mirror of
https://github.com/rust-unofficial/patterns
synced 2024-11-02 21:40:26 +00:00
312 lines
19 KiB
HTML
312 lines
19 KiB
HTML
<!DOCTYPE HTML>
|
|
<html lang="en" class="sidebar-visible no-js rust">
|
|
<head>
|
|
<!-- Book generated using mdBook -->
|
|
<meta charset="UTF-8">
|
|
<title>On-Stack Dynamic Dispatch - Rust Design Patterns</title>
|
|
|
|
|
|
|
|
|
|
<!-- Custom HTML head -->
|
|
|
|
|
|
|
|
<meta content="text/html; charset=utf-8" http-equiv="Content-Type">
|
|
<meta name="description" content="A catalogue of Rust design patterns, anti-patterns and idioms">
|
|
<meta name="viewport" content="width=device-width, initial-scale=1">
|
|
<meta name="theme-color" content="#ffffff" />
|
|
|
|
|
|
<link rel="icon" href="../favicon.svg">
|
|
|
|
|
|
<link rel="shortcut icon" href="../favicon.png">
|
|
|
|
<link rel="stylesheet" href="../css/variables.css">
|
|
<link rel="stylesheet" href="../css/general.css">
|
|
<link rel="stylesheet" href="../css/chrome.css">
|
|
|
|
<link rel="stylesheet" href="../css/print.css" media="print">
|
|
|
|
|
|
<!-- Fonts -->
|
|
<link rel="stylesheet" href="../FontAwesome/css/font-awesome.css">
|
|
|
|
<link rel="stylesheet" href="../fonts/fonts.css">
|
|
|
|
|
|
<!-- Highlight.js Stylesheets -->
|
|
<link rel="stylesheet" href="../highlight.css">
|
|
<link rel="stylesheet" href="../tomorrow-night.css">
|
|
<link rel="stylesheet" href="../ayu-highlight.css">
|
|
|
|
<!-- Custom theme stylesheets -->
|
|
|
|
|
|
|
|
</head>
|
|
<body>
|
|
<!-- Provide site root to javascript -->
|
|
<script type="text/javascript">
|
|
var path_to_root = "../";
|
|
var default_theme = window.matchMedia("(prefers-color-scheme: dark)").matches ? "navy" : "rust";
|
|
</script>
|
|
|
|
<!-- Work around some values being stored in localStorage wrapped in quotes -->
|
|
<script type="text/javascript">
|
|
try {
|
|
var theme = localStorage.getItem('mdbook-theme');
|
|
var sidebar = localStorage.getItem('mdbook-sidebar');
|
|
|
|
if (theme.startsWith('"') && theme.endsWith('"')) {
|
|
localStorage.setItem('mdbook-theme', theme.slice(1, theme.length - 1));
|
|
}
|
|
|
|
if (sidebar.startsWith('"') && sidebar.endsWith('"')) {
|
|
localStorage.setItem('mdbook-sidebar', sidebar.slice(1, sidebar.length - 1));
|
|
}
|
|
} catch (e) { }
|
|
</script>
|
|
|
|
<!-- Set the theme before any content is loaded, prevents flash -->
|
|
<script type="text/javascript">
|
|
var theme;
|
|
try { theme = localStorage.getItem('mdbook-theme'); } catch(e) { }
|
|
if (theme === null || theme === undefined) { theme = default_theme; }
|
|
var html = document.querySelector('html');
|
|
html.classList.remove('no-js')
|
|
html.classList.remove('rust')
|
|
html.classList.add(theme);
|
|
html.classList.add('js');
|
|
</script>
|
|
|
|
<!-- Hide / unhide sidebar before it is displayed -->
|
|
<script type="text/javascript">
|
|
var html = document.querySelector('html');
|
|
var sidebar = 'hidden';
|
|
if (document.body.clientWidth >= 1080) {
|
|
try { sidebar = localStorage.getItem('mdbook-sidebar'); } catch(e) { }
|
|
sidebar = sidebar || 'visible';
|
|
}
|
|
html.classList.remove('sidebar-visible');
|
|
html.classList.add("sidebar-" + sidebar);
|
|
</script>
|
|
|
|
<nav id="sidebar" class="sidebar" aria-label="Table of contents">
|
|
<div class="sidebar-scrollbox">
|
|
<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="../CONTRIBUTING.html"><strong aria-hidden="true">2.</strong> Contributing</a></li><li class="chapter-item expanded "><a href="../idioms/index.html"><strong aria-hidden="true">3.</strong> Idioms</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../idioms/coercion-arguments.html"><strong aria-hidden="true">3.1.</strong> Use borrowed types for arguments</a></li><li class="chapter-item expanded "><a href="../idioms/concat-format.html"><strong aria-hidden="true">3.2.</strong> Concatenating Strings with format!</a></li><li class="chapter-item expanded "><a href="../idioms/ctor.html"><strong aria-hidden="true">3.3.</strong> Constructor</a></li><li class="chapter-item expanded "><a href="../idioms/default.html"><strong aria-hidden="true">3.4.</strong> The Default Trait</a></li><li class="chapter-item expanded "><a href="../idioms/deref.html"><strong aria-hidden="true">3.5.</strong> Collections Are Smart Pointers</a></li><li class="chapter-item expanded "><a href="../idioms/dtor-finally.html"><strong aria-hidden="true">3.6.</strong> Finalisation in Destructors</a></li><li class="chapter-item expanded "><a href="../idioms/mem-replace.html"><strong aria-hidden="true">3.7.</strong> mem::replace(_)</a></li><li class="chapter-item expanded "><a href="../idioms/on-stack-dyn-dispatch.html" class="active"><strong aria-hidden="true">3.8.</strong> On-Stack Dynamic Dispatch</a></li><li class="chapter-item expanded "><a href="../idioms/ffi-intro.html"><strong aria-hidden="true">3.9.</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">3.9.1.</strong> Idiomatic Errors</a></li><li class="chapter-item expanded "><a href="../idioms/ffi-accepting-strings.html"><strong aria-hidden="true">3.9.2.</strong> Accepting Strings</a></li><li class="chapter-item expanded "><a href="../idioms/ffi-passing-strings.html"><strong aria-hidden="true">3.9.3.</strong> Passing Strings</a></li></ol></li><li class="chapter-item expanded "><a href="../idioms/option-iter.html"><strong aria-hidden="true">3.10.</strong> Iterating over an Option</a></li><li class="chapter-item expanded "><a href="../idioms/pass-var-to-closure.html"><strong aria-hidden="true">3.11.</strong> Pass Variables to Closure</a></li><li class="chapter-item expanded "><a href="../idioms/priv-extend.html"><strong aria-hidden="true">3.12.</strong> Privacy For Extensibility</a></li><li class="chapter-item expanded "><a href="../idioms/rustdoc-init.html"><strong aria-hidden="true">3.13.</strong> Easy doc initialization</a></li><li class="chapter-item expanded "><a href="../idioms/temporary-mutability.html"><strong aria-hidden="true">3.14.</strong> Temporary mutability</a></li></ol></li><li class="chapter-item expanded "><a href="../patterns/index.html"><strong aria-hidden="true">4.</strong> Design Patterns</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../patterns/builder.html"><strong aria-hidden="true">4.1.</strong> Builder</a></li><li class="chapter-item expanded "><a href="../patterns/compose-structs.html"><strong aria-hidden="true">4.2.</strong> Compose Structs</a></li><li class="chapter-item expanded "><a href="../patterns/entry.html"><strong aria-hidden="true">4.3.</strong> Entry API</a></li><li class="chapter-item expanded "><a href="../patterns/ffi-intro.html"><strong aria-hidden="true">4.4.</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">4.4.1.</strong> Object-Based APIs</a></li><li class="chapter-item expanded "><a href="../patterns/ffi-wrappers.html"><strong aria-hidden="true">4.4.2.</strong> Type Consolidation into Wrappers</a></li></ol></li><li class="chapter-item expanded "><a href="../patterns/fold.html"><strong aria-hidden="true">4.5.</strong> Fold</a></li><li class="chapter-item expanded "><a href="../patterns/newtype.html"><strong aria-hidden="true">4.6.</strong> Newtype</a></li><li class="chapter-item expanded "><a href="../patterns/RAII.html"><strong aria-hidden="true">4.7.</strong> RAII Guards</a></li><li class="chapter-item expanded "><a href="../patterns/small-crates.html"><strong aria-hidden="true">4.8.</strong> Prefer Small Crates</a></li><li class="chapter-item expanded "><a href="../patterns/unsafe-mods.html"><strong aria-hidden="true">4.9.</strong> Contain unsafety in small modules</a></li><li class="chapter-item expanded "><a href="../patterns/visitor.html"><strong aria-hidden="true">4.10.</strong> Visitor</a></li></ol></li><li class="chapter-item expanded "><a href="../anti_patterns/index.html"><strong aria-hidden="true">5.</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">5.1.</strong> #[deny(warnings)]</a></li><li class="chapter-item expanded "><a href="../anti_patterns/deref.html"><strong aria-hidden="true">5.2.</strong> Deref Polymorphism</a></li></ol></li><li class="chapter-item expanded "><a href="../functional/index.html"><strong aria-hidden="true">6.</strong> Functional Programming</a></li><li class="chapter-item expanded "><a href="../additional_resources.html"><strong aria-hidden="true">7.</strong> Additional Resources</a></li></ol>
|
|
</div>
|
|
<div id="sidebar-resize-handle" class="sidebar-resize-handle"></div>
|
|
</nav>
|
|
|
|
<div id="page-wrapper" class="page-wrapper">
|
|
|
|
<div class="page">
|
|
|
|
<div id="menu-bar-hover-placeholder"></div>
|
|
<div id="menu-bar" class="menu-bar sticky bordered">
|
|
<div class="left-buttons">
|
|
<button id="sidebar-toggle" class="icon-button" type="button" title="Toggle Table of Contents" aria-label="Toggle Table of Contents" aria-controls="sidebar">
|
|
<i class="fa fa-bars"></i>
|
|
</button>
|
|
<button id="theme-toggle" class="icon-button" type="button" title="Change theme" aria-label="Change theme" aria-haspopup="true" aria-expanded="false" aria-controls="theme-list">
|
|
<i class="fa fa-paint-brush"></i>
|
|
</button>
|
|
<ul id="theme-list" class="theme-popup" aria-label="Themes" role="menu">
|
|
<li role="none"><button role="menuitem" class="theme" id="light">Light</button></li>
|
|
<li role="none"><button role="menuitem" class="theme" id="rust">Rust (default)</button></li>
|
|
<li role="none"><button role="menuitem" class="theme" id="coal">Coal</button></li>
|
|
<li role="none"><button role="menuitem" class="theme" id="navy">Navy</button></li>
|
|
<li role="none"><button role="menuitem" class="theme" id="ayu">Ayu</button></li>
|
|
</ul>
|
|
|
|
<button id="search-toggle" class="icon-button" type="button" title="Search. (Shortkey: s)" aria-label="Toggle Searchbar" aria-expanded="false" aria-keyshortcuts="S" aria-controls="searchbar">
|
|
<i class="fa fa-search"></i>
|
|
</button>
|
|
|
|
</div>
|
|
|
|
<h1 class="menu-title">Rust Design Patterns</h1>
|
|
|
|
<div class="right-buttons">
|
|
|
|
<a href="../print.html" title="Print this book" aria-label="Print this book">
|
|
<i id="print-button" class="fa fa-print"></i>
|
|
</a>
|
|
|
|
|
|
<a href="https://github.com/rust-unofficial/patterns" title="Git repository" aria-label="Git repository">
|
|
<i id="git-repository-button" class="fa fa-github"></i>
|
|
</a>
|
|
|
|
</div>
|
|
</div>
|
|
|
|
|
|
<div id="search-wrapper" class="hidden">
|
|
<form id="searchbar-outer" class="searchbar-outer">
|
|
<input type="search" name="search" id="searchbar" name="searchbar" placeholder="Search this book ..." aria-controls="searchresults-outer" aria-describedby="searchresults-header">
|
|
</form>
|
|
<div id="searchresults-outer" class="searchresults-outer hidden">
|
|
<div id="searchresults-header" class="searchresults-header"></div>
|
|
<ul id="searchresults">
|
|
</ul>
|
|
</div>
|
|
</div>
|
|
|
|
|
|
<!-- Apply ARIA attributes after the sidebar and the sidebar toggle button are added to the DOM -->
|
|
<script type="text/javascript">
|
|
document.getElementById('sidebar-toggle').setAttribute('aria-expanded', sidebar === 'visible');
|
|
document.getElementById('sidebar').setAttribute('aria-hidden', sidebar !== 'visible');
|
|
Array.from(document.querySelectorAll('#sidebar a')).forEach(function(link) {
|
|
link.setAttribute('tabIndex', sidebar === 'visible' ? 0 : -1);
|
|
});
|
|
</script>
|
|
|
|
<div id="content" class="content">
|
|
<main>
|
|
<h1><a class="header" href="#on-stack-dynamic-dispatch" id="on-stack-dynamic-dispatch">On-Stack Dynamic Dispatch</a></h1>
|
|
<h2><a class="header" href="#description" id="description">Description</a></h2>
|
|
<p>We can dynamically dispatch over multiple values, however, to do so, we need
|
|
to declare multiple variables to bind differently-typed objects. To extend the
|
|
lifetime as necessary, we can use deferred conditional initialization, as seen
|
|
below:</p>
|
|
<h2><a class="header" href="#example" id="example">Example</a></h2>
|
|
<pre><pre class="playground"><code class="language-rust edition2018">use std::io;
|
|
use std::fs;
|
|
|
|
<span class="boring">fn main() -> Result<(), Box<dyn std::error::Error>> {
|
|
</span><span class="boring">let arg = "-";
|
|
</span>
|
|
// These must live longer than `readable`, and thus are declared first:
|
|
let (mut stdin_read, mut file_read);
|
|
|
|
// We need to ascribe the type to get dynamic dispatch.
|
|
let readable: &mut dyn io::Read = if arg == "-" {
|
|
stdin_read = io::stdin();
|
|
&mut stdin_read
|
|
} else {
|
|
file_read = fs::File::open(arg)?;
|
|
&mut file_read
|
|
};
|
|
|
|
// Read from `readable` here.
|
|
|
|
<span class="boring">Ok(())
|
|
</span><span class="boring">}
|
|
</span></code></pre></pre>
|
|
<h2><a class="header" href="#motivation" id="motivation">Motivation</a></h2>
|
|
<p>Rust monomorphises code by default. This means a copy of the code will be
|
|
generated for each type it is used with and optimized independently. While this
|
|
allows for very fast code on the hot path, it also bloats the code in places
|
|
where performance is not of the essence, thus costing compile time and cache
|
|
usage.</p>
|
|
<p>Luckily, Rust allows us to use dynamic dispatch, but we have to explicitly ask
|
|
for it.</p>
|
|
<h2><a class="header" href="#advantages" id="advantages">Advantages</a></h2>
|
|
<p>We do not need to allocate anything on the heap. Neither do we need to
|
|
initialize something we won't use later, nor do we need to monomorphize the
|
|
whole code that follows to work with both <code>File</code> or <code>Stdin</code>.</p>
|
|
<h2><a class="header" href="#disadvantages" id="disadvantages">Disadvantages</a></h2>
|
|
<p>The code needs more moving parts than the <code>Box</code>-based version:</p>
|
|
<pre><code class="language-rust ignore">// We still need to ascribe the type for dynamic dispatch.
|
|
let readable: Box<dyn io::Read> = if arg == "-" {
|
|
Box::new(io::stdin())
|
|
} else {
|
|
Box::new(fs::File::open(arg)?)
|
|
};
|
|
// Read from `readable` here.
|
|
</code></pre>
|
|
<h2><a class="header" href="#discussion" id="discussion">Discussion</a></h2>
|
|
<p>Rust newcomers will usually learn that Rust requires all variables to be
|
|
initialized <em>before use</em>, so it's easy to overlook the fact that <em>unused</em>
|
|
variables may well be uninitialized. Rust works quite hard to ensure that this
|
|
works out fine and only the initialized values are dropped at the end of their
|
|
scope.</p>
|
|
<p>The example meets all the constraints Rust places on us:</p>
|
|
<ul>
|
|
<li>All variables are initialized before using (in this case borrowing) them</li>
|
|
<li>Each variable only holds values of a single type. In our example, <code>stdin</code> is
|
|
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>
|
|
<li>Each borrowed value outlives all the references borrowed from it</li>
|
|
</ul>
|
|
<h2><a class="header" href="#see-also" id="see-also">See also</a></h2>
|
|
<ul>
|
|
<li><a href="dtor-finally.html">Finalisation in destructors</a> and
|
|
<a href="../patterns/RAII.html">RAII guards</a> can benefit from tight control over lifetimes.</li>
|
|
<li>For conditionally filled <code>Option<&T></code>s of (mutable) references, one can
|
|
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
|
|
optional reference.</li>
|
|
</ul>
|
|
|
|
</main>
|
|
|
|
<nav class="nav-wrapper" aria-label="Page navigation">
|
|
<!-- Mobile navigation buttons -->
|
|
|
|
<a rel="prev" href="../idioms/mem-replace.html" class="mobile-nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left">
|
|
<i class="fa fa-angle-left"></i>
|
|
</a>
|
|
|
|
|
|
|
|
<a rel="next" href="../idioms/ffi-intro.html" class="mobile-nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right">
|
|
<i class="fa fa-angle-right"></i>
|
|
</a>
|
|
|
|
|
|
<div style="clear: both"></div>
|
|
</nav>
|
|
</div>
|
|
</div>
|
|
|
|
<nav class="nav-wide-wrapper" aria-label="Page navigation">
|
|
|
|
<a rel="prev" href="../idioms/mem-replace.html" class="nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left">
|
|
<i class="fa fa-angle-left"></i>
|
|
</a>
|
|
|
|
|
|
|
|
<a rel="next" href="../idioms/ffi-intro.html" class="nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right">
|
|
<i class="fa fa-angle-right"></i>
|
|
</a>
|
|
|
|
</nav>
|
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
<script type="text/javascript">
|
|
window.playground_copyable = true;
|
|
</script>
|
|
|
|
|
|
|
|
|
|
|
|
<script src="../elasticlunr.min.js" type="text/javascript" charset="utf-8"></script>
|
|
<script src="../mark.min.js" type="text/javascript" charset="utf-8"></script>
|
|
<script src="../searcher.js" type="text/javascript" charset="utf-8"></script>
|
|
|
|
|
|
<script src="../clipboard.min.js" type="text/javascript" charset="utf-8"></script>
|
|
<script src="../highlight.js" type="text/javascript" charset="utf-8"></script>
|
|
<script src="../book.js" type="text/javascript" charset="utf-8"></script>
|
|
|
|
<!-- Custom JS scripts -->
|
|
|
|
|
|
|
|
|
|
</body>
|
|
</html>
|