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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" class="active"><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"><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><li class="chapter-item expanded "><a href="../functional/lenses.html"><strong aria-hidden="true">5.3.</strong> Lenses and Prisms</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="constructors"><a class="header" href="#constructors">Constructors</a></h1>
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<h2 id="description"><a class="header" href="#description">Description</a></h2>
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<p>Rust does not have constructors as a language construct. Instead, the
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convention is to use an <a href="https://doc.rust-lang.org/stable/book/ch05-03-method-syntax.html#associated-functions">associated function</a> <code>new</code> to create an object:</p>
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<pre><pre class="playground"><code class="language-rust edition2018"><span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>/// Time in seconds.
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///
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/// # Example
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///
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/// ```
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/// let s = Second::new(42);
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/// assert_eq!(42, s.value());
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/// ```
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pub struct Second {
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value: u64
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}
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impl Second {
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// Constructs a new instance of [`Second`].
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// Note this is an associated function - no self.
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pub fn new(value: u64) -> Self {
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Self { value }
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}
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/// Returns the value in seconds.
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pub fn value(&self) -> u64 {
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self.value
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}
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}
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<span class="boring">}</span></code></pre></pre>
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<h2 id="default-constructors"><a class="header" href="#default-constructors">Default Constructors</a></h2>
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<p>Rust supports default constructors with the <a href="https://doc.rust-lang.org/stable/std/default/trait.Default.html"><code>Default</code></a> trait:</p>
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<pre><pre class="playground"><code class="language-rust edition2018"><span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>/// Time in seconds.
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///
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/// # Example
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///
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/// ```
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/// let s = Second::default();
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/// assert_eq!(0, s.value());
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/// ```
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pub struct Second {
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value: u64
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}
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impl Second {
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/// Returns the value in seconds.
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pub fn value(&self) -> u64 {
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self.value
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}
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}
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impl Default for Second {
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fn default() -> Self {
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Self { value: 0 }
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}
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}
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<span class="boring">}</span></code></pre></pre>
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<p><code>Default</code> can also be derived if all types of all fields implement <code>Default</code>,
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like they do with <code>Second</code>:</p>
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<pre><pre class="playground"><code class="language-rust edition2018"><span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>/// Time in seconds.
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///
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/// # Example
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///
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/// ```
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/// let s = Second::default();
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/// assert_eq!(0, s.value());
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/// ```
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#[derive(Default)]
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pub struct Second {
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value: u64
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}
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impl Second {
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/// Returns the value in seconds.
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pub fn value(&self) -> u64 {
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self.value
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}
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}
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<span class="boring">}</span></code></pre></pre>
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<p><strong>Note:</strong> It is common and expected for types to implement both
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<code>Default</code> and an empty <code>new</code> constructor. <code>new</code> is the constructor
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convention in Rust, and users expect it to exist, so if it is
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reasonable for the basic constructor to take no arguments, then it
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should, even if it is functionally identical to default.</p>
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<p><strong>Hint:</strong> The advantage of implementing or deriving <code>Default</code> is that your type
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can now be used where a <code>Default</code> implementation is required, most prominently,
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any of the <a href="https://doc.rust-lang.org/stable/std/?search=or_default"><code>*or_default</code> functions in the standard library</a>.</p>
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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>
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<p>The <a href="default.html">default idiom</a> for a more in-depth description of the
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<code>Default</code> trait.</p>
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</li>
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<li>
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<p>The <a href="../patterns/creational/builder.html">builder pattern</a> for constructing
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objects where there are multiple configurations.</p>
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</li>
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<li>
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<p><a href="https://rust-lang.github.io/api-guidelines/interoperability.html#types-eagerly-implement-common-traits-c-common-traits">API Guidelines/C-COMMON-TRAITS</a> for
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implementing both, <code>Default</code> and <code>new</code>.</p>
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</li>
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</ul>
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