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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="API documentation for the Rust `from_reader` fn in crate `serde_json`."><meta name="keywords" content="rust, rustlang, rust-lang, from_reader"><title>serde_json::from_reader - Rust</title><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="../dark.css"><link rel="stylesheet" type="text/css" href="../light.css" id="themeStyle"><script src="../storage.js"></script><noscript><link rel="stylesheet" href="../noscript.css"></noscript><link rel="shortcut icon" href="../favicon.ico"><style type="text/css">#crate-search{background-image:url("../down-arrow.svg");}</style></head><body class="rustdoc fn"><!--[if lte IE 8]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="sidebar"><div class="sidebar-menu">☰</div><a href='../serde_json/index.html'><div class='logo-container'><img src='../rust-logo.png' alt='logo'></div></a><div class="sidebar-elems"><p class='location'><a href='index.html'>serde_json</a></p><script>window.sidebarCurrent = {name: 'from_reader', ty: 'fn', relpath: ''};</script><script defer src="sidebar-items.js"></script></div></nav><div class="theme-picker"><button id="theme-picker" aria-label="Pick another theme!"><img src="../brush.svg" width="18" alt="Pick another theme!"></button><div id="theme-choices"></div></div><script src="../theme.js"></script><nav class="sub"><form class="search-form js-only"><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><a id="settings-menu" href="../settings.html"><img src="../wheel.svg" width="18" alt="Change settings"></a></div></form></nav><section id="main" class="content"><h1 class='fqn'><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#2244-2250' title='goto source code'>[src]</a></span><span class='in-band'>Function <a href='index.html'>serde_json</a>::<wbr><a class="fn" href=''>from_reader</a></span></h1><pre class='rust fn'>pub fn from_reader<R, T>(rdr: R) -> <a class="type" href="../serde_json/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/nightly/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></pre><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/nightly/std/io/buffered/struct.BufReader.html" title="`std::io::BufReader`"><code>std::io::BufReader</code></a>.</p>
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<h1 id="example" class="section-header"><a href="#example">Example</a></h1>
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<div class='information'><div class='tooltip edition'>ⓘ<span class='tooltiptext'>This code runs with edition 2018</span></div></div><div class="example-wrap"><pre class="rust rust-example-rendered edition ">
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<span class="kw">use</span> <span class="ident">serde</span>::<span class="ident">Deserialize</span>;
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<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">error</span>::<span class="ident">Error</span>;
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<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">fs</span>::<span class="ident">File</span>;
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<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">io</span>::<span class="ident">BufReader</span>;
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<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">path</span>::<span class="ident">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="op">-</span><span class="op">></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="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</span>::<span class="ident">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</span>::<span class="ident">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</span>::<span class="ident">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="macro">!</span>(<span class="string">"{:#?}"</span>, <span class="ident">u</span>);
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}</pre></div>
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<h1 id="errors" class="section-header"><a href="#errors">Errors</a></h1>
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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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