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<ol class="chapter"><li class="chapter-item expanded "><a href="../intro.html"><strong aria-hidden="true">1.</strong> Introduction</a></li><li class="chapter-item expanded "><a href="../idioms/index.html"><strong aria-hidden="true">2.</strong> Idioms</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../idioms/coercion-arguments.html"><strong aria-hidden="true">2.1.</strong> Use borrowed types for arguments</a></li><li class="chapter-item expanded "><a href="../idioms/concat-format.html"><strong aria-hidden="true">2.2.</strong> Concatenating Strings with format!</a></li><li class="chapter-item expanded "><a href="../idioms/ctor.html"><strong aria-hidden="true">2.3.</strong> Constructor</a></li><li class="chapter-item expanded "><a href="../idioms/default.html"><strong aria-hidden="true">2.4.</strong> The Default Trait</a></li><li class="chapter-item expanded "><a href="../idioms/deref.html"><strong aria-hidden="true">2.5.</strong> Collections Are Smart Pointers</a></li><li class="chapter-item expanded "><a href="../idioms/dtor-finally.html"><strong aria-hidden="true">2.6.</strong> Finalisation in Destructors</a></li><li class="chapter-item expanded "><a href="../idioms/mem-replace.html" class="active"><strong aria-hidden="true">2.7.</strong> mem::{take(), replace()}</a></li><li class="chapter-item expanded "><a href="../idioms/on-stack-dyn-dispatch.html"><strong aria-hidden="true">2.8.</strong> On-Stack Dynamic Dispatch</a></li><li class="chapter-item expanded "><a href="../idioms/ffi/intro.html"><strong aria-hidden="true">2.9.</strong> Foreign function interface (FFI)</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../idioms/ffi/errors.html"><strong aria-hidden="true">2.9.1.</strong> Idiomatic Errors</a></li><li class="chapter-item expanded "><a href="../idioms/ffi/accepting-strings.html"><strong aria-hidden="true">2.9.2.</strong> Accepting Strings</a></li><li class="chapter-item expanded "><a href="../idioms/ffi/passing-strings.html"><strong aria-hidden="true">2.9.3.</strong> Passing Strings</a></li></ol></li><li class="chapter-item expanded "><a href="../idioms/option-iter.html"><strong aria-hidden="true">2.10.</strong> Iterating over an Option</a></li><li class="chapter-item expanded "><a href="../idioms/pass-var-to-closure.html"><strong aria-hidden="true">2.11.</strong> Pass Variables to Closure</a></li><li class="chapter-item expanded "><a href="../idioms/priv-extend.html"><strong aria-hidden="true">2.12.</strong> Privacy For Extensibility</a></li><li class="chapter-item expanded "><a href="../idioms/rustdoc-init.html"><strong aria-hidden="true">2.13.</strong> Easy doc initialization</a></li><li class="chapter-item expanded "><a href="../idioms/temporary-mutability.html"><strong aria-hidden="true">2.14.</strong> Temporary mutability</a></li></ol></li><li class="chapter-item expanded "><a href="../patterns/index.html"><strong aria-hidden="true">3.</strong> Design Patterns</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../patterns/behavioural/intro.html"><strong aria-hidden="true">3.1.</strong> Behavioural</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../patterns/behavioural/command.html"><strong aria-hidden="true">3.1.1.</strong> Command</a></li><li class="chapter-item expanded "><a href="../patterns/behavioural/interpreter.html"><strong aria-hidden="true">3.1.2.</strong> Interpreter</a></li><li class="chapter-item expanded "><a href="../patterns/behavioural/newtype.html"><strong aria-hidden="true">3.1.3.</strong> Newtype</a></li><li class="chapter-item expanded "><a href="../patterns/behavioural/RAII.html"><strong aria-hidden="true">3.1.4.</strong> RAII Guards</a></li><li class="chapter-item expanded "><a href="../patterns/behavioural/strategy.html"><strong aria-hidden="true">3.1.5.</strong> Strategy</a></li><li class="chapter-item expanded "><a href="../patterns/behavioural/visitor.html"><strong aria-hidden="true">3.1.6.</strong> Visitor</a></li></ol></li><li class="chapter-item expanded "><a href="../patterns/creational/intro.html"><strong aria-hidden="true">3.2.</strong> Creational</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../patterns/creational/builder.html"><strong aria-hidden="true">3.2.1.</strong> Builder</a></li><li class="chapter-item expanded "><a href="../patterns/creational/fold.html"><strong aria-hidden="true">3.2.2.</strong> Fold</a></li></ol></li><li class="chapter-item expanded "><a href="../patterns/structural/intro.html"><strong aria-hidden="true">3.3.</strong> Structural</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../patterns/structural/compose-structs.html"><strong aria-hidden="true">3.3.1.</strong> Compose Structs</a></li><li class="chapter-item expanded "><a href="../patterns/structural/small-crates.html"><strong aria-hidden="true">3.3.2.</strong> Prefer Small Crates</a></li><li class="chapter-item expanded "><a href="../patterns/structural/unsafe-mods.html"><strong aria-hidden="true">3.3.3.</strong> Contain unsafety in small modules</a></li></ol></li><li class="chapter-item expanded "><a href="../patterns/ffi/intro.html"><strong aria-hidden="true">3.4.</strong> Foreign function interface (FFI)</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../patterns/ffi/export.html"><strong aria-hidden="true">3.4.1.</strong> Object-Based APIs</a></li><li class="chapter-item expanded "><a href="../patterns/ffi/wrappers.html"><strong aria-hidden="true">3.4.2.</strong> Type Consolidation into Wrappers</a></li></ol></li></ol></li><li class="chapter-item expanded "><a href="../anti_patterns/index.html"><strong aria-hidden="true">4.</strong> Anti-patterns</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../anti_patterns/borrow_clone.html"><strong aria-hidden="true">4.1.</strong> Clone to satisfy the borrow checker</a></li><li class="chapter-item expanded "><a href="../anti_patterns/deny-warnings.html"><strong aria-hidden="true">4.2.</strong> #[deny(warnings)]</a></li><li class="chapter-item expanded "><a href="../anti_patterns/deref.html"><strong aria-hidden="true">4.3.</strong> Deref Polymorphism</a></li></ol></li><li class="chapter-item expanded "><a href="../functional/index.html"><strong aria-hidden="true">5.</strong> Functional Programming</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../functional/paradigms.html"><strong aria-hidden="true">5.1.</strong> Programming paradigms</a></li></ol></li><li class="chapter-item expanded "><a href="../additional_resources/index.html"><strong aria-hidden="true">6.</strong> Additional Resources</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../additional_resources/design-principles.html"><strong aria-hidden="true">6.1.</strong> Design principles</a></li></ol></li></ol>
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<main>
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<h1 id="memtake_-replace_-to-keep-owned-values-in-changed-enums"><a class="header" href="#memtake_-replace_-to-keep-owned-values-in-changed-enums"><code>mem::{take(_), replace(_)}</code> to keep owned values in changed enums</a></h1>
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<h2 id="description"><a class="header" href="#description">Description</a></h2>
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<p>Say we have a <code>&mut MyEnum</code> which has (at least) two variants,
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<code>A { name: String, x: u8 }</code> and <code>B { name: String }</code>. Now we want to change
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<code>MyEnum::A</code> to a <code>B</code> if <code>x</code> is zero, while keeping <code>MyEnum::B</code> intact.</p>
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<p>We can do this without cloning the <code>name</code>.</p>
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<h2 id="example"><a class="header" href="#example">Example</a></h2>
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<pre><pre class="playground"><code class="language-rust edition2018">
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<span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>use std::mem;
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enum MyEnum {
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A { name: String, x: u8 },
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B { name: String }
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}
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fn a_to_b(e: &mut MyEnum) {
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if let MyEnum::A { name, x: 0 } = e {
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// this takes out our `name` and put in an empty String instead
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// (note that empty strings don't allocate).
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// Then, construct the new enum variant (which will
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// be assigned to `*e`).
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*e = MyEnum::B { name: mem::take(name) }
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}
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}
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<span class="boring">}
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</span></code></pre></pre>
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<p>This also works with more variants:</p>
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<pre><pre class="playground"><code class="language-rust edition2018">
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<span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>use std::mem;
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enum MultiVariateEnum {
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A { name: String },
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B { name: String },
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C,
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D
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}
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fn swizzle(e: &mut MultiVariateEnum) {
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use MultiVariateEnum::*;
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*e = match e {
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// Ownership rules do not allow taking `name` by value, but we cannot
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// take the value out of a mutable reference, unless we replace it:
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A { name } => B { name: mem::take(name) },
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B { name } => A { name: mem::take(name) },
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C => D,
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D => C
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}
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}
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<span class="boring">}
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</span></code></pre></pre>
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<h2 id="motivation"><a class="header" href="#motivation">Motivation</a></h2>
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<p>When working with enums, we may want to change an enum value in place, perhaps
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to another variant. This is usually done in two phases to keep the borrow
|
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checker happy. In the first phase, we observe the existing value and look at
|
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its parts to decide what to do next. In the second phase we may conditionally
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change the value (as in the example above).</p>
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<p>The borrow checker won't allow us to take out <code>name</code> of the enum (because
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<em>something</em> must be there. We could of course <code>.clone()</code> name and put the clone
|
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into our <code>MyEnum::B</code>, but that would be an instance of the [Clone to satisfy
|
|
|
the borrow checker] antipattern. Anyway, we can avoid the extra allocation by
|
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changing <code>e</code> with only a mutable borrow.</p>
|
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<p><code>mem::take</code> lets us swap out the value, replacing it with it's default value,
|
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and returning the previous value. For <code>String</code>, the default value is an empty
|
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<code>String</code>, which does not need to allocate. As a result, we get the original
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<code>name</code> <em>as an owned value</em>. We can then wrap this in another enum.</p>
|
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|
<p><strong>NOTE:</strong> <code>mem::replace</code> is very similar, but allows us to specify what to
|
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replace the value with. An equivalent to our <code>mem::take</code> line would be
|
|
|
<code>mem::replace(name, String::new())</code>.</p>
|
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|
<p>Note, however, that if we are using an <code>Option</code> and want to replace its
|
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|
value with a <code>None</code>, <code>Option</code>’s <code>take()</code> method provides a shorter and
|
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|
more idiomatic alternative.</p>
|
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|
<h2 id="advantages"><a class="header" href="#advantages">Advantages</a></h2>
|
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|
<p>Look ma, no allocation! Also you may feel like Indiana Jones while doing it.</p>
|
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<h2 id="disadvantages"><a class="header" href="#disadvantages">Disadvantages</a></h2>
|
|
|
<p>This gets a bit wordy. Getting it wrong repeatedly will make you hate the
|
|
|
borrow checker. The compiler may fail to optimize away the double store,
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resulting in reduced performance as opposed to what you'd do in unsafe
|
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languages.</p>
|
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|
<p>Furthermore, the type you are taking needs to implement the <a href="./default.html"><code>Default</code>
|
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trait</a>. However, if the type you're working with doesn't
|
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implement this, you can instead use <code>mem::replace</code>.</p>
|
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|
<h2 id="discussion"><a class="header" href="#discussion">Discussion</a></h2>
|
|
|
<p>This pattern is only of interest in Rust. In GC'd languages, you'd take the
|
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|
reference to the value by default (and the GC would keep track of refs), and in
|
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|
other low-level languages like C you'd simply alias the pointer and fix things
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later.</p>
|
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<p>However, in Rust, we have to do a little more work to do this. An owned value
|
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may only have one owner, so to take it out, we need to put something back in –
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|
like Indiana Jones, replacing the artifact with a bag of sand.</p>
|
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|
<h2 id="see-also"><a class="header" href="#see-also">See also</a></h2>
|
|
|
<p>This gets rid of the <a href="../anti_patterns/borrow_clone.html">Clone to satisfy the borrow checker</a>
|
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|
antipattern in a specific case.</p>
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