| <!DOCTYPE html><html lang="en"><head><meta charset="utf-8"><meta name="viewport" content="width=device-width, initial-scale=1.0"><meta name="generator" content="rustdoc"><meta name="description" content="Source of the Rust file `/root/.cargo/git/checkouts/tokio-377c595163f99a10/dfe252d/tokio/src/util/once_cell.rs`."><meta name="keywords" content="rust, rustlang, rust-lang"><title>once_cell.rs - source</title><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../SourceSerif4-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../FiraSans-Regular.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../FiraSans-Medium.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../SourceCodePro-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../SourceSerif4-Bold.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../SourceCodePro-Semibold.ttf.woff2"><link rel="stylesheet" href="../../../normalize.css"><link rel="stylesheet" href="../../../rustdoc.css" id="mainThemeStyle"><link rel="stylesheet" href="../../../ayu.css" disabled><link rel="stylesheet" href="../../../dark.css" disabled><link rel="stylesheet" href="../../../light.css" id="themeStyle"><script id="default-settings" ></script><script src="../../../storage.js"></script><script defer src="../../../source-script.js"></script><script defer src="../../../source-files.js"></script><script defer src="../../../main.js"></script><noscript><link rel="stylesheet" href="../../../noscript.css"></noscript><link rel="alternate icon" type="image/png" href="../../../favicon-16x16.png"><link rel="alternate icon" type="image/png" href="../../../favicon-32x32.png"><link rel="icon" type="image/svg+xml" href="../../../favicon.svg"></head><body class="rustdoc source"><!--[if lte IE 11]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="sidebar"><a class="sidebar-logo" href="../../../tokio/index.html"><div class="logo-container"><img class="rust-logo" src="../../../rust-logo.svg" alt="logo"></div></a></nav><main><div class="width-limiter"><nav class="sub"><a class="sub-logo-container" href="../../../tokio/index.html"><img class="rust-logo" src="../../../rust-logo.svg" alt="logo"></a><form class="search-form"><div class="search-container"><span></span><input class="search-input" name="search" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><div id="help-button" title="help" tabindex="-1"><a href="../../../help.html">?</a></div><div id="settings-menu" tabindex="-1"><a href="../../../settings.html" title="settings"><img width="22" height="22" alt="Change settings" src="../../../wheel.svg"></a></div></div></form></nav><section id="main-content" class="content"><div class="example-wrap"><pre class="src-line-numbers"><span id="1">1</span> |
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| </pre><pre class="rust"><code><span class="attribute">#![allow(dead_code)] |
| </span><span class="kw">use </span>std::cell::UnsafeCell; |
| <span class="kw">use </span>std::mem::MaybeUninit; |
| <span class="kw">use </span>std::sync::Once; |
| |
| <span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">struct </span>OnceCell<T> { |
| once: Once, |
| value: UnsafeCell<MaybeUninit<T>>, |
| } |
| |
| <span class="kw">unsafe impl</span><T: Send + Sync> Send <span class="kw">for </span>OnceCell<T> {} |
| <span class="kw">unsafe impl</span><T: Send + Sync> Sync <span class="kw">for </span>OnceCell<T> {} |
| |
| <span class="kw">impl</span><T> OnceCell<T> { |
| <span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">const fn </span>new() -> <span class="self">Self </span>{ |
| <span class="self">Self </span>{ |
| once: Once::new(), |
| value: UnsafeCell::new(MaybeUninit::uninit()), |
| } |
| } |
| |
| <span class="doccomment">/// Get the value inside this cell, intiailizing it using the provided |
| /// function if necessary. |
| /// |
| /// If the `init` closure panics, then the `OnceCell` is poisoned and all |
| /// future calls to `get` will panic. |
| </span><span class="attribute">#[inline] |
| </span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>get(<span class="kw-2">&</span><span class="self">self</span>, init: <span class="kw">impl </span>FnOnce() -> T) -> <span class="kw-2">&</span>T { |
| <span class="kw">if </span>!<span class="self">self</span>.once.is_completed() { |
| <span class="self">self</span>.do_init(init); |
| } |
| |
| <span class="comment">// Safety: The `std::sync::Once` guarantees that we can only reach this |
| // line if a `call_once` closure has been run exactly once and without |
| // panicking. Thus, the value is not uninitialized. |
| // |
| // There is also no race because the only `&self` method that modifies |
| // `value` is `do_init`, but if the `call_once` closure is still |
| // running, then no thread has gotten past the `call_once`. |
| </span><span class="kw">unsafe </span>{ <span class="kw-2">&*</span>(<span class="self">self</span>.value.get() <span class="kw">as </span><span class="kw-2">*const </span>T) } |
| } |
| |
| <span class="attribute">#[cold] |
| </span><span class="kw">fn </span>do_init(<span class="kw-2">&</span><span class="self">self</span>, init: <span class="kw">impl </span>FnOnce() -> T) { |
| <span class="kw">let </span>value_ptr = <span class="self">self</span>.value.get() <span class="kw">as </span><span class="kw-2">*mut </span>T; |
| |
| <span class="self">self</span>.once.call_once(|| { |
| <span class="kw">let </span>set_to = init(); |
| |
| <span class="comment">// Safety: The `std::sync::Once` guarantees that this initialization |
| // will run at most once, and that no thread can get past the |
| // `call_once` until it has run exactly once. Thus, we have |
| // exclusive access to `value`. |
| </span><span class="kw">unsafe </span>{ |
| std::ptr::write(value_ptr, set_to); |
| } |
| }); |
| } |
| } |
| |
| <span class="kw">impl</span><T> Drop <span class="kw">for </span>OnceCell<T> { |
| <span class="kw">fn </span>drop(<span class="kw-2">&mut </span><span class="self">self</span>) { |
| <span class="kw">if </span><span class="self">self</span>.once.is_completed() { |
| <span class="kw">let </span>value_ptr = <span class="self">self</span>.value.get() <span class="kw">as </span><span class="kw-2">*mut </span>T; |
| <span class="kw">unsafe </span>{ |
| std::ptr::drop_in_place(value_ptr); |
| } |
| } |
| } |
| } |
| </code></pre></div> |
| </section></div></main><div id="rustdoc-vars" data-root-path="../../../" data-current-crate="tokio" data-themes="ayu,dark,light" data-resource-suffix="" data-rustdoc-version="1.66.0-nightly (5c8bff74b 2022-10-21)" ></div></body></html> |
