<!DOCTYPE HTML>
< html lang = "zh-CN" class = "light sidebar-visible" dir = "ltr" >
< head >
<!-- Book generated using mdBook -->
< meta charset = "UTF-8" >
< title > 结构体中的自引用 - Rust语言圣经(Rust Course)< / title >
<!-- Custom HTML head -->
< 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-de23e50b.svg" >
< link rel = "shortcut icon" href = "../../favicon-8114d1fc.png" >
< link rel = "stylesheet" href = "../../css/variables-8adf115d.css" >
< link rel = "stylesheet" href = "../../css/general-2459343d.css" >
< link rel = "stylesheet" href = "../../css/chrome-ae938929.css" >
< link rel = "stylesheet" href = "../../css/print-9e4910d8.css" media = "print" >
<!-- Fonts -->
< link rel = "stylesheet" href = "../../fonts/fonts-9644e21d.css" >
<!-- Highlight.js Stylesheets -->
< link rel = "stylesheet" id = "mdbook-highlight-css" href = "../../highlight-493f70e1.css" >
< link rel = "stylesheet" id = "mdbook-tomorrow-night-css" href = "../../tomorrow-night-4c0ae647.css" >
< link rel = "stylesheet" id = "mdbook-ayu-highlight-css" href = "../../ayu-highlight-3fdfc3ac.css" >
<!-- Custom theme stylesheets -->
< link rel = "stylesheet" href = "../../theme/style-234bffbf.css" >
<!-- Provide site root and default themes to javascript -->
< script >
const path_to_root = "../../";
const default_light_theme = "light";
const default_dark_theme = "navy";
window.path_to_searchindex_js = "../../searchindex-4a1a74fa.js";
< / script >
<!-- Start loading toc.js asap -->
< script src = "../../toc-42887675.js" > < / script >
< / head >
< body >
< div id = "mdbook-help-container" >
< div id = "mdbook-help-popup" >
< h2 class = "mdbook-help-title" > Keyboard shortcuts< / h2 >
< div >
< p > Press < kbd > ←< / kbd > or < kbd > →< / kbd > to navigate between chapters< / p >
< p > Press < kbd > S< / kbd > or < kbd > /< / kbd > to search in the book< / p >
< p > Press < kbd > ?< / kbd > to show this help< / p >
< p > Press < kbd > Esc< / kbd > to hide this help< / p >
< / div >
< / div >
< / div >
< div id = "mdbook-body-container" >
<!-- Work around some values being stored in localStorage wrapped in quotes -->
< script >
try {
let theme = localStorage.getItem('mdbook-theme');
let 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 >
const default_theme = window.matchMedia("(prefers-color-scheme: dark)").matches ? default_dark_theme : default_light_theme;
let theme;
try { theme = localStorage.getItem('mdbook-theme'); } catch(e) { }
if (theme === null || theme === undefined) { theme = default_theme; }
const html = document.documentElement;
html.classList.remove('light')
html.classList.add(theme);
html.classList.add("js");
< / script >
< input type = "checkbox" id = "mdbook-sidebar-toggle-anchor" class = "hidden" >
<!-- Hide / unhide sidebar before it is displayed -->
< script >
let sidebar = null;
const sidebar_toggle = document.getElementById("mdbook-sidebar-toggle-anchor");
if (document.body.clientWidth >= 1080) {
try { sidebar = localStorage.getItem('mdbook-sidebar'); } catch(e) { }
sidebar = sidebar || 'visible';
} else {
sidebar = 'hidden';
sidebar_toggle.checked = false;
}
if (sidebar === 'visible') {
sidebar_toggle.checked = true;
} else {
html.classList.remove('sidebar-visible');
}
< / script >
< nav id = "mdbook-sidebar" class = "sidebar" aria-label = "Table of contents" >
<!-- populated by js -->
< mdbook-sidebar-scrollbox class = "sidebar-scrollbox" > < / mdbook-sidebar-scrollbox >
< noscript >
< iframe class = "sidebar-iframe-outer" src = "../../toc.html" > < / iframe >
< / noscript >
< div id = "mdbook-sidebar-resize-handle" class = "sidebar-resize-handle" >
< div class = "sidebar-resize-indicator" > < / div >
< / div >
< / nav >
< div id = "mdbook-page-wrapper" class = "page-wrapper" >
< div class = "page" >
< div id = "mdbook-menu-bar-hover-placeholder" > < / div >
< div id = "mdbook-menu-bar" class = "menu-bar sticky" >
< div class = "left-buttons" >
< label id = "mdbook-sidebar-toggle" class = "icon-button" for = "mdbook-sidebar-toggle-anchor" title = "Toggle Table of Contents" aria-label = "Toggle Table of Contents" aria-controls = "mdbook-sidebar" >
< span class = fa-svg > < svg xmlns = "http://www.w3.org/2000/svg" viewBox = "0 0 448 512" > <!-- ! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --> < path d = "M0 96C0 78.3 14.3 64 32 64H416c17.7 0 32 14.3 32 32s-14.3 32-32 32H32C14.3 128 0 113.7 0 96zM0 256c0-17.7 14.3-32 32-32H416c17.7 0 32 14.3 32 32s-14.3 32-32 32H32c-17.7 0-32-14.3-32-32zM448 416c0 17.7-14.3 32-32 32H32c-17.7 0-32-14.3-32-32s14.3-32 32-32H416c17.7 0 32 14.3 32 32z" / > < / svg > < / span >
< / label >
< button id = "mdbook-theme-toggle" class = "icon-button" type = "button" title = "Change theme" aria-label = "Change theme" aria-haspopup = "true" aria-expanded = "false" aria-controls = "mdbook-theme-list" >
< span class = fa-svg > < svg xmlns = "http://www.w3.org/2000/svg" viewBox = "0 0 576 512" > <!-- ! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --> < path d = "M371.3 367.1c27.3-3.9 51.9-19.4 67.2-42.9L600.2 74.1c12.6-19.5 9.4-45.3-7.6-61.2S549.7-4.4 531.1 9.6L294.4 187.2c-24 18-38.2 46.1-38.4 76.1L371.3 367.1zm-19.6 25.4l-116-104.4C175.9 290.3 128 339.6 128 400c0 3.9 .2 7.8 .6 11.6c1.8 17.5-10.2 36.4-27.8 36.4H96c-17.7 0-32 14.3-32 32s14.3 32 32 32H240c61.9 0 112-50.1 112-112c0-2.5-.1-5-.2-7.5z" / > < / svg > < / span >
< / button >
< ul id = "mdbook-theme-list" class = "theme-popup" aria-label = "Themes" role = "menu" >
< li role = "none" > < button role = "menuitem" class = "theme" id = "mdbook-theme-default_theme" > Auto< / button > < / li >
< li role = "none" > < button role = "menuitem" class = "theme" id = "mdbook-theme-light" > Light< / button > < / li >
< li role = "none" > < button role = "menuitem" class = "theme" id = "mdbook-theme-rust" > Rust< / button > < / li >
< li role = "none" > < button role = "menuitem" class = "theme" id = "mdbook-theme-coal" > Coal< / button > < / li >
< li role = "none" > < button role = "menuitem" class = "theme" id = "mdbook-theme-navy" > Navy< / button > < / li >
< li role = "none" > < button role = "menuitem" class = "theme" id = "mdbook-theme-ayu" > Ayu< / button > < / li >
< / ul >
< button id = "mdbook-search-toggle" class = "icon-button" type = "button" title = "Search (`/`)" aria-label = "Toggle Searchbar" aria-expanded = "false" aria-keyshortcuts = "/ s" aria-controls = "mdbook-searchbar" >
< span class = fa-svg > < svg xmlns = "http://www.w3.org/2000/svg" viewBox = "0 0 512 512" > <!-- ! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --> < path d = "M416 208c0 45.9-14.9 88.3-40 122.7L502.6 457.4c12.5 12.5 12.5 32.8 0 45.3s-32.8 12.5-45.3 0L330.7 376c-34.4 25.2-76.8 40-122.7 40C93.1 416 0 322.9 0 208S93.1 0 208 0S416 93.1 416 208zM208 352c79.5 0 144-64.5 144-144s-64.5-144-144-144S64 128.5 64 208s64.5 144 144 144z" / > < / svg > < / span >
< / button >
< / div >
< h1 class = "menu-title" > Rust语言圣经(Rust Course)< / h1 >
< div class = "right-buttons" >
< a href = "../../print.html" title = "Print this book" aria-label = "Print this book" >
< span class = fa-svg id = "print-button" > < svg xmlns = "http://www.w3.org/2000/svg" viewBox = "0 0 512 512" > <!-- ! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --> < path d = "M128 0C92.7 0 64 28.7 64 64v96h64V64H354.7L384 93.3V160h64V93.3c0-17-6.7-33.3-18.7-45.3L400 18.7C388 6.7 371.7 0 354.7 0H128zM384 352v32 64H128V384 368 352H384zm64 32h32c17.7 0 32-14.3 32-32V256c0-35.3-28.7-64-64-64H64c-35.3 0-64 28.7-64 64v96c0 17.7 14.3 32 32 32H64v64c0 35.3 28.7 64 64 64H384c35.3 0 64-28.7 64-64V384zm-16-88c-13.3 0-24-10.7-24-24s10.7-24 24-24s24 10.7 24 24s-10.7 24-24 24z" / > < / svg > < / span >
< / a >
< a href = "https://github.com/sunface/rust-course" title = "Git repository" aria-label = "Git repository" >
< span class = fa-svg > < svg xmlns = "http://www.w3.org/2000/svg" viewBox = "0 0 496 512" > <!-- ! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --> < path d = "M165.9 397.4c0 2-2.3 3.6-5.2 3.6-3.3.3-5.6-1.3-5.6-3.6 0-2 2.3-3.6 5.2-3.6 3-.3 5.6 1.3 5.6 3.6zm-31.1-4.5c-.7 2 1.3 4.3 4.3 4.9 2.6 1 5.6 0 6.2-2s-1.3-4.3-4.3-5.2c-2.6-.7-5.5.3-6.2 2.3zm44.2-1.7c-2.9.7-4.9 2.6-4.6 4.9.3 2 2.9 3.3 5.9 2.6 2.9-.7 4.9-2.6 4.6-4.6-.3-1.9-3-3.2-5.9-2.9zM244.8 8C106.1 8 0 113.3 0 252c0 110.9 69.8 205.8 169.5 239.2 12.8 2.3 17.3-5.6 17.3-12.1 0-6.2-.3-40.4-.3-61.4 0 0-70 15-84.7-29.8 0 0-11.4-29.1-27.8-36.6 0 0-22.9-15.7 1.6-15.4 0 0 24.9 2 38.6 25.8 21.9 38.6 58.6 27.5 72.9 20.9 2.3-16 8.8-27.1 16-33.7-55.9-6.2-112.3-14.3-112.3-110.5 0-27.5 7.6-41.3 23.6-58.9-2.6-6.5-11.1-33.3 2.6-67.9 20.9-6.5 69 27 69 27 20-5.6 41.5-8.5 62.8-8.5s42.8 2.9 62.8 8.5c0 0 48.1-33.6 69-27 13.7 34.7 5.2 61.4 2.6 67.9 16 17.7 25.8 31.5 25.8 58.9 0 96.5-58.9 104.2-114.8 110.5 9.2 7.9 17 22.9 17 46.4 0 33.7-.3 75.4-.3 83.6 0 6.5 4.6 14.4 17.3 12.1C428.2 457.8 496 362.9 496 252 496 113.3 383.5 8 244.8 8zM97.2 352.9c-1.3 1-1 3.3.7 5.2 1.6 1.6 3.9 2.3 5.2 1 1.3-1 1-3.3-.7-5.2-1.6-1.6-3.9-2.3-5.2-1zm-10.8-8.1c-.7 1.3.3 2.9 2.3 3.9 1.6 1 3.6.7 4.3-.7.7-1.3-.3-2.9-2.3-3.9-2-.6-3.6-.3-4.3.7zm32.4 35.6c-1.6 1.3-1 4.3 1.3 6.2 2.3 2.3 5.2 2.6 6.5 1 1.3-1.3.7-4.3-1.3-6.2-2.2-2.3-5.2-2.6-6.5-1zm-11.4-14.7c-1.6 1-1.6 3.6 0 5.9 1.6 2.3 4.3 3.3 5.6 2.3 1.6-1.3 1.6-3.9 0-6.2-1.4-2.3-4-3.3-5.6-2z" / > < / svg > < / span >
< / a >
< a href = "https://github.com/sunface/rust-course/edit/main/src/advance/circle-self-ref/self-referential.md" title = "Suggest an edit" aria-label = "Suggest an edit" rel = "edit" >
< span class = fa-svg id = "git-edit-button" > < svg xmlns = "http://www.w3.org/2000/svg" viewBox = "0 0 512 512" > <!-- ! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --> < path d = "M421.7 220.3l-11.3 11.3-22.6 22.6-205 205c-6.6 6.6-14.8 11.5-23.8 14.1L30.8 511c-8.4 2.5-17.5 .2-23.7-6.1S-1.5 489.7 1 481.2L38.7 353.1c2.6-9 7.5-17.2 14.1-23.8l205-205 22.6-22.6 11.3-11.3 33.9 33.9 62.1 62.1 33.9 33.9zM96 353.9l-9.3 9.3c-.9 .9-1.6 2.1-2 3.4l-25.3 86 86-25.3c1.3-.4 2.5-1.1 3.4-2l9.3-9.3H112c-8.8 0-16-7.2-16-16V353.9zM453.3 19.3l39.4 39.4c25 25 25 65.5 0 90.5l-14.5 14.5-22.6 22.6-11.3 11.3-33.9-33.9-62.1-62.1L314.3 67.7l11.3-11.3 22.6-22.6 14.5-14.5c25-25 65.5-25 90.5 0z" / > < / svg > < / span >
< / a >
< / div >
< / div >
< div id = "mdbook-search-wrapper" class = "hidden" >
< form id = "mdbook-searchbar-outer" class = "searchbar-outer" >
< div class = "search-wrapper" >
< input type = "search" id = "mdbook-searchbar" name = "searchbar" placeholder = "Search this book ..." aria-controls = "mdbook-searchresults-outer" aria-describedby = "searchresults-header" >
< div class = "spinner-wrapper" >
< span class = fa-svg id = "fa-spin" > < svg xmlns = "http://www.w3.org/2000/svg" viewBox = "0 0 512 512" > <!-- ! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --> < path d = "M304 48c0-26.5-21.5-48-48-48s-48 21.5-48 48s21.5 48 48 48s48-21.5 48-48zm0 416c0-26.5-21.5-48-48-48s-48 21.5-48 48s21.5 48 48 48s48-21.5 48-48zM48 304c26.5 0 48-21.5 48-48s-21.5-48-48-48s-48 21.5-48 48s21.5 48 48 48zm464-48c0-26.5-21.5-48-48-48s-48 21.5-48 48s21.5 48 48 48s48-21.5 48-48zM142.9 437c18.7-18.7 18.7-49.1 0-67.9s-49.1-18.7-67.9 0s-18.7 49.1 0 67.9s49.1 18.7 67.9 0zm0-294.2c18.7-18.7 18.7-49.1 0-67.9S93.7 56.2 75 75s-18.7 49.1 0 67.9s49.1 18.7 67.9 0zM369.1 437c18.7 18.7 49.1 18.7 67.9 0s18.7-49.1 0-67.9s-49.1-18.7-67.9 0s-18.7 49.1 0 67.9z" / > < / svg > < / span >
< / div >
< / div >
< / form >
< div id = "mdbook-searchresults-outer" class = "searchresults-outer hidden" >
< div id = "mdbook-searchresults-header" class = "searchresults-header" > < / div >
< ul id = "mdbook-searchresults" >
< / ul >
< / div >
< / div >
<!-- Apply ARIA attributes after the sidebar and the sidebar toggle button are added to the DOM -->
< script >
document.getElementById('mdbook-sidebar-toggle').setAttribute('aria-expanded', sidebar === 'visible');
document.getElementById('mdbook-sidebar').setAttribute('aria-hidden', sidebar !== 'visible');
Array.from(document.querySelectorAll('#mdbook-sidebar a')).forEach(function(link) {
link.setAttribute('tabIndex', sidebar === 'visible' ? 0 : -1);
});
< / script >
< div id = "mdbook-content" class = "content" >
< main >
< h2 id = "结构体自引用" > < a class = "header" href = "#结构体自引用" > 结构体自引用< / a > < / h2 >
< p > 结构体自引用在 Rust 中是一个众所周知的难题，而且众说纷纭，也没有一篇文章能把相关的话题讲透，那本文就王婆卖瓜，来试试看能不能讲透这一块儿内容，让读者大大们舒心。< / p >
< h2 id = "平平无奇的自引用" > < a class = "header" href = "#平平无奇的自引用" > 平平无奇的自引用< / a > < / h2 >
< p > 可能也有不少人第一次听说自引用结构体，那咱们先来看看它们长啥样。< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > struct SelfRef< 'a> {
value: String,
// 该引用指向上面的value
pointer_to_value: & 'a str,
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 以上就是一个很简单的自引用结构体，看上去好像没什么，那来试着运行下：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > fn main(){
let s = "aaa".to_string();
let v = SelfRef {
value: s,
pointer_to_value: & s
};
}< / code > < / pre >
< p > 编译时报错：< / p >
< pre > < code class = "language-console" > let v = SelfRef {
12 | value: s,
| - value moved here
13 | pointer_to_value: & s
| ^^ value borrowed here after move
< / code > < / pre >
< p > 因为我们试图同时使用值和值的引用，最终所有权转移和借用一起发生了。所以，这个问题貌似并没有那么好解决，不信你可以回想下自己具有的知识，是否可以解决？< / p >
< h2 id = "使用-option" > < a class = "header" href = "#使用-option" > 使用 Option< / a > < / h2 >
< p > 最简单的方式就是使用 < code > Option< / code > 分两步来实现：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > #[derive(Debug)]
struct WhatAboutThis< 'a> {
name: String,
nickname: Option< & 'a str> ,
}
fn main() {
let mut tricky = WhatAboutThis {
name: "Annabelle".to_string(),
nickname: None,
};
tricky.nickname = Some(& tricky.name[..4]);
println!("{:?}", tricky);
}< / code > < / pre >
< p > 在某种程度上来说，< code > Option< / code > 这个方法可以工作，但是这个方法的限制较多，例如从一个函数创建并返回它是不可能的：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > fn creator< 'a> () -> WhatAboutThis< 'a> {
let mut tricky = WhatAboutThis {
name: "Annabelle".to_string(),
nickname: None,
};
tricky.nickname = Some(& tricky.name[..4]);
tricky
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 报错如下：< / p >
< pre > < code class = "language-console" > error[E0515]: cannot return value referencing local data `tricky.name`
--> src/main.rs:24:5
|
22 | tricky.nickname = Some(& tricky.name[..4]);
| ----------- `tricky.name` is borrowed here
23 |
24 | tricky
| ^^^^^^ returns a value referencing data owned by the current function
< / code > < / pre >
< p > 其实从函数签名就能看出来端倪，< code > 'a< / code > 生命周期是凭空产生的！< / p >
< p > 如果是通过方法使用，你需要一个无用 < code > & 'a self< / code > 生命周期标识，一旦有了这个标识，代码将变得更加受限，你将很容易就获得借用错误，就连 NLL 规则都没用：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > #[derive(Debug)]
struct WhatAboutThis< 'a> {
name: String,
nickname: Option< & 'a str> ,
}
impl< 'a> WhatAboutThis< 'a> {
fn tie_the_knot(& 'a mut self) {
self.nickname = Some(& self.name[..4]);
}
}
fn main() {
let mut tricky = WhatAboutThis {
name: "Annabelle".to_string(),
nickname: None,
};
tricky.tie_the_knot();
// cannot borrow `tricky` as immutable because it is also borrowed as mutable
// println!("{:?}", tricky);
}< / code > < / pre >
< h2 id = "unsafe-实现" > < a class = "header" href = "#unsafe-实现" > unsafe 实现< / a > < / h2 >
< p > 既然借用规则妨碍了我们，那就一脚踢开：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > #[derive(Debug)]
struct SelfRef {
value: String,
pointer_to_value: *const String,
}
impl SelfRef {
fn new(txt: & str) -> Self {
SelfRef {
value: String::from(txt),
pointer_to_value: std::ptr::null(),
}
}
fn init(& mut self) {
let self_ref: *const String = & self.value;
self.pointer_to_value = self_ref;
}
fn value(& self) -> & str {
& self.value
}
fn pointer_to_value(& self) -> & String {
assert!(!self.pointer_to_value.is_null(),
"SelfRef::pointer_to_value called without SelfRef::init being called first");
unsafe { & *(self.pointer_to_value) }
}
}
fn main() {
let mut t = SelfRef::new("hello");
t.init();
// 打印值和指针地址
println!("{}, {:p}", t.value(), t.pointer_to_value());
}< / code > < / pre >
< p > 在这里，我们在 < code > pointer_to_value< / code > 中直接存储裸指针，而不是 Rust 的引用，因此不再受到 Rust 借用规则和生命周期的限制，而且实现起来非常清晰、简洁。但是缺点就是，通过指针获取值时需要使用 < code > unsafe< / code > 代码。< / p >
< p > 当然，上面的代码你还能通过裸指针来修改 < code > String< / code > ，但是需要将 < code > *const< / code > 修改为 < code > *mut< / code > ： < / p >
< pre class = "playground" > < code class = "language-rust edition2021" > #[derive(Debug)]
struct SelfRef {
value: String,
pointer_to_value: *mut String,
}
impl SelfRef {
fn new(txt: & str) -> Self {
SelfRef {
value: String::from(txt),
pointer_to_value: std::ptr::null_mut(),
}
}
fn init(& mut self) {
let self_ref: *mut String = & mut self.value;
self.pointer_to_value = self_ref;
}
fn value(& self) -> & str {
& self.value
}
fn pointer_to_value(& self) -> & String {
assert!(!self.pointer_to_value.is_null(),
"SelfRef::pointer_to_value called without SelfRef::init being called first");
unsafe { & *(self.pointer_to_value) }
}
}
fn main() {
let mut t = SelfRef::new("hello");
t.init();
println!("{}, {:p}", t.value(), t.pointer_to_value());
t.value.push_str(", world");
unsafe {
(& mut *t.pointer_to_value).push_str("!");
}
println!("{}, {:p}", t.value(), t.pointer_to_value());
}< / code > < / pre >
< p > 运行后输出：< / p >
< pre > < code class = "language-console" > hello, 0x16f3aec70
hello, world!, 0x16f3aec70
< / code > < / pre >
< p > 上面的 < code > unsafe< / code > 虽然简单好用，但是它不太安全，是否还有其他选择？还真的有，那就是 < code > Pin< / code > 。< / p >
< h2 id = "无法被移动的-pin" > < a class = "header" href = "#无法被移动的-pin" > 无法被移动的 Pin< / a > < / h2 >
< p > < code > Pin< / code > 在后续章节会深入讲解，目前你只需要知道它可以固定住一个值，防止该值在内存中被移动。< / p >
< p > 通过开头我们知道，自引用最麻烦的就是创建引用的同时，值的所有权会被转移，而通过 < code > Pin< / code > 就可以很好的防止这一点：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > use std::marker::PhantomPinned;
use std::pin::Pin;
use std::ptr::NonNull;
// 下面是一个自引用数据结构体，因为 slice 字段是一个指针，指向了 data 字段
// 我们无法使用普通引用来实现，因为违背了 Rust 的编译规则
// 因此，这里我们使用了一个裸指针，通过 NonNull 来确保它不会为 null
struct Unmovable {
data: String,
slice: NonNull< String> ,
_pin: PhantomPinned,
}
impl Unmovable {
// 为了确保函数返回时数据的所有权不会被转移，我们将它放在堆上，唯一的访问方式就是通过指针
fn new(data: String) -> Pin< Box< Self> > {
let res = Unmovable {
data,
// 只有在数据到位时，才创建指针，否则数据会在开始之前就被转移所有权
slice: NonNull::dangling(),
_pin: PhantomPinned,
};
let mut boxed = Box::pin(res);
let slice = NonNull::from(& boxed.data);
// 这里其实安全的，因为修改一个字段不会转移整个结构体的所有权
unsafe {
let mut_ref: Pin< & mut Self> = Pin::as_mut(& mut boxed);
Pin::get_unchecked_mut(mut_ref).slice = slice;
}
boxed
}
}
fn main() {
let unmoved = Unmovable::new("hello".to_string());
// 只要结构体没有被转移，那指针就应该指向正确的位置，而且我们可以随意移动指针
let mut still_unmoved = unmoved;
assert_eq!(still_unmoved.slice, NonNull::from(& still_unmoved.data));
// 因为我们的类型没有实现 `Unpin` 特征，下面这段代码将无法编译
// let mut new_unmoved = Unmovable::new("world".to_string());
// std::mem::swap(& mut *still_unmoved, & mut *new_unmoved);
}< / code > < / pre >
< p > 上面的代码也非常清晰，虽然使用了 < code > unsafe< / code > ，其实更多的是无奈之举，跟之前的 < code > unsafe< / code > 实现完全不可同日而语。< / p >
< p > 其实 < code > Pin< / code > 在这里并没有魔法，它也并不是实现自引用类型的主要原因，最关键的还是里面的裸指针的使用，而 < code > Pin< / code > 起到的作用就是确保我们的值不会被移走，否则指针就会指向一个错误的地址！< / p >
< h2 id = "使用-ouroboros" > < a class = "header" href = "#使用-ouroboros" > 使用 ouroboros< / a > < / h2 >
< p > 对于自引用结构体，三方库也有支持的，其中一个就是 < a href = "https://github.com/joshua-maros/ouroboros" > ouroboros< / a > ，当然它也有自己的限制，我们后面会提到，先来看看该如何使用：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > use ouroboros::self_referencing;
#[self_referencing]
struct SelfRef {
value: String,
#[borrows(value)]
pointer_to_value: & 'this str,
}
fn main(){
let v = SelfRefBuilder {
value: "aaa".to_string(),
pointer_to_value_builder: |value: & String| value,
}.build();
// 借用value值
let s = v.borrow_value();
// 借用指针
let p = v.borrow_pointer_to_value();
// value值和指针指向的值相等
assert_eq!(s, *p);
}< / code > < / pre >
< p > 可以看到，< code > ouroboros< / code > 使用起来并不复杂，就是需要你去按照它的方式创建结构体和引用类型：< code > SelfRef< / code > 变成 < code > SelfRefBuilder< / code > ，引用字段从 < code > pointer_to_value< / code > 变成 < code > pointer_to_value_builder< / code > ，并且连类型都变了。< / p >
< p > 在使用时，通过 < code > borrow_value< / code > 来借用 < code > value< / code > 的值，通过 < code > borrow_pointer_to_value< / code > 来借用 < code > pointer_to_value< / code > 这个指针。< / p >
< p > 看上去很美好对吧？但是你可以尝试着去修改 < code > String< / code > 字符串的值试试，< code > ouroboros< / code > 限制还是较多的，但是对于基本类型依然是支持的不错，以下例子来源于官方：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > use ouroboros::self_referencing;
#[self_referencing]
struct MyStruct {
int_data: i32,
float_data: f32,
#[borrows(int_data)]
int_reference: & 'this i32,
#[borrows(mut float_data)]
float_reference: & 'this mut f32,
}
fn main() {
let mut my_value = MyStructBuilder {
int_data: 42,
float_data: 3.14,
int_reference_builder: |int_data: & i32| int_data,
float_reference_builder: |float_data: & mut f32| float_data,
}.build();
// Prints 42
println!("{:?}", my_value.borrow_int_data());
// Prints 3.14
println!("{:?}", my_value.borrow_float_reference());
// Sets the value of float_data to 84.0
my_value.with_mut(|fields| {
**fields.float_reference = (**fields.int_reference as f32) * 2.0;
});
// We can hold on to this reference...
let int_ref = *my_value.borrow_int_reference();
println!("{:?}", *int_ref);
// As long as the struct is still alive.
drop(my_value);
// This will cause an error!
// println!("{:?}", *int_ref);
}< / code > < / pre >
< p > 总之，使用这个库前，强烈建议看一些官方的例子中支持什么样的类型和 API， ， ， ， < / p >
< p > 只能说，它确实帮助我们解决了问题，但是一个是破坏了原有的结构，另外就是并不是所有数据类型都支持：它需要目标值的内存地址不会改变，因此 < code > Vec< / code > 动态数组就不适合， ， < / p >
< p > 类似的库还有：< / p >
< ul >
< li > < a href = "https://github.com/jpernst/rental" > rental< / a > ， < / li >
< li > < a href = "https://github.com/Kimundi/owning-ref-rs" > owning-ref< / a > ，将所有者和它的引用绑定到一个封装类型< / li >
< / ul >
< p > 这三个库，各有各的特点，也各有各的缺陷，建议大家需要时，一定要仔细调研，并且写 demo 进行测试，不可大意。< / p >
< blockquote >
< p > rental 虽然不怎么维护，但是可能依然是这三个里面最强大的，而且网上的用例也比较多，容易找到参考代码< / p >
< / blockquote >
< h2 id = "rc--refcell-或-arc--mutex" > < a class = "header" href = "#rc--refcell-或-arc--mutex" > Rc + RefCell 或 Arc + Mutex< / a > < / h2 >
< p > 类似于循环引用的解决方式，自引用也可以用这种组合来解决，但是会导致代码的类型标识到处都是，大大的影响了可读性。< / p >
< h2 id = "终极大法" > < a class = "header" href = "#终极大法" > 终极大法< / a > < / h2 >
< p > 如果两个放在一起会报错，那就分开它们。对，终极大法就这么简单，当然思路上的简单不代表实现上的简单，最终结果就是导致代码复杂度的上升。< / p >
< h2 id = "学习一本书如何实现链表" > < a class = "header" href = "#学习一本书如何实现链表" > 学习一本书：如何实现链表< / a > < / h2 >
< p > 最后，推荐一本专门讲如何实现链表的书（真是富有 Rust 特色，链表都能复杂到出书了 o_o） ， < a href = "https://rust-unofficial.github.io/too-many-lists/" > Learn Rust by writing Entirely Too Many Linked Lists< / a > < / p >
< h2 id = "总结" > < a class = "header" href = "#总结" > 总结< / a > < / h2 >
< p > 上面讲了这么多方法，但是我们依然无法正确的告诉你在某个场景应该使用哪个方法，这个需要你自己的判断，因为自引用实在是过于复杂。< / p >
< p > 我们能做的就是告诉你，有这些办法可以解决自引用问题，而这些办法每个都有自己适用的范围，需要你未来去深入的挖掘和发现。< / p >
< p > 偷偷说一句，就算是我，遇到自引用一样挺头疼，好在这种情况真的不常见，往往是实现特定的算法和数据结构时才需要，应用代码中几乎用不到。< / p >
< / main >
< nav class = "nav-wrapper" aria-label = "Page navigation" >
<!-- Mobile navigation buttons -->
< a rel = "prev" href = "../../advance/circle-self-ref/circle-reference.html" class = "mobile-nav-chapters previous" title = "Previous chapter" aria-label = "Previous chapter" aria-keyshortcuts = "Left" >
< span class = fa-svg > < svg xmlns = "http://www.w3.org/2000/svg" viewBox = "0 0 320 512" > <!-- ! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --> < path d = "M41.4 233.4c-12.5 12.5-12.5 32.8 0 45.3l160 160c12.5 12.5 32.8 12.5 45.3 0s12.5-32.8 0-45.3L109.3 256 246.6 118.6c12.5-12.5 12.5-32.8 0-45.3s-32.8-12.5-45.3 0l-160 160z" / > < / svg > < / span >
< / a >
< a rel = "next prefetch" href = "../../advance/concurrency-with-threads/intro.html" class = "mobile-nav-chapters next" title = "Next chapter" aria-label = "Next chapter" aria-keyshortcuts = "Right" >
< span class = fa-svg > < svg xmlns = "http://www.w3.org/2000/svg" viewBox = "0 0 320 512" > <!-- ! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --> < path d = "M278.6 233.4c12.5 12.5 12.5 32.8 0 45.3l-160 160c-12.5 12.5-32.8 12.5-45.3 0s-12.5-32.8 0-45.3L210.7 256 73.4 118.6c-12.5-12.5-12.5-32.8 0-45.3s32.8-12.5 45.3 0l160 160z" / > < / svg > < / span >
< / a >
< div style = "clear: both" > < / div >
< / nav >
< / div >
< / div >
< nav class = "nav-wide-wrapper" aria-label = "Page navigation" >
< a rel = "prev" href = "../../advance/circle-self-ref/circle-reference.html" class = "nav-chapters previous" title = "Previous chapter" aria-label = "Previous chapter" aria-keyshortcuts = "Left" >
< span class = fa-svg > < svg xmlns = "http://www.w3.org/2000/svg" viewBox = "0 0 320 512" > <!-- ! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --> < path d = "M41.4 233.4c-12.5 12.5-12.5 32.8 0 45.3l160 160c12.5 12.5 32.8 12.5 45.3 0s12.5-32.8 0-45.3L109.3 256 246.6 118.6c12.5-12.5 12.5-32.8 0-45.3s-32.8-12.5-45.3 0l-160 160z" / > < / svg > < / span >
< / a >
< a rel = "next prefetch" href = "../../advance/concurrency-with-threads/intro.html" class = "nav-chapters next" title = "Next chapter" aria-label = "Next chapter" aria-keyshortcuts = "Right" >
< span class = fa-svg > < svg xmlns = "http://www.w3.org/2000/svg" viewBox = "0 0 320 512" > <!-- ! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --> < path d = "M278.6 233.4c12.5 12.5 12.5 32.8 0 45.3l-160 160c-12.5 12.5-32.8 12.5-45.3 0s-12.5-32.8 0-45.3L210.7 256 73.4 118.6c-12.5-12.5-12.5-32.8 0-45.3s32.8-12.5 45.3 0l160 160z" / > < / svg > < / span >
< / a >
< / nav >
< / div >
< template id = fa-eye > < span class = fa-svg > < svg xmlns = "http://www.w3.org/2000/svg" viewBox = "0 0 576 512" > <!-- ! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --> < path d = "M288 32c-80.8 0-145.5 36.8-192.6 80.6C48.6 156 17.3 208 2.5 243.7c-3.3 7.9-3.3 16.7 0 24.6C17.3 304 48.6 356 95.4 399.4C142.5 443.2 207.2 480 288 480s145.5-36.8 192.6-80.6c46.8-43.5 78.1-95.4 93-131.1c3.3-7.9 3.3-16.7 0-24.6c-14.9-35.7-46.2-87.7-93-131.1C433.5 68.8 368.8 32 288 32zM432 256c0 79.5-64.5 144-144 144s-144-64.5-144-144s64.5-144 144-144s144 64.5 144 144zM288 192c0 35.3-28.7 64-64 64c-11.5 0-22.3-3-31.6-8.4c-.2 2.8-.4 5.5-.4 8.4c0 53 43 96 96 96s96-43 96-96s-43-96-96-96c-2.8 0-5.6 .1-8.4 .4c5.3 9.3 8.4 20.1 8.4 31.6z" / > < / svg > < / span > < / template >
< template id = fa-eye-slash > < span class = fa-svg > < svg xmlns = "http://www.w3.org/2000/svg" viewBox = "0 0 640 512" > <!-- ! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --> < path d = "M38.8 5.1C28.4-3.1 13.3-1.2 5.1 9.2S-1.2 34.7 9.2 42.9l592 464c10.4 8.2 25.5 6.3 33.7-4.1s6.3-25.5-4.1-33.7L525.6 386.7c39.6-40.6 66.4-86.1 79.9-118.4c3.3-7.9 3.3-16.7 0-24.6c-14.9-35.7-46.2-87.7-93-131.1C465.5 68.8 400.8 32 320 32c-68.2 0-125 26.3-169.3 60.8L38.8 5.1zM223.1 149.5C248.6 126.2 282.7 112 320 112c79.5 0 144 64.5 144 144c0 24.9-6.3 48.3-17.4 68.7L408 294.5c5.2-11.8 8-24.8 8-38.5c0-53-43-96-96-96c-2.8 0-5.6 .1-8.4 .4c5.3 9.3 8.4 20.1 8.4 31.6c0 10.2-2.4 19.8-6.6 28.3l-90.3-70.8zm223.1 298L373 389.9c-16.4 6.5-34.3 10.1-53 10.1c-79.5 0-144-64.5-144-144c0-6.9 .5-13.6 1.4-20.2L83.1 161.5C60.3 191.2 44 220.8 34.5 243.7c-3.3 7.9-3.3 16.7 0 24.6c14.9 35.7 46.2 87.7 93 131.1C174.5 443.2 239.2 480 320 480c47.8 0 89.9-12.9 126.2-32.5z" / > < / svg > < / span > < / template >
< template id = fa-copy > < span class = fa-svg > < svg xmlns = "http://www.w3.org/2000/svg" viewBox = "0 0 512 512" > <!-- ! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --> < path d = "M502.6 70.63l-61.25-61.25C435.4 3.371 427.2 0 418.7 0H255.1c-35.35 0-64 28.66-64 64l.0195 256C192 355.4 220.7 384 256 384h192c35.2 0 64-28.8 64-64V93.25C512 84.77 508.6 76.63 502.6 70.63zM464 320c0 8.836-7.164 16-16 16H255.1c-8.838 0-16-7.164-16-16L239.1 64.13c0-8.836 7.164-16 16-16h128L384 96c0 17.67 14.33 32 32 32h47.1V320zM272 448c0 8.836-7.164 16-16 16H63.1c-8.838 0-16-7.164-16-16L47.98 192.1c0-8.836 7.164-16 16-16H160V128H63.99c-35.35 0-64 28.65-64 64l.0098 256C.002 483.3 28.66 512 64 512h192c35.2 0 64-28.8 64-64v-32h-47.1L272 448z" / > < / svg > < / span > < / template >
< template id = fa-play > < span class = fa-svg > < svg xmlns = "http://www.w3.org/2000/svg" viewBox = "0 0 384 512" > <!-- ! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --> < path d = "M73 39c-14.8-9.1-33.4-9.4-48.5-.9S0 62.6 0 80V432c0 17.4 9.4 33.4 24.5 41.9s33.7 8.1 48.5-.9L361 297c14.3-8.7 23-24.2 23-41s-8.7-32.2-23-41L73 39z" / > < / svg > < / span > < / template >
< template id = fa-clock-rotate-left > < span class = fa-svg > < svg xmlns = "http://www.w3.org/2000/svg" viewBox = "0 0 512 512" > <!-- ! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --> < path d = "M75 75L41 41C25.9 25.9 0 36.6 0 57.9V168c0 13.3 10.7 24 24 24H134.1c21.4 0 32.1-25.9 17-41l-30.8-30.8C155 85.5 203 64 256 64c106 0 192 86 192 192s-86 192-192 192c-40.8 0-78.6-12.7-109.7-34.4c-14.5-10.1-34.4-6.6-44.6 7.9s-6.6 34.4 7.9 44.6C151.2 495 201.7 512 256 512c141.4 0 256-114.6 256-256S397.4 0 256 0C185.3 0 121.3 28.7 75 75zm181 53c-13.3 0-24 10.7-24 24V256c0 6.4 2.5 12.5 7 17l72 72c9.4 9.4 24.6 9.4 33.9 0s9.4-24.6 0-33.9l-65-65V152c0-13.3-10.7-24-24-24z" / > < / svg > < / span > < / template >
< script >
window.playground_copyable = true;
< / script >
< script src = "../../ace-2a3cd908.js" > < / script >
< script src = "../../mode-rust-2c9d5c9a.js" > < / script >
< script src = "../../editor-16ca416c.js" > < / script >
< script src = "../../theme-dawn-4493f9c8.js" > < / script >
< script src = "../../theme-tomorrow_night-9dbe62a9.js" > < / script >
< script src = "../../elasticlunr-ef4e11c1.min.js" > < / script >
< script src = "../../mark-09e88c2c.min.js" > < / script >
< script src = "../../searcher-c2a407aa.js" > < / script >
< script src = "../../clipboard-1626706a.min.js" > < / script >
< script src = "../../highlight-abc7f01d.js" > < / script >
< script src = "../../book-a0b12cfe.js" > < / script >
<!-- Custom JS scripts -->
< script src = "../../assets/custom-690c9de5.js" > < / script >
< script src = "../../assets/bigPicture-be03f9c8.js" > < / script >
< / div >
< / body >
< / html >
