You cannot select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
340 lines
20 KiB
HTML
340 lines
20 KiB
HTML
<!DOCTYPE HTML>
|
|
<html lang="en" class="sidebar-visible no-js rust">
|
|
<head>
|
|
<!-- Book generated using mdBook -->
|
|
<meta charset="UTF-8">
|
|
<title>Deref Polymorphism - 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="../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::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 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/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 href="../patterns/fold.html"><strong aria-hidden="true">3.5.</strong> Fold</a></li><li class="chapter-item expanded "><a href="../patterns/newtype.html"><strong aria-hidden="true">3.6.</strong> Newtype</a></li><li class="chapter-item expanded "><a href="../patterns/RAII.html"><strong aria-hidden="true">3.7.</strong> RAII Guards</a></li><li class="chapter-item expanded "><a href="../patterns/small-crates.html"><strong aria-hidden="true">3.8.</strong> Prefer Small Crates</a></li><li class="chapter-item expanded "><a href="../patterns/unsafe-mods.html"><strong aria-hidden="true">3.9.</strong> Contain unsafety in small modules</a></li><li class="chapter-item expanded "><a href="../patterns/visitor.html"><strong aria-hidden="true">3.10.</strong> Visitor</a></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/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" class="active"><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>
|
|
</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="#deref-polymorphism" id="deref-polymorphism"><code>Deref</code> polymorphism</a></h1>
|
|
<h2><a class="header" href="#description" id="description">Description</a></h2>
|
|
<p>Abuse the <code>Deref</code> trait to emulate inheritance between structs, and thus reuse
|
|
methods.</p>
|
|
<h2><a class="header" href="#example" id="example">Example</a></h2>
|
|
<p>Sometimes we want to emulate the following common pattern from OO languages such
|
|
as Java:</p>
|
|
<pre><code class="language-java">class Foo {
|
|
void m() { ... }
|
|
}
|
|
|
|
class Bar extends Foo {}
|
|
|
|
public static void main(String[] args) {
|
|
Bar b = new Bar();
|
|
b.m();
|
|
}
|
|
</code></pre>
|
|
<p>We can use the deref polymorphism anti-pattern to do so:</p>
|
|
<pre><code class="language-rust ignore">use std::ops::Deref;
|
|
|
|
struct Foo {}
|
|
|
|
impl Foo {
|
|
fn m(&self) {
|
|
//..
|
|
}
|
|
|
|
}
|
|
|
|
struct Bar {
|
|
f: Foo
|
|
}
|
|
|
|
impl Deref for Bar {
|
|
type Target = Foo;
|
|
fn deref(&self) -> &Foo {
|
|
&self.f
|
|
}
|
|
}
|
|
|
|
fn main() {
|
|
let b = Bar { Foo {} };
|
|
b.m();
|
|
}
|
|
</code></pre>
|
|
<p>There is no struct inheritance in Rust. Instead we use composition and include
|
|
an instance of <code>Foo</code> in <code>Bar</code> (since the field is a value, it is stored inline,
|
|
so if there were fields, they would have the same layout in memory as the Java
|
|
version (probably, you should use <code>#[repr(C)]</code> if you want to be sure)).</p>
|
|
<p>In order to make the method call work we implement <code>Deref</code> for <code>Bar</code> with <code>Foo</code>
|
|
as the target (returning the embedded <code>Foo</code> field). That means that when we
|
|
dereference a <code>Bar</code> (for example, using <code>*</code>) then we will get a <code>Foo</code>. That is
|
|
pretty weird. Dereferencing usually gives a <code>T</code> from a reference to <code>T</code>, here we
|
|
have two unrelated types. However, since the dot operator does implicit
|
|
dereferencing, it means that the method call will search for methods on <code>Foo</code> as
|
|
well as <code>Bar</code>.</p>
|
|
<h2><a class="header" href="#advantages" id="advantages">Advantages</a></h2>
|
|
<p>You save a little boilerplate, e.g.,</p>
|
|
<pre><code class="language-rust ignore">impl Bar {
|
|
fn m(&self) {
|
|
self.f.m()
|
|
}
|
|
}
|
|
</code></pre>
|
|
<h2><a class="header" href="#disadvantages" id="disadvantages">Disadvantages</a></h2>
|
|
<p>Most importantly this is a surprising idiom - future programmers reading this in
|
|
code will not expect this to happen. That's because we are abusing the <code>Deref</code>
|
|
trait rather than using it as intended (and documented, etc.). It's also because
|
|
the mechanism here is completely implicit.</p>
|
|
<p>This pattern does not introduce subtyping between <code>Foo</code> and <code>Bar</code> like
|
|
inheritance in Java or C++ does. Furthermore, traits implemented by <code>Foo</code> are
|
|
not automatically implemented for <code>Bar</code>, so this pattern interacts badly with
|
|
bounds checking and thus generic programming.</p>
|
|
<p>Using this pattern gives subtly different semantics from most OO languages with
|
|
regards to <code>self</code>. Usually it remains a reference to the sub-class, with this
|
|
pattern it will be the 'class' where the method is defined.</p>
|
|
<p>Finally, this pattern only supports single inheritance, and has no notion of
|
|
interfaces, class-based privacy, or other inheritance-related features. So, it
|
|
gives an experience that will be subtly surprising to programmers used to Java
|
|
inheritance, etc.</p>
|
|
<h2><a class="header" href="#discussion" id="discussion">Discussion</a></h2>
|
|
<p>There is no one good alternative. Depending on the exact circumstances it might
|
|
be better to re-implement using traits or to write out the facade methods to
|
|
dispatch to <code>Foo</code> manually. We do intend to add a mechanism for inheritance
|
|
similar to this to Rust, but it is likely to be some time before it reaches
|
|
stable Rust. See these <a href="http://aturon.github.io/blog/2015/09/18/reuse/">blog</a>
|
|
<a href="http://smallcultfollowing.com/babysteps/blog/2015/10/08/virtual-structs-part-4-extended-enums-and-thin-traits/">posts</a>
|
|
and this <a href="https://github.com/rust-lang/rfcs/issues/349">RFC issue</a> for more details.</p>
|
|
<p>The <code>Deref</code> trait is designed for the implementation of custom pointer types.
|
|
The intention is that it will take a pointer-to-<code>T</code> to a <code>T</code>, not convert
|
|
between different types. It is a shame that this isn't (probably cannot be)
|
|
enforced by the trait definition.</p>
|
|
<p>Rust tries to strike a careful balance between explicit and implicit mechanisms,
|
|
favouring explicit conversions between types. Automatic dereferencing in the dot
|
|
operator is a case where the ergonomics strongly favour an implicit mechanism,
|
|
but the intention is that this is limited to degrees of indirection, not
|
|
conversion between arbitrary types.</p>
|
|
<h2><a class="header" href="#see-also" id="see-also">See also</a></h2>
|
|
<p><a href="../idioms/deref.html">Collections are smart pointers idiom</a>.</p>
|
|
<p><a href="https://doc.rust-lang.org/std/ops/trait.Deref.html">Documentation for <code>Deref</code> trait</a>.</p>
|
|
|
|
</main>
|
|
|
|
<nav class="nav-wrapper" aria-label="Page navigation">
|
|
<!-- Mobile navigation buttons -->
|
|
|
|
<a rel="prev" href="../anti_patterns/deny-warnings.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="../functional/index.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="../anti_patterns/deny-warnings.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="../functional/index.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>
|