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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" class="active"><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::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 usage</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 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Patterns</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../patterns/builder.html"><strong aria-hidden="true">3.1.</strong> Builder</a></li><li class="chapter-item expanded "><a href="../patterns/compose-structs.html"><strong aria-hidden="true">3.2.</strong> Compose Structs</a></li><li class="chapter-item expanded "><a href="../patterns/entry.html"><strong aria-hidden="true">3.3.</strong> Entry API</a></li><li class="chapter-item expanded "><a href="../patterns/ffi-intro.html"><strong aria-hidden="true">3.4.</strong> Foreign function interface usage</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><li class="chapter-item expanded "><a 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aria-hidden="true">4.</strong> Anti-patterns</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../anti_patterns/deny-warnings.html"><strong aria-hidden="true">4.1.</strong> #[deny(warnings)]</a></li><li class="chapter-item expanded "><a href="../anti_patterns/deref.html"><strong aria-hidden="true">4.2.</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 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><a class="header" href="#collections-are-smart-pointers" id="collections-are-smart-pointers">Collections are smart pointers</a></h1>
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<h2><a class="header" href="#description" id="description">Description</a></h2>
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<p>Use the <code>Deref</code> trait to treat collections like smart pointers, offering owning
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and borrowed views of data.</p>
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<h2><a class="header" href="#example" id="example">Example</a></h2>
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<pre><code class="language-rust ignore">use std::ops::Deref;
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struct Vec<T> {
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data: T,
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//..
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}
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impl<T> Deref for Vec<T> {
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type Target = [T];
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fn deref(&self) -> &[T] {
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//..
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}
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}
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</code></pre>
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<p>A <code>Vec<T></code> is an owning collection of <code>T</code>s, a slice (<code>&[T]</code>) is a borrowed
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collection of <code>T</code>s. Implementing <code>Deref</code> for <code>Vec</code> allows implicit dereferencing
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from <code>&Vec<T></code> to <code>&[T]</code> and includes the relationship in auto-derefencing
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searches. Most methods you might expect to be implemented for <code>Vec</code>s are instead
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implemented for slices.</p>
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<p>See also <code>String</code> and <code>&str</code>.</p>
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<h2><a class="header" href="#motivation" id="motivation">Motivation</a></h2>
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<p>Ownership and borrowing are key aspects of the Rust language. Data structures
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must account for these semantics properly in order to give a good user
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experience. When implementing a data structure which owns its data, offering a
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borrowed view of that data allows for more flexible APIs.</p>
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<h2><a class="header" href="#advantages" id="advantages">Advantages</a></h2>
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<p>Most methods can be implemented only for the borrowed view, they are then
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implicitly available for the owning view.</p>
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<p>Gives clients a choice between borrowing or taking ownership of data.</p>
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<h2><a class="header" href="#disadvantages" id="disadvantages">Disadvantages</a></h2>
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<p>Methods and traits only available via dereferencing are not taken into account
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when bounds checking, so generic programming with data structures using this
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pattern can get complex (see the <code>Borrow</code> and <code>AsRef</code> traits, etc.).</p>
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<h2><a class="header" href="#discussion" id="discussion">Discussion</a></h2>
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<p>Smart pointers and collections are analogous: a smart pointer points to a single
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object, whereas a collection points to many objects. From the point of view of
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the type system there is little difference between the two. A collection owns
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its data if the only way to access each datum is via the collection and the
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collection is responsible for deleting the data (even in cases of shared
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ownership, some kind of borrowed view may be appropriate). If a collection owns
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its data, it is usually useful to provide a view of the data as borrowed so that
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it can be multiply referenced.</p>
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<p>Most smart pointers (e.g., <code>Foo<T></code>) implement <code>Deref<Target=T></code>. However,
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collections will usually dereference to a custom type. <code>[T]</code> and <code>str</code> have some
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language support, but in the general case, this is not necessary. <code>Foo<T></code> can
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implement <code>Deref<Target=Bar<T>></code> where <code>Bar</code> is a dynamically sized type and
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<code>&Bar<T></code> is a borrowed view of the data in <code>Foo<T></code>.</p>
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<p>Commonly, ordered collections will implement <code>Index</code> for <code>Range</code>s to provide
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slicing syntax. The target will be the borrowed view.</p>
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<h2><a class="header" href="#see-also" id="see-also">See also</a></h2>
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<p><a href="../anti_patterns/deref.html">Deref polymorphism anti-pattern</a>.</p>
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<p><a href="https://doc.rust-lang.org/std/ops/trait.Deref.html">Documentation for <code>Deref</code> trait</a>.</p>
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