<!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 `writeln` macro in crate `std`."><meta name="keywords" content="rust, rustlang, rust-lang, writeln"><title>std::writeln - 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><link rel="shortcut icon" href="https://doc.rust-lang.org/favicon.ico"></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='https://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png' alt='logo' width='100'></a><div class="sidebar-elems"><p class='location'><a href='index.html'>std</a></p><script>window.sidebarCurrent = {name: 'writeln', 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="../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"><input class="search-input" name="search" autocomplete="off" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><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='in-band'>Macro <a href='index.html'>std</a>::<wbr><a class="macro" href=''>writeln</a></span><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/core/macros.rs.html#399-412' title='goto source code'>[src]</a></span></h1><div class="docblock type-decl"><pre class="rust macro">
<span class="macro">macro_rules</span><span class="macro">!</span> <span class="ident">writeln</span> {
( $ <span class="ident">dst</span> : <span class="ident">expr</span> ) <span class="op">=></span> { ... };
( $ <span class="ident">dst</span> : <span class="ident">expr</span> , ) <span class="op">=></span> { ... };
( $ <span class="ident">dst</span> : <span class="ident">expr</span> , $ <span class="ident">fmt</span> : <span class="ident">expr</span> ) <span class="op">=></span> { ... };
(
$ <span class="ident">dst</span> : <span class="ident">expr</span> , $ <span class="ident">fmt</span> : <span class="ident">expr</span> , $ ( $ <span class="ident">arg</span> : <span class="ident">tt</span> ) <span class="op">*</span> ) <span class="op">=></span> { ... };
}</pre>
</div><div class='docblock'><p>Write formatted data into a buffer, with a newline appended.</p>
<p>On all platforms, the newline is the LINE FEED character (<code>\n</code>/<code>U+000A</code>) alone
(no additional CARRIAGE RETURN (<code>\r</code>/<code>U+000D</code>).</p>
<p>For more information, see <a href="macro.write.html"><code>write!</code></a>. For information on the format string syntax, see
<a href="../std/fmt/index.html"><code>std::fmt</code></a>.</p>
<h1 id="examples" class="section-header"><a href="#examples">Examples</a></h1>
<pre class="rust rust-example-rendered">
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">io</span>::<span class="ident">Write</span>;
<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">w</span> <span class="op">=</span> <span class="ident">Vec</span>::<span class="ident">new</span>();
<span class="macro">writeln</span><span class="macro">!</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">w</span>).<span class="ident">unwrap</span>();
<span class="macro">writeln</span><span class="macro">!</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">w</span>, <span class="string">"test"</span>).<span class="ident">unwrap</span>();
<span class="macro">writeln</span><span class="macro">!</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">w</span>, <span class="string">"formatted {}"</span>, <span class="string">"arguments"</span>).<span class="ident">unwrap</span>();
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="kw-2">&</span><span class="ident">w</span>[..], <span class="string">"\ntest\nformatted arguments\n"</span>.<span class="ident">as_bytes</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%3Aio%3A%3AWrite%3B%0A%0Alet%20mut%20w%20%3D%20Vec%3A%3Anew()%3B%0Awriteln!(%26mut%20w).unwrap()%3B%0Awriteln!(%26mut%20w%2C%20%22test%22).unwrap()%3B%0Awriteln!(%26mut%20w%2C%20%22formatted%20%7B%7D%22%2C%20%22arguments%22).unwrap()%3B%0A%0Aassert_eq!(%26w%5B..%5D%2C%20%22%5Cntest%5Cnformatted%20arguments%5Cn%22.as_bytes())%3B%0A%7D">Run</a></pre>
<p>A module can import both <code>std::fmt::Write</code> and <code>std::io::Write</code> and call <code>write!</code> on objects
implementing either, as objects do not typically implement both. However, the module must
import the traits qualified so their names do not conflict:</p>
<pre class="rust rust-example-rendered">
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">fmt</span>::<span class="ident">Write</span> <span class="kw">as</span> <span class="ident">FmtWrite</span>;
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">io</span>::<span class="ident">Write</span> <span class="kw">as</span> <span class="ident">IoWrite</span>;
<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">s</span> <span class="op">=</span> <span class="ident">String</span>::<span class="ident">new</span>();
<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">v</span> <span class="op">=</span> <span class="ident">Vec</span>::<span class="ident">new</span>();
<span class="macro">writeln</span><span class="macro">!</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">s</span>, <span class="string">"{} {}"</span>, <span class="string">"abc"</span>, <span class="number">123</span>).<span class="ident">unwrap</span>(); <span class="comment">// uses fmt::Write::write_fmt</span>
<span class="macro">writeln</span><span class="macro">!</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">v</span>, <span class="string">"s = {:?}"</span>, <span class="ident">s</span>).<span class="ident">unwrap</span>(); <span class="comment">// uses io::Write::write_fmt</span>
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">v</span>, <span class="string">b"s = \"abc 123\\n\"\n"</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%3A%3AWrite%20as%20FmtWrite%3B%0Ause%20std%3A%3Aio%3A%3AWrite%20as%20IoWrite%3B%0A%0Alet%20mut%20s%20%3D%20String%3A%3Anew()%3B%0Alet%20mut%20v%20%3D%20Vec%3A%3Anew()%3B%0Awriteln!(%26mut%20s%2C%20%22%7B%7D%20%7B%7D%22%2C%20%22abc%22%2C%20123).unwrap()%3B%20%2F%2F%20uses%20fmt%3A%3AWrite%3A%3Awrite_fmt%0Awriteln!(%26mut%20v%2C%20%22s%20%3D%20%7B%3A%3F%7D%22%2C%20s).unwrap()%3B%20%2F%2F%20uses%20io%3A%3AWrite%3A%3Awrite_fmt%0Aassert_eq!(v%2C%20b%22s%20%3D%20%5C%22abc%20123%5C%5Cn%5C%22%5Cn%22)%3B%0A%7D">Run</a></pre>
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