="true">10.6.</strong> Trait and lifetime bounds</a></li><li><a href="type-coercions.html"><strong aria-hidden="true">10.7.</strong> Type coercions</a></li><li><a href="destructors.html"><strong aria-hidden="true">10.8.</strong> Destructors</a></li><li><a href="lifetime-elision.html"><strong aria-hidden="true">10.9.</strong> Lifetime elision</a></li></ol></li><li><a href="special-types-and-traits.html"><strong aria-hidden="true">11.</strong> Special types and traits</a></li><li><a href="memory-model.html"><strong aria-hidden="true">12.</strong> Memory model</a></li><li><ol class="section"><li><a href="memory-allocation-and-lifetime.html"><strong aria-hidden="true">12.1.</strong> Memory allocation and lifetime</a></li><li><a href="memory-ownership.html"><strong aria-hidden="true">12.2.</strong> Memory ownership</a></li><li><a href="variables.html"><strong aria-hidden="true">12.3.</strong> Variables</a></li></ol></li><li><a href="linkage.html"><strong aria-hidden="true">13.</strong> Linkage</a></li><li><a href="unsafety.html"><strong aria-hidden="true">14.</strong> Unsafety</a></li><li><ol class="section"><li><a href="unsafe-functions.html"><strong aria-hidden="true">14.1.</strong> Unsafe functions</a></li><li><a href="unsafe-blocks.html"><strong aria-hidden="true">14.2.</strong> Unsafe blocks</a></li><li><a href="behavior-considered-undefined.html"><strong aria-hidden="true">14.3.</strong> Behavior considered undefined</a></li><li><a href="behavior-not-considered-unsafe.html"><strong aria-hidden="true">14.4.</strong> Behavior not considered unsafe</a></li></ol></li><li><a href="const_eval.html"><strong aria-hidden="true">15.</strong> Constant Evaluation</a></li><li><a href="abi.html"><strong aria-hidden="true">16.</strong> Application Binary Interface</a></li><li><a href="runtime.html"><strong aria-hidden="true">17.</strong> The Rust runtime</a></li><li><a href="appendices.html"><strong aria-hidden="true">18.</strong> Appendices</a></li><li><ol class="section"><li><a href="macro-ambiguity.html"><strong aria-hidden="true">18.1.</strong> Macro Follow-Set Ambiguity Formal Specification</a></li><li><a href="influences.html"><strong aria-hidden="true">18.2.</strong> Influences</a></li><li><a href="glossary.html"><strong aria-hidden="true">18.3.</strong> Glossary</a></li></ol></li></ol>
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<a class="header" href="types/trait-object.html#trait-objects" id="trait-objects"><h1>Trait objects</h1></a>
<blockquote>
<p><strong><sup>Syntax</sup></strong><br />
<em>TraitObjectType</em> :<br />
<code>dyn</code><sup>?</sup> <a href="types.html#type-expressions"><em>TypeParamBounds</em></a></p>
<p><em>TraitObjectTypeOneBound</em> :<br />
<code>dyn</code><sup>?</sup> <a href="trait-bounds.html"><em>TraitBound</em></a></p>
</blockquote>
<p>A <em>trait object</em> is an opaque value of another type that implements a set of
traits. The set of traits is made up of an <a href="items/traits.html#object-safety">object safe</a> <em>base trait</em> plus any
number of <a href="special-types-and-traits.html#auto-traits">auto traits</a>.</p>
<p>Trait objects implement the base trait, its auto traits, and any <a href="items/traits.html#supertraits">supertraits</a>
of the base trait.</p>
<p>Trait objects are written as the optional keyword <code>dyn</code> followed by a set of
trait bounds, but with the following restrictions on the trait bounds. All
traits except the first trait must be auto traits, there may not be more than
one lifetime, and opt-out bounds (e.g. <code>?Sized</code>) are not allowed. Furthermore,
paths to traits may be parenthesized.</p>
<p>For example, given a trait <code>Trait</code>, the following are all trait objects:</p>
<ul>
<li><code>Trait</code></li>
<li><code>dyn Trait</code></li>
<li><code>dyn Trait + Send</code></li>
<li><code>dyn Trait + Send + Sync</code></li>
<li><code>dyn Trait + 'static</code></li>
<li><code>dyn Trait + Send + 'static</code></li>
<li><code>dyn Trait +</code></li>
<li><code>dyn 'static + Trait</code>.</li>
<li><code>dyn (Trait)</code></li>
</ul>
<blockquote>
<p><strong>Edition Differences</strong>: In the 2015 edition, if the first bound of the
trait object is a path that starts with <code>::</code>, then the <code>dyn</code> will be treated
as a part of the path. The first path can be put in parenthesis to get
around this. As such, if you want a trait object with the trait
<code>::your_module::Trait</code>, you should write it as <code>dyn (::your_module::Trait)</code>.</p>
<p>Beginning in the 2018 edition, <code>dyn</code> is a true keyword and is not allowed in
paths, so the parentheses are not necessary.</p>
</blockquote>
<blockquote>
<p>Note: For clarity, it is recommended to always use the <code>dyn</code> keyword on your
trait objects unless your codebase supports compiling with Rust 1.26 or lower.</p>
</blockquote>
<p>Two trait object types alias each other if the base traits alias each other and
if the sets of auto traits are the same and the lifetime bounds are the same.
For example, <code>dyn Trait + Send + UnwindSafe</code> is the same as
<code>dyn Trait + Unwindsafe + Send</code>.</p>
<div class="warning">
<p><strong><em>Warning:</em></strong> With two trait object types, even when the complete set of traits
is the same, if the base traits differ, the type is different. For example,
<code>dyn Send + Sync</code> is a different type from <code>dyn Sync + Send</code>. See <a href="https://github.com/rust-lang/rust/issues/33140">issue 33140</a>.</p>
</div>
<p>Due to the opaqueness of which concrete type the value is of, trait objects are
<a href="dynamically-sized-types.html">dynamically sized types</a>. Like all
<abbr title="dynamically sized types">DSTs</abbr>, trait objects are used
behind some type of pointer; for example <code>&dyn SomeTrait</code> or
<code>Box<dyn SomeTrait></code>. Each instance of a pointer to a trait object includes:</p>
<ul>
<li>a pointer to an instance of a type <code>T</code> that implements <code>SomeTrait</code></li>
<li>a <em>virtual method table</em>, often just called a <em>vtable</em>, which contains, for
each method of <code>SomeTrait</code> and its <a href="items/traits.html#supertraits">supertraits</a> that <code>T</code> implements, a
pointer to <code>T</code>'s implementation (i.e. a function pointer).</li>
</ul>
<p>The purpose of trait objects is to permit "late binding" of methods. Calling a
method on a trait object results in virtual dispatch at runtime: that is, a
function pointer is loaded from the trait object vtable and invoked indirectly.
The actual implementation for each vtable entry can vary on an object-by-object
basis.</p>
<p>An example of a trait object:</p>
<pre><pre class="playpen"><code class="language-rust">trait Printable {
fn stringify(&self) -> String;
}
impl Printable for i32 {
fn stringify(&self) -> String { self.to_string() }
}
fn print(a: Box<dyn Printable>) {
println!("{}", a.stringify());
}
fn main() {
print(Box::new(10) as Box<dyn Printable>);
}
</code></pre></pre>
<p>In this example, the trait <code>Printable</code> occurs as a trait object in both the
type signature of <code>print</code>, and the cast expression in <code>main</code>.</p>
<a class="header" href="types/trait-object.html#trait-object-lifetime-bounds" id="trait-object-lifetime-bounds"><h2>Trait Object Lifetime Bounds</h2></a>
<p>Since a trait object can contain references, the lifetimes of those references
need to be expressed as part of the trait object. This lifetime is written as
<code>Trait + 'a</code>. There are <a href="lifetime-elision.html#default-trait-object-lifetimes">defaults</a> that allow this lifetime to usually be
inferred with a sensible choice.</p>
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