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<h1 class='fqn'><span class='in-band'>Macro <a href='index.html'>std</a>::<wbr><a class="macro" href=''>write</a></span><span class='out-of-band'><span id='render-detail'>
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<pre class="rust macro">
<span class="macro">macro_rules</span><span class="macro">!</span> <span class="ident">write</span> {
( $ <span class="ident">dst</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 class='docblock'><p>Write formatted data into a buffer.</p>
<p>This macro accepts a format string, a list of arguments, and a 'writer'. Arguments will be
formatted according to the specified format string and the result will be passed to the writer.
The writer may be any value with a <code>write_fmt</code> method; generally this comes from an
implementation of either the <a href="../std/fmt/trait.Write.html"><code>std::fmt::Write</code></a> or the <a href="../std/io/trait.Write.html"><code>std::io::Write</code></a> trait. The macro
returns whatever the <code>write_fmt</code> method returns; commonly a <a href="../std/fmt/type.Result.html"><code>std::fmt::Result</code></a>, or an
<a href="../std/io/type.Result.html"><code>io::Result</code></a>.</p>
<p>See <a href="../std/fmt/index.html"><code>std::fmt</code></a> for more information on the format string syntax.</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">write</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">write</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="ident">w</span>, <span class="string">b"testformatted arguments"</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%0Awrite!(%26mut%20w%2C%20%22test%22).unwrap()%3B%0Awrite!(%26mut%20w%2C%20%22formatted%20%7B%7D%22%2C%20%22arguments%22).unwrap()%3B%0A%0Aassert_eq!(w%2C%20b%22testformatted%20arguments%22)%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">write</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">write</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\""</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%0Awrite!(%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%0Awrite!(%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%22%22)%3B%0A%7D">Run</a></pre>
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