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<meta name="description" content="API documentation for the Rust `boxed` mod in crate `bitflags`.">
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<title>bitflags::__core::boxed - Rust</title>
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<p class='location'><a href='../../index.html'>bitflags</a>::<wbr><a href='../index.html'>__core</a></p><script>window.sidebarCurrent = {name: 'boxed', ty: 'mod', relpath: '../'};</script><script defer src="../sidebar-items.js"></script>
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<section id='main' class="content mod">
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<h1 class='fqn'><span class='in-band'>Module <a href='../../index.html'>bitflags</a>::<wbr><a href='../index.html'>__core</a>::<wbr><a class='mod' href=''>boxed</a></span><span class='out-of-band'><span id='render-detail'>
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<a id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs">
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[<span class='inner'>−</span>]
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</a>
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</span><a id='src-72' class='srclink' href='https://doc.rust-lang.org/nightly/alloc/boxed/index.html?gotosrc=72' title='goto source code'>[src]</a></span></h1>
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<div class='docblock'><p>A pointer type for heap allocation.</p>
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<p><code>Box<T></code>, casually referred to as a 'box', provides the simplest form of
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heap allocation in Rust. Boxes provide ownership for this allocation, and
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drop their contents when they go out of scope.</p>
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<h1 id='examples' class='section-header'><a href='#examples'>Examples</a></h1>
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<p>Creating a box:</p>
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<pre class='rust rust-example-rendered'>
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<span class='kw'>let</span> <span class='ident'>x</span> <span class='op'>=</span> <span class='ident'>Box</span>::<span class='ident'>new</span>(<span class='number'>5</span>);</pre>
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<p>Creating a recursive data structure:</p>
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<pre class='rust rust-example-rendered'>
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<span class='attribute'>#[<span class='ident'>derive</span>(<span class='ident'>Debug</span>)]</span>
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<span class='kw'>enum</span> <span class='ident'>List</span><span class='op'><</span><span class='ident'>T</span><span class='op'>></span> {
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<span class='ident'>Cons</span>(<span class='ident'>T</span>, <span class='ident'>Box</span><span class='op'><</span><span class='ident'>List</span><span class='op'><</span><span class='ident'>T</span><span class='op'>>></span>),
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<span class='ident'>Nil</span>,
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}
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<span class='kw'>fn</span> <span class='ident'>main</span>() {
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<span class='kw'>let</span> <span class='ident'>list</span>: <span class='ident'>List</span><span class='op'><</span><span class='ident'>i32</span><span class='op'>></span> <span class='op'>=</span> <span class='ident'>List</span>::<span class='ident'>Cons</span>(<span class='number'>1</span>, <span class='ident'>Box</span>::<span class='ident'>new</span>(<span class='ident'>List</span>::<span class='ident'>Cons</span>(<span class='number'>2</span>, <span class='ident'>Box</span>::<span class='ident'>new</span>(<span class='ident'>List</span>::<span class='ident'>Nil</span>))));
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<span class='macro'>println</span><span class='macro'>!</span>(<span class='string'>"{:?}"</span>, <span class='ident'>list</span>);
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}</pre>
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<p>This will print <code>Cons(1, Cons(2, Nil))</code>.</p>
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<p>Recursive structures must be boxed, because if the definition of <code>Cons</code>
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looked like this:</p>
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<pre class='rust rust-example-rendered'>
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<span class='ident'>Cons</span>(<span class='ident'>T</span>, <span class='ident'>List</span><span class='op'><</span><span class='ident'>T</span><span class='op'>></span>),</pre>
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<p>It wouldn't work. This is because the size of a <code>List</code> depends on how many
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elements are in the list, and so we don't know how much memory to allocate
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for a <code>Cons</code>. By introducing a <code>Box</code>, which has a defined size, we know how
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big <code>Cons</code> needs to be.</p>
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</div><h2 id='structs' class='section-header'><a href="#structs">Structs</a></h2>
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<table>
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<tr class=' module-item'>
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<td><a class='struct' href='struct.Box.html'
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title='bitflags::__core::boxed::Box'>Box</a></td>
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<td class='docblock short'>
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<p>A pointer type for heap allocation.</p>
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</td>
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</tr>
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<tr class='unstable module-item'>
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<td><a class='struct' href='struct.ExchangeHeapSingleton.html'
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title='bitflags::__core::boxed::ExchangeHeapSingleton'>ExchangeHeapSingleton</a></td>
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<td class='docblock short'>
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[<em class='stab unstable'>Unstable</em>] <p>This the singleton type used solely for <code>boxed::HEAP</code>.</p>
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</td>
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</tr>
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<tr class='unstable module-item'>
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<td><a class='struct' href='struct.IntermediateBox.html'
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title='bitflags::__core::boxed::IntermediateBox'>IntermediateBox</a></td>
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<td class='docblock short'>
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[<em class='stab unstable'>Unstable</em>] <p><code>IntermediateBox</code> represents uninitialized backing storage for <code>Box</code>.</p>
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</td>
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</tr></table><h2 id='constants' class='section-header'><a href="#constants">Constants</a></h2>
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<table>
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<tr class='unstable module-item'>
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<td><a class='constant' href='constant.HEAP.html'
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title='bitflags::__core::boxed::HEAP'>HEAP</a></td>
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<td class='docblock short'>
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[<em class='stab unstable'>Unstable</em>] <p>A value that represents the heap. This is the default place that the <code>box</code>
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keyword allocates into when no place is supplied.</p>
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</td>
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</tr></table><h2 id='traits' class='section-header'><a href="#traits">Traits</a></h2>
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<table>
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<tr class='unstable module-item'>
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<td><a class='trait' href='trait.FnBox.html'
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title='bitflags::__core::boxed::FnBox'>FnBox</a></td>
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<td class='docblock short'>
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[<em class='stab unstable'>Unstable</em>] <p><code>FnBox</code> is a version of the <code>FnOnce</code> intended for use with boxed
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closure objects. The idea is that where one would normally store a
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<code>Box<FnOnce()></code> in a data structure, you should use
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<code>Box<FnBox()></code>. The two traits behave essentially the same, except
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that a <code>FnBox</code> closure can only be called if it is boxed. (Note
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that <code>FnBox</code> may be deprecated in the future if <code>Box<FnOnce()></code>
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closures become directly usable.)</p>
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</td>
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