<!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 `size_of` fn in crate `std`."><meta name="keywords" content="rust, rustlang, rust-lang, size_of"><title>std::mem::size_of - 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><link rel="shortcut icon" href="https://doc.rust-lang.org/favicon.ico"></head><body class="rustdoc fn"><!--[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='../../std/index.html'><img src='https://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png' alt='logo' width='100'></a><div class="sidebar-elems"><p class='location'><a href='../index.html'>std</a>::<wbr><a href='index.html'>mem</a></p><script>window.sidebarCurrent = {name: 'size_of', ty: 'fn', 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 js-only"><div class="search-container"><input class="search-input" name="search" autocomplete="off" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><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='in-band'>Function <a href='../index.html'>std</a>::<wbr><a href='index.html'>mem</a>::<wbr><a class="fn" href=''>size_of</a></span><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='../../src/core/mem.rs.html#317-319' title='goto source code'>[src]</a></span></h1><pre class='rust fn'>pub const fn size_of<T>() -> <a class="primitive" href="../primitive.usize.html">usize</a></pre><div class='docblock'><p>Returns the size of a type in bytes.</p>
<p>More specifically, this is the offset in bytes between successive elements
in an array with that item type including alignment padding. Thus, for any
type <code>T</code> and length <code>n</code>, <code>[T; n]</code> has a size of <code>n * size_of::<T>()</code>.</p>
<p>In general, the size of a type is not stable across compilations, but
specific types such as primitives are.</p>
<p>The following table gives the size for primitives.</p>
<table><thead><tr><th>Type </th><th> size_of::<Type>()</th></tr></thead><tbody>
<tr><td>() </td><td> 0</td></tr>
<tr><td>bool </td><td> 1</td></tr>
<tr><td>u8 </td><td> 1</td></tr>
<tr><td>u16 </td><td> 2</td></tr>
<tr><td>u32 </td><td> 4</td></tr>
<tr><td>u64 </td><td> 8</td></tr>
<tr><td>u128 </td><td> 16</td></tr>
<tr><td>i8 </td><td> 1</td></tr>
<tr><td>i16 </td><td> 2</td></tr>
<tr><td>i32 </td><td> 4</td></tr>
<tr><td>i64 </td><td> 8</td></tr>
<tr><td>i128 </td><td> 16</td></tr>
<tr><td>f32 </td><td> 4</td></tr>
<tr><td>f64 </td><td> 8</td></tr>
<tr><td>char </td><td> 4</td></tr>
</tbody></table>
<p>Furthermore, <code>usize</code> and <code>isize</code> have the same size.</p>
<p>The types <code>*const T</code>, <code>&T</code>, <code>Box<T></code>, <code>Option<&T></code>, and <code>Option<Box<T>></code> all have
the same size. If <code>T</code> is Sized, all of those types have the same size as <code>usize</code>.</p>
<p>The mutability of a pointer does not change its size. As such, <code>&T</code> and <code>&mut T</code>
have the same size. Likewise for <code>*const T</code> and <code>*mut T</code>.</p>
<h1 id="size-of-reprc-items" class="section-header"><a href="#size-of-reprc-items">Size of <code>#[repr(C)]</code> items</a></h1>
<p>The <code>C</code> representation for items has a defined layout. With this layout,
the size of items is also stable as long as all fields have a stable size.</p>
<h2 id="size-of-structs" class="section-header"><a href="#size-of-structs">Size of Structs</a></h2>
<p>For <code>structs</code>, the size is determined by the following algorithm.</p>
<p>For each field in the struct ordered by declaration order:</p>
<ol>
<li>Add the size of the field.</li>
<li>Round up the current size to the nearest multiple of the next field's <a href="./fn.align_of.html">alignment</a>.</li>
</ol>
<p>Finally, round the size of the struct to the nearest multiple of its <a href="./fn.align_of.html">alignment</a>.</p>
<p>Unlike <code>C</code>, zero sized structs are not rounded up to one byte in size.</p>
<h2 id="size-of-enums" class="section-header"><a href="#size-of-enums">Size of Enums</a></h2>
<p>Enums that carry no data other than the descriminant have the same size as C enums
on the platform they are compiled for.</p>
<h2 id="size-of-unions" class="section-header"><a href="#size-of-unions">Size of Unions</a></h2>
<p>The size of a union is the size of its largest field.</p>
<p>Unlike <code>C</code>, zero sized unions are not rounded up to one byte in size.</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">mem</span>;
<span class="comment">// Some primitives</span>
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="number">4</span>, <span class="ident">mem</span>::<span class="ident">size_of</span>::<span class="op"><</span><span class="ident">i32</span><span class="op">></span>());
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="number">8</span>, <span class="ident">mem</span>::<span class="ident">size_of</span>::<span class="op"><</span><span class="ident">f64</span><span class="op">></span>());
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="number">0</span>, <span class="ident">mem</span>::<span class="ident">size_of</span>::<span class="op"><</span>()<span class="op">></span>());
<span class="comment">// Some arrays</span>
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="number">8</span>, <span class="ident">mem</span>::<span class="ident">size_of</span>::<span class="op"><</span>[<span class="ident">i32</span>; <span class="number">2</span>]<span class="op">></span>());
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="number">12</span>, <span class="ident">mem</span>::<span class="ident">size_of</span>::<span class="op"><</span>[<span class="ident">i32</span>; <span class="number">3</span>]<span class="op">></span>());
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="number">0</span>, <span class="ident">mem</span>::<span class="ident">size_of</span>::<span class="op"><</span>[<span class="ident">i32</span>; <span class="number">0</span>]<span class="op">></span>());
<span class="comment">// Pointer size equality</span>
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">mem</span>::<span class="ident">size_of</span>::<span class="op"><</span><span class="kw-2">&</span><span class="ident">i32</span><span class="op">></span>(), <span class="ident">mem</span>::<span class="ident">size_of</span>::<span class="op"><</span><span class="kw-2">*</span><span class="kw">const</span> <span class="ident">i32</span><span class="op">></span>());
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">mem</span>::<span class="ident">size_of</span>::<span class="op"><</span><span class="kw-2">&</span><span class="ident">i32</span><span class="op">></span>(), <span class="ident">mem</span>::<span class="ident">size_of</span>::<span class="op"><</span><span class="ident">Box</span><span class="op"><</span><span class="ident">i32</span><span class="op">>></span>());
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">mem</span>::<span class="ident">size_of</span>::<span class="op"><</span><span class="kw-2">&</span><span class="ident">i32</span><span class="op">></span>(), <span class="ident">mem</span>::<span class="ident">size_of</span>::<span class="op"><</span><span class="prelude-ty">Option</span><span class="op"><</span><span class="kw-2">&</span><span class="ident">i32</span><span class="op">>></span>());
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">mem</span>::<span class="ident">size_of</span>::<span class="op"><</span><span class="ident">Box</span><span class="op"><</span><span class="ident">i32</span><span class="op">>></span>(), <span class="ident">mem</span>::<span class="ident">size_of</span>::<span class="op"><</span><span class="prelude-ty">Option</span><span class="op"><</span><span class="ident">Box</span><span class="op"><</span><span class="ident">i32</span><span class="op">>></span><span class="op">></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%3Amem%3B%0A%0A%2F%2F%20Some%20primitives%0Aassert_eq!(4%2C%20mem%3A%3Asize_of%3A%3A%3Ci32%3E())%3B%0Aassert_eq!(8%2C%20mem%3A%3Asize_of%3A%3A%3Cf64%3E())%3B%0Aassert_eq!(0%2C%20mem%3A%3Asize_of%3A%3A%3C()%3E())%3B%0A%0A%2F%2F%20Some%20arrays%0Aassert_eq!(8%2C%20mem%3A%3Asize_of%3A%3A%3C%5Bi32%3B%202%5D%3E())%3B%0Aassert_eq!(12%2C%20mem%3A%3Asize_of%3A%3A%3C%5Bi32%3B%203%5D%3E())%3B%0Aassert_eq!(0%2C%20mem%3A%3Asize_of%3A%3A%3C%5Bi32%3B%200%5D%3E())%3B%0A%0A%0A%2F%2F%20Pointer%20size%20equality%0Aassert_eq!(mem%3A%3Asize_of%3A%3A%3C%26i32%3E()%2C%20mem%3A%3Asize_of%3A%3A%3C*const%20i32%3E())%3B%0Aassert_eq!(mem%3A%3Asize_of%3A%3A%3C%26i32%3E()%2C%20mem%3A%3Asize_of%3A%3A%3CBox%3Ci32%3E%3E())%3B%0Aassert_eq!(mem%3A%3Asize_of%3A%3A%3C%26i32%3E()%2C%20mem%3A%3Asize_of%3A%3A%3COption%3C%26i32%3E%3E())%3B%0Aassert_eq!(mem%3A%3Asize_of%3A%3A%3CBox%3Ci32%3E%3E()%2C%20mem%3A%3Asize_of%3A%3A%3COption%3CBox%3Ci32%3E%3E%3E())%3B%0A%7D">Run</a></pre>
<p>Using <code>#[repr(C)]</code>.</p>
<pre class="rust rust-example-rendered">
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">mem</span>;
<span class="attribute">#[<span class="ident">repr</span>(<span class="ident">C</span>)]</span>
<span class="kw">struct</span> <span class="ident">FieldStruct</span> {
<span class="ident">first</span>: <span class="ident">u8</span>,
<span class="ident">second</span>: <span class="ident">u16</span>,
<span class="ident">third</span>: <span class="ident">u8</span>
}
<span class="comment">// The size of the first field is 1, so add 1 to the size. Size is 1.</span>
<span class="comment">// The alignment of the second field is 2, so add 1 to the size for padding. Size is 2.</span>
<span class="comment">// The size of the second field is 2, so add 2 to the size. Size is 4.</span>
<span class="comment">// The alignment of the third field is 1, so add 0 to the size for padding. Size is 4.</span>
<span class="comment">// The size of the third field is 1, so add 1 to the size. Size is 5.</span>
<span class="comment">// Finally, the alignment of the struct is 2, so add 1 to the size for padding. Size is 6.</span>
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="number">6</span>, <span class="ident">mem</span>::<span class="ident">size_of</span>::<span class="op"><</span><span class="ident">FieldStruct</span><span class="op">></span>());
<span class="attribute">#[<span class="ident">repr</span>(<span class="ident">C</span>)]</span>
<span class="kw">struct</span> <span class="ident">TupleStruct</span>(<span class="ident">u8</span>, <span class="ident">u16</span>, <span class="ident">u8</span>);
<span class="comment">// Tuple structs follow the same rules.</span>
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="number">6</span>, <span class="ident">mem</span>::<span class="ident">size_of</span>::<span class="op"><</span><span class="ident">TupleStruct</span><span class="op">></span>());
<span class="comment">// Note that reordering the fields can lower the size. We can remove both padding bytes</span>
<span class="comment">// by putting `third` before `second`.</span>
<span class="attribute">#[<span class="ident">repr</span>(<span class="ident">C</span>)]</span>
<span class="kw">struct</span> <span class="ident">FieldStructOptimized</span> {
<span class="ident">first</span>: <span class="ident">u8</span>,
<span class="ident">third</span>: <span class="ident">u8</span>,
<span class="ident">second</span>: <span class="ident">u16</span>
}
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="number">4</span>, <span class="ident">mem</span>::<span class="ident">size_of</span>::<span class="op"><</span><span class="ident">FieldStructOptimized</span><span class="op">></span>());
<span class="comment">// Union size is the size of the largest field.</span>
<span class="attribute">#[<span class="ident">repr</span>(<span class="ident">C</span>)]</span>
<span class="ident">union</span> <span class="ident">ExampleUnion</span> {
<span class="ident">smaller</span>: <span class="ident">u8</span>,
<span class="ident">larger</span>: <span class="ident">u16</span>
}
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="number">2</span>, <span class="ident">mem</span>::<span class="ident">size_of</span>::<span class="op"><</span><span class="ident">ExampleUnion</span><span class="op">></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%3Amem%3B%0A%0A%23%5Brepr(C)%5D%0Astruct%20FieldStruct%20%7B%0A%20%20%20%20first%3A%20u8%2C%0A%20%20%20%20second%3A%20u16%2C%0A%20%20%20%20third%3A%20u8%0A%7D%0A%0A%2F%2F%20The%20size%20of%20the%20first%20field%20is%201%2C%20so%20add%201%20to%20the%20size.%20Size%20is%201.%0A%2F%2F%20The%20alignment%20of%20the%20second%20field%20is%202%2C%20so%20add%201%20to%20the%20size%20for%20padding.%20Size%20is%202.%0A%2F%2F%20The%20size%20of%20the%20second%20field%20is%202%2C%20so%20add%202%20to%20the%20size.%20Size%20is%204.%0A%2F%2F%20The%20alignment%20of%20the%20third%20field%20is%201%2C%20so%20add%200%20to%20the%20size%20for%20padding.%20Size%20is%204.%0A%2F%2F%20The%20size%20of%20the%20third%20field%20is%201%2C%20so%20add%201%20to%20the%20size.%20Size%20is%205.%0A%2F%2F%20Finally%2C%20the%20alignment%20of%20the%20struct%20is%202%2C%20so%20add%201%20to%20the%20size%20for%20padding.%20Size%20is%206.%0Aassert_eq!(6%2C%20mem%3A%3Asize_of%3A%3A%3CFieldStruct%3E())%3B%0A%0A%23%5Brepr(C)%5D%0Astruct%20TupleStruct(u8%2C%20u16%2C%20u8)%3B%0A%0A%2F%2F%20Tuple%20structs%20follow%20the%20same%20rules.%0Aassert_eq!(6%2C%20mem%3A%3Asize_of%3A%3A%3CTupleStruct%3E())%3B%0A%0A%2F%2F%20Note%20that%20reordering%20the%20fields%20can%20lower%20the%20size.%20We%20can%20remove%20both%20padding%20bytes%0A%2F%2F%20by%20putting%20%60third%60%20before%20%60second%60.%0A%23%5Brepr(C)%5D%0Astruct%20FieldStructOptimized%20%7B%0A%20%20%20%20first%3A%20u8%2C%0A%20%20%20%20third%3A%20u8%2C%0A%20%20%20%20second%3A%20u16%0A%7D%0A%0Aassert_eq!(4%2C%20mem%3A%3Asize_of%3A%3A%3CFieldStructOptimized%3E())%3B%0A%0A%2F%2F%20Union%20size%20is%20the%20size%20of%20the%20largest%20field.%0A%23%5Brepr(C)%5D%0Aunion%20ExampleUnion%20%7B%0A%20%20%20%20smaller%3A%20u8%2C%0A%20%20%20%20larger%3A%20u16%0A%7D%0A%0Aassert_eq!(2%2C%20mem%3A%3Asize_of%3A%3A%3CExampleUnion%3E())%3B%0A%7D">Run</a></pre>
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