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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><ol class="section"><li class="chapter-item expanded "><a href="../translations.html"><strong aria-hidden="true">1.1.</strong> Translations</a></li></ol></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" class="active"><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"><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><li class="chapter-item expanded "><a href="../functional/generics-type-classes.html"><strong aria-hidden="true">5.2.</strong> Generics as Type Classes</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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<h1 class="menu-title">Rust Design Patterns</h1>
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<h1 id="the-default-trait"><a class="header" href="#the-default-trait">The <code>Default</code> Trait</a></h1>
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<h2 id="description"><a class="header" href="#description">Description</a></h2>
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<p>Many types in Rust have a <a href="ctor.html">constructor</a>. However, this is <em>specific</em> to the
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type; Rust cannot abstract over "everything that has a <code>new()</code> method". To
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allow this, the <a href="https://doc.rust-lang.org/stable/std/default/trait.Default.html"><code>Default</code></a> trait was conceived, which can be used with
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containers and other generic types (e.g. see <a href="https://doc.rust-lang.org/stable/std/option/enum.Option.html#method.unwrap_or_default"><code>Option::unwrap_or_default()</code></a>).
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Notably, some containers already implement it where applicable.</p>
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<p>Not only do one-element containers like <code>Cow</code>, <code>Box</code> or <code>Arc</code> implement
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<code>Default</code> for contained <code>Default</code> types, one can automatically
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<code>#[derive(Default)]</code> for structs whose fields all implement it, so the more
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types implement <code>Default</code>, the more useful it becomes.</p>
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<p>On the other hand, constructors can take multiple arguments, while the
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<code>default()</code> method does not. There can even be multiple constructors with
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different names, but there can only be one <code>Default</code> implementation per type.</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">use std::{path::PathBuf, time::Duration};
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// note that we can simply auto-derive Default here.
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#[derive(Default, Debug, PartialEq)]
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struct MyConfiguration {
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// Option defaults to None
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output: Option<PathBuf>,
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// Vecs default to empty vector
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search_path: Vec<PathBuf>,
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// Duration defaults to zero time
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timeout: Duration,
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// bool defaults to false
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check: bool,
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}
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impl MyConfiguration {
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// add setters here
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}
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fn main() {
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// construct a new instance with default values
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let mut conf = MyConfiguration::default();
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// do something with conf here
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conf.check = true;
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println!("conf = {:#?}", conf);
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// partial initialization with default values, creates the same instance
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let conf1 = MyConfiguration {
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check: true,
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..Default::default()
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};
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assert_eq!(conf, conf1);
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}
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</code></pre></pre>
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<h2 id="see-also"><a class="header" href="#see-also">See also</a></h2>
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<ul>
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<li>The <a href="ctor.html">constructor</a> idiom is another way to generate instances that may or may
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not be "default"</li>
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<li>The <a href="https://doc.rust-lang.org/stable/std/default/trait.Default.html"><code>Default</code></a> documentation (scroll down for the list of implementors)</li>
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<li><a href="https://doc.rust-lang.org/stable/std/option/enum.Option.html#method.unwrap_or_default"><code>Option::unwrap_or_default()</code></a></li>
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<li><a href="https://crates.io/crates/derive-new/"><code>derive(new)</code></a></li>
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