2021-01-01 11:28:10 +00:00
<!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" >
2021-03-30 20:49:44 +00:00
< 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" > < 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" class = "active" > < 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
2021-01-01 11:28:10 +00:00
< / 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 >
2021-03-02 21:12:13 +00:00
< h1 id = "on-stack-dynamic-dispatch" > < a class = "header" href = "#on-stack-dynamic-dispatch" > On-Stack Dynamic Dispatch< / a > < / h1 >
< h2 id = "description" > < a class = "header" href = "#description" > Description< / a > < / h2 >
2021-01-01 11:28:10 +00:00
< 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 >
2021-03-02 21:12:13 +00:00
< h2 id = "example" > < a class = "header" href = "#example" > Example< / a > < / h2 >
2021-01-03 07:40:16 +00:00
< pre > < pre class = "playground" > < code class = "language-rust edition2018" > use std::io;
use std::fs;
2021-01-02 11:41:18 +00:00
2021-01-03 07:40:16 +00:00
< span class = "boring" > fn main() -> Result< (), Box< dyn std::error::Error> > {
< / span > < span class = "boring" > let arg = " -" ;
< / span >
2021-01-02 11:41:18 +00:00
// These must live longer than `readable`, and thus are declared first:
2021-01-01 11:28:10 +00:00
let (mut stdin_read, mut file_read);
// We need to ascribe the type to get dynamic dispatch.
2021-01-03 07:40:16 +00:00
let readable: & mut dyn io::Read = if arg == " -" {
2021-01-01 11:28:10 +00:00
stdin_read = io::stdin();
& mut stdin_read
} else {
file_read = fs::File::open(arg)?;
& mut file_read
};
// Read from `readable` here.
2021-01-03 07:40:16 +00:00
< span class = "boring" > Ok(())
< / span > < span class = "boring" > }
< / span > < / code > < / pre > < / pre >
2021-03-02 21:12:13 +00:00
< h2 id = "motivation" > < a class = "header" href = "#motivation" > Motivation< / a > < / h2 >
2021-01-01 11:28:10 +00:00
< 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 >
2021-03-02 21:12:13 +00:00
< h2 id = "advantages" > < a class = "header" href = "#advantages" > Advantages< / a > < / h2 >
2021-01-01 11:28:10 +00:00
< 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
2021-01-03 02:26:25 +00:00
whole code that follows to work with both < code > File< / code > or < code > Stdin< / code > .< / p >
2021-03-02 21:12:13 +00:00
< h2 id = "disadvantages" > < a class = "header" href = "#disadvantages" > Disadvantages< / a > < / h2 >
2021-01-01 11:28:10 +00:00
< p > The code needs more moving parts than the < code > Box< / code > -based version:< / p >
2021-01-02 11:41:18 +00:00
< pre > < code class = "language-rust ignore" > // We still need to ascribe the type for dynamic dispatch.
2021-01-01 11:28:10 +00:00
let readable: Box< dyn io::Read> = if arg == " -" {
Box::new(io::stdin())
} else {
Box::new(fs::File::open(arg)?)
};
// Read from `readable` here.
2021-01-02 11:41:18 +00:00
< / code > < / pre >
2021-03-02 21:12:13 +00:00
< h2 id = "discussion" > < a class = "header" href = "#discussion" > Discussion< / a > < / h2 >
2021-01-01 11:28:10 +00:00
< 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 >
2021-03-02 21:12:13 +00:00
< h2 id = "see-also" > < a class = "header" href = "#see-also" > See also< / a > < / h2 >
2021-01-01 11:28:10 +00:00
< ul >
2021-01-05 14:36:44 +00:00
< li > < a href = "dtor-finally.html" > Finalisation in destructors< / a > and
2021-01-01 11:28:10 +00:00
< 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 >
2021-02-25 19:56:58 +00:00
< a rel = "next" href = "../idioms/ffi/intro.html" class = "mobile-nav-chapters next" title = "Next chapter" aria-label = "Next chapter" aria-keyshortcuts = "Right" >
2021-01-01 11:28:10 +00:00
< 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 >
2021-02-25 19:56:58 +00:00
< a rel = "next" href = "../idioms/ffi/intro.html" class = "nav-chapters next" title = "Next chapter" aria-label = "Next chapter" aria-keyshortcuts = "Right" >
2021-01-01 11:28:10 +00:00
< 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 >
