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<title>bitflags::__core::io::BufWriter - Rust</title>
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<p class='location'><a href='../../index.html'>bitflags</a>::<wbr><a href='../index.html'>__core</a>::<wbr><a href='index.html'>io</a></p><script>window.sidebarCurrent = {name: 'BufWriter', ty: 'struct', relpath: ''};</script><script defer src="sidebar-items.js"></script>
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<section id='main' class="content struct">
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<h1 class='fqn'><span class='in-band'>Struct <a href='../../index.html'>bitflags</a>::<wbr><a href='../index.html'>__core</a>::<wbr><a href='index.html'>io</a>::<wbr><a class='struct' href=''>BufWriter</a></span><span class='out-of-band'><span id='render-detail'>
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[<span class='inner'>−</span>]
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</span><a id='src-4044' class='srclink' href='https://doc.rust-lang.org/nightly/std/io/buffered/struct.BufWriter.html?gotosrc=4044' title='goto source code'>[src]</a></span></h1>
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<pre class='rust struct'>pub struct BufWriter<W> <span class='where'>where W: <a class='trait' href='../../../bitflags/__core/io/trait.Write.html' title='bitflags::__core::io::Write'>Write</a></span> {
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// some fields omitted
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}</pre><span class="since">1.0.0</span><div class='docblock'><p>Wraps a writer and buffers its output.</p>
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<p>It can be excessively inefficient to work directly with something that
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implements <code>Write</code>. For example, every call to <code>write</code> on <code>TcpStream</code>
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results in a system call. A <code>BufWriter</code> keeps an in-memory buffer of data
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and writes it to an underlying writer in large, infrequent batches.</p>
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<p>The buffer will be written out when the writer is dropped.</p>
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<h1 id='examples' class='section-header'><a href='#examples'>Examples</a></h1>
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<p>Let's write the numbers one through ten to a <code>TcpStream</code>:</p>
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<pre class='rust rust-example-rendered'>
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<span class='kw'>use</span> <span class='ident'>std</span>::<span class='ident'>io</span>::<span class='ident'>prelude</span>::<span class='op'>*</span>;
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<span class='kw'>use</span> <span class='ident'>std</span>::<span class='ident'>net</span>::<span class='ident'>TcpStream</span>;
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<span class='kw'>let</span> <span class='kw-2'>mut</span> <span class='ident'>stream</span> <span class='op'>=</span> <span class='ident'>TcpStream</span>::<span class='ident'>connect</span>(<span class='string'>"127.0.0.1:34254"</span>).<span class='ident'>unwrap</span>();
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<span class='kw'>for</span> <span class='ident'>i</span> <span class='kw'>in</span> <span class='number'>1</span>..<span class='number'>10</span> {
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<span class='ident'>stream</span>.<span class='ident'>write</span>(<span class='kw-2'>&</span>[<span class='ident'>i</span>]).<span class='ident'>unwrap</span>();
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}</pre>
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<p>Because we're not buffering, we write each one in turn, incurring the
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overhead of a system call per byte written. We can fix this with a
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<code>BufWriter</code>:</p>
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<pre class='rust rust-example-rendered'>
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<span class='kw'>use</span> <span class='ident'>std</span>::<span class='ident'>io</span>::<span class='ident'>prelude</span>::<span class='op'>*</span>;
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<span class='kw'>use</span> <span class='ident'>std</span>::<span class='ident'>io</span>::<span class='ident'>BufWriter</span>;
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<span class='kw'>use</span> <span class='ident'>std</span>::<span class='ident'>net</span>::<span class='ident'>TcpStream</span>;
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<span class='kw'>let</span> <span class='kw-2'>mut</span> <span class='ident'>stream</span> <span class='op'>=</span> <span class='ident'>BufWriter</span>::<span class='ident'>new</span>(<span class='ident'>TcpStream</span>::<span class='ident'>connect</span>(<span class='string'>"127.0.0.1:34254"</span>).<span class='ident'>unwrap</span>());
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<span class='kw'>for</span> <span class='ident'>i</span> <span class='kw'>in</span> <span class='number'>1</span>..<span class='number'>10</span> {
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<span class='ident'>stream</span>.<span class='ident'>write</span>(<span class='kw-2'>&</span>[<span class='ident'>i</span>]).<span class='ident'>unwrap</span>();
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}</pre>
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<p>By wrapping the stream with a <code>BufWriter</code>, these ten writes are all grouped
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together by the buffer, and will all be written out in one system call when
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the <code>stream</code> is dropped.</p>
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