| <!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="`DeserializeSeed` is the stateful form of the `Deserialize` trait. If you ever find yourself looking for a way to pass data into a `Deserialize` impl, this trait is the way to do it."><meta name="keywords" content="rust, rustlang, rust-lang, DeserializeSeed"><title>DeserializeSeed in serde::de - Rust</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="sidebar-items.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 trait"><!--[if lte IE 11]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="mobile-topbar"><button class="sidebar-menu-toggle">☰</button><a class="sidebar-logo" href="../../serde/index.html"><div class="logo-container"><img class="rust-logo" src="../../rust-logo.svg" alt="logo"></div></a><h2></h2></nav><nav class="sidebar"><a class="sidebar-logo" href="../../serde/index.html"><div class="logo-container"><img class="rust-logo" src="../../rust-logo.svg" alt="logo"></div></a><h2 class="location"><a href="#">DeserializeSeed</a></h2><div class="sidebar-elems"><section><h3><a href="#required-associated-types">Required Associated Types</a></h3><ul class="block"><li><a href="#associatedtype.Value">Value</a></li></ul><h3><a href="#required-methods">Required Methods</a></h3><ul class="block"><li><a href="#tymethod.deserialize">deserialize</a></li></ul><h3><a href="#foreign-impls">Implementations on Foreign Types</a></h3><ul class="block"><li><a href="#impl-DeserializeSeed%3C%27de%3E-for-PhantomData%3CT%3E">PhantomData<T></a></li></ul><h3><a href="#implementors">Implementors</a></h3></section><h2><a href="index.html">In serde::de</a></h2></div></nav><main><div class="width-limiter"><nav class="sub"><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="main-heading"><h1 class="fqn">Trait <a href="../index.html">serde</a>::<wbr><a href="index.html">de</a>::<wbr><a class="trait" href="#">DeserializeSeed</a><button id="copy-path" onclick="copy_path(this)" title="Copy item path to clipboard"><img src="../../clipboard.svg" width="19" height="18" alt="Copy item path"></button></h1><span class="out-of-band"><a class="srclink" href="../../src/serde/de/mod.rs.html#769-778">source</a> · <a id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs">[<span class="inner">−</span>]</a></span></div><div class="item-decl"><pre class="rust trait"><code>pub trait DeserializeSeed<'de>: <a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Sized.html" title="trait core::marker::Sized">Sized</a> { |
| type <a href="#associatedtype.Value" class="associatedtype">Value</a>; |
| |
| fn <a href="#tymethod.deserialize" class="fnname">deserialize</a><D>(self, deserializer: D) -> <a class="enum" href="https://doc.rust-lang.org/nightly/core/result/enum.Result.html" title="enum core::result::Result">Result</a><Self::<a class="associatedtype" href="trait.DeserializeSeed.html#associatedtype.Value" title="type serde::de::DeserializeSeed::Value">Value</a>, D::<a class="associatedtype" href="../trait.Deserializer.html#associatedtype.Error" title="type serde::Deserializer::Error">Error</a>><br> <span class="where">where<br> D: <a class="trait" href="../trait.Deserializer.html" title="trait serde::Deserializer">Deserializer</a><'de></span>; |
| }</code></pre></div><details class="rustdoc-toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p><code>DeserializeSeed</code> is the stateful form of the <code>Deserialize</code> trait. If you |
| ever find yourself looking for a way to pass data into a <code>Deserialize</code> impl, |
| this trait is the way to do it.</p> |
| <p>As one example of stateful deserialization consider deserializing a JSON |
| array into an existing buffer. Using the <code>Deserialize</code> trait we could |
| deserialize a JSON array into a <code>Vec<T></code> but it would be a freshly allocated |
| <code>Vec<T></code>; there is no way for <code>Deserialize</code> to reuse a previously allocated |
| buffer. Using <code>DeserializeSeed</code> instead makes this possible as in the |
| example code below.</p> |
| <p>The canonical API for stateless deserialization looks like this:</p> |
| |
| <div class="example-wrap edition"><div class='tooltip' data-edition="2018">ⓘ</div><pre class="rust rust-example-rendered"><code><span class="kw">fn </span>func<<span class="lifetime">'de</span>, T: Deserialize<<span class="lifetime">'de</span>>>() -> <span class="prelude-ty">Result</span><T, Error></code></pre></div> |
| <p>Adjusting an API like this to support stateful deserialization is a matter |
| of accepting a seed as input:</p> |
| |
| <div class="example-wrap edition"><div class='tooltip' data-edition="2018">ⓘ</div><pre class="rust rust-example-rendered"><code><span class="kw">fn </span>func_seed<<span class="lifetime">'de</span>, T: DeserializeSeed<<span class="lifetime">'de</span>>>(seed: T) -> <span class="prelude-ty">Result</span><T::Value, Error></code></pre></div> |
| <p>In practice the majority of deserialization is stateless. An API expecting a |
| seed can be appeased by passing <code>std::marker::PhantomData</code> as a seed in the |
| case of stateless deserialization.</p> |
| <h2 id="lifetime"><a href="#lifetime">Lifetime</a></h2> |
| <p>The <code>'de</code> lifetime of this trait is the lifetime of data that may be |
| borrowed by <code>Self::Value</code> when deserialized. See the page <a href="https://serde.rs/lifetimes.html">Understanding |
| deserializer lifetimes</a> for a more detailed explanation of these lifetimes.</p> |
| <h2 id="example"><a href="#example">Example</a></h2> |
| <p>Suppose we have JSON that looks like <code>[[1, 2], [3, 4, 5], [6]]</code> and we need |
| to deserialize it into a flat representation like <code>vec![1, 2, 3, 4, 5, 6]</code>. |
| Allocating a brand new <code>Vec<T></code> for each subarray would be slow. Instead we |
| would like to allocate a single <code>Vec<T></code> and then deserialize each subarray |
| into it. This requires stateful deserialization using the <code>DeserializeSeed</code> |
| trait.</p> |
| |
| <div class="example-wrap edition"><div class='tooltip' data-edition="2018">ⓘ</div><pre class="rust rust-example-rendered"><code><span class="kw">use </span>std::fmt; |
| <span class="kw">use </span>std::marker::PhantomData; |
| |
| <span class="kw">use </span>serde::de::{Deserialize, DeserializeSeed, Deserializer, SeqAccess, Visitor}; |
| |
| <span class="comment">// A DeserializeSeed implementation that uses stateful deserialization to |
| // append array elements onto the end of an existing vector. The preexisting |
| // state ("seed") in this case is the Vec<T>. The `deserialize` method of |
| // `ExtendVec` will be traversing the inner arrays of the JSON input and |
| // appending each integer into the existing Vec. |
| </span><span class="kw">struct </span>ExtendVec<<span class="lifetime">'a</span>, T: <span class="lifetime">'a</span>>(<span class="kw-2">&</span><span class="lifetime">'a </span><span class="kw-2">mut </span>Vec<T>); |
| |
| <span class="kw">impl</span><<span class="lifetime">'de</span>, <span class="lifetime">'a</span>, T> DeserializeSeed<<span class="lifetime">'de</span>> <span class="kw">for </span>ExtendVec<<span class="lifetime">'a</span>, T> |
| <span class="kw">where |
| </span>T: Deserialize<<span class="lifetime">'de</span>>, |
| { |
| <span class="comment">// The return type of the `deserialize` method. This implementation |
| // appends onto an existing vector but does not create any new data |
| // structure, so the return type is (). |
| </span><span class="kw">type </span>Value = (); |
| |
| <span class="kw">fn </span>deserialize<D>(<span class="self">self</span>, deserializer: D) -> <span class="prelude-ty">Result</span><<span class="self">Self</span>::Value, D::Error> |
| <span class="kw">where |
| </span>D: Deserializer<<span class="lifetime">'de</span>>, |
| { |
| <span class="comment">// Visitor implementation that will walk an inner array of the JSON |
| // input. |
| </span><span class="kw">struct </span>ExtendVecVisitor<<span class="lifetime">'a</span>, T: <span class="lifetime">'a</span>>(<span class="kw-2">&</span><span class="lifetime">'a </span><span class="kw-2">mut </span>Vec<T>); |
| |
| <span class="kw">impl</span><<span class="lifetime">'de</span>, <span class="lifetime">'a</span>, T> Visitor<<span class="lifetime">'de</span>> <span class="kw">for </span>ExtendVecVisitor<<span class="lifetime">'a</span>, T> |
| <span class="kw">where |
| </span>T: Deserialize<<span class="lifetime">'de</span>>, |
| { |
| <span class="kw">type </span>Value = (); |
| |
| <span class="kw">fn </span>expecting(<span class="kw-2">&</span><span class="self">self</span>, formatter: <span class="kw-2">&mut </span>fmt::Formatter) -> fmt::Result { |
| <span class="macro">write!</span>(formatter, <span class="string">"an array of integers"</span>) |
| } |
| |
| <span class="kw">fn </span>visit_seq<A>(<span class="self">self</span>, <span class="kw-2">mut </span>seq: A) -> <span class="prelude-ty">Result</span><(), A::Error> |
| <span class="kw">where |
| </span>A: SeqAccess<<span class="lifetime">'de</span>>, |
| { |
| <span class="comment">// Decrease the number of reallocations if there are many elements |
| </span><span class="kw">if let </span><span class="prelude-val">Some</span>(size_hint) = seq.size_hint() { |
| <span class="self">self</span>.<span class="number">0</span>.reserve(size_hint); |
| } |
| |
| <span class="comment">// Visit each element in the inner array and push it onto |
| // the existing vector. |
| </span><span class="kw">while let </span><span class="prelude-val">Some</span>(elem) = seq.next_element()<span class="question-mark">? </span>{ |
| <span class="self">self</span>.<span class="number">0</span>.push(elem); |
| } |
| <span class="prelude-val">Ok</span>(()) |
| } |
| } |
| |
| deserializer.deserialize_seq(ExtendVecVisitor(<span class="self">self</span>.<span class="number">0</span>)) |
| } |
| } |
| |
| <span class="comment">// Visitor implementation that will walk the outer array of the JSON input. |
| </span><span class="kw">struct </span>FlattenedVecVisitor<T>(PhantomData<T>); |
| |
| <span class="kw">impl</span><<span class="lifetime">'de</span>, T> Visitor<<span class="lifetime">'de</span>> <span class="kw">for </span>FlattenedVecVisitor<T> |
| <span class="kw">where |
| </span>T: Deserialize<<span class="lifetime">'de</span>>, |
| { |
| <span class="comment">// This Visitor constructs a single Vec<T> to hold the flattened |
| // contents of the inner arrays. |
| </span><span class="kw">type </span>Value = Vec<T>; |
| |
| <span class="kw">fn </span>expecting(<span class="kw-2">&</span><span class="self">self</span>, formatter: <span class="kw-2">&mut </span>fmt::Formatter) -> fmt::Result { |
| <span class="macro">write!</span>(formatter, <span class="string">"an array of arrays"</span>) |
| } |
| |
| <span class="kw">fn </span>visit_seq<A>(<span class="self">self</span>, <span class="kw-2">mut </span>seq: A) -> <span class="prelude-ty">Result</span><Vec<T>, A::Error> |
| <span class="kw">where |
| </span>A: SeqAccess<<span class="lifetime">'de</span>>, |
| { |
| <span class="comment">// Create a single Vec to hold the flattened contents. |
| </span><span class="kw">let </span><span class="kw-2">mut </span>vec = Vec::new(); |
| |
| <span class="comment">// Each iteration through this loop is one inner array. |
| </span><span class="kw">while let </span><span class="prelude-val">Some</span>(()) = seq.next_element_seed(ExtendVec(<span class="kw-2">&mut </span>vec))<span class="question-mark">? </span>{ |
| <span class="comment">// Nothing to do; inner array has been appended into `vec`. |
| </span>} |
| |
| <span class="comment">// Return the finished vec. |
| </span><span class="prelude-val">Ok</span>(vec) |
| } |
| } |
| |
| <span class="kw">let </span>visitor = FlattenedVecVisitor(PhantomData); |
| <span class="kw">let </span>flattened: Vec<u64> = deserializer.deserialize_seq(visitor)<span class="question-mark">?</span>;</code></pre></div> |
| </div></details><h2 id="required-associated-types" class="small-section-header">Required Associated Types<a href="#required-associated-types" class="anchor"></a></h2><div class="methods"><details class="rustdoc-toggle method-toggle" open><summary><section id="associatedtype.Value" class="method has-srclink"><a class="srclink rightside" href="../../src/serde/de/mod.rs.html#771">source</a><h4 class="code-header">type <a href="#associatedtype.Value" class="associatedtype">Value</a></h4></section></summary><div class="docblock"><p>The type produced by using this seed.</p> |
| </div></details></div><h2 id="required-methods" class="small-section-header">Required Methods<a href="#required-methods" class="anchor"></a></h2><div class="methods"><details class="rustdoc-toggle method-toggle" open><summary><section id="tymethod.deserialize" class="method has-srclink"><a class="srclink rightside" href="../../src/serde/de/mod.rs.html#775-777">source</a><h4 class="code-header">fn <a href="#tymethod.deserialize" class="fnname">deserialize</a><D>(self, deserializer: D) -> <a class="enum" href="https://doc.rust-lang.org/nightly/core/result/enum.Result.html" title="enum core::result::Result">Result</a><Self::<a class="associatedtype" href="trait.DeserializeSeed.html#associatedtype.Value" title="type serde::de::DeserializeSeed::Value">Value</a>, D::<a class="associatedtype" href="../trait.Deserializer.html#associatedtype.Error" title="type serde::Deserializer::Error">Error</a>><span class="where fmt-newline">where<br> D: <a class="trait" href="../trait.Deserializer.html" title="trait serde::Deserializer">Deserializer</a><'de>,</span></h4></section></summary><div class="docblock"><p>Equivalent to the more common <code>Deserialize::deserialize</code> method, except |
| with some initial piece of data (the seed) passed in.</p> |
| </div></details></div><h2 id="foreign-impls" class="small-section-header">Implementations on Foreign Types<a href="#foreign-impls" class="anchor"></a></h2><details class="rustdoc-toggle implementors-toggle"><summary><section id="impl-DeserializeSeed%3C%27de%3E-for-PhantomData%3CT%3E" class="impl has-srclink"><a class="srclink rightside" href="../../src/serde/de/mod.rs.html#780-793">source</a><a href="#impl-DeserializeSeed%3C%27de%3E-for-PhantomData%3CT%3E" class="anchor"></a><h3 class="code-header">impl<'de, T> <a class="trait" href="trait.DeserializeSeed.html" title="trait serde::de::DeserializeSeed">DeserializeSeed</a><'de> for <a class="struct" href="https://doc.rust-lang.org/nightly/core/marker/struct.PhantomData.html" title="struct core::marker::PhantomData">PhantomData</a><T><span class="where fmt-newline">where<br> T: <a class="trait" href="../trait.Deserialize.html" title="trait serde::Deserialize">Deserialize</a><'de>,</span></h3></section></summary><div class="impl-items"><section id="associatedtype.Value-1" class="associatedtype trait-impl has-srclink"><a href="#associatedtype.Value-1" class="anchor"></a><h4 class="code-header">type <a href="#associatedtype.Value" class="associatedtype">Value</a> = T</h4></section><section id="method.deserialize" class="method trait-impl has-srclink"><a class="srclink rightside" href="../../src/serde/de/mod.rs.html#787-792">source</a><a href="#method.deserialize" class="anchor"></a><h4 class="code-header">fn <a href="#tymethod.deserialize" class="fnname">deserialize</a><D>(self, deserializer: D) -> <a class="enum" href="https://doc.rust-lang.org/nightly/core/result/enum.Result.html" title="enum core::result::Result">Result</a><T, D::<a class="associatedtype" href="../trait.Deserializer.html#associatedtype.Error" title="type serde::Deserializer::Error">Error</a>><span class="where fmt-newline">where<br> D: <a class="trait" href="../trait.Deserializer.html" title="trait serde::Deserializer">Deserializer</a><'de>,</span></h4></section></div></details><h2 id="implementors" class="small-section-header">Implementors<a href="#implementors" class="anchor"></a></h2><div id="implementors-list"></div><script src="../../implementors/serde/de/trait.DeserializeSeed.js" data-ignore-extern-crates="core" async></script></section></div></main><div id="rustdoc-vars" data-root-path="../../" data-current-crate="serde" data-themes="ayu,dark,light" data-resource-suffix="" data-rustdoc-version="1.66.0-nightly (5c8bff74b 2022-10-21)" ></div></body></html> |
