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<h1 class='fqn'><span class='in-band'>Function <a href='../index.html'>core</a>::<wbr><a href='index.html'>mem</a>::<wbr><a class="fn" href=''>replace</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'>
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[<span class='inner'>−</span>]
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</span><a class='srclink' href='../../src/core/mem.rs.html#690-693' title='goto source code'>[src]</a></span></h1>
<pre class='rust fn'>pub fn replace<T>(dest: &mut T, src: T) -> T</pre><div class='docblock'><p>Replaces the value at a mutable location with a new one, returning the old value, without
deinitializing either one.</p>
<h1 id="examples" class="section-header"><a href="#examples">Examples</a></h1>
<p>A simple example:</p>
<pre class="rust rust-example-rendered">
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">mem</span>;
<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">v</span>: <span class="ident">Vec</span><span class="op"><</span><span class="ident">i32</span><span class="op">></span> <span class="op">=</span> <span class="macro">vec</span><span class="macro">!</span>[<span class="number">1</span>, <span class="number">2</span>];
<span class="kw">let</span> <span class="ident">old_v</span> <span class="op">=</span> <span class="ident">mem</span>::<span class="ident">replace</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">v</span>, <span class="macro">vec</span><span class="macro">!</span>[<span class="number">3</span>, <span class="number">4</span>, <span class="number">5</span>]);
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="number">2</span>, <span class="ident">old_v</span>.<span class="ident">len</span>());
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="number">3</span>, <span class="ident">v</span>.<span class="ident">len</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%0Alet%20mut%20v%3A%20Vec%3Ci32%3E%20%3D%20vec!%5B1%2C%202%5D%3B%0A%0Alet%20old_v%20%3D%20mem%3A%3Areplace(%26mut%20v%2C%20vec!%5B3%2C%204%2C%205%5D)%3B%0Aassert_eq!(2%2C%20old_v.len())%3B%0Aassert_eq!(3%2C%20v.len())%3B%0A%7D">Run</a></pre>
<p><code>replace</code> allows consumption of a struct field by replacing it with another value.
Without <code>replace</code> you can run into issues like these:</p>
<div class='information'><div class='tooltip compile_fail'>ⓘ<span class='tooltiptext'>This example deliberately fails to compile</span></div></div><pre class="rust rust-example-rendered compile_fail">
<span class="kw">struct</span> <span class="ident">Buffer</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> { <span class="ident">buf</span>: <span class="ident">Vec</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> }
<span class="kw">impl</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> <span class="ident">Buffer</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
<span class="kw">fn</span> <span class="ident">get_and_reset</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">self</span>) <span class="op">-></span> <span class="ident">Vec</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
<span class="comment">// error: cannot move out of dereference of `&mut`-pointer</span>
<span class="kw">let</span> <span class="ident">buf</span> <span class="op">=</span> <span class="self">self</span>.<span class="ident">buf</span>;
<span class="self">self</span>.<span class="ident">buf</span> <span class="op">=</span> <span class="ident">Vec</span>::<span class="ident">new</span>();
<span class="ident">buf</span>
}
}<a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=%23!%5Ballow(unused)%5D%0Afn%20main()%20%7B%0Astruct%20Buffer%3CT%3E%20%7B%20buf%3A%20Vec%3CT%3E%20%7D%0A%0Aimpl%3CT%3E%20Buffer%3CT%3E%20%7B%0A%20%20%20%20fn%20get_and_reset(%26mut%20self)%20-%3E%20Vec%3CT%3E%20%7B%0A%20%20%20%20%20%20%20%20%2F%2F%20error%3A%20cannot%20move%20out%20of%20dereference%20of%20%60%26mut%60-pointer%0A%20%20%20%20%20%20%20%20let%20buf%20%3D%20self.buf%3B%0A%20%20%20%20%20%20%20%20self.buf%20%3D%20Vec%3A%3Anew()%3B%0A%20%20%20%20%20%20%20%20buf%0A%20%20%20%20%7D%0A%7D%0A%7D">Run</a></pre>
<p>Note that <code>T</code> does not necessarily implement <a href="../../std/clone/trait.Clone.html"><code>Clone</code></a>, so it can't even clone and reset
<code>self.buf</code>. But <code>replace</code> can be used to disassociate the original value of <code>self.buf</code> from
<code>self</code>, allowing it to be returned:</p>
<pre class="rust rust-example-rendered">
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">mem</span>;
<span class="kw">impl</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> <span class="ident">Buffer</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
<span class="kw">fn</span> <span class="ident">get_and_reset</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">self</span>) <span class="op">-></span> <span class="ident">Vec</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
<span class="ident">mem</span>::<span class="ident">replace</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">self</span>.<span class="ident">buf</span>, <span class="ident">Vec</span>::<span class="ident">new</span>())
}
}<a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=%23!%5Ballow(unused)%5D%0A%23!%5Ballow(dead_code)%5D%0Afn%20main()%20%7B%0Ause%20std%3A%3Amem%3B%0A%0Astruct%20Buffer%3CT%3E%20%7B%20buf%3A%20Vec%3CT%3E%20%7D%0Aimpl%3CT%3E%20Buffer%3CT%3E%20%7B%0A%20%20%20%20fn%20get_and_reset(%26mut%20self)%20-%3E%20Vec%3CT%3E%20%7B%0A%20%20%20%20%20%20%20%20mem%3A%3Areplace(%26mut%20self.buf%2C%20Vec%3A%3Anew())%0A%20%20%20%20%7D%0A%7D%0A%7D">Run</a></pre>
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