<!DOCTYPE html><htmllang="en"><head><metacharset="utf-8"><metaname="viewport"content="width=device-width, initial-scale=1.0"><metaname="generator"content="rustdoc"><metaname="description"content="API documentation for the Rust `Once` struct in crate `parking_lot`."><metaname="keywords"content="rust, rustlang, rust-lang, Once"><title>parking_lot::Once - Rust</title><linkrel="stylesheet"type="text/css"href="../normalize.css"><linkrel="stylesheet"type="text/css"href="../rustdoc.css"id="mainThemeStyle"><linkrel="stylesheet"type="text/css"href="../dark.css"><linkrel="stylesheet"type="text/css"href="../light.css"id="themeStyle"><scriptsrc="../storage.js"></script><noscript><linkrel="stylesheet"href="../noscript.css"></noscript><linkrel="shortcut icon"href="../favicon.ico"><styletype="text/css">#crate-search{background-image:url("../down-arrow.svg");}</style></head><bodyclass="rustdoc struct"><!--[if lte IE 8]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><navclass="sidebar"><divclass="sidebar-menu">☰</div><ahref='../parking_lot/index.html'><divclass='logo-container'><imgsrc='../rust-logo.png'alt='logo'></div></a><pclass='location'>Struct Once</p><divclass="sidebar-elems"><divclass="block items"><aclass="sidebar-title"href="#methods">Methods</a><divclass="sidebar-links"><ahref="#method.call_once">call_once</a><ahref="#method.call_once_force">call_once_force</a><ahref="#method.new">new</a><ahref="#method.state">state</a></div><aclass="sidebar-title"href="#implementations">Trait Implementations</a><divclass="sidebar-links"><ahref="#impl-Debug">Debug</a><ahref="#impl-Default">Default</a></div><aclass="sidebar-title"href="#synthetic-implementations">Auto Trait Implementations</a><divclass="sidebar-links"><ahref="#impl-RefUnwindSafe">RefUnwindSafe</a><ahref="#impl-Send">Send</a><ahref="#impl-Sync">Sync</a><ahref="#impl-Unpin">Unpin</a><ahref="#impl-UnwindSafe">UnwindSafe</a></div><aclass="sidebar-title"href="#blanket-implementations">Blanket Implementations</a><divclass="sidebar-links"><ahref="#impl-Any">Any</a><ahref="#impl-Borrow%3CT%3E">Borrow<T></a><ahref="#impl-BorrowMut%3CT%3E">BorrowMut<T></a><ahref="#impl-From%3CT%3E">From<T></a><ahref="#impl-Into%3CU%3E">Into<U></a><ahref="#impl-TryFrom%3CU%3E">TryFrom<U></a><ahref="#impl-TryInto%3CU%3E">TryInto<U></a></div></div><pclass='location'><ahref='index.html'>parking_lot</a></p><script>window.sidebarCurrent={name:'Once',ty:'struct',relpath:''};</script><scriptdefersrc="sidebar-items.js"></script></div></nav><divclass="theme-picker"><buttonid="theme-picker"aria-label="Pick another theme!"><imgsrc="../brush.svg"width="18"alt="Pick another theme!"></button><divid="theme-choices"></div></div><scriptsrc="../theme.js"></script><navclass="sub"><formclass="search-form js-only"><divclass="search-container"><div><selectid="crate-search"><optionvalue="All crates">All crates</option></select><inputclass="search-input"name="search"autocomplete="off"spellcheck="false"placeholder="Click or press ‘S’ to search, ‘?’ for more options…"type="search"></div><aid="settings-menu"href="../settings.html"><imgsrc="../wheel.svg"width="18"alt="Change settings"></a></div></form></nav><sectionid="main"class="content"><h1class='fqn'><spanclass='out-of-band'><spanid='render-detail'><aid="toggle-all-docs"href="javascript:void(0)"title="collapse all docs">[<spanclass='inner'>−</span>]</a></span><aclass='srclink'href='../src/parking_lot/once.rs.html#92'title='goto source code'>[src]</a></span><spanclass='in-band'>Struct <ahref='index.html'>parking_lot</a>::<wbr><aclass="struct"href=''>Once</a></span></h1><divclass="docblock type-decl hidden-by-usual-hider"><preclass='rust struct'>pub struct Once(_);</pre></div><divclass='docblock'><p>A synchronization primitive which can be used to run a one-time
initialization. Useful for one-time initialization for globals, FFI or
related functionality.</p>
<h1id="differences-from-the-standard-library-once"class="section-header"><ahref="#differences-from-the-standard-library-once">Differences from the standard library <code>Once</code></a></h1>
<ul>
<li>Only requires 1 byte of space, instead of 1 word.</li>
<li>Not required to be <code>'static</code>.</li>
<li>Relaxed memory barriers in the fast path, which can significantly improve
performance on some architectures.</li>
<li>Efficient handling of micro-contention using adaptive spinning.</li>
<spanclass="comment">// run initialization here</span>
});</pre></div>
</div><h2id='methods'class='small-section-header'>Methods<ahref='#methods'class='anchor'></a></h2><h3id='impl'class='impl'><codeclass='in-band'>impl <aclass="struct"href="../parking_lot/struct.Once.html"title="struct parking_lot::Once">Once</a></code><ahref='#impl'class='anchor'></a><aclass='srclink'href='../src/parking_lot/once.rs.html#94-319'title='goto source code'>[src]</a></h3><divclass='impl-items'><h4id='method.new'class="method"><codeid='new.v'>pub const fn <ahref='#method.new'class='fnname'>new</a>() -><aclass="struct"href="../parking_lot/struct.Once.html"title="struct parking_lot::Once">Once</a></code><aclass='srclink'href='../src/parking_lot/once.rs.html#97-99'title='goto source code'>[src]</a></h4><divclass='docblock'><p>Creates a new <code>Once</code> value.</p>
</div><h4id='method.state'class="method"><codeid='state.v'>pub fn <ahref='#method.state'class='fnname'>state</a>(&self) -><aclass="enum"href="../parking_lot/enum.OnceState.html"title="enum parking_lot::OnceState">OnceState</a></code><aclass='srclink'href='../src/parking_lot/once.rs.html#103-114'title='goto source code'>[src]</a></h4><divclass='docblock'><p>Returns the current state of this <code>Once</code>.</p>
</div><h4id='method.call_once'class="method"><codeid='call_once.v'>pub fn <ahref='#method.call_once'class='fnname'>call_once</a><F>(&self, f: F) <spanclass="where fmt-newline">where<br> F: <aclass="trait"href="https://doc.rust-lang.org/nightly/core/ops/function/trait.FnOnce.html"title="trait core::ops::function::FnOnce">FnOnce</a>(), </span></code><aclass='srclink'href='../src/parking_lot/once.rs.html#166-176'title='goto source code'>[src]</a></h4><divclass='docblock'><p>Performs an initialization routine once and only once. The given closure
will be executed if this is the first time <code>call_once</code> has been called,
and otherwise the routine will <em>not</em> be invoked.</p>
<p>This method will block the calling thread if another initialization
routine is currently running.</p>
<p>When this function returns, it is guaranteed that some initialization
has run and completed (it may not be the closure specified). It is also
guaranteed that any memory writes performed by the executed closure can
be reliably observed by other threads at this point (there is a
happens-before relation between the closure and code executing after the
<p>The closure <code>f</code> will only be executed once if this is called
concurrently amongst many threads. If that closure panics, however, then
it will <em>poison</em> this <code>Once</code> instance, causing all future invocations of
<code>call_once</code> to also panic.</p>
</div><h4id='method.call_once_force'class="method"><codeid='call_once_force.v'>pub fn <ahref='#method.call_once_force'class='fnname'>call_once_force</a><F>(&self, f: F) <spanclass="where fmt-newline">where<br> F: <aclass="trait"href="https://doc.rust-lang.org/nightly/core/ops/function/trait.FnOnce.html"title="trait core::ops::function::FnOnce">FnOnce</a>(<aclass="enum"href="../parking_lot/enum.OnceState.html"title="enum parking_lot::OnceState">OnceState</a>), </span></code><aclass='srclink'href='../src/parking_lot/once.rs.html#188-198'title='goto source code'>[src]</a></h4><divclass='docblock'><p>Performs the same function as <code>call_once</code> except ignores poisoning.</p>
<p>If this <code>Once</code> has been poisoned (some initialization panicked) then
this function will continue to attempt to call initialization functions
until one of them doesn't panic.</p>
<p>The closure <code>f</code> is yielded a structure which can be used to query the
state of this <code>Once</code> (whether initialization has previously panicked or
not).</p>
</div></div><h2id='implementations'class='small-section-header'>Trait Implementations<ahref='#implementations'class='anchor'></a></h2><divid='implementations-list'><h3id='impl-Default'class='impl'><codeclass='in-band'>impl <aclass="trait"href="https://doc.rust-lang.org/nightly/core/default/trait.Default.html"title="trait core::default::Default">Default</a> for <aclass="struct"href="../parking_lot/struct.Once.html"title="struct parking_lot::Once">Once</a></code><ahref='#impl-Default'class='anchor'></a><aclass='srclink'href='../src/parking_lot/once.rs.html#321-326'title='goto source code'>[src]</a></h3><divclass='impl-items'><h4id='method.default'class="method hidden"><codeid='default.v'>fn <ahref='https://doc.rust-lang.org/nightly/core/default/trait.Default.html#tymethod.default'class='fnname'>default</a>() -><aclass="struct"href="../parking_lot/struct.Once.html"title="struct parking_lot::Once">Once</a></code><aclass='srclink'href='../src/parking_lot/once.rs.html#323-325'title='goto source code'>[src]</a></h4><divclass='docblock hidden'><p>Returns the "default value" for a type. <ahref="https://doc.rust-lang.org/nightly/core/default/trait.Default.html#tymethod.default">Read more</a></p>
</div></div><h3id='impl-Debug'class='impl'><codeclass='in-band'>impl <aclass="trait"href="https://doc.rust-lang.org/nightly/core/fmt/trait.Debug.html"title="trait core::fmt::Debug">Debug</a> for <aclass="struct"href="../parking_lot/struct.Once.html"title="struct parking_lot::Once">Once</a></code><ahref='#impl-Debug'class='anchor'></a><aclass='srclink'href='../src/parking_lot/once.rs.html#328-332'title='goto source code'>[src]</a></h3><divclass='impl-items'><h4id='method.fmt'class="method hidden"><codeid='fmt.v'>fn <ahref='https://doc.rust-lang.org/nightly/core/fmt/trait.Debug.html#tymethod.fmt'class='fnname'>fmt</a>(&self, f: &mut <aclass="struct"href="https://doc.rust-lang.org/nightly/core/fmt/struct.Formatter.html"title="struct core::fmt::Formatter">Formatter</a>) -><aclass="type"href="https://doc.rust-lang.org/nightly/core/fmt/type.Result.html"title="type core::fmt::Result">Result</a></code><aclass='srclink'href='../src/parking_lot/once.rs.html#329-331'title='goto source code'>[src]</a></h4><divclass='docblock hidden'><p>Formats the value using the given formatter. <ahref="https://doc.rust-lang.org/nightly/core/fmt/trait.Debug.html#tymethod.fmt">Read more</a></p>
</div></div><h3id='impl-From%3CT%3E'class='impl'><codeclass='in-band'>impl<T><aclass="trait"href="https://doc.rust-lang.org/nightly/core/convert/trait.From.html"title="trait core::convert::From">From</a><T> for T</code><ahref='#impl-From%3CT%3E'class='anchor'></a><aclass='srclink'href='https://doc.rust-lang.org/nightly/src/core/convert.rs.html#552-554'title='goto source code'>[src]</a></h3><divclass='impl-items'><h4id='method.from'class="method hidden"><codeid='from.v'>fn <ahref='https://doc.rust-lang.org/nightly/core/convert/trait.From.html#tymethod.from'class='fnname'>from</a>(t: T) -> T</code><aclass='srclink'href='https://doc.rust-lang.org/nightly/src/core/convert.rs.html#553'title='goto source code'>[src]</a></h4><divclass='docblock hidden'><p>Performs the conversion.</p>
</div></div><h3id='impl-TryFrom%3CU%3E'class='impl'><codeclass='in-band'>impl<T, U><aclass="trait"href="https://doc.rust-lang.org/nightly/core/convert/trait.TryFrom.html"title="trait core::convert::TryFrom">TryFrom</a><U> for T <spanclass="where fmt-newline">where<br> U: <aclass="trait"href="https://doc.rust-lang.org/nightly/core/convert/trait.Into.html"title="trait core::convert::Into">Into</a><T>, </span></code><ahref='#impl-TryFrom%3CU%3E'class='anchor'></a><aclass='srclink'href='https://doc.rust-lang.org/nightly/src/core/convert.rs.html#571-577'title='goto source code'>[src]</a></h3><divclass='impl-items'><h4id='associatedtype.Error'class="type"><codeid='Error.t'>type <ahref='https://doc.rust-lang.org/nightly/core/convert/trait.TryFrom.html#associatedtype.Error'class="type">Error</a> = <aclass="enum"href="https://doc.rust-lang.org/nightly/core/convert/enum.Infallible.html"title="enum core::convert::Infallible">Infallible</a></code></h4><divclass='docblock'><p>The type returned in the event of a conversion error.</p>
</div><h4id='method.try_from'class="method hidden"><codeid='try_from.v'>fn <ahref='https://doc.rust-lang.org/nightly/core/convert/trait.TryFrom.html#tymethod.try_from'class='fnname'>try_from</a>(value: U) -><aclass="enum"href="https://doc.rust-lang.org/nightly/core/result/enum.Result.html"title="enum core::result::Result">Result</a><T, <T as <aclass="trait"href="https://doc.rust-lang.org/nightly/core/convert/trait.TryFrom.html"title="trait core::convert::TryFrom">TryFrom</a><U>>::<aclass="type"href="https://doc.rust-lang.org/nightly/core/convert/trait.TryFrom.html#associatedtype.Error"title="type core::convert::TryFrom::Error">Error</a>></code><aclass='srclink'href='https://doc.rust-lang.org/nightly/src/core/convert.rs.html#574-576'title='goto source code'>[src]</a></h4><divclass='docblock hidden'><p>Performs the conversion.</p>
</div></div><h3id='impl-TryInto%3CU%3E'class='impl'><codeclass='in-band'>impl<T, U><aclass="trait"href="https://doc.rust-lang.org/nightly/core/convert/trait.TryInto.html"title="trait core::convert::TryInto">TryInto</a><U> for T <spanclass="where fmt-newline">where<br> U: <aclass="trait"href="https://doc.rust-lang.org/nightly/core/convert/trait.TryFrom.html"title="trait core::convert::TryFrom">TryFrom</a><T>, </span></code><ahref='#impl-TryInto%3CU%3E'class='anchor'></a><aclass='srclink'href='https://doc.rust-lang.org/nightly/src/core/convert.rs.html#559-566'title='goto source code'>[src]</a></h3><divclass='impl-items'><h4id='associatedtype.Error-1'class="type"><codeid='Error.t-1'>type <ahref='https://doc.rust-lang.org/nightly/core/convert/trait.TryInto.html#associatedtype.Error'class="type">Error</a> = <U as <aclass="trait"href="https://doc.rust-lang.org/nightly/core/convert/trait.TryFrom.html"title="trait core::convert::TryFrom">TryFrom</a><T>>::<aclass="type"href="https://doc.rust-lang.org/nightly/core/convert/trait.TryFrom.html#associatedtype.Error"title="type core::convert::TryFrom::Error">Error</a></code></h4><divclass='docblock'><p>The type returned in the event of a conversion error.</p>
</div><h4id='method.try_into'class="method hidden"><codeid='try_into.v'>fn <ahref='https://doc.rust-lang.org/nightly/core/convert/trait.TryInto.html#tymethod.try_into'class='fnname'>try_into</a>(self) -><aclass="enum"href="https://doc.rust-lang.org/nightly/core/result/enum.Result.html"title="enum core::result::Result">Result</a><U, <U as <aclass="trait"href="https://doc.rust-lang.org/nightly/core/convert/trait.TryFrom.html"title="trait core::convert::TryFrom">TryFrom</a><T>>::<aclass="type"href="https://doc.rust-lang.org/nightly/core/convert/trait.TryFrom.html#associatedtype.Error"title="type core::convert::TryFrom::Error">Error</a>></code><aclass='srclink'href='https://doc.rust-lang.org/nightly/src/core/convert.rs.html#563-565'title='goto source code'>[src]</a></h4><divclass='docblock hidden'><p>Performs the conversion.</p>
</div></div><h3id='impl-BorrowMut%3CT%3E'class='impl'><codeclass='in-band'>impl<T><aclass="trait"href="https://doc.rust-lang.org/nightly/core/borrow/trait.BorrowMut.html"title="trait core::borrow::BorrowMut">BorrowMut</a><T> for T <spanclass="where fmt-newline">where<br> T: ?<aclass="trait"href="https://doc.rust-lang.org/nightly/core/marker/trait.Sized.html"title="trait core::marker::Sized">Sized</a>, </span></code><ahref='#impl-BorrowMut%3CT%3E'class='anchor'></a><aclass='srclink'href='https://doc.rust-lang.org/nightly/src/core/borrow.rs.html#218-220'title='goto source code'>[src]</a></h3><divclass='impl-items'><h4id='method.borrow_mut'class="method hidden"><codeid='borrow_mut.v'>fn <ahref='https://doc.rust-lang.org/nightly/core/borrow/trait.BorrowMut.html#tymethod.borrow_mut'class='fnname'>borrow_mut</a>(&mut self) -><aclass="primitive"href="https://doc.rust-lang.org/nightly/std/primitive.reference.html">&mut </a>T</code><aclass='srclink'href='https://doc.rust-lang.org/nightly/src/core/borrow.rs.html#219'title='goto source code'>[src]</a></h4><divclass='docblock hidden'><p>Mutably borrows from an owned value. <ahref="https://doc.rust-lang.org/nightly/core/borrow/trait.BorrowMut.html#tymethod.borrow_mut">Read more</a></p>
</div></div><h3id='impl-Borrow%3CT%3E'class='impl'><codeclass='in-band'>impl<T><aclass="trait"href="https://doc.rust-lang.org/nightly/core/borrow/trait.Borrow.html"title="trait core::borrow::Borrow">Borrow</a><T> for T <spanclass="where fmt-newline">where<br> T: ?<aclass="trait"href="https://doc.rust-lang.org/nightly/core/marker/trait.Sized.html"title="trait core::marker::Sized">Sized</a>, </span></code><ahref='#impl-Borrow%3CT%3E'class='anchor'></a><aclass='srclink'href='https://doc.rust-lang.org/nightly/src/core/borrow.rs.html#213-215'title='goto source code'>[src]</a></h3><divclass='impl-items'><h4id='method.borrow'class="method hidden"><codeid='borrow.v'>fn <ahref='https://doc.rust-lang.org/nightly/core/borrow/trait.Borrow.html#tymethod.borrow'class='fnname'>borrow</a>(&self) -><aclass="primitive"href="https://doc.rust-lang.org/nightly/std/primitive.reference.html">&</a>T</code><aclass='srclink'href='https://doc.rust-lang.org/nightly/src/core/borrow.rs.html#214'title='goto source code'>[src]</a></h4><divclass='docblock hidden'><p>Immutably borrows from an owned value. <ahref="https://doc.rust-lang.org/nightly/core/borrow/trait.Borrow.html#tymethod.borrow">Read more</a></p>
</div></div><h3id='impl-Any'class='impl'><codeclass='in-band'>impl<T><aclass="trait"href="https://doc.rust-lang.org/nightly/core/any/trait.Any.html"title="trait core::any::Any">Any</a> for T <spanclass="where fmt-newline">where<br> T: 'static + ?<aclass="trait"href="https://doc.rust-lang.org/nightly/core/marker/trait.Sized.html"title="trait core::marker::Sized">Sized</a>, </span></code><ahref='#impl-Any'class='anchor'></a><aclass='srclink'href='https://doc.rust-lang.org/nightly/src/core/any.rs.html#100-102'title='goto source code'>[src]</a></h3><divclass='impl-items'><h4id='method.type_id'class="method hidden"><codeid='type_id.v'>fn <ahref='https://doc.rust-lang.org/nightly/core/any/trait.Any.html#tymethod.type_id'class='fnname'>type_id</a>(&self) -><aclass="struct"href="https://doc.rust-lang.org/nightly/core/any/struct.TypeId.html"title="struct core::any::TypeId">TypeId</a></code><aclass='srclink'href='https://doc.rust-lang.org/nightly/src/core/any.rs.html#101'title='goto source code'>[src]</a></h4><divclass='docblock hidden'><p>Gets the <code>TypeId</code> of <code>self</code>. <ahref="https://doc.rust-lang.org/nightly/core/any/trait.Any.html#tymethod.type_id">Read more</a></p>
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