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oxipng/doc/bitflags/__core/mem/fn.replace.html
Joshua Holmer 193e6496bd Version 0.7.0
2016-05-04 10:41:29 -04:00

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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=''>replace</a></span><span class='out-of-band'><span id='render-detail'>
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</span><a id='src-5616' class='srclink' href='https://doc.rust-lang.org/nightly/core/mem/fn.replace.html?gotosrc=5616' title='goto source code'>[src]</a></span></h1>
<pre class='rust fn'>pub fn replace&lt;T&gt;(dest: &amp;mut T, src: T) -&gt; T</pre><span class="since">1.0.0</span><div class='docblock'><p>Replaces the value at a mutable location with a new one, returning the old value, without
deinitializing or copying either one.</p>
<p>This is primarily used for transferring and swapping ownership of a value in a mutable
location.</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'>&lt;</span><span class='ident'>i32</span><span class='op'>&gt;</span> <span class='op'>=</span> <span class='ident'>Vec</span>::<span class='ident'>new</span>();
<span class='ident'>mem</span>::<span class='ident'>replace</span>(<span class='kw-2'>&amp;</span><span class='kw-2'>mut</span> <span class='ident'>v</span>, <span class='ident'>Vec</span>::<span class='ident'>new</span>());</pre>
<p>This function allows consumption of one field of a struct by replacing it with another value.
The normal approach doesn&#39;t always work:</p>
<pre class='rust rust-example-rendered'>
<span class='kw'>struct</span> <span class='ident'>Buffer</span><span class='op'>&lt;</span><span class='ident'>T</span><span class='op'>&gt;</span> { <span class='ident'>buf</span>: <span class='ident'>Vec</span><span class='op'>&lt;</span><span class='ident'>T</span><span class='op'>&gt;</span> }
<span class='kw'>impl</span><span class='op'>&lt;</span><span class='ident'>T</span><span class='op'>&gt;</span> <span class='ident'>Buffer</span><span class='op'>&lt;</span><span class='ident'>T</span><span class='op'>&gt;</span> {
<span class='kw'>fn</span> <span class='ident'>get_and_reset</span>(<span class='kw-2'>&amp;</span><span class='kw-2'>mut</span> <span class='self'>self</span>) <span class='op'>-&gt;</span> <span class='ident'>Vec</span><span class='op'>&lt;</span><span class='ident'>T</span><span class='op'>&gt;</span> {
<span class='comment'>// error: cannot move out of dereference of `&amp;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>
}
}</pre>
<p>Note that <code>T</code> does not necessarily implement <code>Clone</code>, so it can&#39;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'>&lt;</span><span class='ident'>T</span><span class='op'>&gt;</span> <span class='ident'>Buffer</span><span class='op'>&lt;</span><span class='ident'>T</span><span class='op'>&gt;</span> {
<span class='kw'>fn</span> <span class='ident'>get_and_reset</span>(<span class='kw-2'>&amp;</span><span class='kw-2'>mut</span> <span class='self'>self</span>) <span class='op'>-&gt;</span> <span class='ident'>Vec</span><span class='op'>&lt;</span><span class='ident'>T</span><span class='op'>&gt;</span> {
<span class='ident'>mem</span>::<span class='ident'>replace</span>(<span class='kw-2'>&amp;</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>())
}
}</pre>
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