<!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 `format_args` macro in crate `std`."><meta name="keywords" content="rust, rustlang, rust-lang, format_args"><title>std::format_args - Rust</title><link rel="stylesheet" type="text/css" href="../normalize1.35.0.css"><link rel="stylesheet" type="text/css" href="../rustdoc1.35.0.css" id="mainThemeStyle"><link rel="stylesheet" type="text/css" href="../dark1.35.0.css"><link rel="stylesheet" type="text/css" href="../light1.35.0.css" id="themeStyle"><script src="../storage1.35.0.js"></script><noscript><link rel="stylesheet" href="../noscript1.35.0.css"></noscript><link rel="shortcut icon" href="../favicon1.35.0.ico"><style type="text/css">#crate-search{background-image:url("../down-arrow1.35.0.svg");}</style></head><body class="rustdoc macro"><!--[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='../std/index.html'><img src='../rust-logo1.35.0.png' alt='logo' width='100'></a><div class="sidebar-elems"><p class='location'><a href='index.html'>std</a></p><script>window.sidebarCurrent = {name: 'format_args', ty: 'macro', 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="../brush1.35.0.svg" width="18" alt="Pick another theme!"></button><div id="theme-choices"></div></div><script src="../theme1.35.0.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="../wheel1.35.0.svg" width="18" alt="Change settings"></a></div></form></nav><section id="main" class="content"><h1 class='fqn'><span class='out-of-band'><span class='since' title='Stable since Rust version 1.0.0'>1.0.0</span><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/std/macros.rs.html#523-526' title='goto source code'>[src]</a></span><span class='in-band'>Macro <a href='index.html'>std</a>::<wbr><a class="macro" href=''>format_args</a></span></h1><div class="docblock type-decl hidden-by-usual-hider"><div class="example-wrap"><pre class="rust macro">
<span class="macro">macro_rules</span><span class="macro">!</span> <span class="ident">format_args</span> {
(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">fmt</span>:<span class="ident">expr</span>) <span class="op">=></span> { ... };
(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">fmt</span>:<span class="ident">expr</span>, $(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">args</span>:<span class="ident">tt</span>)<span class="kw-2">*</span>) <span class="op">=></span> { ... };
}</pre></div>
</div><div class='docblock'><p>Constructs parameters for the other string-formatting macros.</p>
<p>This macro functions by taking a formatting string literal containing
<code>{}</code> for each additional argument passed. <code>format_args!</code> prepares the
additional parameters to ensure the output can be interpreted as a string
and canonicalizes the arguments into a single type. Any value that implements
the <a href="../std/fmt/trait.Display.html"><code>Display</code></a> trait can be passed to <code>format_args!</code>, as can any
<a href="../std/fmt/trait.Debug.html"><code>Debug</code></a> implementation be passed to a <code>{:?}</code> within the formatting string.</p>
<p>This macro produces a value of type <a href="../std/fmt/struct.Arguments.html"><code>fmt::Arguments</code></a>. This value can be
passed to the macros within <a href="../std/fmt/index.html"><code>std::fmt</code></a> for performing useful redirection.
All other formatting macros (<a href="../std/macro.format.html"><code>format!</code></a>, <a href="../std/macro.write.html"><code>write!</code></a>, <a href="../std/macro.println.html"><code>println!</code></a>, etc) are
proxied through this one. <code>format_args!</code>, unlike its derived macros, avoids
heap allocations.</p>
<p>You can use the <a href="../std/fmt/struct.Arguments.html"><code>fmt::Arguments</code></a> value that <code>format_args!</code> returns
in <code>Debug</code> and <code>Display</code> contexts as seen below. The example also shows
that <code>Debug</code> and <code>Display</code> format to the same thing: the interpolated
format string in <code>format_args!</code>.</p>
<div class="example-wrap"><pre class="rust rust-example-rendered">
<span class="kw">let</span> <span class="ident">debug</span> <span class="op">=</span> <span class="macro">format</span><span class="macro">!</span>(<span class="string">"{:?}"</span>, <span class="macro">format_args</span><span class="macro">!</span>(<span class="string">"{} foo {:?}"</span>, <span class="number">1</span>, <span class="number">2</span>));
<span class="kw">let</span> <span class="ident">display</span> <span class="op">=</span> <span class="macro">format</span><span class="macro">!</span>(<span class="string">"{}"</span>, <span class="macro">format_args</span><span class="macro">!</span>(<span class="string">"{} foo {:?}"</span>, <span class="number">1</span>, <span class="number">2</span>));
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="string">"1 foo 2"</span>, <span class="ident">display</span>);
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">display</span>, <span class="ident">debug</span>);<a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=%23!%5Ballow(unused)%5D%0Afn%20main()%20%7B%0Alet%20debug%20%3D%20format!(%22%7B%3A%3F%7D%22%2C%20format_args!(%22%7B%7D%20foo%20%7B%3A%3F%7D%22%2C%201%2C%202))%3B%0Alet%20display%20%3D%20format!(%22%7B%7D%22%2C%20format_args!(%22%7B%7D%20foo%20%7B%3A%3F%7D%22%2C%201%2C%202))%3B%0Aassert_eq!(%221%20foo%202%22%2C%20display)%3B%0Aassert_eq!(display%2C%20debug)%3B%0A%7D">Run</a></pre></div>
<p>For more information, see the documentation in <a href="../std/fmt/index.html"><code>std::fmt</code></a>.</p>
<h1 id="examples" class="section-header"><a href="#examples">Examples</a></h1>
<div class="example-wrap"><pre class="rust rust-example-rendered">
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">fmt</span>;
<span class="kw">let</span> <span class="ident">s</span> <span class="op">=</span> <span class="ident">fmt</span>::<span class="ident">format</span>(<span class="macro">format_args</span><span class="macro">!</span>(<span class="string">"hello {}"</span>, <span class="string">"world"</span>));
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">s</span>, <span class="macro">format</span><span class="macro">!</span>(<span class="string">"hello {}"</span>, <span class="string">"world"</span>));<a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=%23!%5Ballow(unused)%5D%0Afn%20main()%20%7B%0Ause%20std%3A%3Afmt%3B%0A%0Alet%20s%20%3D%20fmt%3A%3Aformat(format_args!(%22hello%20%7B%7D%22%2C%20%22world%22))%3B%0Aassert_eq!(s%2C%20format!(%22hello%20%7B%7D%22%2C%20%22world%22))%3B%0A%7D">Run</a></pre></div>
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