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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"><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" class="active"><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="unsized-types.html#unsized-types" id="unsized-types"><h1>Unsized Types</h1></a>
<p>Most types have a particular size, in bytes, that is knowable at compile time.
For example, an <code>i32</code> is thirty-two bits big, or four bytes. However, there are
some types which are useful to express, but do not have a defined size. These are
called ‘unsized’ or ‘dynamically sized’ types. One example is <code>[T]</code>. This type
represents a certain number of <code>T</code> in sequence. But we don’t know how many
there are, so the size is not known.</p>
<p>Rust understands a few of these types, but they have some restrictions. There
are three:</p>
<ol>
<li>We can only manipulate an instance of an unsized type via a pointer. An
<code>&[T]</code> works fine, but a <code>[T]</code> does not.</li>
<li>Variables and arguments cannot have dynamically sized types.</li>
<li>Only the last field in a <code>struct</code> may have a dynamically sized type; the
other fields must not. Enum variants must not have dynamically sized types as
data.</li>
</ol>
<p>So why bother? Well, because <code>[T]</code> can only be used behind a pointer, if we
didn’t have language support for unsized types, it would be impossible to write
this:</p>
<pre><code class="language-rust ignore">impl Foo for str {
</code></pre>
<p>or</p>
<pre><code class="language-rust ignore">impl<T> Foo for [T] {
</code></pre>
<p>Instead, you would have to write:</p>
<pre><code class="language-rust ignore">impl Foo for &str {
</code></pre>
<p>Meaning, this implementation would only work for <a href="references-and-borrowing.html">references</a>, and not
other types of pointers. With the <code>impl for str</code>, all pointers, including (at
some point, there are some bugs to fix first) user-defined custom smart
pointers, can use this <code>impl</code>.</p>
<a class="header" href="unsized-types.html#sized" id="sized"><h1>?Sized</h1></a>
<p>If you want to write a function that accepts a dynamically sized type, you
can use the special bound syntax, <code>?Sized</code>:</p>
<pre><pre class="playpen"><code class="language-rust"># #![allow(unused_variables)]
#fn main() {
struct Foo<T: ?Sized> {
f: T,
}
#}</code></pre></pre>
<p>This <code>?Sized</code>, read as “T may or may not be <code>Sized</code>”, which allows us to match
both sized and unsized types. All generic type parameters implicitly
have the <code>Sized</code> bound, so the <code>?Sized</code> can be used to opt-out of the implicit
bound.</p>
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