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oxipng/doc/crossbeam/struct.Scope.html
Joshua Holmer 56e0e2de01 Add interface for rustdoc to github pages 
Closes #28
2016-03-11 12:13:13 -05:00

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<section id='main' class="content struct">
<h1 class='fqn'><span class='in-band'>Struct <a href='index.html'>crossbeam</a>::<wbr><a class='struct' href=''>Scope</a></span><span class='out-of-band'><span id='render-detail'>
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<pre class='rust struct'>pub struct Scope&lt;'a&gt; {
// some fields omitted
}</pre><h2 id='methods'>Methods</h2><h3 class='impl'><code>impl&lt;'a&gt; <a class='struct' href='../crossbeam/struct.Scope.html' title='crossbeam::Scope'>Scope</a>&lt;'a&gt;</code></h3><div class='impl-items'><h4 id='method.defer' class='method'><code>fn <a href='#method.defer' class='fnname'>defer</a>&lt;F&gt;(&amp;self, f: F) <span class='where'>where F: <a class='trait' href='https://doc.rust-lang.org/nightly/core/ops/trait.FnOnce.html' title='core::ops::FnOnce'>FnOnce</a>() + 'a</span></code></h4>
<div class='docblock'><p>Schedule code to be executed when exiting the scope.</p>
<p>This is akin to having a destructor on the stack, except that it is
<em>guaranteed</em> to be run.</p>
</div><h4 id='method.spawn' class='method'><code>fn <a href='#method.spawn' class='fnname'>spawn</a>&lt;F, T&gt;(&amp;self, f: F) -&gt; <a class='struct' href='../crossbeam/struct.ScopedJoinHandle.html' title='crossbeam::ScopedJoinHandle'>ScopedJoinHandle</a>&lt;T&gt; <span class='where'>where F: <a class='trait' href='https://doc.rust-lang.org/nightly/core/ops/trait.FnOnce.html' title='core::ops::FnOnce'>FnOnce</a>() -&gt; T + <a class='trait' href='https://doc.rust-lang.org/nightly/core/marker/trait.Send.html' title='core::marker::Send'>Send</a> + 'a, T: <a class='trait' href='https://doc.rust-lang.org/nightly/core/marker/trait.Send.html' title='core::marker::Send'>Send</a> + 'a</span></code></h4>
<div class='docblock'><p>Create a scoped thread.</p>
<p><code>spawn</code> is similar to the <a href="http://doc.rust-lang.org/std/thread/fn.spawn.html"><code>spawn</code></a> function in Rust&#39;s standard library. The
difference is that this thread is scoped, meaning that it&#39;s guaranteed to terminate
before the current stack frame goes away, allowing you to reference the parent stack frame
directly. This is ensured by having the parent thread join on the child thread before the
scope exits.</p>
<h1 id='examples' class='section-header'><a href='#examples'>Examples</a></h1>
<p>A basic scoped thread:</p>
<pre class='rust rust-example-rendered'>
<span class='ident'>crossbeam</span>::<span class='ident'>scope</span>(<span class='op'>|</span><span class='ident'>scope</span><span class='op'>|</span> {
<span class='ident'>scope</span>.<span class='ident'>spawn</span>(<span class='op'>||</span> {
<span class='macro'>println</span><span class='macro'>!</span>(<span class='string'>&quot;Hello from a scoped thread!&quot;</span>);
});
});</pre>
<p>When writing concurrent Rust programs, you&#39;ll sometimes see a pattern like this, using
<a href="http://doc.rust-lang.org/std/thread/fn.spawn.html"><code>std::thread::spawn</code></a>:</p>
<pre class='rust rust-example-rendered'>
<span class='kw'>let</span> <span class='ident'>array</span> <span class='op'>=</span> [<span class='number'>1</span>, <span class='number'>2</span>, <span class='number'>3</span>];
<span class='kw'>let</span> <span class='kw-2'>mut</span> <span class='ident'>guards</span> <span class='op'>=</span> <span class='macro'>vec</span><span class='macro'>!</span>[];
<span class='kw'>for</span> <span class='ident'>i</span> <span class='kw'>in</span> <span class='kw-2'>&amp;</span><span class='ident'>array</span> {
<span class='kw'>let</span> <span class='ident'>guard</span> <span class='op'>=</span> <span class='ident'>std</span>::<span class='ident'>thread</span>::<span class='ident'>spawn</span>(<span class='kw'>move</span> <span class='op'>||</span> {
<span class='macro'>println</span><span class='macro'>!</span>(<span class='string'>&quot;element: {}&quot;</span>, <span class='ident'>i</span>);
});
<span class='ident'>guards</span>.<span class='ident'>push</span>(<span class='ident'>guard</span>);
}
<span class='kw'>for</span> <span class='ident'>guard</span> <span class='kw'>in</span> <span class='ident'>guards</span> {
<span class='ident'>guard</span>.<span class='ident'>join</span>().<span class='ident'>unwrap</span>();
}</pre>
<p>The basic pattern is:</p>
<ol>
<li>Iterate over some collection.</li>
<li>Spin up a thread to operate on each part of the collection.</li>
<li>Join all the threads.</li>
</ol>
<p>However, this code actually gives an error:</p>
<pre><code class="language-text">error: `array` does not live long enough
for i in &amp;array {
^~~~~
in expansion of for loop expansion
note: expansion site
note: reference must be valid for the static lifetime...
note: ...but borrowed value is only valid for the block suffix following statement 0 at ...
let array = [1, 2, 3];
let mut guards = vec![];
for i in &amp;array {
let guard = std::thread::spawn(move || {
println!(&quot;element: {}&quot;, i);
...
error: aborting due to previous error
</code></pre>
<p>Because <a href="http://doc.rust-lang.org/std/thread/fn.spawn.html"><code>std::thread::spawn</code></a> doesn&#39;t know about this scope, it requires a
<code>&#39;static</code> lifetime. One way of giving it a proper lifetime is to use an <a href="http://doc.rust-lang.org/stable/std/sync/struct.Arc.html"><code>Arc</code></a>:</p>
<pre class='rust rust-example-rendered'>
<span class='kw'>use</span> <span class='ident'>std</span>::<span class='ident'>sync</span>::<span class='ident'>Arc</span>;
<span class='kw'>let</span> <span class='ident'>array</span> <span class='op'>=</span> <span class='ident'>Arc</span>::<span class='ident'>new</span>([<span class='number'>1</span>, <span class='number'>2</span>, <span class='number'>3</span>]);
<span class='kw'>let</span> <span class='kw-2'>mut</span> <span class='ident'>guards</span> <span class='op'>=</span> <span class='macro'>vec</span><span class='macro'>!</span>[];
<span class='kw'>for</span> <span class='ident'>i</span> <span class='kw'>in</span> (<span class='number'>0</span>..<span class='ident'>array</span>.<span class='ident'>len</span>()) {
<span class='kw'>let</span> <span class='ident'>a</span> <span class='op'>=</span> <span class='ident'>array</span>.<span class='ident'>clone</span>();
<span class='kw'>let</span> <span class='ident'>guard</span> <span class='op'>=</span> <span class='ident'>std</span>::<span class='ident'>thread</span>::<span class='ident'>spawn</span>(<span class='kw'>move</span> <span class='op'>||</span> {
<span class='macro'>println</span><span class='macro'>!</span>(<span class='string'>&quot;element: {}&quot;</span>, <span class='ident'>a</span>[<span class='ident'>i</span>]);
});
<span class='ident'>guards</span>.<span class='ident'>push</span>(<span class='ident'>guard</span>);
}
<span class='kw'>for</span> <span class='ident'>guard</span> <span class='kw'>in</span> <span class='ident'>guards</span> {
<span class='ident'>guard</span>.<span class='ident'>join</span>().<span class='ident'>unwrap</span>();
}</pre>
<p>But this introduces unnecessary allocation, as <code>Arc&lt;T&gt;</code> puts its data on the heap, and we
also end up dealing with reference counts. We know that we&#39;re joining the threads before
our function returns, so just taking a reference <em>should</em> be safe. Rust can&#39;t know that,
though.</p>
<p>Enter scoped threads. Here&#39;s our original example, using <code>spawn</code> from crossbeam rather
than from <code>std::thread</code>:</p>
<pre class='rust rust-example-rendered'>
<span class='kw'>let</span> <span class='ident'>array</span> <span class='op'>=</span> [<span class='number'>1</span>, <span class='number'>2</span>, <span class='number'>3</span>];
<span class='ident'>crossbeam</span>::<span class='ident'>scope</span>(<span class='op'>|</span><span class='ident'>scope</span><span class='op'>|</span> {
<span class='kw'>for</span> <span class='ident'>i</span> <span class='kw'>in</span> <span class='kw-2'>&amp;</span><span class='ident'>array</span> {
<span class='ident'>scope</span>.<span class='ident'>spawn</span>(<span class='kw'>move</span> <span class='op'>||</span> {
<span class='macro'>println</span><span class='macro'>!</span>(<span class='string'>&quot;element: {}&quot;</span>, <span class='ident'>i</span>);
});
}
});</pre>
<p>Much more straightforward.</p>
</div></div><h2 id='implementations'>Trait Implementations</h2><h3 class='impl'><code>impl&lt;'a&gt; <a class='trait' href='https://doc.rust-lang.org/nightly/core/ops/trait.Drop.html' title='core::ops::Drop'>Drop</a> for <a class='struct' href='../crossbeam/struct.Scope.html' title='crossbeam::Scope'>Scope</a>&lt;'a&gt;</code></h3><div class='impl-items'><h4 id='method.drop' class='method'><code>fn <a href='https://doc.rust-lang.org/nightly/core/ops/trait.Drop.html#method.drop' class='fnname'>drop</a>(&amp;mut self)</code></h4>
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