2020-12-25 14:16:02 +00:00
<!DOCTYPE HTML>
< html lang = "en" class = "sidebar-visible no-js light" >
< head >
<!-- Book generated using mdBook -->
< meta charset = "UTF-8" >
< title > More on references - Easy Rust< / title >
<!-- Custom HTML head -->
< 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 = "icon" href = "favicon.svg" >
< 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 rel = "stylesheet" href = "fonts/fonts.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 -->
< / 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 ? "navy" : "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 class = "sidebar-scrollbox" >
2021-05-23 04:18:41 +00:00
< ol class = "chapter" > < li class = "chapter-item expanded " > < a href = "Chapter_0.html" > < strong aria-hidden = "true" > 1.< / strong > Updates< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_1.html" > < strong aria-hidden = "true" > 2.< / strong > Introduction< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_2.html" > < strong aria-hidden = "true" > 3.< / strong > Who am I?< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_3.html" > < strong aria-hidden = "true" > 4.< / strong > Writing Rust in Easy English< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_4.html" > < strong aria-hidden = "true" > 5.< / strong > Rust Playground< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_5.html" > < strong aria-hidden = "true" > 6.< / strong > 🚧 and ⚠️< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_6.html" > < strong aria-hidden = "true" > 7.< / strong > Comments< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_7.html" > < strong aria-hidden = "true" > 8.< / strong > Types< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_8.html" > < strong aria-hidden = "true" > 9.< / strong > Type inference< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_9.html" > < strong aria-hidden = "true" > 10.< / strong > Printing 'hello, world!'< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_10.html" > < strong aria-hidden = "true" > 11.< / strong > Display and debug< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_11.html" > < strong aria-hidden = "true" > 12.< / strong > Mutability (changing)< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_12.html" > < strong aria-hidden = "true" > 13.< / strong > The stack, the heap, and pointers< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_13.html" > < strong aria-hidden = "true" > 14.< / strong > More about printing< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_14.html" > < strong aria-hidden = "true" > 15.< / strong > Strings< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_15.html" > < strong aria-hidden = "true" > 16.< / strong > const and static< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_16.html" class = "active" > < strong aria-hidden = "true" > 17.< / strong > More on references< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_17.html" > < strong aria-hidden = "true" > 18.< / strong > Mutable references< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_18.html" > < strong aria-hidden = "true" > 19.< / strong > Giving references to functions< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_19.html" > < strong aria-hidden = "true" > 20.< / strong > Copy types< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_20.html" > < strong aria-hidden = "true" > 21.< / strong > Collection types< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_21.html" > < strong aria-hidden = "true" > 22.< / strong > Vectors< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_22.html" > < strong aria-hidden = "true" > 23.< / strong > Tuples< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_23.html" > < strong aria-hidden = "true" > 24.< / strong > Control flow< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_24.html" > < strong aria-hidden = "true" > 25.< / strong > Structs< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_25.html" > < strong aria-hidden = "true" > 26.< / strong > Enums< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_26.html" > < strong aria-hidden = "true" > 27.< / strong > Loops< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_27.html" > < strong aria-hidden = "true" > 28.< / strong > Implementing structs and enums< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_28.html" > < strong aria-hidden = "true" > 29.< / strong > Destructuring< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_29.html" > < strong aria-hidden = "true" > 30.< / strong > References and the dot operator< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_30.html" > < strong aria-hidden = "true" > 31.< / strong > Generics< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_31.html" > < strong aria-hidden = "true" > 32.< / strong > Option and Result< / a > < / li > < li class = "chapter-item expanded " > < a href = "Chapter_32.html"
2020-12-25 14:16:02 +00:00
< / 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-hover-placeholder" > < / div >
< div id = "menu-bar" class = "menu-bar sticky bordered" >
< 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" > Easy Rust< / 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 >
2021-05-23 04:18:41 +00:00
2020-12-25 14:16:02 +00:00
< / div >
< / div >
< div id = "search-wrapper" class = "hidden" >
< form id = "searchbar-outer" class = "searchbar-outer" >
2021-05-23 04:18:41 +00:00
< input type = "search" id = "searchbar" name = "searchbar" placeholder = "Search this book ..." aria-controls = "searchresults-outer" aria-describedby = "searchresults-header" >
2020-12-25 14:16:02 +00:00
< / 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 >
2021-02-24 06:13:19 +00:00
< h2 id = "more-on-references" > < a class = "header" href = "#more-on-references" > More on references< / a > < / h2 >
2021-02-05 07:42:46 +00:00
< p > < strong > < a href = "https://youtu.be/R13sQ8SNoEQ" > See this chapter on YouTube< / a > < / strong > < / p >
2020-12-25 14:16:02 +00:00
< p > References are very important in Rust. Rust uses references to make sure that all memory access is safe. We know that we use < code > & < / code > to create a reference:< / p >
< pre > < pre class = "playground" > < code class = "language-rust" > fn main() {
let country = String::from(" Austria" );
let ref_one = & country;
let ref_two = & country;
println!(" {}" , ref_one);
}
< / code > < / pre > < / pre >
< p > This prints < code > Austria< / code > .< / p >
< p > In the code, < code > country< / code > is a < code > String< / code > . We then created two references to < code > country< / code > . They have the type < code > & String< / code > , which you say is a " reference to a String" . We could create three references or one hundred references to < code > country< / code > and it would be no problem.< / p >
< p > But this is a problem:< / p >
< pre > < pre class = "playground" > < code class = "language-rust" > fn return_str() -> & str {
let country = String::from(" Austria" );
let country_ref = & country;
country_ref // ⚠️
}
fn main() {
let country = return_str();
}
< / code > < / pre > < / pre >
< p > The function < code > return_str()< / code > creates a String, then it creates a reference to the String. Then it tries to return the reference. But the String < code > country< / code > only lives inside the function, and then it dies. Once a variable is gone, the computer will clean up the memory and use it for something else. So after the function is over, < code > country_ref< / code > is referring to memory that is already gone, and that's not okay. Rust prevents us from making a mistake with memory here.< / p >
< p > This is the important part about the " owned" type that we talked about above. Because you own a < code > String< / code > , you can pass it around. But a < code > & String< / code > will die if its < code > String< / code > dies, so you don't pass around " ownership" with it.< / p >
< / main >
< nav class = "nav-wrapper" aria-label = "Page navigation" >
<!-- Mobile navigation buttons -->
< a rel = "prev" href = "Chapter_15.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 = "Chapter_17.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 rel = "prev" href = "Chapter_15.html" class = "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 = "Chapter_17.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.playground_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 -->
< / body >
< / html >
