<!DOCTYPE HTML> <html lang="en"> <head> <meta charset="UTF-8"> <title>Generic Types, Traits, and Lifetimes - 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" /> <base href=""> <link rel="stylesheet" href="book.css"> <link href="https://fonts.googleapis.com/css?family=Open+Sans:300italic,400italic,600italic,700italic,800italic,400,300,600,700,800" rel="stylesheet" type="text/css"> <link href="https://fonts.googleapis.com/css?family=Source+Code+Pro:500" rel="stylesheet" type="text/css"> <link rel="shortcut icon" href="favicon.png"> <!-- Font Awesome --> <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/font-awesome/4.3.0/css/font-awesome.min.css"> <link rel="stylesheet" href="highlight.css"> <link rel="stylesheet" href="tomorrow-night.css"> <link rel="stylesheet" href="ayu-highlight.css"> <!-- Custom theme --> <!-- Fetch Clipboard.js from CDN but have a local fallback --> <script src="https://cdn.jsdelivr.net/clipboard.js/1.6.1/clipboard.min.js"></script> <script> if (typeof Clipboard == 'undefined') { document.write(unescape("%3Cscript src='clipboard.min.js'%3E%3C/script%3E")); } </script> </head> <body class="light"> <!-- 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 = 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href="ch01-03-how-rust-is-made-and-nightly-rust.html"><strong aria-hidden="true">1.3.</strong> How Rust is Made and “Nightly Rust”</a></li></ol></li><li><a href="ch02-00-guessing-game-tutorial.html"><strong aria-hidden="true">2.</strong> Guessing Game Tutorial</a></li><li><a href="ch03-00-common-programming-concepts.html"><strong aria-hidden="true">3.</strong> Common Programming Concepts</a></li><li><ol class="section"><li><a href="ch03-01-variables-and-mutability.html"><strong aria-hidden="true">3.1.</strong> Variables and Mutability</a></li><li><a href="ch03-02-data-types.html"><strong aria-hidden="true">3.2.</strong> Data Types</a></li><li><a href="ch03-03-how-functions-work.html"><strong aria-hidden="true">3.3.</strong> How Functions Work</a></li><li><a href="ch03-04-comments.html"><strong aria-hidden="true">3.4.</strong> Comments</a></li><li><a href="ch03-05-control-flow.html"><strong aria-hidden="true">3.5.</strong> Control Flow</a></li></ol></li><li><a href="ch04-00-understanding-ownership.html"><strong aria-hidden="true">4.</strong> Understanding Ownership</a></li><li><ol class="section"><li><a href="ch04-01-what-is-ownership.html"><strong aria-hidden="true">4.1.</strong> What is Ownership?</a></li><li><a href="ch04-02-references-and-borrowing.html"><strong aria-hidden="true">4.2.</strong> References & Borrowing</a></li><li><a href="ch04-03-slices.html"><strong aria-hidden="true">4.3.</strong> Slices</a></li></ol></li><li><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><a href="ch05-01-defining-structs.html"><strong aria-hidden="true">5.1.</strong> Defining and Instantiating Structs</a></li><li><a href="ch05-02-example-structs.html"><strong aria-hidden="true">5.2.</strong> An Example Program Using Structs</a></li><li><a href="ch05-03-method-syntax.html"><strong aria-hidden="true">5.3.</strong> Method Syntax</a></li></ol></li><li><a href="ch06-00-enums.html"><strong aria-hidden="true">6.</strong> Enums and Pattern Matching</a></li><li><ol class="section"><li><a href="ch06-01-defining-an-enum.html"><strong aria-hidden="true">6.1.</strong> Defining an Enum</a></li><li><a href="ch06-02-match.html"><strong aria-hidden="true">6.2.</strong> The match Control Flow Operator</a></li><li><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><a href="ch07-00-modules.html"><strong aria-hidden="true">7.</strong> Modules</a></li><li><ol class="section"><li><a href="ch07-01-mod-and-the-filesystem.html"><strong aria-hidden="true">7.1.</strong> mod and the Filesystem</a></li><li><a href="ch07-02-controlling-visibility-with-pub.html"><strong aria-hidden="true">7.2.</strong> Controlling Visibility with pub</a></li><li><a href="ch07-03-importing-names-with-use.html"><strong aria-hidden="true">7.3.</strong> Referring to Names in Different Modules</a></li></ol></li><li><a href="ch08-00-common-collections.html"><strong aria-hidden="true">8.</strong> Common Collections</a></li><li><ol class="section"><li><a href="ch08-01-vectors.html"><strong aria-hidden="true">8.1.</strong> Vectors</a></li><li><a href="ch08-02-strings.html"><strong aria-hidden="true">8.2.</strong> Strings</a></li><li><a href="ch08-03-hash-maps.html"><strong aria-hidden="true">8.3.</strong> Hash Maps</a></li></ol></li><li><a href="ch09-00-error-handling.html"><strong aria-hidden="true">9.</strong> Error Handling</a></li><li><ol class="section"><li><a href="ch09-01-unrecoverable-errors-with-panic.html"><strong aria-hidden="true">9.1.</strong> Unrecoverable Errors with panic!</a></li><li><a href="ch09-02-recoverable-errors-with-result.html"><strong aria-hidden="true">9.2.</strong> Recoverable Errors with Result</a></li><li><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><a href="ch10-00-generics.html" class="active"><strong aria-hidden="true">10.</strong> Generic Types, Traits, and Lifetimes</a></li><li><ol class="section"><li><a href="ch10-01-syntax.html"><strong aria-hidden="true">10.1.</strong> Generic Data Types</a></li><li><a href="ch10-02-traits.html"><strong aria-hidden="true">10.2.</strong> Traits: Defining Shared Behavior</a></li><li><a href="ch10-03-lifetime-syntax.html"><strong aria-hidden="true">10.3.</strong> Validating References with Lifetimes</a></li></ol></li><li><a href="ch11-00-testing.html"><strong aria-hidden="true">11.</strong> Testing</a></li><li><ol class="section"><li><a href="ch11-01-writing-tests.html"><strong aria-hidden="true">11.1.</strong> Writing tests</a></li><li><a href="ch11-02-running-tests.html"><strong aria-hidden="true">11.2.</strong> Running tests</a></li><li><a href="ch11-03-test-organization.html"><strong aria-hidden="true">11.3.</strong> Test Organization</a></li></ol></li><li><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><a href="ch12-01-accepting-command-line-arguments.html"><strong aria-hidden="true">12.1.</strong> Accepting Command Line Arguments</a></li><li><a href="ch12-02-reading-a-file.html"><strong aria-hidden="true">12.2.</strong> Reading a File</a></li><li><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><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><a href="ch12-05-working-with-environment-variables.html"><strong aria-hidden="true">12.5.</strong> Working with Environment Variables</a></li><li><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><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><a href="ch13-01-closures.html"><strong aria-hidden="true">13.1.</strong> Closures: Anonymous Functions that Can Capture Their Environment</a></li><li><a href="ch13-02-iterators.html"><strong aria-hidden="true">13.2.</strong> Processing a Series of Items with Iterators</a></li><li><a href="ch13-03-improving-our-io-project.html"><strong aria-hidden="true">13.3.</strong> Improving Our I/O Project</a></li><li><a href="ch13-04-performance.html"><strong aria-hidden="true">13.4.</strong> Comparing Performance: Loops vs. Iterators</a></li></ol></li><li><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><a href="ch14-01-release-profiles.html"><strong aria-hidden="true">14.1.</strong> Customizing Builds with Release Profiles</a></li><li><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><a href="ch14-03-cargo-workspaces.html"><strong aria-hidden="true">14.3.</strong> Cargo Workspaces</a></li><li><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><a href="ch14-05-extending-cargo.html"><strong aria-hidden="true">14.5.</strong> Extending Cargo with Custom Commands</a></li></ol></li><li><a href="ch15-00-smart-pointers.html"><strong aria-hidden="true">15.</strong> Smart Pointers</a></li><li><ol class="section"><li><a href="ch15-01-box.html"><strong aria-hidden="true">15.1.</strong> Box<T> Points to Data on the Heap and Has a Known Size</a></li><li><a href="ch15-02-deref.html"><strong aria-hidden="true">15.2.</strong> The Deref Trait Allows Access to the Data Through a Reference</a></li><li><a href="ch15-03-drop.html"><strong aria-hidden="true">15.3.</strong> The Drop Trait Runs Code on Cleanup</a></li><li><a href="ch15-04-rc.html"><strong aria-hidden="true">15.4.</strong> Rc<T>, the Reference Counted Smart Pointer</a></li><li><a href="ch15-05-interior-mutability.html"><strong aria-hidden="true">15.5.</strong> RefCell<T> and the Interior Mutability Pattern</a></li><li><a href="ch15-06-reference-cycles.html"><strong aria-hidden="true">15.6.</strong> Creating Reference Cycles and Leaking Memory is Safe</a></li></ol></li><li><a href="ch16-00-concurrency.html"><strong aria-hidden="true">16.</strong> Fearless Concurrency</a></li><li><ol class="section"><li><a href="ch16-01-threads.html"><strong aria-hidden="true">16.1.</strong> Threads</a></li><li><a href="ch16-02-message-passing.html"><strong aria-hidden="true">16.2.</strong> Message Passing</a></li><li><a href="ch16-03-shared-state.html"><strong aria-hidden="true">16.3.</strong> Shared State</a></li><li><a href="ch16-04-extensible-concurrency-sync-and-send.html"><strong aria-hidden="true">16.4.</strong> Extensible Concurrency: Sync and Send</a></li></ol></li><li><a href="ch17-00-oop.html"><strong aria-hidden="true">17.</strong> Is Rust an Object-Oriented Programming Language?</a></li><li><ol class="section"><li><a href="ch17-01-what-is-oo.html"><strong aria-hidden="true">17.1.</strong> What Does Object-Oriented Mean?</a></li><li><a href="ch17-02-trait-objects.html"><strong aria-hidden="true">17.2.</strong> Trait Objects for Using Values of Different Types</a></li><li><a href="ch17-03-oo-design-patterns.html"><strong aria-hidden="true">17.3.</strong> Object-Oriented Design Pattern Implementations</a></li></ol></li><li><a href="ch18-00-patterns.html"><strong aria-hidden="true">18.</strong> Patterns Match the Structure of Values</a></li><li><ol class="section"><li><a href="ch18-01-all-the-places-for-patterns.html"><strong aria-hidden="true">18.1.</strong> All the Places Patterns May be Used</a></li><li><a href="ch18-02-refutability.html"><strong aria-hidden="true">18.2.</strong> Refutability: Whether a Pattern Might Fail to Match</a></li><li><a href="ch18-03-pattern-syntax.html"><strong aria-hidden="true">18.3.</strong> All the Pattern Syntax</a></li></ol></li><li><a href="ch19-00-advanced-features.html"><strong aria-hidden="true">19.</strong> Advanced Features</a></li><li><ol class="section"><li><a href="ch19-01-unsafe-rust.html"><strong aria-hidden="true">19.1.</strong> Unsafe Rust</a></li><li><a href="ch19-02-advanced-lifetimes.html"><strong aria-hidden="true">19.2.</strong> Advanced Lifetimes</a></li><li><a href="ch19-03-advanced-traits.html"><strong aria-hidden="true">19.3.</strong> Advanced Traits</a></li><li><a href="ch19-04-advanced-types.html"><strong aria-hidden="true">19.4.</strong> Advanced Types</a></li><li><a href="ch19-05-advanced-functions-and-closures.html"><strong aria-hidden="true">19.5.</strong> Advanced Functions & Closures</a></li></ol></li><li><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><a href="ch20-01-single-threaded.html"><strong aria-hidden="true">20.1.</strong> A Single Threaded Web Server</a></li><li><a href="ch20-02-slow-requests.html"><strong aria-hidden="true">20.2.</strong> How Slow Requests Affect Throughput</a></li><li><a href="ch20-03-designing-the-interface.html"><strong aria-hidden="true">20.3.</strong> Designing the Thread Pool Interface</a></li><li><a href="ch20-04-storing-threads.html"><strong aria-hidden="true">20.4.</strong> Creating the Thread Pool and Storing Threads</a></li><li><a href="ch20-05-sending-requests-via-channels.html"><strong aria-hidden="true">20.5.</strong> Sending Requests to Threads Via Channels</a></li><li><a href="ch20-06-graceful-shutdown-and-cleanup.html"><strong aria-hidden="true">20.6.</strong> Graceful Shutdown and Cleanup</a></li></ol></li><li><a href="appendix-00.html"><strong aria-hidden="true">21.</strong> Appendix</a></li><li><ol class="section"><li><a href="appendix-01-keywords.html"><strong aria-hidden="true">21.1.</strong> A - Keywords</a></li><li><a href="appendix-02-operators.html"><strong aria-hidden="true">21.2.</strong> B - Operators and Symbols</a></li><li><a href="appendix-03-derivable-traits.html"><strong aria-hidden="true">21.3.</strong> C - Derivable Traits</a></li><li><a href="appendix-04-macros.html"><strong aria-hidden="true">21.4.</strong> D - Macros</a></li><li><a href="appendix-05-translation.html"><strong aria-hidden="true">21.5.</strong> E - Translations</a></li><li><a href="appendix-06-newest-features.html"><strong aria-hidden="true">21.6.</strong> F - Newest Features</a></li></ol></li></ol> </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="submenu"> <li><button class="theme" id="light">Light <span class="default">(default)</span></button></li> <li><button class="theme" id="rust">Rust</button></li> <li><button class="theme" id="coal">Coal</button></li> <li><button class="theme" id="navy">Navy</button></li> <li><button class="theme" id="ayu">Ayu</button></li> </ul> </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> <!-- 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> <a class="header" href="ch10-00-generics.html#generic-types-traits-and-lifetimes" id="generic-types-traits-and-lifetimes"><h1>Generic Types, Traits, and Lifetimes</h1></a> <p>Every programming language has tools to deal effectively with duplication of concepts; in Rust, one of those tools is <em>generics</em>. Generics are abstract stand-ins for concrete types or other properties. When we’re writing and compiling the code we can express properties of generics, such as their behavior or how they relate to other generics, without needing to know what will actually be in their place.</p> <p>In the same way that a function takes parameters whose value we don’t know in order to write code once that will be run on multiple concrete values, we can write functions that take parameters of some generic type instead of a concrete type like <code>i32</code> or <code>String</code>. We’ve already used generics in Chapter 6 with <code>Option<T></code>, Chapter 8 with <code>Vec<T></code> and <code>HashMap<K, V></code>, and Chapter 9 with <code>Result<T, E></code>. In this chapter, we’ll explore how to define our own types, functions, and methods with generics!</p> <p>First, we’re going to review the mechanics of extracting a function that reduces code duplication. Then we’ll use the same mechanics to make a generic function out of two functions that only differ in the types of their parameters. We’ll go over using generic types in struct and enum definitions too.</p> <p>After that, we’ll discuss <em>traits</em>, which are a way to define behavior in a generic way. Traits can be combined with generic types in order to constrain a generic type to those types that have a particular behavior, rather than any type at all.</p> <p>Finally, we’ll discuss <em>lifetimes</em>, which are a kind of generic that let us give the compiler information about how references are related to each other. Lifetimes are the feature in Rust that allow us to borrow values in many situations and still have the compiler check that references will be valid.</p> <a class="header" href="ch10-00-generics.html#removing-duplication-by-extracting-a-function" id="removing-duplication-by-extracting-a-function"><h2>Removing Duplication by Extracting a Function</h2></a> <p>Before getting into generics syntax, let’s first review a technique for dealing with duplication that doesn’t use generic types: extracting a function. Once that’s fresh in our minds, we’ll use the same mechanics with generics to extract a generic function! In the same way that you recognize duplicated code to extract into a function, you’ll start to recognize duplicated code that can use generics.</p> <p>Consider a small program that finds the largest number in a list, shown in Listing 10-1:</p> <p><span class="filename">Filename: src/main.rs</span></p> <pre><pre class="playpen"><code class="language-rust">fn main() { let number_list = vec![34, 50, 25, 100, 65]; let mut largest = number_list[0]; for number in number_list { if number > largest { largest = number; } } println!("The largest number is {}", largest); # assert_eq!(largest, 100); } </code></pre></pre> <p><span class="caption">Listing 10-1: Code to find the largest number in a list of numbers</span></p> <p>This code takes a list of integers, stored here in the variable <code>number_list</code>. It puts the first item in the list in a variable named <code>largest</code>. Then it iterates through all the numbers in the list, and if the current value is greater than the number stored in <code>largest</code>, it replaces the value in <code>largest</code>. If the current value is smaller than the largest value seen so far, <code>largest</code> is not changed. When all the items in the list have been considered, <code>largest</code> will hold the largest value, which in this case is 100.</p> <p>If we needed to find the largest number in two different lists of numbers, we could duplicate the code in Listing 10-1 and have the same logic exist in two places in the program, as in Listing 10-2:</p> <p><span class="filename">Filename: src/main.rs</span></p> <pre><pre class="playpen"><code class="language-rust">fn main() { let number_list = vec![34, 50, 25, 100, 65]; let mut largest = number_list[0]; for number in number_list { if number > largest { largest = number; } } println!("The largest number is {}", largest); let number_list = vec![102, 34, 6000, 89, 54, 2, 43, 8]; let mut largest = number_list[0]; for number in number_list { if number > largest { largest = number; } } println!("The largest number is {}", largest); } </code></pre></pre> <p><span class="caption">Listing 10-2: Code to find the largest number in <em>two</em> lists of numbers</span></p> <p>While this code works, duplicating code is tedious and error-prone, and means we have multiple places to update the logic if we need to change it.</p> <!-- Are we safe assuming the reader will be familiar with the term "abstraction" in this context, or do we want to give a brief definition? --> <!-- Yes, our audience will be familiar with this term. /Carol --> <p>To eliminate this duplication, we can create an abstraction, which in this case will be in the form of a function that operates on any list of integers given to the function in a parameter. This will increase the clarity of our code and let us communicate and reason about the concept of finding the largest number in a list independently of the specific places this concept is used.</p> <p>In the program in Listing 10-3, we’ve extracted the code that finds the largest number into a function named <code>largest</code>. This program can find the largest number in two different lists of numbers, but the code from Listing 10-1 only exists in one spot:</p> <p><span class="filename">Filename: src/main.rs</span></p> <pre><pre class="playpen"><code class="language-rust">fn largest(list: &[i32]) -> i32 { let mut largest = list[0]; for &item in list.iter() { if item > largest { largest = item; } } largest } fn main() { let number_list = vec![34, 50, 25, 100, 65]; let result = largest(&number_list); println!("The largest number is {}", result); # assert_eq!(result, 100); let number_list = vec![102, 34, 6000, 89, 54, 2, 43, 8]; let result = largest(&number_list); println!("The largest number is {}", result); # assert_eq!(result, 6000); } </code></pre></pre> <p><span class="caption">Listing 10-3: Abstracted code to find the largest number in two lists</span></p> <p>The function has a parameter, <code>list</code>, which represents any concrete slice of <code>i32</code> values that we might pass into the function. The code in the function definition operates on the <code>list</code> representation of any <code>&[i32]</code>. When we call the <code>largest</code> function, the code actually runs on the specific values that we pass in.</p> <p>The mechanics we went through to get from Listing 10-2 to Listing 10-3 were these steps:</p> <ol> <li>We noticed there was duplicate code.</li> <li>We extracted the duplicate code into the body of the function, and specified the inputs and return values of that code in the function signature.</li> <li>We replaced the two concrete places that had the duplicated code to call the function instead.</li> </ol> <p>We can use these same steps with generics to reduce code duplication in different ways in different scenarios. In the same way that the function body is now operating on an abstract <code>list</code> instead of concrete values, code using generics will operate on abstract types. The concepts powering generics are the same concepts you already know that power functions, just applied in different ways.</p> <p>What if we had two functions, one that found the largest item in a slice of <code>i32</code> values and one that found the largest item in a slice of <code>char</code> values? How would we get rid of that duplication? 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