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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="Serde JSON"><title>serde_json - Rust</title><link rel="preload" as="font" type="font/woff2" crossorigin href="../static.files/SourceSerif4-Regular-46f98efaafac5295.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../static.files/FiraSans-Regular-018c141bf0843ffd.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../static.files/FiraSans-Medium-8f9a781e4970d388.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../static.files/SourceCodePro-Regular-562dcc5011b6de7d.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../static.files/SourceCodePro-Semibold-d899c5a5c4aeb14a.ttf.woff2"><link rel="stylesheet" href="../static.files/normalize-76eba96aa4d2e634.css"><link rel="stylesheet" href="../static.files/rustdoc-ac92e1bbe349e143.css"><meta name="rustdoc-vars" data-root-path="../" data-static-root-path="../static.files/" data-current-crate="serde_json" data-themes="" data-resource-suffix="" data-rustdoc-version="1.76.0 (07dca489a 2024-02-04)" data-channel="1.76.0" data-search-js="search-2b6ce74ff89ae146.js" data-settings-js="settings-4313503d2e1961c2.js" ><script src="../static.files/storage-f2adc0d6ca4d09fb.js"></script><script defer src="../crates.js"></script><script defer src="../static.files/main-305769736d49e732.js"></script><noscript><link rel="stylesheet" href="../static.files/noscript-feafe1bb7466e4bd.css"></noscript><link rel="alternate icon" type="image/png" href="../static.files/favicon-16x16-8b506e7a72182f1c.png"><link rel="alternate icon" type="image/png" href="../static.files/favicon-32x32-422f7d1d52889060.png"><link rel="icon" type="image/svg+xml" href="../static.files/favicon-2c020d218678b618.svg"></head><body class="rustdoc mod crate"><!--[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></nav><nav class="sidebar"><div class="sidebar-crate"><h2><a href="../serde_json/index.html">serde_json</a><span class="version">1.0.107</span></h2></div><div class="sidebar-elems"><ul class="block">
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<li><a id="all-types" href="all.html">All Items</a></li></ul><section><ul class="block"><li><a href="#modules">Modules</a></li><li><a href="#macros">Macros</a></li><li><a href="#structs">Structs</a></li><li><a href="#enums">Enums</a></li><li><a href="#functions">Functions</a></li><li><a href="#types">Type Aliases</a></li></ul></section></div></nav><div class="sidebar-resizer"></div>
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<main><div class="width-limiter"><nav class="sub"><form class="search-form"><span></span><div id="sidebar-button" tabindex="-1"><a href="../serde_json/all.html" title="show sidebar"></a></div><input class="search-input" name="search" aria-label="Run search in the documentation" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><div id="help-button" tabindex="-1"><a href="../help.html" title="help">?</a></div><div id="settings-menu" tabindex="-1"><a href="../settings.html" title="settings"><img width="22" height="22" alt="Change settings" src="../static.files/wheel-7b819b6101059cd0.svg"></a></div></form></nav><section id="main-content" class="content"><div class="main-heading"><h1>Crate <a class="mod" href="#">serde_json</a><button id="copy-path" title="Copy item path to clipboard"><img src="../static.files/clipboard-7571035ce49a181d.svg" width="19" height="18" alt="Copy item path"></button></h1><span class="out-of-band"><a class="src" href="../src/serde_json/lib.rs.html#1-417">source</a> · <button id="toggle-all-docs" title="collapse all docs">[<span>−</span>]</button></span></div><details class="toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><h2 id="serde-json"><a href="#serde-json">Serde JSON</a></h2>
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<p>JSON is a ubiquitous open-standard format that uses human-readable text to
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transmit data objects consisting of key-value pairs.</p>
|
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<div class="example-wrap"><pre class="language-json"><code>{
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"name": "John Doe",
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"age": 43,
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"address": {
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"street": "10 Downing Street",
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"city": "London"
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},
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"phones": [
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"+44 1234567",
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"+44 2345678"
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]
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}
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</code></pre></div>
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<p>There are three common ways that you might find yourself needing to work
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with JSON data in Rust.</p>
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<ul>
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<li><strong>As text data.</strong> An unprocessed string of JSON data that you receive on
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an HTTP endpoint, read from a file, or prepare to send to a remote
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server.</li>
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||
<li><strong>As an untyped or loosely typed representation.</strong> Maybe you want to
|
||
check that some JSON data is valid before passing it on, but without
|
||
knowing the structure of what it contains. Or you want to do very basic
|
||
manipulations like insert a key in a particular spot.</li>
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<li><strong>As a strongly typed Rust data structure.</strong> When you expect all or most
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of your data to conform to a particular structure and want to get real
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||
work done without JSON’s loosey-goosey nature tripping you up.</li>
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||
</ul>
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<p>Serde JSON provides efficient, flexible, safe ways of converting data
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between each of these representations.</p>
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<h2 id="operating-on-untyped-json-values"><a href="#operating-on-untyped-json-values">Operating on untyped JSON values</a></h2>
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<p>Any valid JSON data can be manipulated in the following recursive enum
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||
representation. This data structure is <a href="enum.Value.html" title="enum serde_json::Value"><code>serde_json::Value</code></a>.</p>
|
||
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">enum </span>Value {
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Null,
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Bool(bool),
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Number(Number),
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String(String),
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Array(Vec<Value>),
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Object(Map<String, Value>),
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}</code></pre></div>
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<p>A string of JSON data can be parsed into a <code>serde_json::Value</code> by the
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<a href="fn.from_str.html" title="fn serde_json::from_str"><code>serde_json::from_str</code></a> function. There is also <a href="fn.from_slice.html" title="fn serde_json::from_slice"><code>from_slice</code></a>
|
||
for parsing from a byte slice &[u8] and <a href="fn.from_reader.html" title="fn serde_json::from_reader"><code>from_reader</code></a> for parsing from
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||
any <code>io::Read</code> like a File or a TCP stream.</p>
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||
|
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>serde_json::{<span class="prelude-ty">Result</span>, Value};
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||
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<span class="kw">fn </span>untyped_example() -> <span class="prelude-ty">Result</span><()> {
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<span class="comment">// Some JSON input data as a &str. Maybe this comes from the user.
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</span><span class="kw">let </span>data = <span class="string">r#"
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{
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"name": "John Doe",
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"age": 43,
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"phones": [
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"+44 1234567",
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"+44 2345678"
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]
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}"#</span>;
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||
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<span class="comment">// Parse the string of data into serde_json::Value.
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</span><span class="kw">let </span>v: Value = serde_json::from_str(data)<span class="question-mark">?</span>;
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||
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<span class="comment">// Access parts of the data by indexing with square brackets.
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</span><span class="macro">println!</span>(<span class="string">"Please call {} at the number {}"</span>, v[<span class="string">"name"</span>], v[<span class="string">"phones"</span>][<span class="number">0</span>]);
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<span class="prelude-val">Ok</span>(())
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}</code></pre></div>
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<p>The result of square bracket indexing like <code>v["name"]</code> is a borrow of the
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data at that index, so the type is <code>&Value</code>. A JSON map can be indexed with
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string keys, while a JSON array can be indexed with integer keys. If the
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type of the data is not right for the type with which it is being indexed,
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or if a map does not contain the key being indexed, or if the index into a
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vector is out of bounds, the returned element is <code>Value::Null</code>.</p>
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<p>When a <code>Value</code> is printed, it is printed as a JSON string. So in the code
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above, the output looks like <code>Please call "John Doe" at the number "+44 1234567"</code>. The quotation marks appear because <code>v["name"]</code> is a <code>&Value</code>
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containing a JSON string and its JSON representation is <code>"John Doe"</code>.
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Printing as a plain string without quotation marks involves converting from
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||
a JSON string to a Rust string with <a href="enum.Value.html#method.as_str" title="method serde_json::Value::as_str"><code>as_str()</code></a> or avoiding the use of
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<code>Value</code> as described in the following section.</p>
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<p>The <code>Value</code> representation is sufficient for very basic tasks but can be
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tedious to work with for anything more significant. Error handling is
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verbose to implement correctly, for example imagine trying to detect the
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presence of unrecognized fields in the input data. The compiler is powerless
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to help you when you make a mistake, for example imagine typoing <code>v["name"]</code>
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as <code>v["nmae"]</code> in one of the dozens of places it is used in your code.</p>
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<h2 id="parsing-json-as-strongly-typed-data-structures"><a href="#parsing-json-as-strongly-typed-data-structures">Parsing JSON as strongly typed data structures</a></h2>
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<p>Serde provides a powerful way of mapping JSON data into Rust data structures
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largely automatically.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>serde::{Deserialize, Serialize};
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<span class="kw">use </span>serde_json::Result;
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<span class="attr">#[derive(Serialize, Deserialize)]
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</span><span class="kw">struct </span>Person {
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name: String,
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age: u8,
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phones: Vec<String>,
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}
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<span class="kw">fn </span>typed_example() -> <span class="prelude-ty">Result</span><()> {
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<span class="comment">// Some JSON input data as a &str. Maybe this comes from the user.
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</span><span class="kw">let </span>data = <span class="string">r#"
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{
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"name": "John Doe",
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"age": 43,
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"phones": [
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"+44 1234567",
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"+44 2345678"
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]
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}"#</span>;
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<span class="comment">// Parse the string of data into a Person object. This is exactly the
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// same function as the one that produced serde_json::Value above, but
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// now we are asking it for a Person as output.
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</span><span class="kw">let </span>p: Person = serde_json::from_str(data)<span class="question-mark">?</span>;
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<span class="comment">// Do things just like with any other Rust data structure.
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</span><span class="macro">println!</span>(<span class="string">"Please call {} at the number {}"</span>, p.name, p.phones[<span class="number">0</span>]);
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<span class="prelude-val">Ok</span>(())
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}</code></pre></div>
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<p>This is the same <code>serde_json::from_str</code> function as before, but this time we
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assign the return value to a variable of type <code>Person</code> so Serde will
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automatically interpret the input data as a <code>Person</code> and produce informative
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error messages if the layout does not conform to what a <code>Person</code> is expected
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to look like.</p>
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<p>Any type that implements Serde’s <code>Deserialize</code> trait can be deserialized
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||
this way. This includes built-in Rust standard library types like <code>Vec<T></code>
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||
and <code>HashMap<K, V></code>, as well as any structs or enums annotated with
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<code>#[derive(Deserialize)]</code>.</p>
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||
<p>Once we have <code>p</code> of type <code>Person</code>, our IDE and the Rust compiler can help us
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use it correctly like they do for any other Rust code. The IDE can
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autocomplete field names to prevent typos, which was impossible in the
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<code>serde_json::Value</code> representation. And the Rust compiler can check that
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when we write <code>p.phones[0]</code>, then <code>p.phones</code> is guaranteed to be a
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<code>Vec<String></code> so indexing into it makes sense and produces a <code>String</code>.</p>
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<h2 id="constructing-json-values"><a href="#constructing-json-values">Constructing JSON values</a></h2>
|
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<p>Serde JSON provides a <a href="macro.json.html" title="macro serde_json::json"><code>json!</code> macro</a> to build <code>serde_json::Value</code>
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||
objects with very natural JSON syntax.</p>
|
||
|
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>serde_json::json;
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<span class="kw">fn </span>main() {
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<span class="comment">// The type of `john` is `serde_json::Value`
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</span><span class="kw">let </span>john = <span class="macro">json!</span>({
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<span class="string">"name"</span>: <span class="string">"John Doe"</span>,
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<span class="string">"age"</span>: <span class="number">43</span>,
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<span class="string">"phones"</span>: [
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<span class="string">"+44 1234567"</span>,
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<span class="string">"+44 2345678"
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</span>]
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});
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<span class="macro">println!</span>(<span class="string">"first phone number: {}"</span>, john[<span class="string">"phones"</span>][<span class="number">0</span>]);
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<span class="comment">// Convert to a string of JSON and print it out
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</span><span class="macro">println!</span>(<span class="string">"{}"</span>, john.to_string());
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||
}</code></pre></div>
|
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<p>The <code>Value::to_string()</code> function converts a <code>serde_json::Value</code> into a
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||
<code>String</code> of JSON text.</p>
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||
<p>One neat thing about the <code>json!</code> macro is that variables and expressions can
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||
be interpolated directly into the JSON value as you are building it. Serde
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||
will check at compile time that the value you are interpolating is able to
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||
be represented as JSON.</p>
|
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">let </span>full_name = <span class="string">"John Doe"</span>;
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<span class="kw">let </span>age_last_year = <span class="number">42</span>;
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<span class="comment">// The type of `john` is `serde_json::Value`
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</span><span class="kw">let </span>john = <span class="macro">json!</span>({
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<span class="string">"name"</span>: full_name,
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<span class="string">"age"</span>: age_last_year + <span class="number">1</span>,
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<span class="string">"phones"</span>: [
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<span class="macro">format!</span>(<span class="string">"+44 {}"</span>, random_phone())
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]
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||
});</code></pre></div>
|
||
<p>This is amazingly convenient, but we have the problem we had before with
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||
<code>Value</code>: the IDE and Rust compiler cannot help us if we get it wrong. Serde
|
||
JSON provides a better way of serializing strongly-typed data structures
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||
into JSON text.</p>
|
||
<h2 id="creating-json-by-serializing-data-structures"><a href="#creating-json-by-serializing-data-structures">Creating JSON by serializing data structures</a></h2>
|
||
<p>A data structure can be converted to a JSON string by
|
||
<a href="fn.to_string.html" title="fn serde_json::to_string"><code>serde_json::to_string</code></a>. There is also
|
||
<a href="fn.to_vec.html" title="fn serde_json::to_vec"><code>serde_json::to_vec</code></a> which serializes to a <code>Vec<u8></code> and
|
||
<a href="fn.to_writer.html" title="fn serde_json::to_writer"><code>serde_json::to_writer</code></a> which serializes to any <code>io::Write</code>
|
||
such as a File or a TCP stream.</p>
|
||
|
||
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>serde::{Deserialize, Serialize};
|
||
<span class="kw">use </span>serde_json::Result;
|
||
|
||
<span class="attr">#[derive(Serialize, Deserialize)]
|
||
</span><span class="kw">struct </span>Address {
|
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street: String,
|
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city: String,
|
||
}
|
||
|
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<span class="kw">fn </span>print_an_address() -> <span class="prelude-ty">Result</span><()> {
|
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<span class="comment">// Some data structure.
|
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</span><span class="kw">let </span>address = Address {
|
||
street: <span class="string">"10 Downing Street"</span>.to_owned(),
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city: <span class="string">"London"</span>.to_owned(),
|
||
};
|
||
|
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<span class="comment">// Serialize it to a JSON string.
|
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</span><span class="kw">let </span>j = serde_json::to_string(<span class="kw-2">&</span>address)<span class="question-mark">?</span>;
|
||
|
||
<span class="comment">// Print, write to a file, or send to an HTTP server.
|
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</span><span class="macro">println!</span>(<span class="string">"{}"</span>, j);
|
||
|
||
<span class="prelude-val">Ok</span>(())
|
||
}</code></pre></div>
|
||
<p>Any type that implements Serde’s <code>Serialize</code> trait can be serialized this
|
||
way. This includes built-in Rust standard library types like <code>Vec<T></code> and
|
||
<code>HashMap<K, V></code>, as well as any structs or enums annotated with
|
||
<code>#[derive(Serialize)]</code>.</p>
|
||
<h2 id="no-std-support"><a href="#no-std-support">No-std support</a></h2>
|
||
<p>As long as there is a memory allocator, it is possible to use serde_json
|
||
without the rest of the Rust standard library. Disable the default “std”
|
||
feature and enable the “alloc” feature:</p>
|
||
<div class="example-wrap"><pre class="language-toml"><code>[dependencies]
|
||
serde_json = { version = "1.0", default-features = false, features = ["alloc"] }
|
||
</code></pre></div>
|
||
<p>For JSON support in Serde without a memory allocator, please see the
|
||
<a href="https://github.com/rust-embedded-community/serde-json-core"><code>serde-json-core</code></a> crate.</p>
|
||
</div></details><h2 id="modules" class="section-header"><a href="#modules">Modules</a></h2><ul class="item-table"><li><div class="item-name"><a class="mod" href="de/index.html" title="mod serde_json::de">de</a></div><div class="desc docblock-short">Deserialize JSON data to a Rust data structure.</div></li><li><div class="item-name"><a class="mod" href="error/index.html" title="mod serde_json::error">error</a></div><div class="desc docblock-short">When serializing or deserializing JSON goes wrong.</div></li><li><div class="item-name"><a class="mod" href="map/index.html" title="mod serde_json::map">map</a></div><div class="desc docblock-short">A map of String to serde_json::Value.</div></li><li><div class="item-name"><a class="mod" href="ser/index.html" title="mod serde_json::ser">ser</a></div><div class="desc docblock-short">Serialize a Rust data structure into JSON data.</div></li><li><div class="item-name"><a class="mod" href="value/index.html" title="mod serde_json::value">value</a></div><div class="desc docblock-short">The Value enum, a loosely typed way of representing any valid JSON value.</div></li></ul><h2 id="macros" class="section-header"><a href="#macros">Macros</a></h2><ul class="item-table"><li><div class="item-name"><a class="macro" href="macro.json.html" title="macro serde_json::json">json</a></div><div class="desc docblock-short">Construct a <code>serde_json::Value</code> from a JSON literal.</div></li></ul><h2 id="structs" class="section-header"><a href="#structs">Structs</a></h2><ul class="item-table"><li><div class="item-name"><a class="struct" href="struct.Deserializer.html" title="struct serde_json::Deserializer">Deserializer</a></div><div class="desc docblock-short">A structure that deserializes JSON into Rust values.</div></li><li><div class="item-name"><a class="struct" href="struct.Error.html" title="struct serde_json::Error">Error</a></div><div class="desc docblock-short">This type represents all possible errors that can occur when serializing or
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deserializing JSON data.</div></li><li><div class="item-name"><a class="struct" href="struct.Map.html" title="struct serde_json::Map">Map</a></div><div class="desc docblock-short">Represents a JSON key/value type.</div></li><li><div class="item-name"><a class="struct" href="struct.Number.html" title="struct serde_json::Number">Number</a></div><div class="desc docblock-short">Represents a JSON number, whether integer or floating point.</div></li><li><div class="item-name"><a class="struct" href="struct.Serializer.html" title="struct serde_json::Serializer">Serializer</a></div><div class="desc docblock-short">A structure for serializing Rust values into JSON.</div></li><li><div class="item-name"><a class="struct" href="struct.StreamDeserializer.html" title="struct serde_json::StreamDeserializer">StreamDeserializer</a></div><div class="desc docblock-short">Iterator that deserializes a stream into multiple JSON values.</div></li></ul><h2 id="enums" class="section-header"><a href="#enums">Enums</a></h2><ul class="item-table"><li><div class="item-name"><a class="enum" href="enum.Value.html" title="enum serde_json::Value">Value</a></div><div class="desc docblock-short">Represents any valid JSON value.</div></li></ul><h2 id="functions" class="section-header"><a href="#functions">Functions</a></h2><ul class="item-table"><li><div class="item-name"><a class="fn" href="fn.from_reader.html" title="fn serde_json::from_reader">from_reader</a></div><div class="desc docblock-short">Deserialize an instance of type <code>T</code> from an I/O stream of JSON.</div></li><li><div class="item-name"><a class="fn" href="fn.from_slice.html" title="fn serde_json::from_slice">from_slice</a></div><div class="desc docblock-short">Deserialize an instance of type <code>T</code> from bytes of JSON text.</div></li><li><div class="item-name"><a class="fn" href="fn.from_str.html" title="fn serde_json::from_str">from_str</a></div><div class="desc docblock-short">Deserialize an instance of type <code>T</code> from a string of JSON text.</div></li><li><div class="item-name"><a class="fn" href="fn.from_value.html" title="fn serde_json::from_value">from_value</a></div><div class="desc docblock-short">Interpret a <code>serde_json::Value</code> as an instance of type <code>T</code>.</div></li><li><div class="item-name"><a class="fn" href="fn.to_string.html" title="fn serde_json::to_string">to_string</a></div><div class="desc docblock-short">Serialize the given data structure as a String of JSON.</div></li><li><div class="item-name"><a class="fn" href="fn.to_string_pretty.html" title="fn serde_json::to_string_pretty">to_string_pretty</a></div><div class="desc docblock-short">Serialize the given data structure as a pretty-printed String of JSON.</div></li><li><div class="item-name"><a class="fn" href="fn.to_value.html" title="fn serde_json::to_value">to_value</a></div><div class="desc docblock-short">Convert a <code>T</code> into <code>serde_json::Value</code> which is an enum that can represent
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any valid JSON data.</div></li><li><div class="item-name"><a class="fn" href="fn.to_vec.html" title="fn serde_json::to_vec">to_vec</a></div><div class="desc docblock-short">Serialize the given data structure as a JSON byte vector.</div></li><li><div class="item-name"><a class="fn" href="fn.to_vec_pretty.html" title="fn serde_json::to_vec_pretty">to_vec_pretty</a></div><div class="desc docblock-short">Serialize the given data structure as a pretty-printed JSON byte vector.</div></li><li><div class="item-name"><a class="fn" href="fn.to_writer.html" title="fn serde_json::to_writer">to_writer</a></div><div class="desc docblock-short">Serialize the given data structure as JSON into the I/O stream.</div></li><li><div class="item-name"><a class="fn" href="fn.to_writer_pretty.html" title="fn serde_json::to_writer_pretty">to_writer_pretty</a></div><div class="desc docblock-short">Serialize the given data structure as pretty-printed JSON into the I/O
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stream.</div></li></ul><h2 id="types" class="section-header"><a href="#types">Type Aliases</a></h2><ul class="item-table"><li><div class="item-name"><a class="type" href="type.Result.html" title="type serde_json::Result">Result</a></div><div class="desc docblock-short">Alias for a <code>Result</code> with the error type <code>serde_json::Error</code>.</div></li></ul></section></div></main></body></html> |