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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'>mem</a></p><script>window.sidebarCurrent = {name: 'uninitialized', ty: 'fn', relpath: ''};</script><script defer src="sidebar-items.js"></script>
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<section id='main' class="content fn">
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<h1 class='fqn'><span class='in-band'>Function <a href='../../index.html'>bitflags</a>::<wbr><a href='../index.html'>__core</a>::<wbr><a href='index.html'>mem</a>::<wbr><a class='fn' href=''>uninitialized</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-5609' class='srclink' href='https://doc.rust-lang.org/nightly/core/mem/fn.uninitialized.html?gotosrc=5609' title='goto source code'>[src]</a></span></h1>
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<pre class='rust fn'>pub unsafe fn uninitialized<T>() -> T</pre><span class="since">1.0.0</span><div class='docblock'><p>Bypasses Rust's normal memory-initialization checks by pretending to
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produce a value of type T, while doing nothing at all.</p>
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<p><strong>This is incredibly dangerous, and should not be done lightly. Deeply
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consider initializing your memory with a default value instead.</strong></p>
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<p>This is useful for FFI functions and initializing arrays sometimes,
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but should generally be avoided.</p>
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<h1 id='undefined-behavior' class='section-header'><a href='#undefined-behavior'>Undefined Behavior</a></h1>
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<p>It is Undefined Behavior to read uninitialized memory. Even just an
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uninitialized boolean. For instance, if you branch on the value of such
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a boolean your program may take one, both, or neither of the branches.</p>
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<p>Note that this often also includes <em>writing</em> to the uninitialized value.
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Rust believes the value is initialized, and will therefore try to Drop
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the uninitialized value and its fields if you try to overwrite the memory
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in a normal manner. The only way to safely initialize an arbitrary
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uninitialized value is with one of the <code>ptr</code> functions: <code>write</code>, <code>copy</code>, or
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<code>copy_nonoverlapping</code>. This isn't necessary if <code>T</code> is a primitive
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or otherwise only contains types that don't implement Drop.</p>
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<p>If this value <em>does</em> need some kind of Drop, it must be initialized before
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it goes out of scope (and therefore would be dropped). Note that this
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includes a <code>panic</code> occurring and unwinding the stack suddenly.</p>
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<h1 id='examples' class='section-header'><a href='#examples'>Examples</a></h1>
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<p>Here's how to safely initialize an array of <code>Vec</code>s.</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'>mem</span>;
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<span class='kw'>use</span> <span class='ident'>std</span>::<span class='ident'>ptr</span>;
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<span class='comment'>// Only declare the array. This safely leaves it</span>
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<span class='comment'>// uninitialized in a way that Rust will track for us.</span>
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<span class='comment'>// However we can't initialize it element-by-element</span>
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<span class='comment'>// safely, and we can't use the `[value; 1000]`</span>
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<span class='comment'>// constructor because it only works with `Copy` data.</span>
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<span class='kw'>let</span> <span class='kw-2'>mut</span> <span class='ident'>data</span>: [<span class='ident'>Vec</span><span class='op'><</span><span class='ident'>u32</span><span class='op'>></span>; <span class='number'>1000</span>];
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<span class='kw'>unsafe</span> {
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<span class='comment'>// So we need to do this to initialize it.</span>
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<span class='ident'>data</span> <span class='op'>=</span> <span class='ident'>mem</span>::<span class='ident'>uninitialized</span>();
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<span class='comment'>// DANGER ZONE: if anything panics or otherwise</span>
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<span class='comment'>// incorrectly reads the array here, we will have</span>
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<span class='comment'>// Undefined Behavior.</span>
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<span class='comment'>// It's ok to mutably iterate the data, since this</span>
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<span class='comment'>// doesn't involve reading it at all.</span>
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<span class='comment'>// (ptr and len are statically known for arrays)</span>
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<span class='kw'>for</span> <span class='ident'>elem</span> <span class='kw'>in</span> <span class='kw-2'>&</span><span class='kw-2'>mut</span> <span class='ident'>data</span>[..] {
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<span class='comment'>// *elem = Vec::new() would try to drop the</span>
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<span class='comment'>// uninitialized memory at `elem` -- bad!</span>
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<span class='comment'>//</span>
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<span class='comment'>// Vec::new doesn't allocate or do really</span>
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<span class='comment'>// anything. It's only safe to call here</span>
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<span class='comment'>// because we know it won't panic.</span>
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<span class='ident'>ptr</span>::<span class='ident'>write</span>(<span class='ident'>elem</span>, <span class='ident'>Vec</span>::<span class='ident'>new</span>());
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}
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<span class='comment'>// SAFE ZONE: everything is initialized.</span>
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}
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<span class='macro'>println</span><span class='macro'>!</span>(<span class='string'>"{:?}"</span>, <span class='kw-2'>&</span><span class='ident'>data</span>[<span class='number'>0</span>]);</pre>
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<p>This example emphasizes exactly how delicate and dangerous doing this is.
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Note that the <code>vec!</code> macro <em>does</em> let you initialize every element with a
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value that is only <code>Clone</code>, so the following is semantically equivalent and
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vastly less dangerous, as long as you can live with an extra heap
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allocation:</p>
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<pre class='rust rust-example-rendered'>
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<span class='kw'>let</span> <span class='ident'>data</span>: <span class='ident'>Vec</span><span class='op'><</span><span class='ident'>Vec</span><span class='op'><</span><span class='ident'>u32</span><span class='op'>>></span> <span class='op'>=</span> <span class='macro'>vec</span><span class='macro'>!</span>[<span class='ident'>Vec</span>::<span class='ident'>new</span>(); <span class='number'>1000</span>];
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<span class='macro'>println</span><span class='macro'>!</span>(<span class='string'>"{:?}"</span>, <span class='kw-2'>&</span><span class='ident'>data</span>[<span class='number'>0</span>]);</pre>
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