366 lines
37 KiB
HTML
366 lines
37 KiB
HTML
<!DOCTYPE HTML>
|
||
<html lang="en" class="sidebar-visible no-js light">
|
||
<head>
|
||
<!-- Book generated using mdBook -->
|
||
<meta charset="UTF-8">
|
||
<title>C - Derivable Traits - The Rust Programming Language</title>
|
||
|
||
|
||
<meta content="text/html; charset=utf-8" http-equiv="Content-Type">
|
||
<meta name="description" content="">
|
||
<meta name="viewport" content="width=device-width, initial-scale=1">
|
||
<meta name="theme-color" content="#ffffff" />
|
||
|
||
<link rel="shortcut icon" href="favicon.png">
|
||
<link rel="stylesheet" href="css/variables.css">
|
||
<link rel="stylesheet" href="css/general.css">
|
||
<link rel="stylesheet" href="css/chrome.css">
|
||
<link rel="stylesheet" href="css/print.css" media="print">
|
||
|
||
<!-- Fonts -->
|
||
<link rel="stylesheet" href="FontAwesome/css/font-awesome.css">
|
||
<link href="googleFonts/css.css" rel="stylesheet" type="text/css">
|
||
|
||
<!-- Highlight.js Stylesheets -->
|
||
<link rel="stylesheet" href="highlight.css">
|
||
<link rel="stylesheet" href="tomorrow-night.css">
|
||
<link rel="stylesheet" href="ayu-highlight.css">
|
||
|
||
<!-- Custom theme stylesheets -->
|
||
|
||
<link rel="stylesheet" href="ferris.css">
|
||
|
||
<link rel="stylesheet" href="theme/2018-edition.css">
|
||
|
||
|
||
|
||
</head>
|
||
<body>
|
||
<!-- Provide site root to javascript -->
|
||
<script type="text/javascript">
|
||
var path_to_root = "";
|
||
var default_theme = window.matchMedia("(prefers-color-scheme: dark)").matches ? "light" : "light";
|
||
</script>
|
||
|
||
<!-- Work around some values being stored in localStorage wrapped in quotes -->
|
||
<script type="text/javascript">
|
||
try {
|
||
var theme = localStorage.getItem('mdbook-theme');
|
||
var sidebar = localStorage.getItem('mdbook-sidebar');
|
||
|
||
if (theme.startsWith('"') && theme.endsWith('"')) {
|
||
localStorage.setItem('mdbook-theme', theme.slice(1, theme.length - 1));
|
||
}
|
||
|
||
if (sidebar.startsWith('"') && sidebar.endsWith('"')) {
|
||
localStorage.setItem('mdbook-sidebar', sidebar.slice(1, sidebar.length - 1));
|
||
}
|
||
} catch (e) { }
|
||
</script>
|
||
|
||
<!-- Set the theme before any content is loaded, prevents flash -->
|
||
<script type="text/javascript">
|
||
var theme;
|
||
try { theme = localStorage.getItem('mdbook-theme'); } catch(e) { }
|
||
if (theme === null || theme === undefined) { theme = default_theme; }
|
||
var html = document.querySelector('html');
|
||
html.classList.remove('no-js')
|
||
html.classList.remove('light')
|
||
html.classList.add(theme);
|
||
html.classList.add('js');
|
||
</script>
|
||
|
||
<!-- Hide / unhide sidebar before it is displayed -->
|
||
<script type="text/javascript">
|
||
var html = document.querySelector('html');
|
||
var sidebar = 'hidden';
|
||
if (document.body.clientWidth >= 1080) {
|
||
try { sidebar = localStorage.getItem('mdbook-sidebar'); } catch(e) { }
|
||
sidebar = sidebar || 'visible';
|
||
}
|
||
html.classList.remove('sidebar-visible');
|
||
html.classList.add("sidebar-" + sidebar);
|
||
</script>
|
||
|
||
<nav id="sidebar" class="sidebar" aria-label="Table of contents">
|
||
<div id="sidebar-scrollbox" class="sidebar-scrollbox">
|
||
<ol class="chapter"><li class="expanded affix "><a href="title-page.html">The Rust Programming Language</a></li><li class="expanded affix "><a href="foreword.html">Foreword</a></li><li class="expanded affix "><a href="ch00-00-introduction.html">Introduction</a></li><li class="expanded "><a href="ch01-00-getting-started.html"><strong aria-hidden="true">1.</strong> Getting Started</a></li><li><ol class="section"><li class="expanded "><a href="ch01-01-installation.html"><strong aria-hidden="true">1.1.</strong> Installation</a></li><li class="expanded "><a href="ch01-02-hello-world.html"><strong aria-hidden="true">1.2.</strong> Hello, World!</a></li><li class="expanded "><a href="ch01-03-hello-cargo.html"><strong aria-hidden="true">1.3.</strong> Hello, Cargo!</a></li></ol></li><li class="expanded "><a href="ch02-00-guessing-game-tutorial.html"><strong aria-hidden="true">2.</strong> Programming a Guessing Game</a></li><li class="expanded "><a href="ch03-00-common-programming-concepts.html"><strong aria-hidden="true">3.</strong> Common Programming Concepts</a></li><li><ol class="section"><li class="expanded "><a href="ch03-01-variables-and-mutability.html"><strong aria-hidden="true">3.1.</strong> Variables and Mutability</a></li><li class="expanded "><a href="ch03-02-data-types.html"><strong aria-hidden="true">3.2.</strong> Data Types</a></li><li class="expanded "><a href="ch03-03-how-functions-work.html"><strong aria-hidden="true">3.3.</strong> Functions</a></li><li class="expanded "><a href="ch03-04-comments.html"><strong aria-hidden="true">3.4.</strong> Comments</a></li><li class="expanded "><a href="ch03-05-control-flow.html"><strong aria-hidden="true">3.5.</strong> Control Flow</a></li></ol></li><li class="expanded "><a href="ch04-00-understanding-ownership.html"><strong aria-hidden="true">4.</strong> Understanding Ownership</a></li><li><ol class="section"><li class="expanded "><a href="ch04-01-what-is-ownership.html"><strong aria-hidden="true">4.1.</strong> What is Ownership?</a></li><li class="expanded "><a href="ch04-02-references-and-borrowing.html"><strong aria-hidden="true">4.2.</strong> References and Borrowing</a></li><li class="expanded "><a href="ch04-03-slices.html"><strong aria-hidden="true">4.3.</strong> The Slice Type</a></li></ol></li><li class="expanded "><a href="ch05-00-structs.html"><strong aria-hidden="true">5.</strong> Using Structs to Structure Related Data</a></li><li><ol class="section"><li class="expanded "><a href="ch05-01-defining-structs.html"><strong aria-hidden="true">5.1.</strong> Defining and Instantiating Structs</a></li><li class="expanded "><a href="ch05-02-example-structs.html"><strong aria-hidden="true">5.2.</strong> An Example Program Using Structs</a></li><li class="expanded "><a href="ch05-03-method-syntax.html"><strong aria-hidden="true">5.3.</strong> Method Syntax</a></li></ol></li><li class="expanded "><a href="ch06-00-enums.html"><strong aria-hidden="true">6.</strong> Enums and Pattern Matching</a></li><li><ol class="section"><li class="expanded "><a href="ch06-01-defining-an-enum.html"><strong aria-hidden="true">6.1.</strong> Defining an Enum</a></li><li class="expanded "><a href="ch06-02-match.html"><strong aria-hidden="true">6.2.</strong> The match Control Flow Operator</a></li><li class="expanded "><a href="ch06-03-if-let.html"><strong aria-hidden="true">6.3.</strong> Concise Control Flow with if let</a></li></ol></li><li class="expanded "><a href="ch07-00-managing-growing-projects-with-packages-crates-and-modules.html"><strong aria-hidden="true">7.</strong> Managing Growing Projects with Packages, Crates, and Modules</a></li><li><ol class="section"><li class="expanded "><a href="ch07-01-packages-and-crates.html"><strong aria-hidden="true">7.1.</strong> Packages and Crates</a></li><li class="expanded "><a href="ch07-02-defining-modules-to-control-scope-and-privacy.html"><strong aria-hidden="true">7.2.</strong> Defining Modules to Control Scope and Privacy</a></li><li class="expanded "><a href="ch07-03-paths-for-referring-to-an-item-in-the-module-tree.html"><strong aria-hidden="true">7.3.</strong> Paths for Referring to an Item in the Module Tree</a></li><li class="expanded "><a href="ch07-04-bringing-paths-into-scope-with-the-use-keyword.html"><strong aria-hidden="true">7.4.</strong> Bringing Paths Into Scope with the use Keyword</a></li><li class="expanded "><a href="ch07-05-separating-modules-into-different-files.html"><strong aria-hidden="true">7.5.</strong> Separating Modules into Different Files</a></li></ol></li><li class="expanded "><a href="ch08-00-common-collections.html"><strong aria-hidden="true">8.</strong> Common Collections</a></li><li><ol class="section"><li class="expanded "><a href="ch08-01-vectors.html"><strong aria-hidden="true">8.1.</strong> Storing Lists of Values with Vectors</a></li><li class="expanded "><a href="ch08-02-strings.html"><strong aria-hidden="true">8.2.</strong> Storing UTF-8 Encoded Text with Strings</a></li><li class="expanded "><a href="ch08-03-hash-maps.html"><strong aria-hidden="true">8.3.</strong> Storing Keys with Associated Values in Hash Maps</a></li></ol></li><li class="expanded "><a href="ch09-00-error-handling.html"><strong aria-hidden="true">9.</strong> Error Handling</a></li><li><ol class="section"><li class="expanded "><a href="ch09-01-unrecoverable-errors-with-panic.html"><strong aria-hidden="true">9.1.</strong> Unrecoverable Errors with panic!</a></li><li class="expanded "><a href="ch09-02-recoverable-errors-with-result.html"><strong aria-hidden="true">9.2.</strong> Recoverable Errors with Result</a></li><li class="expanded "><a href="ch09-03-to-panic-or-not-to-panic.html"><strong aria-hidden="true">9.3.</strong> To panic! or Not To panic!</a></li></ol></li><li class="expanded "><a href="ch10-00-generics.html"><strong aria-hidden="true">10.</strong> Generic Types, Traits, and Lifetimes</a></li><li><ol class="section"><li class="expanded "><a href="ch10-01-syntax.html"><strong aria-hidden="true">10.1.</strong> Generic Data Types</a></li><li class="expanded "><a href="ch10-02-traits.html"><strong aria-hidden="true">10.2.</strong> Traits: Defining Shared Behavior</a></li><li class="expanded "><a href="ch10-03-lifetime-syntax.html"><strong aria-hidden="true">10.3.</strong> Validating References with Lifetimes</a></li></ol></li><li class="expanded "><a href="ch11-00-testing.html"><strong aria-hidden="true">11.</strong> Writing Automated Tests</a></li><li><ol class="section"><li class="expanded "><a href="ch11-01-writing-tests.html"><strong aria-hidden="true">11.1.</strong> How to Write Tests</a></li><li class="expanded "><a href="ch11-02-running-tests.html"><strong aria-hidden="true">11.2.</strong> Controlling How Tests Are Run</a></li><li class="expanded "><a href="ch11-03-test-organization.html"><strong aria-hidden="true">11.3.</strong> Test Organization</a></li></ol></li><li class="expanded "><a href="ch12-00-an-io-project.html"><strong aria-hidden="true">12.</strong> An I/O Project: Building a Command Line Program</a></li><li><ol class="section"><li class="expanded "><a href="ch12-01-accepting-command-line-arguments.html"><strong aria-hidden="true">12.1.</strong> Accepting Command Line Arguments</a></li><li class="expanded "><a href="ch12-02-reading-a-file.html"><strong aria-hidden="true">12.2.</strong> Reading a File</a></li><li class="expanded "><a href="ch12-03-improving-error-handling-and-modularity.html"><strong aria-hidden="true">12.3.</strong> Refactoring to Improve Modularity and Error Handling</a></li><li class="expanded "><a href="ch12-04-testing-the-librarys-functionality.html"><strong aria-hidden="true">12.4.</strong> Developing the Library’s Functionality with Test Driven Development</a></li><li class="expanded "><a href="ch12-05-working-with-environment-variables.html"><strong aria-hidden="true">12.5.</strong> Working with Environment Variables</a></li><li class="expanded "><a href="ch12-06-writing-to-stderr-instead-of-stdout.html"><strong aria-hidden="true">12.6.</strong> Writing Error Messages to Standard Error Instead of Standard Output</a></li></ol></li><li class="expanded "><a href="ch13-00-functional-features.html"><strong aria-hidden="true">13.</strong> Functional Language Features: Iterators and Closures</a></li><li><ol class="section"><li class="expanded "><a href="ch13-01-closures.html"><strong aria-hidden="true">13.1.</strong> Closures: Anonymous Functions that Can Capture Their Environment</a></li><li class="expanded "><a href="ch13-02-iterators.html"><strong aria-hidden="true">13.2.</strong> Processing a Series of Items with Iterators</a></li><li class="expanded "><a href="ch13-03-improving-our-io-project.html"><strong aria-hidden="true">13.3.</strong> Improving Our I/O Project</a></li><li class="expanded "><a href="ch13-04-performance.html"><strong aria-hidden="true">13.4.</strong> Comparing Performance: Loops vs. Iterators</a></li></ol></li><li class="expanded "><a href="ch14-00-more-about-cargo.html"><strong aria-hidden="true">14.</strong> More about Cargo and Crates.io</a></li><li><ol class="section"><li class="expanded "><a href="ch14-01-release-profiles.html"><strong aria-hidden="true">14.1.</strong> Customizing Builds with Release Profiles</a></li><li class="expanded "><a href="ch14-02-publishing-to-crates-io.html"><strong aria-hidden="true">14.2.</strong> Publishing a Crate to Crates.io</a></li><li class="expanded "><a href="ch14-03-cargo-workspaces.html"><strong aria-hidden="true">14.3.</strong> Cargo Workspaces</a></li><li class="expanded "><a href="ch14-04-installing-binaries.html"><strong aria-hidden="true">14.4.</strong> Installing Binaries from Crates.io with cargo install</a></li><li class="expanded "><a href="ch14-05-extending-cargo.html"><strong aria-hidden="true">14.5.</strong> Extending Cargo with Custom Commands</a></li></ol></li><li class="expanded "><a href="ch15-00-smart-pointers.html"><strong aria-hidden="true">15.</strong> Smart Pointers</a></li><li><ol class="section"><li class="expanded "><a href="ch15-01-box.html"><strong aria-hidden="true">15.1.</strong> Using Box<T> to Point to Data on the Heap</a></li><li class="expanded "><a href="ch15-02-deref.html"><strong aria-hidden="true">15.2.</strong> Treating Smart Pointers Like Regular References with the Deref Trait</a></li><li class="expanded "><a href="ch15-03-drop.html"><strong aria-hidden="true">15.3.</strong> Running Code on Cleanup with the Drop Trait</a></li><li class="expanded "><a href="ch15-04-rc.html"><strong aria-hidden="true">15.4.</strong> Rc<T>, the Reference Counted Smart Pointer</a></li><li class="expanded "><a href="ch15-05-interior-mutability.html"><strong aria-hidden="true">15.5.</strong> RefCell<T> and the Interior Mutability Pattern</a></li><li class="expanded "><a href="ch15-06-reference-cycles.html"><strong aria-hidden="true">15.6.</strong> Reference Cycles Can Leak Memory</a></li></ol></li><li class="expanded "><a href="ch16-00-concurrency.html"><strong aria-hidden="true">16.</strong> Fearless Concurrency</a></li><li><ol class="section"><li class="expanded "><a href="ch16-01-threads.html"><strong aria-hidden="true">16.1.</strong> Using Threads to Run Code Simultaneously</a></li><li class="expanded "><a href="ch16-02-message-passing.html"><strong aria-hidden="true">16.2.</strong> Using Message Passing to Transfer Data Between Threads</a></li><li class="expanded "><a href="ch16-03-shared-state.html"><strong aria-hidden="true">16.3.</strong> Shared-State Concurrency</a></li><li class="expanded "><a href="ch16-04-extensible-concurrency-sync-and-send.html"><strong aria-hidden="true">16.4.</strong> Extensible Concurrency with the Sync and Send Traits</a></li></ol></li><li class="expanded "><a href="ch17-00-oop.html"><strong aria-hidden="true">17.</strong> Object Oriented Programming Features of Rust</a></li><li><ol class="section"><li class="expanded "><a href="ch17-01-what-is-oo.html"><strong aria-hidden="true">17.1.</strong> Characteristics of Object-Oriented Languages</a></li><li class="expanded "><a href="ch17-02-trait-objects.html"><strong aria-hidden="true">17.2.</strong> Using Trait Objects That Allow for Values of Different Types</a></li><li class="expanded "><a href="ch17-03-oo-design-patterns.html"><strong aria-hidden="true">17.3.</strong> Implementing an Object-Oriented Design Pattern</a></li></ol></li><li class="expanded "><a href="ch18-00-patterns.html"><strong aria-hidden="true">18.</strong> Patterns and Matching</a></li><li><ol class="section"><li class="expanded "><a href="ch18-01-all-the-places-for-patterns.html"><strong aria-hidden="true">18.1.</strong> All the Places Patterns Can Be Used</a></li><li class="expanded "><a href="ch18-02-refutability.html"><strong aria-hidden="true">18.2.</strong> Refutability: Whether a Pattern Might Fail to Match</a></li><li class="expanded "><a href="ch18-03-pattern-syntax.html"><strong aria-hidden="true">18.3.</strong> Pattern Syntax</a></li></ol></li><li class="expanded "><a href="ch19-00-advanced-features.html"><strong aria-hidden="true">19.</strong> Advanced Features</a></li><li><ol class="section"><li class="expanded "><a href="ch19-01-unsafe-rust.html"><strong aria-hidden="true">19.1.</strong> Unsafe Rust</a></li><li class="expanded "><a href="ch19-03-advanced-traits.html"><strong aria-hidden="true">19.2.</strong> Advanced Traits</a></li><li class="expanded "><a href="ch19-04-advanced-types.html"><strong aria-hidden="true">19.3.</strong> Advanced Types</a></li><li class="expanded "><a href="ch19-05-advanced-functions-and-closures.html"><strong aria-hidden="true">19.4.</strong> Advanced Functions and Closures</a></li><li class="expanded "><a href="ch19-06-macros.html"><strong aria-hidden="true">19.5.</strong> Macros</a></li></ol></li><li class="expanded "><a href="ch20-00-final-project-a-web-server.html"><strong aria-hidden="true">20.</strong> Final Project: Building a Multithreaded Web Server</a></li><li><ol class="section"><li class="expanded "><a href="ch20-01-single-threaded.html"><strong aria-hidden="true">20.1.</strong> Building a Single-Threaded Web Server</a></li><li class="expanded "><a href="ch20-02-multithreaded.html"><strong aria-hidden="true">20.2.</strong> Turning Our Single-Threaded Server into a Multithreaded Server</a></li><li class="expanded "><a href="ch20-03-graceful-shutdown-and-cleanup.html"><strong aria-hidden="true">20.3.</strong> Graceful Shutdown and Cleanup</a></li></ol></li><li class="expanded "><a href="appendix-00.html"><strong aria-hidden="true">21.</strong> Appendix</a></li><li><ol class="section"><li class="expanded "><a href="appendix-01-keywords.html"><strong aria-hidden="true">21.1.</strong> A - Keywords</a></li><li class="expanded "><a href="appendix-02-operators.html"><strong aria-hidden="true">21.2.</strong> B - Operators and Symbols</a></li><li class="expanded "><a href="appendix-03-derivable-traits.html" class="active"><strong aria-hidden="true">21.3.</strong> C - Derivable Traits</a></li><li class="expanded "><a href="appendix-04-useful-development-tools.html"><strong aria-hidden="true">21.4.</strong> D - Useful Development Tools</a></li><li class="expanded "><a href="appendix-05-editions.html"><strong aria-hidden="true">21.5.</strong> E - Editions</a></li><li class="expanded "><a href="appendix-06-translation.html"><strong aria-hidden="true">21.6.</strong> F - Translations of the Book</a></li><li class="expanded "><a href="appendix-07-nightly-rust.html"><strong aria-hidden="true">21.7.</strong> G - How Rust is Made and “Nightly Rust”</a></li></ol></li></ol>
|
||
</div>
|
||
<div id="sidebar-resize-handle" class="sidebar-resize-handle"></div>
|
||
</nav>
|
||
|
||
<div id="page-wrapper" class="page-wrapper">
|
||
|
||
<div class="page">
|
||
|
||
<div id="menu-bar" class="menu-bar">
|
||
<div id="menu-bar-sticky-container">
|
||
<div class="left-buttons">
|
||
<button id="sidebar-toggle" class="icon-button" type="button" title="Toggle Table of Contents" aria-label="Toggle Table of Contents" aria-controls="sidebar">
|
||
<i class="fa fa-bars"></i>
|
||
</button>
|
||
<button id="theme-toggle" class="icon-button" type="button" title="Change theme" aria-label="Change theme" aria-haspopup="true" aria-expanded="false" aria-controls="theme-list">
|
||
<i class="fa fa-paint-brush"></i>
|
||
</button>
|
||
<ul id="theme-list" class="theme-popup" aria-label="Themes" role="menu">
|
||
<li role="none"><button role="menuitem" class="theme" id="light">Light (default)</button></li>
|
||
<li role="none"><button role="menuitem" class="theme" id="rust">Rust</button></li>
|
||
<li role="none"><button role="menuitem" class="theme" id="coal">Coal</button></li>
|
||
<li role="none"><button role="menuitem" class="theme" id="navy">Navy</button></li>
|
||
<li role="none"><button role="menuitem" class="theme" id="ayu">Ayu</button></li>
|
||
</ul>
|
||
|
||
<button id="search-toggle" class="icon-button" type="button" title="Search. (Shortkey: s)" aria-label="Toggle Searchbar" aria-expanded="false" aria-keyshortcuts="S" aria-controls="searchbar">
|
||
<i class="fa fa-search"></i>
|
||
</button>
|
||
|
||
</div>
|
||
|
||
<h1 class="menu-title">The Rust Programming Language</h1>
|
||
|
||
<div class="right-buttons">
|
||
<a href="print.html" title="Print this book" aria-label="Print this book">
|
||
<i id="print-button" class="fa fa-print"></i>
|
||
</a>
|
||
|
||
</div>
|
||
</div>
|
||
</div>
|
||
|
||
|
||
<div id="search-wrapper" class="hidden">
|
||
<form id="searchbar-outer" class="searchbar-outer">
|
||
<input type="search" name="search" id="searchbar" name="searchbar" placeholder="Search this book ..." aria-controls="searchresults-outer" aria-describedby="searchresults-header">
|
||
</form>
|
||
<div id="searchresults-outer" class="searchresults-outer hidden">
|
||
<div id="searchresults-header" class="searchresults-header"></div>
|
||
<ul id="searchresults">
|
||
</ul>
|
||
</div>
|
||
</div>
|
||
|
||
|
||
<!-- Apply ARIA attributes after the sidebar and the sidebar toggle button are added to the DOM -->
|
||
<script type="text/javascript">
|
||
document.getElementById('sidebar-toggle').setAttribute('aria-expanded', sidebar === 'visible');
|
||
document.getElementById('sidebar').setAttribute('aria-hidden', sidebar !== 'visible');
|
||
Array.from(document.querySelectorAll('#sidebar a')).forEach(function(link) {
|
||
link.setAttribute('tabIndex', sidebar === 'visible' ? 0 : -1);
|
||
});
|
||
</script>
|
||
|
||
<div id="content" class="content">
|
||
<main>
|
||
<h2><a class="header" href="#appendix-c-derivable-traits" id="appendix-c-derivable-traits">Appendix C: Derivable Traits</a></h2>
|
||
<p>In various places in the book, we’ve discussed the <code>derive</code> attribute, which
|
||
you can apply to a struct or enum definition. The <code>derive</code> attribute generates
|
||
code that will implement a trait with its own default implementation on the
|
||
type you’ve annotated with the <code>derive</code> syntax.</p>
|
||
<p>In this appendix, we provide a reference of all the traits in the standard
|
||
library that you can use with <code>derive</code>. Each section covers:</p>
|
||
<ul>
|
||
<li>What operators and methods deriving this trait will enable</li>
|
||
<li>What the implementation of the trait provided by <code>derive</code> does</li>
|
||
<li>What implementing the trait signifies about the type</li>
|
||
<li>The conditions in which you’re allowed or not allowed to implement the trait</li>
|
||
<li>Examples of operations that require the trait</li>
|
||
</ul>
|
||
<p>If you want different behavior from that provided by the <code>derive</code> attribute,
|
||
consult the <a href="../std/index.html">standard library documentation</a><!-- ignore -->
|
||
for each trait for details of how to manually implement them.</p>
|
||
<p>The rest of the traits defined in the standard library can’t be implemented on
|
||
your types using <code>derive</code>. These traits don’t have sensible default behavior,
|
||
so it’s up to you to implement them in the way that makes sense for what you’re
|
||
trying to accomplish.</p>
|
||
<p>An example of a trait that can’t be derived is <code>Display</code>, which handles
|
||
formatting for end users. You should always consider the appropriate way to
|
||
display a type to an end user. What parts of the type should an end user be
|
||
allowed to see? What parts would they find relevant? What format of the data
|
||
would be most relevant to them? The Rust compiler doesn’t have this insight, so
|
||
it can’t provide appropriate default behavior for you.</p>
|
||
<p>The list of derivable traits provided in this appendix is not comprehensive:
|
||
libraries can implement <code>derive</code> for their own traits, making the list of
|
||
traits you can use <code>derive</code> with truly open-ended. Implementing <code>derive</code>
|
||
involves using a procedural macro, which is covered in the
|
||
<a href="ch19-06-macros.html#macros">“Macros”</a><!-- ignore --> section of Chapter 19.</p>
|
||
<h3><a class="header" href="#debug-for-programmer-output" id="debug-for-programmer-output"><code>Debug</code> for Programmer Output</a></h3>
|
||
<p>The <code>Debug</code> trait enables debug formatting in format strings, which you
|
||
indicate by adding <code>:?</code> within <code>{}</code> placeholders.</p>
|
||
<p>The <code>Debug</code> trait allows you to print instances of a type for debugging
|
||
purposes, so you and other programmers using your type can inspect an instance
|
||
at a particular point in a program’s execution.</p>
|
||
<p>The <code>Debug</code> trait is required, for example, in use of the <code>assert_eq!</code> macro.
|
||
This macro prints the values of instances given as arguments if the equality
|
||
assertion fails so programmers can see why the two instances weren’t equal.</p>
|
||
<h3><a class="header" href="#partialeq-and-eq-for-equality-comparisons" id="partialeq-and-eq-for-equality-comparisons"><code>PartialEq</code> and <code>Eq</code> for Equality Comparisons</a></h3>
|
||
<p>The <code>PartialEq</code> trait allows you to compare instances of a type to check for
|
||
equality and enables use of the <code>==</code> and <code>!=</code> operators.</p>
|
||
<p>Deriving <code>PartialEq</code> implements the <code>eq</code> method. When <code>PartialEq</code> is derived on
|
||
structs, two instances are equal only if <em>all</em> fields are equal, and the
|
||
instances are not equal if any fields are not equal. When derived on enums,
|
||
each variant is equal to itself and not equal to the other variants.</p>
|
||
<p>The <code>PartialEq</code> trait is required, for example, with the use of the
|
||
<code>assert_eq!</code> macro, which needs to be able to compare two instances of a type
|
||
for equality.</p>
|
||
<p>The <code>Eq</code> trait has no methods. Its purpose is to signal that for every value of
|
||
the annotated type, the value is equal to itself. The <code>Eq</code> trait can only be
|
||
applied to types that also implement <code>PartialEq</code>, although not all types that
|
||
implement <code>PartialEq</code> can implement <code>Eq</code>. One example of this is floating point
|
||
number types: the implementation of floating point numbers states that two
|
||
instances of the not-a-number (<code>NaN</code>) value are not equal to each other.</p>
|
||
<p>An example of when <code>Eq</code> is required is for keys in a <code>HashMap<K, V></code> so the
|
||
<code>HashMap<K, V></code> can tell whether two keys are the same.</p>
|
||
<h3><a class="header" href="#partialord-and-ord-for-ordering-comparisons" id="partialord-and-ord-for-ordering-comparisons"><code>PartialOrd</code> and <code>Ord</code> for Ordering Comparisons</a></h3>
|
||
<p>The <code>PartialOrd</code> trait allows you to compare instances of a type for sorting
|
||
purposes. A type that implements <code>PartialOrd</code> can be used with the <code><</code>, <code>></code>,
|
||
<code><=</code>, and <code>>=</code> operators. You can only apply the <code>PartialOrd</code> trait to types
|
||
that also implement <code>PartialEq</code>.</p>
|
||
<p>Deriving <code>PartialOrd</code> implements the <code>partial_cmp</code> method, which returns an
|
||
<code>Option<Ordering></code> that will be <code>None</code> when the values given don’t produce an
|
||
ordering. An example of a value that doesn’t produce an ordering, even though
|
||
most values of that type can be compared, is the not-a-number (<code>NaN</code>) floating
|
||
point value. Calling <code>partial_cmp</code> with any floating point number and the <code>NaN</code>
|
||
floating point value will return <code>None</code>.</p>
|
||
<p>When derived on structs, <code>PartialOrd</code> compares two instances by comparing the
|
||
value in each field in the order in which the fields appear in the struct
|
||
definition. When derived on enums, variants of the enum declared earlier in the
|
||
enum definition are considered less than the variants listed later.</p>
|
||
<p>The <code>PartialOrd</code> trait is required, for example, for the <code>gen_range</code> method
|
||
from the <code>rand</code> crate that generates a random value in the range specified by a
|
||
low value and a high value.</p>
|
||
<p>The <code>Ord</code> trait allows you to know that for any two values of the annotated
|
||
type, a valid ordering will exist. The <code>Ord</code> trait implements the <code>cmp</code> method,
|
||
which returns an <code>Ordering</code> rather than an <code>Option<Ordering></code> because a valid
|
||
ordering will always be possible. You can only apply the <code>Ord</code> trait to types
|
||
that also implement <code>PartialOrd</code> and <code>Eq</code> (and <code>Eq</code> requires <code>PartialEq</code>). When
|
||
derived on structs and enums, <code>cmp</code> behaves the same way as the derived
|
||
implementation for <code>partial_cmp</code> does with <code>PartialOrd</code>.</p>
|
||
<p>An example of when <code>Ord</code> is required is when storing values in a <code>BTreeSet<T></code>,
|
||
a data structure that stores data based on the sort order of the values.</p>
|
||
<h3><a class="header" href="#clone-and-copy-for-duplicating-values" id="clone-and-copy-for-duplicating-values"><code>Clone</code> and <code>Copy</code> for Duplicating Values</a></h3>
|
||
<p>The <code>Clone</code> trait allows you to explicitly create a deep copy of a value, and
|
||
the duplication process might involve running arbitrary code and copying heap
|
||
data. See the <a href="ch04-01-what-is-ownership.html#ways-variables-and-data-interact-clone">“Ways Variables and Data Interact:
|
||
Clone”</a><!-- ignore --> section in
|
||
Chapter 4 for more information on <code>Clone</code>.</p>
|
||
<p>Deriving <code>Clone</code> implements the <code>clone</code> method, which when implemented for the
|
||
whole type, calls <code>clone</code> on each of the parts of the type. This means all the
|
||
fields or values in the type must also implement <code>Clone</code> to derive <code>Clone</code>.</p>
|
||
<p>An example of when <code>Clone</code> is required is when calling the <code>to_vec</code> method on a
|
||
slice. The slice doesn’t own the type instances it contains, but the vector
|
||
returned from <code>to_vec</code> will need to own its instances, so <code>to_vec</code> calls
|
||
<code>clone</code> on each item. Thus, the type stored in the slice must implement <code>Clone</code>.</p>
|
||
<p>The <code>Copy</code> trait allows you to duplicate a value by only copying bits stored on
|
||
the stack; no arbitrary code is necessary. See the <a href="ch04-01-what-is-ownership.html#stack-only-data-copy">“Stack-Only Data:
|
||
Copy”</a><!-- ignore --> section in Chapter 4 for more
|
||
information on <code>Copy</code>.</p>
|
||
<p>The <code>Copy</code> trait doesn’t define any methods to prevent programmers from
|
||
overloading those methods and violating the assumption that no arbitrary code
|
||
is being run. That way, all programmers can assume that copying a value will be
|
||
very fast.</p>
|
||
<p>You can derive <code>Copy</code> on any type whose parts all implement <code>Copy</code>. You can
|
||
only apply the <code>Copy</code> trait to types that also implement <code>Clone</code>, because a
|
||
type that implements <code>Copy</code> has a trivial implementation of <code>Clone</code> that
|
||
performs the same task as <code>Copy</code>.</p>
|
||
<p>The <code>Copy</code> trait is rarely required; types that implement <code>Copy</code> have
|
||
optimizations available, meaning you don’t have to call <code>clone</code>, which makes
|
||
the code more concise.</p>
|
||
<p>Everything possible with <code>Copy</code> you can also accomplish with <code>Clone</code>, but the
|
||
code might be slower or have to use <code>clone</code> in places.</p>
|
||
<h3><a class="header" href="#hash-for-mapping-a-value-to-a-value-of-fixed-size" id="hash-for-mapping-a-value-to-a-value-of-fixed-size"><code>Hash</code> for Mapping a Value to a Value of Fixed Size</a></h3>
|
||
<p>The <code>Hash</code> trait allows you to take an instance of a type of arbitrary size and
|
||
map that instance to a value of fixed size using a hash function. Deriving
|
||
<code>Hash</code> implements the <code>hash</code> method. The derived implementation of the <code>hash</code>
|
||
method combines the result of calling <code>hash</code> on each of the parts of the type,
|
||
meaning all fields or values must also implement <code>Hash</code> to derive <code>Hash</code>.</p>
|
||
<p>An example of when <code>Hash</code> is required is in storing keys in a <code>HashMap<K, V></code>
|
||
to store data efficiently.</p>
|
||
<h3><a class="header" href="#default-for-default-values" id="default-for-default-values"><code>Default</code> for Default Values</a></h3>
|
||
<p>The <code>Default</code> trait allows you to create a default value for a type. Deriving
|
||
<code>Default</code> implements the <code>default</code> function. The derived implementation of the
|
||
<code>default</code> function calls the <code>default</code> function on each part of the type,
|
||
meaning all fields or values in the type must also implement <code>Default</code> to
|
||
derive <code>Default</code>.</p>
|
||
<p>The <code>Default::default</code> function is commonly used in combination with the struct
|
||
update syntax discussed in the <a href="ch05-01-defining-structs.html#creating-instances-from-other-instances-with-struct-update-syntax">“Creating Instances From Other Instances With
|
||
Struct Update
|
||
Syntax”</a><!-- ignore -->
|
||
section in Chapter 5. You can customize a few fields of a struct and then
|
||
set and use a default value for the rest of the fields by using
|
||
<code>..Default::default()</code>.</p>
|
||
<p>The <code>Default</code> trait is required when you use the method <code>unwrap_or_default</code> on
|
||
<code>Option<T></code> instances, for example. If the <code>Option<T></code> is <code>None</code>, the method
|
||
<code>unwrap_or_default</code> will return the result of <code>Default::default</code> for the type
|
||
<code>T</code> stored in the <code>Option<T></code>.</p>
|
||
|
||
</main>
|
||
|
||
<nav class="nav-wrapper" aria-label="Page navigation">
|
||
<!-- Mobile navigation buttons -->
|
||
|
||
<a rel="prev" href="appendix-02-operators.html" class="mobile-nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left">
|
||
<i class="fa fa-angle-left"></i>
|
||
</a>
|
||
|
||
|
||
|
||
<a rel="next" href="appendix-04-useful-development-tools.html" class="mobile-nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right">
|
||
<i class="fa fa-angle-right"></i>
|
||
</a>
|
||
|
||
|
||
<div style="clear: both"></div>
|
||
</nav>
|
||
</div>
|
||
</div>
|
||
|
||
<nav class="nav-wide-wrapper" aria-label="Page navigation">
|
||
|
||
<a href="appendix-02-operators.html" class="nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left">
|
||
<i class="fa fa-angle-left"></i>
|
||
</a>
|
||
|
||
|
||
|
||
<a href="appendix-04-useful-development-tools.html" class="nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right">
|
||
<i class="fa fa-angle-right"></i>
|
||
</a>
|
||
|
||
</nav>
|
||
|
||
</div>
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
<script type="text/javascript">
|
||
window.playpen_copyable = true;
|
||
</script>
|
||
|
||
|
||
|
||
|
||
|
||
<script src="elasticlunr.min.js" type="text/javascript" charset="utf-8"></script>
|
||
<script src="mark.min.js" type="text/javascript" charset="utf-8"></script>
|
||
<script src="searcher.js" type="text/javascript" charset="utf-8"></script>
|
||
|
||
|
||
<script src="clipboard.min.js" type="text/javascript" charset="utf-8"></script>
|
||
<script src="highlight.js" type="text/javascript" charset="utf-8"></script>
|
||
<script src="book.js" type="text/javascript" charset="utf-8"></script>
|
||
|
||
<!-- Custom JS scripts -->
|
||
|
||
<script type="text/javascript" src="ferris.js"></script>
|
||
|
||
|
||
|
||
|
||
</body>
|
||
</html>
|