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<!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 `boxed` mod in crate `nom`."><meta name="keywords" content="rust, rustlang, rust-lang, boxed"><title>nom::lib::std::boxed - 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><noscript><link rel="stylesheet" href="../../../../noscript.css"></noscript><link rel="shortcut icon" href="../../../../favicon.ico"><style type="text/css">#crate-search{background-image:url("../../../../down-arrow.svg");}</style></head><body class="rustdoc mod"><!--[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='../../../../nom/index.html'><div class='logo-container'><img src='../../../../rust-logo.png' alt='logo'></div></a><p class='location'>Module boxed</p><div class="sidebar-elems"><div class="block items"><ul><li><a href="#structs">Structs</a></li></ul></div><p class='location'><a href='../../../index.html'>nom</a>::<wbr><a href='../../index.html'>lib</a>::<wbr><a href='../index.html'>std</a></p><script>window.sidebarCurrent = {name: 'boxed', ty: 'mod', 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"><div class="search-container"><div><select id="crate-search"><option value="All crates">All crates</option></select><input class="search-input" name="search" disabled autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"></div><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='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='https://doc.rust-lang.org/nightly/src/alloc/lib.rs.html#147' title='goto source code'>[src]</a></span><span class='in-band'>Module <a href='../../../index.html'>nom</a>::<wbr><a href='../../index.html'>lib</a>::<wbr><a href='../index.html'>std</a>::<wbr><a class="mod" href=''>boxed</a></span></h1><div class='docblock'><p>A pointer type for heap allocation.</p>
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<p><a href="struct.Box.html"><code>Box<T></code></a>, 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. Boxes also ensure that they
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never allocate more than <code>isize::MAX</code> bytes.</p>
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<h1 id="examples" class="section-header"><a href="#examples">Examples</a></h1>
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<p>Move a value from the stack to the heap by creating a <a href="struct.Box.html"><code>Box</code></a>:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="kw">let</span> <span class="ident">val</span>: <span class="ident">u8</span> <span class="op">=</span> <span class="number">5</span>;
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<span class="kw">let</span> <span class="ident">boxed</span>: <span class="ident">Box</span><span class="op"><</span><span class="ident">u8</span><span class="op">></span> <span class="op">=</span> <span class="ident">Box</span>::<span class="ident">new</span>(<span class="ident">val</span>);</pre></div>
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<p>Move a value from a <a href="struct.Box.html"><code>Box</code></a> back to the stack by <a href="../../std/ops/trait.Deref.html">dereferencing</a>:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="kw">let</span> <span class="ident">boxed</span>: <span class="ident">Box</span><span class="op"><</span><span class="ident">u8</span><span class="op">></span> <span class="op">=</span> <span class="ident">Box</span>::<span class="ident">new</span>(<span class="number">5</span>);
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<span class="kw">let</span> <span class="ident">val</span>: <span class="ident">u8</span> <span class="op">=</span> <span class="kw-2">*</span><span class="ident">boxed</span>;</pre></div>
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<p>Creating a recursive data structure:</p>
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<div class="example-wrap"><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><span class="op">></span>),
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<span class="ident">Nil</span>,
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}
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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>);</pre></div>
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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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<div class='information'><div class='tooltip compile_fail'>ⓘ<span class='tooltiptext'>This example deliberately fails to compile</span></div></div><div class="example-wrap"><pre class="rust rust-example-rendered compile_fail">
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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></div>
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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 <a href="struct.Box.html"><code>Box<T></code></a>, which has a defined size, we know how
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big <code>Cons</code> needs to be.</p>
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<h1 id="memory-layout" class="section-header"><a href="#memory-layout">Memory layout</a></h1>
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<p>For non-zero-sized values, a <a href="struct.Box.html"><code>Box</code></a> will use the <a href="../alloc/struct.Global.html"><code>Global</code></a> allocator for
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its allocation. It is valid to convert both ways between a <a href="struct.Box.html"><code>Box</code></a> and a
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raw pointer allocated with the <a href="../alloc/struct.Global.html"><code>Global</code></a> allocator, given that the
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<a href="../alloc/struct.Layout.html"><code>Layout</code></a> used with the allocator is correct for the type. More precisely,
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a <code>value: *mut T</code> that has been allocated with the <a href="../alloc/struct.Global.html"><code>Global</code></a> allocator
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with <code>Layout::for_value(&*value)</code> may be converted into a box using
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<a href="struct.Box.html#method.from_raw"><code>Box::<T>::from_raw(value)</code></a>. Conversely, the memory backing a <code>value: *mut T</code> obtained from <a href="struct.Box.html#method.into_raw"><code>Box::<T>::into_raw</code></a> may be deallocated using the
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<a href="../alloc/struct.Global.html"><code>Global</code></a> allocator with <a href="../alloc/struct.Layout.html#method.for_value"><code>Layout::for_value(&*value)</code></a>.</p>
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<p>So long as <code>T: Sized</code>, a <code>Box<T></code> is guaranteed to be represented
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as a single pointer and is also ABI-compatible with C pointers
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(i.e. the C type <code>T*</code>). This means that if you have extern "C"
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Rust functions that will be called from C, you can define those
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Rust functions using <code>Box<T></code> types, and use <code>T*</code> as corresponding
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type on the C side. As an example, consider this C header which
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declares functions that create and destroy some kind of <code>Foo</code>
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value:</p>
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<pre><code class="language-c">/* C header */
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/* Returns ownership to the caller */
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struct Foo* foo_new(void);
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/* Takes ownership from the caller; no-op when invoked with NULL */
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void foo_delete(struct Foo*);
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</code></pre>
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<p>These two functions might be implemented in Rust as follows. Here, the
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<code>struct Foo*</code> type from C is translated to <code>Box<Foo></code>, which captures
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the ownership constraints. Note also that the nullable argument to
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<code>foo_delete</code> is represented in Rust as <code>Option<Box<Foo>></code>, since <code>Box<Foo></code>
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cannot be null.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="attribute">#[<span class="ident">repr</span>(<span class="ident">C</span>)]</span>
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<span class="kw">pub</span> <span class="kw">struct</span> <span class="ident">Foo</span>;
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<span class="attribute">#[<span class="ident">no_mangle</span>]</span>
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<span class="kw">pub</span> <span class="kw">extern</span> <span class="string">"C"</span> <span class="kw">fn</span> <span class="ident">foo_new</span>() <span class="op">-</span><span class="op">></span> <span class="ident">Box</span><span class="op"><</span><span class="ident">Foo</span><span class="op">></span> {
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<span class="ident">Box</span>::<span class="ident">new</span>(<span class="ident">Foo</span>)
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}
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<span class="attribute">#[<span class="ident">no_mangle</span>]</span>
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<span class="kw">pub</span> <span class="kw">extern</span> <span class="string">"C"</span> <span class="kw">fn</span> <span class="ident">foo_delete</span>(<span class="kw">_</span>: <span class="prelude-ty">Option</span><span class="op"><</span><span class="ident">Box</span><span class="op"><</span><span class="ident">Foo</span><span class="op">></span><span class="op">></span>) {}</pre></div>
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<p>Even though <code>Box<T></code> has the same representation and C ABI as a C pointer,
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this does not mean that you can convert an arbitrary <code>T*</code> into a <code>Box<T></code>
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and expect things to work. <code>Box<T></code> values will always be fully aligned,
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non-null pointers. Moreover, the destructor for <code>Box<T></code> will attempt to
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free the value with the global allocator. In general, the best practice
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is to only use <code>Box<T></code> for pointers that originated from the global
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allocator.</p>
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<p><strong>Important.</strong> At least at present, you should avoid using
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<code>Box<T></code> types for functions that are defined in C but invoked
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from Rust. In those cases, you should directly mirror the C types
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as closely as possible. Using types like <code>Box<T></code> where the C
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definition is just using <code>T*</code> can lead to undefined behavior, as
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described in <a href="https://github.com/rust-lang/unsafe-code-guidelines/issues/198">rust-lang/unsafe-code-guidelines#198</a>.</p>
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</div><h2 id='structs' class='section-header'><a href="#structs">Structs</a></h2>
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<table><tr class='module-item'><td><a class="struct" href="struct.Box.html" title='nom::lib::std::boxed::Box struct'>Box</a></td><td class='docblock-short'><p>A pointer type for heap allocation.</p>
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</td></tr></table></section><section id="search" class="content hidden"></section><section class="footer"></section><script>window.rootPath = "../../../../";window.currentCrate = "nom";</script><script src="../../../../aliases.js"></script><script src="../../../../main.js"></script><script defer src="../../../../search-index.js"></script></body></html> |