<!DOCTYPE html> <html lang="en"> <head> <meta charset="utf-8"> <meta name="viewport" content="width=device-width, initial-scale=1.0"> <meta name="generator" content="rustdoc"> <title>Unsized Types</title> <link rel="stylesheet" type="text/css" href="../rust.css"> <link rel="shortcut icon" href="https://www.rust-lang.org/favicon.ico"> </head> <body class="rustdoc"> <!--[if lte IE 8]> <div class="warning"> This old browser is unsupported and will most likely display funky things. </div> <![endif]--> <div id="versioninfo"> <img src="https://www.rust-lang.org/logos/rust-logo-32x32-blk.png" width="32" height="32" alt="Rust logo"><br> <span class="white-sticker"><a href="https://www.rust-lang.org">Rust</a> 1.25.0</span><br> <a href="https://github.com/rust-lang/rust/commit/" class="hash white-sticker"></a> </div> <h1 class="title">Unsized Types</h1> <nav id="TOC"><ul></ul></nav><p><small>There is a new edition of the book and this is an old link.</small></p> <blockquote> <p>Sometimes referred to as ‘DSTs’ or ‘unsized types’, these types let us talk about types whose size we can only know at runtime. The <code>Sized</code> trait is automatically implemented for everything the compiler knows the size of at compile time. A trait bound on <code>?Sized</code> is the opposite of a trait bound on <code>Sized</code>; that is, we would read this as “<code>T</code> may or may not be <code>Sized</code>”.</p> </blockquote> <div class='information'><div class='tooltip ignore'>ⓘ<span class='tooltiptext'>This example is not tested</span></div></div><pre class="rust rust-example-rendered ignore"> <span class="kw">fn</span> <span class="ident">generic</span><span class="op"><</span><span class="ident">T</span>: <span class="question-mark">?</span><span class="ident">Sized</span><span class="op">></span>(<span class="ident">t</span>: <span class="kw-2">&</span><span class="ident">T</span>) { <span class="comment">// ...snip...</span> }<a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=%23!%5Ballow(unused)%5D%0Afn%20main()%20%7B%0Afn%20generic%3CT%3A%20%3FSized%3E(t%3A%20%26T)%20%7B%0A%20%20%20%20%2F%2F%20...snip...%0A%7D%0A%7D">Run</a></pre> <hr /> <p>Here are the relevant sections in the new and old books:</p> <ul> <li><strong><a href="second-edition/ch19-04-advanced-types.html#dynamically-sized-types--sized">In the second edition: Ch 19.04 — Advanced Types, section Dynamically Sized Types</a></strong></li> <li><small><a href="first-edition/unsized-types.html">In the first edition: Ch 3.31 — Unsized Types</a></small></li> </ul> <footer><p> Copyright © 2011 The Rust Project Developers. Licensed under the <a href="http://www.apache.org/licenses/LICENSE-2.0">Apache License, Version 2.0</a> or the <a href="https://opensource.org/licenses/MIT">MIT license</a>, at your option. </p><p> This file may not be copied, modified, or distributed except according to those terms. </p></footer> </body> </html>