424 lines
36 KiB
HTML
424 lines
36 KiB
HTML
<!DOCTYPE HTML>
|
||
<html lang="en" class="sidebar-visible no-js light">
|
||
<head>
|
||
<!-- Book generated using mdBook -->
|
||
<meta charset="UTF-8">
|
||
<title>An Example Program Using Structs - 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" class="active"><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"><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="#an-example-program-using-structs" id="an-example-program-using-structs">An Example Program Using Structs</a></h2>
|
||
<p>To understand when we might want to use structs, let’s write a program that
|
||
calculates the area of a rectangle. We’ll start with single variables, and then
|
||
refactor the program until we’re using structs instead.</p>
|
||
<p>Let’s make a new binary project with Cargo called <em>rectangles</em> that will take
|
||
the width and height of a rectangle specified in pixels and calculate the area
|
||
of the rectangle. Listing 5-8 shows a short program with one way of doing
|
||
exactly that in our project’s <em>src/main.rs</em>.</p>
|
||
<p><span class="filename">Filename: src/main.rs</span></p>
|
||
<pre><pre class="playpen"><code class="language-rust">fn main() {
|
||
let width1 = 30;
|
||
let height1 = 50;
|
||
|
||
println!(
|
||
"The area of the rectangle is {} square pixels.",
|
||
area(width1, height1)
|
||
);
|
||
}
|
||
|
||
fn area(width: u32, height: u32) -> u32 {
|
||
width * height
|
||
}
|
||
</code></pre></pre>
|
||
<p><span class="caption">Listing 5-8: Calculating the area of a rectangle
|
||
specified by separate width and height variables</span></p>
|
||
<p>Now, run this program using <code>cargo run</code>:</p>
|
||
<pre><code class="language-text">The area of the rectangle is 1500 square pixels.
|
||
</code></pre>
|
||
<p>Even though Listing 5-8 works and figures out the area of the rectangle by
|
||
calling the <code>area</code> function with each dimension, we can do better. The width
|
||
and the height are related to each other because together they describe one
|
||
rectangle.</p>
|
||
<p>The issue with this code is evident in the signature of <code>area</code>:</p>
|
||
<pre><code class="language-rust ignore">fn area(width: u32, height: u32) -> u32 {
|
||
</code></pre>
|
||
<p>The <code>area</code> function is supposed to calculate the area of one rectangle, but the
|
||
function we wrote has two parameters. The parameters are related, but that’s
|
||
not expressed anywhere in our program. It would be more readable and more
|
||
manageable to group width and height together. We’ve already discussed one way
|
||
we might do that in <a href="ch03-02-data-types.html#the-tuple-type">“The Tuple Type”</a><!-- ignore --> section
|
||
of Chapter 3: by using tuples.</p>
|
||
<h3><a class="header" href="#refactoring-with-tuples" id="refactoring-with-tuples">Refactoring with Tuples</a></h3>
|
||
<p>Listing 5-9 shows another version of our program that uses tuples.</p>
|
||
<p><span class="filename">Filename: src/main.rs</span></p>
|
||
<pre><pre class="playpen"><code class="language-rust">fn main() {
|
||
let rect1 = (30, 50);
|
||
|
||
println!(
|
||
"The area of the rectangle is {} square pixels.",
|
||
area(rect1)
|
||
);
|
||
}
|
||
|
||
fn area(dimensions: (u32, u32)) -> u32 {
|
||
dimensions.0 * dimensions.1
|
||
}
|
||
</code></pre></pre>
|
||
<p><span class="caption">Listing 5-9: Specifying the width and height of the
|
||
rectangle with a tuple</span></p>
|
||
<p>In one way, this program is better. Tuples let us add a bit of structure, and
|
||
we’re now passing just one argument. But in another way, this version is less
|
||
clear: tuples don’t name their elements, so our calculation has become more
|
||
confusing because we have to index into the parts of the tuple.</p>
|
||
<p>It doesn’t matter if we mix up width and height for the area calculation, but
|
||
if we want to draw the rectangle on the screen, it would matter! We would have
|
||
to keep in mind that <code>width</code> is the tuple index <code>0</code> and <code>height</code> is the tuple
|
||
index <code>1</code>. If someone else worked on this code, they would have to figure this
|
||
out and keep it in mind as well. It would be easy to forget or mix up these
|
||
values and cause errors, because we haven’t conveyed the meaning of our data in
|
||
our code.</p>
|
||
<h3><a class="header" href="#refactoring-with-structs-adding-more-meaning" id="refactoring-with-structs-adding-more-meaning">Refactoring with Structs: Adding More Meaning</a></h3>
|
||
<p>We use structs to add meaning by labeling the data. We can transform the tuple
|
||
we’re using into a data type with a name for the whole as well as names for the
|
||
parts, as shown in Listing 5-10.</p>
|
||
<p><span class="filename">Filename: src/main.rs</span></p>
|
||
<pre><pre class="playpen"><code class="language-rust">struct Rectangle {
|
||
width: u32,
|
||
height: u32,
|
||
}
|
||
|
||
fn main() {
|
||
let rect1 = Rectangle { width: 30, height: 50 };
|
||
|
||
println!(
|
||
"The area of the rectangle is {} square pixels.",
|
||
area(&rect1)
|
||
);
|
||
}
|
||
|
||
fn area(rectangle: &Rectangle) -> u32 {
|
||
rectangle.width * rectangle.height
|
||
}
|
||
</code></pre></pre>
|
||
<p><span class="caption">Listing 5-10: Defining a <code>Rectangle</code> struct</span></p>
|
||
<p>Here we’ve defined a struct and named it <code>Rectangle</code>. Inside the curly
|
||
brackets, we defined the fields as <code>width</code> and <code>height</code>, both of which have
|
||
type <code>u32</code>. Then in <code>main</code>, we created a particular instance of <code>Rectangle</code>
|
||
that has a width of 30 and a height of 50.</p>
|
||
<p>Our <code>area</code> function is now defined with one parameter, which we’ve named
|
||
<code>rectangle</code>, whose type is an immutable borrow of a struct <code>Rectangle</code>
|
||
instance. As mentioned in Chapter 4, we want to borrow the struct rather than
|
||
take ownership of it. This way, <code>main</code> retains its ownership and can continue
|
||
using <code>rect1</code>, which is the reason we use the <code>&</code> in the function signature and
|
||
where we call the function.</p>
|
||
<p>The <code>area</code> function accesses the <code>width</code> and <code>height</code> fields of the <code>Rectangle</code>
|
||
instance. Our function signature for <code>area</code> now says exactly what we mean:
|
||
calculate the area of <code>Rectangle</code>, using its <code>width</code> and <code>height</code> fields. This
|
||
conveys that the width and height are related to each other, and it gives
|
||
descriptive names to the values rather than using the tuple index values of <code>0</code>
|
||
and <code>1</code>. This is a win for clarity.</p>
|
||
<h3><a class="header" href="#adding-useful-functionality-with-derived-traits" id="adding-useful-functionality-with-derived-traits">Adding Useful Functionality with Derived Traits</a></h3>
|
||
<p>It’d be nice to be able to print an instance of <code>Rectangle</code> while we’re
|
||
debugging our program and see the values for all its fields. Listing 5-11 tries
|
||
using the <code>println!</code> macro as we have used in previous chapters. This won’t
|
||
work, however.</p>
|
||
<p><span class="filename">Filename: src/main.rs</span></p>
|
||
<pre><code class="language-rust ignore does_not_compile">struct Rectangle {
|
||
width: u32,
|
||
height: u32,
|
||
}
|
||
|
||
fn main() {
|
||
let rect1 = Rectangle { width: 30, height: 50 };
|
||
|
||
println!("rect1 is {}", rect1);
|
||
}
|
||
</code></pre>
|
||
<p><span class="caption">Listing 5-11: Attempting to print a <code>Rectangle</code>
|
||
instance</span></p>
|
||
<p>When we compile this code, we get an error with this core message:</p>
|
||
<pre><code class="language-text">error[E0277]: `Rectangle` doesn't implement `std::fmt::Display`
|
||
</code></pre>
|
||
<p>The <code>println!</code> macro can do many kinds of formatting, and by default, the curly
|
||
brackets tell <code>println!</code> to use formatting known as <code>Display</code>: output intended
|
||
for direct end user consumption. The primitive types we’ve seen so far
|
||
implement <code>Display</code> by default, because there’s only one way you’d want to show
|
||
a <code>1</code> or any other primitive type to a user. But with structs, the way
|
||
<code>println!</code> should format the output is less clear because there are more
|
||
display possibilities: Do you want commas or not? Do you want to print the
|
||
curly brackets? Should all the fields be shown? Due to this ambiguity, Rust
|
||
doesn’t try to guess what we want, and structs don’t have a provided
|
||
implementation of <code>Display</code>.</p>
|
||
<p>If we continue reading the errors, we’ll find this helpful note:</p>
|
||
<pre><code class="language-text">= help: the trait `std::fmt::Display` is not implemented for `Rectangle`
|
||
= note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead
|
||
</code></pre>
|
||
<p>Let’s try it! The <code>println!</code> macro call will now look like <code>println!("rect1 is {:?}", rect1);</code>. Putting the specifier <code>:?</code> inside the curly brackets tells
|
||
<code>println!</code> we want to use an output format called <code>Debug</code>. The <code>Debug</code> trait
|
||
enables us to print our struct in a way that is useful for developers so we can
|
||
see its value while we’re debugging our code.</p>
|
||
<p>Compile the code with this change. Drat! We still get an error:</p>
|
||
<pre><code class="language-text">error[E0277]: `Rectangle` doesn't implement `std::fmt::Debug`
|
||
</code></pre>
|
||
<p>But again, the compiler gives us a helpful note:</p>
|
||
<pre><code class="language-text">= help: the trait `std::fmt::Debug` is not implemented for `Rectangle`
|
||
= note: add `#[derive(Debug)]` or manually implement `std::fmt::Debug`
|
||
</code></pre>
|
||
<p>Rust <em>does</em> include functionality to print out debugging information, but we
|
||
have to explicitly opt in to make that functionality available for our struct.
|
||
To do that, we add the annotation <code>#[derive(Debug)]</code> just before the struct
|
||
definition, as shown in Listing 5-12.</p>
|
||
<p><span class="filename">Filename: src/main.rs</span></p>
|
||
<pre><pre class="playpen"><code class="language-rust">#[derive(Debug)]
|
||
struct Rectangle {
|
||
width: u32,
|
||
height: u32,
|
||
}
|
||
|
||
fn main() {
|
||
let rect1 = Rectangle { width: 30, height: 50 };
|
||
|
||
println!("rect1 is {:?}", rect1);
|
||
}
|
||
</code></pre></pre>
|
||
<p><span class="caption">Listing 5-12: Adding the annotation to derive the <code>Debug</code>
|
||
trait and printing the <code>Rectangle</code> instance using debug formatting</span></p>
|
||
<p>Now when we run the program, we won’t get any errors, and we’ll see the
|
||
following output:</p>
|
||
<pre><code class="language-text">rect1 is Rectangle { width: 30, height: 50 }
|
||
</code></pre>
|
||
<p>Nice! It’s not the prettiest output, but it shows the values of all the fields
|
||
for this instance, which would definitely help during debugging. When we have
|
||
larger structs, it’s useful to have output that’s a bit easier to read; in
|
||
those cases, we can use <code>{:#?}</code> instead of <code>{:?}</code> in the <code>println!</code> string.
|
||
When we use the <code>{:#?}</code> style in the example, the output will look like this:</p>
|
||
<pre><code class="language-text">rect1 is Rectangle {
|
||
width: 30,
|
||
height: 50
|
||
}
|
||
</code></pre>
|
||
<p>Rust has provided a number of traits for us to use with the <code>derive</code> annotation
|
||
that can add useful behavior to our custom types. Those traits and their
|
||
behaviors are listed in Appendix C. We’ll cover how to implement these traits
|
||
with custom behavior as well as how to create your own traits in Chapter 10.</p>
|
||
<p>Our <code>area</code> function is very specific: it only computes the area of rectangles.
|
||
It would be helpful to tie this behavior more closely to our <code>Rectangle</code>
|
||
struct, because it won’t work with any other type. Let’s look at how we can
|
||
continue to refactor this code by turning the <code>area</code> function into an <code>area</code>
|
||
<em>method</em> defined on our <code>Rectangle</code> type.</p>
|
||
|
||
</main>
|
||
|
||
<nav class="nav-wrapper" aria-label="Page navigation">
|
||
<!-- Mobile navigation buttons -->
|
||
|
||
<a rel="prev" href="ch05-01-defining-structs.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="ch05-03-method-syntax.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="ch05-01-defining-structs.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="ch05-03-method-syntax.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>
|