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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"><strong aria-hidden="true">2.4.</strong> 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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 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<h1 id="on-stack-dynamic-dispatch"><a class="header" href="#on-stack-dynamic-dispatch">On-Stack Dynamic Dispatch</a></h1>
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<h2 id="description"><a class="header" href="#description">Description</a></h2>
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<p>We can dynamically dispatch over multiple values, however, to do so, we need
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to declare multiple variables to bind differently-typed objects. To extend the
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lifetime as necessary, we can use deferred conditional initialization, as seen
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below:</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::io;
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use std::fs;
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<span class="boring">fn main() -> Result<(), Box<dyn std::error::Error>> {
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</span><span class="boring">let arg = "-";
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</span>
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// These must live longer than `readable`, and thus are declared first:
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let (mut stdin_read, mut file_read);
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// We need to ascribe the type to get dynamic dispatch.
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let readable: &mut dyn io::Read = if arg == "-" {
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stdin_read = io::stdin();
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&mut stdin_read
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} else {
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file_read = fs::File::open(arg)?;
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&mut file_read
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};
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// Read from `readable` here.
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<span class="boring">Ok(())
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</span><span class="boring">}</span></code></pre></pre>
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<h2 id="motivation"><a class="header" href="#motivation">Motivation</a></h2>
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<p>Rust monomorphises code by default. This means a copy of the code will be
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generated for each type it is used with and optimized independently. While this
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allows for very fast code on the hot path, it also bloats the code in places
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where performance is not of the essence, thus costing compile time and cache
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usage.</p>
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<p>Luckily, Rust allows us to use dynamic dispatch, but we have to explicitly ask
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for it.</p>
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<h2 id="advantages"><a class="header" href="#advantages">Advantages</a></h2>
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<p>We do not need to allocate anything on the heap. Neither do we need to
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initialize something we won't use later, nor do we need to monomorphize the
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whole code that follows to work with both <code>File</code> or <code>Stdin</code>.</p>
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<h2 id="disadvantages"><a class="header" href="#disadvantages">Disadvantages</a></h2>
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<p>The code needs more moving parts than the <code>Box</code>-based version:</p>
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<pre><code class="language-rust ignore">// We still need to ascribe the type for dynamic dispatch.
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let readable: Box<dyn io::Read> = if arg == "-" {
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Box::new(io::stdin())
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} else {
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Box::new(fs::File::open(arg)?)
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};
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// Read from `readable` here.</code></pre>
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<h2 id="discussion"><a class="header" href="#discussion">Discussion</a></h2>
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<p>Rust newcomers will usually learn that Rust requires all variables to be
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initialized <em>before use</em>, so it's easy to overlook the fact that <em>unused</em>
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variables may well be uninitialized. Rust works quite hard to ensure that this
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works out fine and only the initialized values are dropped at the end of their
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scope.</p>
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<p>The example meets all the constraints Rust places on us:</p>
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<ul>
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<li>All variables are initialized before using (in this case borrowing) them</li>
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<li>Each variable only holds values of a single type. In our example, <code>stdin</code> is
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of type <code>Stdin</code>, <code>file</code> is of type <code>File</code> and <code>readable</code> is of type <code>&mut dyn Read</code></li>
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<li>Each borrowed value outlives all the references borrowed from it</li>
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</ul>
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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><a href="dtor-finally.html">Finalisation in destructors</a> and
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<a href="../patterns/behavioural/RAII.html">RAII guards</a> can benefit from tight control over
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lifetimes.</li>
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<li>For conditionally filled <code>Option<&T></code>s of (mutable) references, one can
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initialize an <code>Option<T></code> directly and use its <a href="https://doc.rust-lang.org/std/option/enum.Option.html#method.as_ref"><code>.as_ref()</code></a> method to get an
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optional reference.</li>
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</ul>
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