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<!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="Deserialize an instance of type `T` from an IO stream of JSON."><meta name="keywords" content="rust, rustlang, rust-lang, from_reader"><title>from_reader in serde_json::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" type="text/css" href="../../normalize.css"><link rel="stylesheet" type="text/css" href="../../rustdoc.css" id="mainThemeStyle"><link rel="stylesheet" type="text/css" href="../../ayu.css" disabled><link rel="stylesheet" type="text/css" href="../../dark.css" disabled><link rel="stylesheet" type="text/css" href="../../light.css" id="themeStyle"><script id="default-settings" ></script><script src="../../storage.js"></script><script src="../../crates.js"></script><script defer src="../../main.js"></script>
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</a><div class="sidebar-elems"><h2 class="location">Other items in<br><a href="../index.html">serde_json</a>::<wbr><a href="index.html">de</a></h2><div id="sidebar-vars" data-name="from_reader" data-ty="fn" data-relpath=""></div><script defer src="sidebar-items.js"></script></div></nav><main><div class="width-limiter"><div class="sub-container"><a class="sub-logo-container" href="../../serde_json/index.html"><img class="rust-logo" src="../../rust-logo.png" alt="logo"></a><nav class="sub"><div class="theme-picker"><button id="theme-picker" aria-label="Pick another theme!" aria-haspopup="menu" title="themes"><img width="18" height="18" alt="Pick another theme!" src="../../brush.svg"></button><div id="theme-choices" role="menu"></div></div><form class="search-form"><div class="search-container"><div><select id="crate-search"><option value="All crates">All crates</option></select><input class="search-input" name="search" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"></div><button type="button" id="help-button" title="help">?</button><a id="settings-menu" href="../../settings.html" title="settings"><img width="18" height="18" alt="Change settings" src="../../wheel.svg"></a></div></form></nav></div><section id="main-content" class="content"><h1 class="fqn"><span class="in-band">Function <a href="../index.html">serde_json</a>::<wbr><a href="index.html">de</a>::<wbr><a class="fn" href="#">from_reader</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></span><span class="out-of-band"><span id="render-detail"><a id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs">[<span class="inner">−</span>]</a></span><a class="srclink" href="../../src/serde_json/de.rs.html#2524-2530" title="goto source code">[src]</a></span></h1><div class="docblock item-decl"><pre class="rust fn"><code>pub fn from_reader<R, T>(rdr: R) -> <a class="type" href="../type.Result.html" title="type serde_json::Result">Result</a><T> <span class="where fmt-newline">where<br> R: <a class="trait" href="https://doc.rust-lang.org/1.59.0/std/io/trait.Read.html" title="trait std::io::Read">Read</a>,<br> T: <a class="trait" href="../../serde/de/trait.DeserializeOwned.html" title="trait serde::de::DeserializeOwned">DeserializeOwned</a>, </span></code></pre></div><details class="rustdoc-toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>Deserialize an instance of type <code>T</code> from an IO stream of JSON.</p>
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<p>The content of the IO stream is deserialized directly from the stream
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without being buffered in memory by serde_json.</p>
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<p>When reading from a source against which short reads are not efficient, such
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as a <a href="https://doc.rust-lang.org/std/fs/struct.File.html"><code>File</code></a>, you will want to apply your own buffering because serde_json
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will not buffer the input. See <a href="https://doc.rust-lang.org/std/io/struct.BufReader.html"><code>std::io::BufReader</code></a>.</p>
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<p>It is expected that the input stream ends after the deserialized object.
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If the stream does not end, such as in the case of a persistent socket connection,
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this function will not return. It is possible instead to deserialize from a prefix of an input
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stream without looking for EOF by managing your own <a href="../struct.Deserializer.html" title="Deserializer"><code>Deserializer</code></a>.</p>
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<p>Note that counter to intuition, this function is usually slower than
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reading a file completely into memory and then applying <a href="./fn.from_str.html"><code>from_str</code></a>
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or <a href="./fn.from_slice.html"><code>from_slice</code></a> on it. See <a href="https://github.com/serde-rs/json/issues/160">issue #160</a>.</p>
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<h2 id="example" class="section-header"><a href="#example">Example</a></h2>
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<p>Reading the contents of a file.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use</span> <span class="ident">serde::Deserialize</span>;
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<span class="kw">use</span> <span class="ident">std::error::Error</span>;
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<span class="kw">use</span> <span class="ident">std::fs::File</span>;
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<span class="kw">use</span> <span class="ident">std::io::BufReader</span>;
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<span class="kw">use</span> <span class="ident">std::path::Path</span>;
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<span class="attribute">#[<span class="ident">derive</span>(<span class="ident">Deserialize</span>, <span class="ident">Debug</span>)]</span>
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<span class="kw">struct</span> <span class="ident">User</span> {
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<span class="ident">fingerprint</span>: <span class="ident">String</span>,
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<span class="ident">location</span>: <span class="ident">String</span>,
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}
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<span class="kw">fn</span> <span class="ident">read_user_from_file</span><span class="op"><</span><span class="ident">P</span>: <span class="ident">AsRef</span><span class="op"><</span><span class="ident">Path</span><span class="op">></span><span class="op">></span>(<span class="ident">path</span>: <span class="ident">P</span>) -> <span class="prelude-ty">Result</span><span class="op"><</span><span class="ident">User</span>, <span class="ident">Box</span><span class="op"><</span><span class="kw">dyn</span> <span class="ident">Error</span><span class="op">></span><span class="op">></span> {
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<span class="comment">// Open the file in read-only mode with buffer.</span>
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<span class="kw">let</span> <span class="ident">file</span> <span class="op">=</span> <span class="ident">File::open</span>(<span class="ident">path</span>)<span class="question-mark">?</span>;
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<span class="kw">let</span> <span class="ident">reader</span> <span class="op">=</span> <span class="ident">BufReader::new</span>(<span class="ident">file</span>);
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<span class="comment">// Read the JSON contents of the file as an instance of `User`.</span>
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<span class="kw">let</span> <span class="ident">u</span> <span class="op">=</span> <span class="ident">serde_json::from_reader</span>(<span class="ident">reader</span>)<span class="question-mark">?</span>;
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<span class="comment">// Return the `User`.</span>
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<span class="prelude-val">Ok</span>(<span class="ident">u</span>)
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}
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<span class="kw">fn</span> <span class="ident">main</span>() {
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<span class="kw">let</span> <span class="ident">u</span> <span class="op">=</span> <span class="ident">read_user_from_file</span>(<span class="string">"test.json"</span>).<span class="ident">unwrap</span>();
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<span class="macro">println!</span>(<span class="string">"{:#?}"</span>, <span class="ident">u</span>);
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}</code></pre></div>
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<p>Reading from a persistent socket connection.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use</span> <span class="ident">serde::Deserialize</span>;
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<span class="kw">use</span> <span class="ident">std::error::Error</span>;
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<span class="kw">use</span> <span class="ident">std::net</span>::{<span class="ident">TcpListener</span>, <span class="ident">TcpStream</span>};
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<span class="attribute">#[<span class="ident">derive</span>(<span class="ident">Deserialize</span>, <span class="ident">Debug</span>)]</span>
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<span class="kw">struct</span> <span class="ident">User</span> {
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<span class="ident">fingerprint</span>: <span class="ident">String</span>,
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<span class="ident">location</span>: <span class="ident">String</span>,
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}
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<span class="kw">fn</span> <span class="ident">read_user_from_stream</span>(<span class="ident">tcp_stream</span>: <span class="ident">TcpStream</span>) -> <span class="prelude-ty">Result</span><span class="op"><</span><span class="ident">User</span>, <span class="ident">Box</span><span class="op"><</span><span class="kw">dyn</span> <span class="ident">Error</span><span class="op">></span><span class="op">></span> {
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<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">de</span> <span class="op">=</span> <span class="ident">serde_json::Deserializer::from_reader</span>(<span class="ident">tcp_stream</span>);
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<span class="kw">let</span> <span class="ident">u</span> <span class="op">=</span> <span class="ident">User::deserialize</span>(<span class="kw-2">&mut</span> <span class="ident">de</span>)<span class="question-mark">?</span>;
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<span class="prelude-val">Ok</span>(<span class="ident">u</span>)
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}
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<span class="kw">fn</span> <span class="ident">main</span>() {
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<span class="kw">let</span> <span class="ident">listener</span> <span class="op">=</span> <span class="ident">TcpListener::bind</span>(<span class="string">"127.0.0.1:4000"</span>).<span class="ident">unwrap</span>();
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<span class="kw">for</span> <span class="ident">stream</span> <span class="kw">in</span> <span class="ident">listener</span>.<span class="ident">incoming</span>() {
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<span class="macro">println!</span>(<span class="string">"{:#?}"</span>, <span class="ident">read_user_from_stream</span>(<span class="ident">stream</span>.<span class="ident">unwrap</span>()));
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}
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}</code></pre></div>
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<h2 id="errors" class="section-header"><a href="#errors">Errors</a></h2>
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<p>This conversion can fail if the structure of the input does not match the
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structure expected by <code>T</code>, for example if <code>T</code> is a struct type but the input
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contains something other than a JSON map. It can also fail if the structure
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is correct but <code>T</code>’s implementation of <code>Deserialize</code> decides that something
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is wrong with the data, for example required struct fields are missing from
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the JSON map or some number is too big to fit in the expected primitive
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type.</p>
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