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 </pre><pre class="rust"><code><span class="attribute">#![deny(unsafe_code)]

 </span><span class="doccomment">//! Caching handle into the [ArcSwapAny].
 //!
 //! The [Cache] keeps a copy of the internal [Arc] for faster access.
 //!
 //! [Arc]: std::sync::Arc

 </span><span class="kw">use </span>std::ops::Deref;
 <span class="kw">use </span>std::sync::atomic::Ordering;

 <span class="kw">use </span><span class="kw">super</span>::ref_cnt::RefCnt;
 <span class="kw">use </span><span class="kw">super</span>::strategy::Strategy;
 <span class="kw">use </span><span class="kw">super</span>::ArcSwapAny;

 <span class="doccomment">/// Generalization of caches providing access to `T`.
 ///
 /// This abstracts over all kinds of caches that can provide a cheap access to values of type `T`.
 /// This is useful in cases where some code doesn&#39;t care if the `T` is the whole structure or just
 /// a part of it.
 ///
 /// See the example at [`Cache::map`].
 </span><span class="kw">pub trait </span>Access&lt;T&gt; {
     <span class="doccomment">/// Loads the value from cache.
     ///
     /// This revalidates the value in the cache, then provides the access to the cached value.
     </span><span class="kw">fn </span>load(<span class="kw-2">&amp;mut </span><span class="self">self</span>) -&gt; <span class="kw-2">&amp;</span>T;
 }

 <span class="doccomment">/// Caching handle for [`ArcSwapAny`][ArcSwapAny].
 ///
 /// Instead of loading the [`Arc`][Arc] on every request from the shared storage, this keeps
 /// another copy inside itself. Upon request it only cheaply revalidates it is up to
 /// date. If it is, access is significantly faster. If it is stale, the [load_full] is done and the
 /// cache value is replaced. Under a read-heavy loads, the measured speedup are 10-25 times,
 /// depending on the architecture.
 ///
 /// There are, however, downsides:
 ///
 /// * The handle needs to be kept around by the caller (usually, one per thread). This is fine if
 ///   there&#39;s one global `ArcSwapAny`, but starts being tricky with eg. data structures build from
 ///   them.
 /// * As it keeps a copy of the [Arc] inside the cache, the old value may be kept alive for longer
 ///   period of time ‒ it is replaced by the new value on [load][Cache::load]. You may not want to
 ///   use this if dropping the old value in timely manner is important (possibly because of
 ///   releasing large amount of RAM or because of closing file handles).
 ///
 /// # Examples
 ///
 /// ```rust
 /// # fn do_something&lt;V&gt;(_v: V) { }
 /// use std::sync::Arc;
 /// use std::sync::atomic::{AtomicBool, Ordering};
 ///
 /// use arc_swap::{ArcSwap, Cache};
 ///
 /// let shared = Arc::new(ArcSwap::from_pointee(42));
 /// # let mut threads = Vec::new();
 /// let terminate = Arc::new(AtomicBool::new(false));
 /// // Start 10 worker threads...
 /// for _ in 0..10 {
 ///     let mut cache = Cache::new(Arc::clone(&amp;shared));
 ///     let terminate = Arc::clone(&amp;terminate);
 ///     # let thread =
 ///     std::thread::spawn(move || {
 ///         // Keep loading it like mad..
 ///         while !terminate.load(Ordering::Relaxed) {
 ///             let value = cache.load();
 ///             do_something(value);
 ///         }
 ///     });
 ///     # threads.push(thread);
 /// }
 /// shared.store(Arc::new(12));
 /// # terminate.store(true, Ordering::Relaxed);
 /// # for thread in threads { thread.join().unwrap() }
 /// ```
 ///
 /// Another one with using a thread local storage and explicit types:
 ///
 /// ```rust
 /// # use std::sync::Arc;
 /// # use std::ops::Deref;
 /// # use std::cell::RefCell;
 /// #
 /// # use arc_swap::ArcSwap;
 /// # use arc_swap::cache::Cache;
 /// # use once_cell::sync::Lazy;
 /// #
 /// # #[derive(Debug, Default)]
 /// # struct Config;
 /// #
 /// static CURRENT_CONFIG: Lazy&lt;ArcSwap&lt;Config&gt;&gt; = Lazy::new(|| ArcSwap::from_pointee(Config::default()));
 ///
 /// thread_local! {
 ///     static CACHE: RefCell&lt;Cache&lt;&amp;&#39;static ArcSwap&lt;Config&gt;, Arc&lt;Config&gt;&gt;&gt; = RefCell::new(Cache::from(CURRENT_CONFIG.deref()));
 /// }
 ///
 /// CACHE.with(|c| {
 ///     // * RefCell needed, because load on cache is `&amp;mut`.
 ///     // * You want to operate inside the `with` ‒ cloning the Arc is comparably expensive as
 ///     //   ArcSwap::load itself and whatever you&#39;d save by the cache would be lost on that.
 ///     println!(&quot;{:?}&quot;, c.borrow_mut().load());
 /// });
 /// ```
 ///
 /// [Arc]: std::sync::Arc
 /// [load_full]: ArcSwapAny::load_full
 </span><span class="attribute">#[derive(Clone, Debug)]
 </span><span class="kw">pub struct </span>Cache&lt;A, T&gt; {
     arc_swap: A,
     cached: T,
 }

 <span class="kw">impl</span>&lt;A, T, S&gt; Cache&lt;A, T&gt;
 <span class="kw">where
     </span>A: Deref&lt;Target = ArcSwapAny&lt;T, S&gt;&gt;,
     T: RefCnt,
     S: Strategy&lt;T&gt;,
 {
     <span class="doccomment">/// Creates a new caching handle.
     ///
     /// The parameter is something dereferencing into an [`ArcSwapAny`] (eg. either to [`ArcSwap`]
     /// or [`ArcSwapOption`]). That can be [`ArcSwapAny`] itself, but that&#39;s not very useful. But
     /// it also can be a reference to it or `Arc`, which makes it possible to share the
     /// [`ArcSwapAny`] with multiple caches or access it in non-cached way too.
     ///
     /// [`ArcSwapOption`]: crate::ArcSwapOption
     /// [`ArcSwap`]: crate::ArcSwap
     </span><span class="kw">pub fn </span>new(arc_swap: A) -&gt; <span class="self">Self </span>{
         <span class="kw">let </span>cached = arc_swap.load_full();
         <span class="self">Self </span>{ arc_swap, cached }
     }

     <span class="doccomment">/// Gives access to the (possibly shared) cached [`ArcSwapAny`].
     </span><span class="kw">pub fn </span>arc_swap(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; <span class="kw-2">&amp;</span>A::Target {
         <span class="kw-2">&amp;</span><span class="self">self</span>.arc_swap
     }

     <span class="doccomment">/// Loads the currently held value.
     ///
     /// This first checks if the cached value is up to date. This check is very cheap.
     ///
     /// If it is up to date, the cached value is simply returned without additional costs. If it is
     /// outdated, a load is done on the underlying shared storage. The newly loaded value is then
     /// stored in the cache and returned.
     </span><span class="attribute">#[inline]
     </span><span class="kw">pub fn </span>load(<span class="kw-2">&amp;mut </span><span class="self">self</span>) -&gt; <span class="kw-2">&amp;</span>T {
         <span class="self">self</span>.revalidate();
         <span class="self">self</span>.load_no_revalidate()
     }

     <span class="attribute">#[inline]
     </span><span class="kw">fn </span>load_no_revalidate(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; <span class="kw-2">&amp;</span>T {
         <span class="kw-2">&amp;</span><span class="self">self</span>.cached
     }

     <span class="attribute">#[inline]
     </span><span class="kw">fn </span>revalidate(<span class="kw-2">&amp;mut </span><span class="self">self</span>) {
         <span class="kw">let </span>cached_ptr = RefCnt::as_ptr(<span class="kw-2">&amp;</span><span class="self">self</span>.cached);
         <span class="comment">// Node: Relaxed here is fine. We do not synchronize any data through this, we already have
         // it synchronized in self.cache. We just want to check if it changed, if it did, the
         // load_full will be responsible for any synchronization needed.
         </span><span class="kw">let </span>shared_ptr = <span class="self">self</span>.arc_swap.ptr.load(Ordering::Relaxed);
         <span class="kw">if </span>cached_ptr != shared_ptr {
             <span class="self">self</span>.cached = <span class="self">self</span>.arc_swap.load_full();
         }
     }

     <span class="doccomment">/// Turns this cache into a cache with a projection inside the cached value.
     ///
     /// You&#39;d use this in case when some part of code needs access to fresh values of `U`, however
     /// a bigger structure containing `U` is provided by this cache. The possibility of giving the
     /// whole structure to the part of the code falls short in terms of reusability (the part of
     /// the code could be used within multiple contexts, each with a bigger different structure
     /// containing `U`) and code separation (the code shouldn&#39;t needs to know about the big
     /// structure).
     ///
     /// # Warning
     ///
     /// As the provided `f` is called inside every [`load`][Access::load], this one should be
     /// cheap. Most often it is expected to be just a closure taking reference of some inner field.
     ///
     /// For the same reasons, it should not have side effects and should never panic (these will
     /// not break Rust&#39;s safety rules, but might produce behaviour you don&#39;t expect).
     ///
     /// # Examples
     ///
     /// ```rust
     /// use arc_swap::ArcSwap;
     /// use arc_swap::cache::{Access, Cache};
     ///
     /// struct InnerCfg {
     ///     answer: usize,
     /// }
     ///
     /// struct FullCfg {
     ///     inner: InnerCfg,
     /// }
     ///
     /// fn use_inner&lt;A: Access&lt;InnerCfg&gt;&gt;(cache: &amp;mut A) {
     ///     let value = cache.load();
     ///     println!(&quot;The answer is: {}&quot;, value.answer);
     /// }
     ///
     /// let full_cfg = ArcSwap::from_pointee(FullCfg {
     ///     inner: InnerCfg {
     ///         answer: 42,
     ///     }
     /// });
     /// let cache = Cache::new(&amp;full_cfg);
     /// use_inner(&amp;mut cache.map(|full| &amp;full.inner));
     ///
     /// let inner_cfg = ArcSwap::from_pointee(InnerCfg { answer: 24 });
     /// let mut inner_cache = Cache::new(&amp;inner_cfg);
     /// use_inner(&amp;mut inner_cache);
     /// ```
     </span><span class="kw">pub fn </span>map&lt;F, U&gt;(<span class="self">self</span>, f: F) -&gt; MapCache&lt;A, T, F&gt;
     <span class="kw">where
         </span>F: FnMut(<span class="kw-2">&amp;</span>T) -&gt; <span class="kw-2">&amp;</span>U,
     {
         MapCache {
             inner: <span class="self">self</span>,
             projection: f,
         }
     }
 }

 <span class="kw">impl</span>&lt;A, T, S&gt; Access&lt;T::Target&gt; <span class="kw">for </span>Cache&lt;A, T&gt;
 <span class="kw">where
     </span>A: Deref&lt;Target = ArcSwapAny&lt;T, S&gt;&gt;,
     T: Deref&lt;Target = &lt;T <span class="kw">as </span>RefCnt&gt;::Base&gt; + RefCnt,
     S: Strategy&lt;T&gt;,
 {
     <span class="kw">fn </span>load(<span class="kw-2">&amp;mut </span><span class="self">self</span>) -&gt; <span class="kw-2">&amp;</span>T::Target {
         <span class="self">self</span>.load().deref()
     }
 }

 <span class="kw">impl</span>&lt;A, T, S&gt; From&lt;A&gt; <span class="kw">for </span>Cache&lt;A, T&gt;
 <span class="kw">where
     </span>A: Deref&lt;Target = ArcSwapAny&lt;T, S&gt;&gt;,
     T: RefCnt,
     S: Strategy&lt;T&gt;,
 {
     <span class="kw">fn </span>from(arc_swap: A) -&gt; <span class="self">Self </span>{
         <span class="self">Self</span>::new(arc_swap)
     }
 }

 <span class="doccomment">/// An implementation of a cache with a projection into the accessed value.
 ///
 /// This is the implementation structure for [`Cache::map`]. It can&#39;t be created directly and it
 /// should be used through the [`Access`] trait.
 </span><span class="attribute">#[derive(Clone, Debug)]
 </span><span class="kw">pub struct </span>MapCache&lt;A, T, F&gt; {
     inner: Cache&lt;A, T&gt;,
     projection: F,
 }

 <span class="kw">impl</span>&lt;A, T, S, F, U&gt; Access&lt;U&gt; <span class="kw">for </span>MapCache&lt;A, T, F&gt;
 <span class="kw">where
     </span>A: Deref&lt;Target = ArcSwapAny&lt;T, S&gt;&gt;,
     T: RefCnt,
     S: Strategy&lt;T&gt;,
     F: FnMut(<span class="kw-2">&amp;</span>T) -&gt; <span class="kw-2">&amp;</span>U,
 {
     <span class="kw">fn </span>load(<span class="kw-2">&amp;mut </span><span class="self">self</span>) -&gt; <span class="kw-2">&amp;</span>U {
         (<span class="self">self</span>.projection)(<span class="self">self</span>.inner.load())
     }
 }

 <span class="attribute">#[cfg(test)]
 </span><span class="kw">mod </span>tests {
     <span class="kw">use </span>std::sync::Arc;

     <span class="kw">use super</span>::<span class="kw-2">*</span>;
     <span class="kw">use crate</span>::{ArcSwap, ArcSwapOption};

     <span class="attribute">#[test]
     </span><span class="kw">fn </span>cached_value() {
         <span class="kw">let </span>a = ArcSwap::from_pointee(<span class="number">42</span>);
         <span class="kw">let </span><span class="kw-2">mut </span>c1 = Cache::new(<span class="kw-2">&amp;</span>a);
         <span class="kw">let </span><span class="kw-2">mut </span>c2 = Cache::new(<span class="kw-2">&amp;</span>a);

         <span class="macro">assert_eq!</span>(<span class="number">42</span>, <span class="kw-2">**</span>c1.load());
         <span class="macro">assert_eq!</span>(<span class="number">42</span>, <span class="kw-2">**</span>c2.load());

         a.store(Arc::new(<span class="number">43</span>));
         <span class="macro">assert_eq!</span>(<span class="number">42</span>, <span class="kw-2">**</span>c1.load_no_revalidate());
         <span class="macro">assert_eq!</span>(<span class="number">43</span>, <span class="kw-2">**</span>c1.load());
     }

     <span class="attribute">#[test]
     </span><span class="kw">fn </span>cached_through_arc() {
         <span class="kw">let </span>a = Arc::new(ArcSwap::from_pointee(<span class="number">42</span>));
         <span class="kw">let </span><span class="kw-2">mut </span>c = Cache::new(Arc::clone(<span class="kw-2">&amp;</span>a));
         <span class="macro">assert_eq!</span>(<span class="number">42</span>, <span class="kw-2">**</span>c.load());
         a.store(Arc::new(<span class="number">0</span>));
         drop(a); <span class="comment">// A is just one handle, the ArcSwap is kept alive by the cache.
     </span>}

     <span class="attribute">#[test]
     </span><span class="kw">fn </span>cache_option() {
         <span class="kw">let </span>a = ArcSwapOption::from_pointee(<span class="number">42</span>);
         <span class="kw">let </span><span class="kw-2">mut </span>c = Cache::new(<span class="kw-2">&amp;</span>a);

         <span class="macro">assert_eq!</span>(<span class="number">42</span>, <span class="kw-2">**</span>c.load().as_ref().unwrap());
         a.store(<span class="prelude-val">None</span>);
         <span class="macro">assert!</span>(c.load().is_none());
     }

     <span class="kw">struct </span>Inner {
         answer: usize,
     }

     <span class="kw">struct </span>Outer {
         inner: Inner,
     }

     <span class="attribute">#[test]
     </span><span class="kw">fn </span>map_cache() {
         <span class="kw">let </span>a = ArcSwap::from_pointee(Outer {
             inner: Inner { answer: <span class="number">42 </span>},
         });

         <span class="kw">let </span><span class="kw-2">mut </span>cache = Cache::new(<span class="kw-2">&amp;</span>a);
         <span class="kw">let </span><span class="kw-2">mut </span>inner = cache.clone().map(|outer| <span class="kw-2">&amp;</span>outer.inner);
         <span class="kw">let </span><span class="kw-2">mut </span>answer = cache.clone().map(|outer| <span class="kw-2">&amp;</span>outer.inner.answer);

         <span class="macro">assert_eq!</span>(<span class="number">42</span>, cache.load().inner.answer);
         <span class="macro">assert_eq!</span>(<span class="number">42</span>, inner.load().answer);
         <span class="macro">assert_eq!</span>(<span class="number">42</span>, <span class="kw-2">*</span>answer.load());

         a.store(Arc::new(Outer {
             inner: Inner { answer: <span class="number">24 </span>},
         }));

         <span class="macro">assert_eq!</span>(<span class="number">24</span>, cache.load().inner.answer);
         <span class="macro">assert_eq!</span>(<span class="number">24</span>, inner.load().answer);
         <span class="macro">assert_eq!</span>(<span class="number">24</span>, <span class="kw-2">*</span>answer.load());
     }
 }
 </code></pre></div>
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