537 lines
43 KiB
HTML
537 lines
43 KiB
HTML
<!DOCTYPE HTML>
|
||
<html lang="en" class="sidebar-visible no-js light">
|
||
<head>
|
||
<!-- Book generated using mdBook -->
|
||
<meta charset="UTF-8">
|
||
<title>Paths for Referring to an Item in the Module Tree - 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" class="active"><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="#paths-for-referring-to-an-item-in-the-module-tree" id="paths-for-referring-to-an-item-in-the-module-tree">Paths for Referring to an Item in the Module Tree</a></h2>
|
||
<p>To show Rust where to find an item in a module tree, we use a path in the same
|
||
way we use a path when navigating a filesystem. If we want to call a function,
|
||
we need to know its path.</p>
|
||
<p>A path can take two forms:</p>
|
||
<ul>
|
||
<li>An <em>absolute path</em> starts from a crate root by using a crate name or a
|
||
literal <code>crate</code>.</li>
|
||
<li>A <em>relative path</em> starts from the current module and uses <code>self</code>, <code>super</code>, or
|
||
an identifier in the current module.</li>
|
||
</ul>
|
||
<p>Both absolute and relative paths are followed by one or more identifiers
|
||
separated by double colons (<code>::</code>).</p>
|
||
<p>Let’s return to the example in Listing 7-1. How do we call the
|
||
<code>add_to_waitlist</code> function? This is the same as asking, what’s the path of the
|
||
<code>add_to_waitlist</code> function? In Listing 7-3, we simplified our code a bit by
|
||
removing some of the modules and functions. We’ll show two ways to call the
|
||
<code>add_to_waitlist</code> function from a new function <code>eat_at_restaurant</code> defined in
|
||
the crate root. The <code>eat_at_restaurant</code> function is part of our library crate’s
|
||
public API, so we mark it with the <code>pub</code> keyword. In the <a href="ch07-03-paths-for-referring-to-an-item-in-the-module-tree.html#exposing-paths-with-the-pub-keyword">”Exposing Paths with
|
||
the <code>pub</code> Keyword”</a><!-- ignore --> section, we’ll go into more detail
|
||
about <code>pub</code>. Note that this example won’t compile just yet; we’ll explain why
|
||
in a bit.</p>
|
||
<p><span class="filename">Filename: src/lib.rs</span></p>
|
||
<pre><code class="language-rust ignore does_not_compile">mod front_of_house {
|
||
mod hosting {
|
||
fn add_to_waitlist() {}
|
||
}
|
||
}
|
||
|
||
pub fn eat_at_restaurant() {
|
||
// Absolute path
|
||
crate::front_of_house::hosting::add_to_waitlist();
|
||
|
||
// Relative path
|
||
front_of_house::hosting::add_to_waitlist();
|
||
}
|
||
</code></pre>
|
||
<p><span class="caption">Listing 7-3: Calling the <code>add_to_waitlist</code> function using
|
||
absolute and relative paths</span></p>
|
||
<p>The first time we call the <code>add_to_waitlist</code> function in <code>eat_at_restaurant</code>,
|
||
we use an absolute path. The <code>add_to_waitlist</code> function is defined in the same
|
||
crate as <code>eat_at_restaurant</code>, which means we can use the <code>crate</code> keyword to
|
||
start an absolute path.</p>
|
||
<p>After <code>crate</code>, we include each of the successive modules until we make our way
|
||
to <code>add_to_waitlist</code>. You can imagine a filesystem with the same structure, and
|
||
we’d specify the path <code>/front_of_house/hosting/add_to_waitlist</code> to run the
|
||
<code>add_to_waitlist</code> program; using the <code>crate</code> name to start from the crate root
|
||
is like using <code>/</code> to start from the filesystem root in your shell.</p>
|
||
<p>The second time we call <code>add_to_waitlist</code> in <code>eat_at_restaurant</code>, we use a
|
||
relative path. The path starts with <code>front_of_house</code>, the name of the module
|
||
defined at the same level of the module tree as <code>eat_at_restaurant</code>. Here the
|
||
filesystem equivalent would be using the path
|
||
<code>front_of_house/hosting/add_to_waitlist</code>. Starting with a name means that the
|
||
path is relative.</p>
|
||
<p>Choosing whether to use a relative or absolute path is a decision you’ll make
|
||
based on your project. The decision should depend on whether you’re more likely
|
||
to move item definition code separately from or together with the code that
|
||
uses the item. For example, if we move the <code>front_of_house</code> module and the
|
||
<code>eat_at_restaurant</code> function into a module named <code>customer_experience</code>, we’d
|
||
need to update the absolute path to <code>add_to_waitlist</code>, but the relative path
|
||
would still be valid. However, if we moved the <code>eat_at_restaurant</code> function
|
||
separately into a module named <code>dining</code>, the absolute path to the
|
||
<code>add_to_waitlist</code> call would stay the same, but the relative path would need to
|
||
be updated. Our preference is to specify absolute paths because it’s more
|
||
likely to move code definitions and item calls independently of each other.</p>
|
||
<p>Let’s try to compile Listing 7-3 and find out why it won’t compile yet! The
|
||
error we get is shown in Listing 7-4.</p>
|
||
<pre><code class="language-text">$ cargo build
|
||
Compiling restaurant v0.1.0 (file:///projects/restaurant)
|
||
error[E0603]: module `hosting` is private
|
||
--> src/lib.rs:9:28
|
||
|
|
||
9 | crate::front_of_house::hosting::add_to_waitlist();
|
||
| ^^^^^^^
|
||
|
||
error[E0603]: module `hosting` is private
|
||
--> src/lib.rs:12:21
|
||
|
|
||
12 | front_of_house::hosting::add_to_waitlist();
|
||
| ^^^^^^^
|
||
</code></pre>
|
||
<p><span class="caption">Listing 7-4: Compiler errors from building the code in
|
||
Listing 7-3</span></p>
|
||
<p>The error messages say that module <code>hosting</code> is private. In other words, we
|
||
have the correct paths for the <code>hosting</code> module and the <code>add_to_waitlist</code>
|
||
function, but Rust won’t let us use them because it doesn’t have access to the
|
||
private sections.</p>
|
||
<p>Modules aren’t useful only for organizing your code. They also define Rust’s
|
||
<em>privacy boundary</em>: the line that encapsulates the implementation details
|
||
external code isn’t allowed to know about, call, or rely on. So, if you want to
|
||
make an item like a function or struct private, you put it in a module.</p>
|
||
<p>The way privacy works in Rust is that all items (functions, methods, structs,
|
||
enums, modules, and constants) are private by default. Items in a parent module
|
||
can’t use the private items inside child modules, but items in child modules
|
||
can use the items in their ancestor modules. The reason is that child modules
|
||
wrap and hide their implementation details, but the child modules can see the
|
||
context in which they’re defined. To continue with the restaurant metaphor,
|
||
think of the privacy rules as being like the back office of a restaurant: what
|
||
goes on in there is private to restaurant customers, but office managers can
|
||
see and do everything in the restaurant in which they operate.</p>
|
||
<p>Rust chose to have the module system function this way so that hiding inner
|
||
implementation details is the default. That way, you know which parts of the
|
||
inner code you can change without breaking outer code. But you can expose inner
|
||
parts of child modules code to outer ancestor modules by using the <code>pub</code>
|
||
keyword to make an item public.</p>
|
||
<h3><a class="header" href="#exposing-paths-with-the-pub-keyword" id="exposing-paths-with-the-pub-keyword">Exposing Paths with the <code>pub</code> Keyword</a></h3>
|
||
<p>Let’s return to the error in Listing 7-4 that told us the <code>hosting</code> module is
|
||
private. We want the <code>eat_at_restaurant</code> function in the parent module to have
|
||
access to the <code>add_to_waitlist</code> function in the child module, so we mark the
|
||
<code>hosting</code> module with the <code>pub</code> keyword, as shown in Listing 7-5.</p>
|
||
<p><span class="filename">Filename: src/lib.rs</span></p>
|
||
<pre><code class="language-rust ignore does_not_compile">mod front_of_house {
|
||
pub mod hosting {
|
||
fn add_to_waitlist() {}
|
||
}
|
||
}
|
||
|
||
pub fn eat_at_restaurant() {
|
||
// Absolute path
|
||
crate::front_of_house::hosting::add_to_waitlist();
|
||
|
||
// Relative path
|
||
front_of_house::hosting::add_to_waitlist();
|
||
}
|
||
</code></pre>
|
||
<p><span class="caption">Listing 7-5: Declaring the <code>hosting</code> module as <code>pub</code> to
|
||
use it from <code>eat_at_restaurant</code></span></p>
|
||
<p>Unfortunately, the code in Listing 7-5 still results in an error, as shown in
|
||
Listing 7-6.</p>
|
||
<pre><code class="language-text">$ cargo build
|
||
Compiling restaurant v0.1.0 (file:///projects/restaurant)
|
||
error[E0603]: function `add_to_waitlist` is private
|
||
--> src/lib.rs:9:37
|
||
|
|
||
9 | crate::front_of_house::hosting::add_to_waitlist();
|
||
| ^^^^^^^^^^^^^^^
|
||
|
||
error[E0603]: function `add_to_waitlist` is private
|
||
--> src/lib.rs:12:30
|
||
|
|
||
12 | front_of_house::hosting::add_to_waitlist();
|
||
| ^^^^^^^^^^^^^^^
|
||
</code></pre>
|
||
<p><span class="caption">Listing 7-6: Compiler errors from building the code in
|
||
Listing 7-5</span></p>
|
||
<p>What happened? Adding the <code>pub</code> keyword in front of <code>mod hosting</code> makes the
|
||
module public. With this change, if we can access <code>front_of_house</code>, we can
|
||
access <code>hosting</code>. But the <em>contents</em> of <code>hosting</code> are still private; making the
|
||
module public doesn’t make its contents public. The <code>pub</code> keyword on a module
|
||
only lets code in its ancestor modules refer to it.</p>
|
||
<p>The errors in Listing 7-6 say that the <code>add_to_waitlist</code> function is private.
|
||
The privacy rules apply to structs, enums, functions, and methods as well as
|
||
modules.</p>
|
||
<p>Let’s also make the <code>add_to_waitlist</code> function public by adding the <code>pub</code>
|
||
keyword before its definition, as in Listing 7-7.</p>
|
||
<p><span class="filename">Filename: src/lib.rs</span></p>
|
||
<pre><pre class="playpen"><code class="language-rust">mod front_of_house {
|
||
pub mod hosting {
|
||
pub fn add_to_waitlist() {}
|
||
}
|
||
}
|
||
|
||
pub fn eat_at_restaurant() {
|
||
// Absolute path
|
||
crate::front_of_house::hosting::add_to_waitlist();
|
||
|
||
// Relative path
|
||
front_of_house::hosting::add_to_waitlist();
|
||
}
|
||
<span class="boring">fn main() {}
|
||
</span></code></pre></pre>
|
||
<p><span class="caption">Listing 7-7: Adding the <code>pub</code> keyword to <code>mod hosting</code>
|
||
and <code>fn add_to_waitlist</code> lets us call the function from
|
||
<code>eat_at_restaurant</code></span></p>
|
||
<p>Now the code will compile! Let’s look at the absolute and the relative path and
|
||
double-check why adding the <code>pub</code> keyword lets us use these paths in
|
||
<code>add_to_waitlist</code> with respect to the privacy rules.</p>
|
||
<p>In the absolute path, we start with <code>crate</code>, the root of our crate’s module
|
||
tree. Then the <code>front_of_house</code> module is defined in the crate root. The
|
||
<code>front_of_house</code> module isn’t public, but because the <code>eat_at_restaurant</code>
|
||
function is defined in the same module as <code>front_of_house</code> (that is,
|
||
<code>eat_at_restaurant</code> and <code>front_of_house</code> are siblings), we can refer to
|
||
<code>front_of_house</code> from <code>eat_at_restaurant</code>. Next is the <code>hosting</code> module marked
|
||
with <code>pub</code>. We can access the parent module of <code>hosting</code>, so we can access
|
||
<code>hosting</code>. Finally, the <code>add_to_waitlist</code> function is marked with <code>pub</code> and we
|
||
can access its parent module, so this function call works!</p>
|
||
<p>In the relative path, the logic is the same as the absolute path except for the
|
||
first step: rather than starting from the crate root, the path starts from
|
||
<code>front_of_house</code>. The <code>front_of_house</code> module is defined within the same module
|
||
as <code>eat_at_restaurant</code>, so the relative path starting from the module in which
|
||
<code>eat_at_restaurant</code> is defined works. Then, because <code>hosting</code> and
|
||
<code>add_to_waitlist</code> are marked with <code>pub</code>, the rest of the path works, and this
|
||
function call is valid!</p>
|
||
<h3><a class="header" href="#starting-relative-paths-with-super" id="starting-relative-paths-with-super">Starting Relative Paths with <code>super</code></a></h3>
|
||
<p>We can also construct relative paths that begin in the parent module by using
|
||
<code>super</code> at the start of the path. This is like starting a filesystem path with
|
||
the <code>..</code> syntax. Why would we want to do this?</p>
|
||
<p>Consider the code in Listing 7-8 that models the situation in which a chef
|
||
fixes an incorrect order and personally brings it out to the customer. The
|
||
function <code>fix_incorrect_order</code> calls the function <code>serve_order</code> by specifying
|
||
the path to <code>serve_order</code> starting with <code>super</code>:</p>
|
||
<p><span class="filename">Filename: src/lib.rs</span></p>
|
||
<pre><pre class="playpen"><code class="language-rust">fn serve_order() {}
|
||
|
||
mod back_of_house {
|
||
fn fix_incorrect_order() {
|
||
cook_order();
|
||
super::serve_order();
|
||
}
|
||
|
||
fn cook_order() {}
|
||
}
|
||
<span class="boring">fn main() {}
|
||
</span></code></pre></pre>
|
||
<p><span class="caption">Listing 7-8: Calling a function using a relative path
|
||
starting with <code>super</code></span></p>
|
||
<p>The <code>fix_incorrect_order</code> function is in the <code>back_of_house</code> module, so we can
|
||
use <code>super</code> to go to the parent module of <code>back_of_house</code>, which in this case
|
||
is <code>crate</code>, the root. From there, we look for <code>serve_order</code> and find it.
|
||
Success! We think the <code>back_of_house</code> module and the <code>serve_order</code> function are
|
||
likely to stay in the same relationship to each other and get moved together
|
||
should we decide to reorganize the crate’s module tree. Therefore, we used
|
||
<code>super</code> so we’ll have fewer places to update code in the future if this code
|
||
gets moved to a different module.</p>
|
||
<h3><a class="header" href="#making-structs-and-enums-public" id="making-structs-and-enums-public">Making Structs and Enums Public</a></h3>
|
||
<p>We can also use <code>pub</code> to designate structs and enums as public, but there are a
|
||
few extra details. If we use <code>pub</code> before a struct definition, we make the
|
||
struct public, but the struct’s fields will still be private. We can make each
|
||
field public or not on a case-by-case basis. In Listing 7-9, we’ve defined a
|
||
public <code>back_of_house::Breakfast</code> struct with a public <code>toast</code> field but a
|
||
private <code>seasonal_fruit</code> field. This models the case in a restaurant where the
|
||
customer can pick the type of bread that comes with a meal, but the chef
|
||
decides which fruit accompanies the meal based on what’s in season and in
|
||
stock. The available fruit changes quickly, so customers can’t choose the fruit
|
||
or even see which fruit they’ll get.</p>
|
||
<p><span class="filename">Filename: src/lib.rs</span></p>
|
||
<pre><pre class="playpen"><code class="language-rust">
|
||
<span class="boring">#![allow(unused_variables)]
|
||
</span><span class="boring">fn main() {
|
||
</span>mod back_of_house {
|
||
pub struct Breakfast {
|
||
pub toast: String,
|
||
seasonal_fruit: String,
|
||
}
|
||
|
||
impl Breakfast {
|
||
pub fn summer(toast: &str) -> Breakfast {
|
||
Breakfast {
|
||
toast: String::from(toast),
|
||
seasonal_fruit: String::from("peaches"),
|
||
}
|
||
}
|
||
}
|
||
}
|
||
|
||
pub fn eat_at_restaurant() {
|
||
// Order a breakfast in the summer with Rye toast
|
||
let mut meal = back_of_house::Breakfast::summer("Rye");
|
||
// Change our mind about what bread we'd like
|
||
meal.toast = String::from("Wheat");
|
||
println!("I'd like {} toast please", meal.toast);
|
||
|
||
// The next line won't compile if we uncomment it; we're not allowed
|
||
// to see or modify the seasonal fruit that comes with the meal
|
||
// meal.seasonal_fruit = String::from("blueberries");
|
||
}
|
||
<span class="boring">}
|
||
</span></code></pre></pre>
|
||
<p><span class="caption">Listing 7-9: A struct with some public fields and some
|
||
private fields</span></p>
|
||
<p>Because the <code>toast</code> field in the <code>back_of_house::Breakfast</code> struct is public,
|
||
in <code>eat_at_restaurant</code> we can write and read to the <code>toast</code> field using dot
|
||
notation. Notice that we can’t use the <code>seasonal_fruit</code> field in
|
||
<code>eat_at_restaurant</code> because <code>seasonal_fruit</code> is private. Try uncommenting the
|
||
line modifying the <code>seasonal_fruit</code> field value to see what error you get!</p>
|
||
<p>Also, note that because <code>back_of_house::Breakfast</code> has a private field, the
|
||
struct needs to provide a public associated function that constructs an
|
||
instance of <code>Breakfast</code> (we’ve named it <code>summer</code> here). If <code>Breakfast</code> didn’t
|
||
have such a function, we couldn’t create an instance of <code>Breakfast</code> in
|
||
<code>eat_at_restaurant</code> because we couldn’t set the value of the private
|
||
<code>seasonal_fruit</code> field in <code>eat_at_restaurant</code>.</p>
|
||
<p>In contrast, if we make an enum public, all of its variants are then public. We
|
||
only need the <code>pub</code> before the <code>enum</code> keyword, as shown in Listing 7-10.</p>
|
||
<p><span class="filename">Filename: src/lib.rs</span></p>
|
||
<pre><pre class="playpen"><code class="language-rust">
|
||
<span class="boring">#![allow(unused_variables)]
|
||
</span><span class="boring">fn main() {
|
||
</span>mod back_of_house {
|
||
pub enum Appetizer {
|
||
Soup,
|
||
Salad,
|
||
}
|
||
}
|
||
|
||
pub fn eat_at_restaurant() {
|
||
let order1 = back_of_house::Appetizer::Soup;
|
||
let order2 = back_of_house::Appetizer::Salad;
|
||
}
|
||
<span class="boring">}
|
||
</span></code></pre></pre>
|
||
<p><span class="caption">Listing 7-10: Designating an enum as public makes all its
|
||
variants public</span></p>
|
||
<p>Because we made the <code>Appetizer</code> enum public, we can use the <code>Soup</code> and <code>Salad</code>
|
||
variants in <code>eat_at_restaurant</code>. Enums aren’t very useful unless their variants
|
||
are public; it would be annoying to have to annotate all enum variants with
|
||
<code>pub</code> in every case, so the default for enum variants is to be public. Structs
|
||
are often useful without their fields being public, so struct fields follow the
|
||
general rule of everything being private by default unless annotated with <code>pub</code>.</p>
|
||
<p>There’s one more situation involving <code>pub</code> that we haven’t covered, and that is
|
||
our last module system feature: the <code>use</code> keyword. We’ll cover <code>use</code> by itself
|
||
first, and then we’ll show how to combine <code>pub</code> and <code>use</code>.</p>
|
||
|
||
</main>
|
||
|
||
<nav class="nav-wrapper" aria-label="Page navigation">
|
||
<!-- Mobile navigation buttons -->
|
||
|
||
<a rel="prev" href="ch07-02-defining-modules-to-control-scope-and-privacy.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="ch07-04-bringing-paths-into-scope-with-the-use-keyword.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="ch07-02-defining-modules-to-control-scope-and-privacy.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="ch07-04-bringing-paths-into-scope-with-the-use-keyword.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>
|