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<!DOCTYPE html><html lang="en"><head><meta charset="utf-8"><meta name="viewport" content="width=device-width, initial-scale=1.0"><meta name="generator" content="rustdoc"><meta name="description" content="API documentation for the Rust `downcast_rs` crate."><meta name="keywords" content="rust, rustlang, rust-lang, downcast_rs"><title>downcast_rs - Rust</title><link rel="stylesheet" type="text/css" href="../normalize.css"><link rel="stylesheet" type="text/css" href="../rustdoc.css" id="mainThemeStyle"><link rel="stylesheet" type="text/css" href="../dark.css"><link rel="stylesheet" type="text/css" href="../light.css" id="themeStyle"><script src="../storage.js"></script><noscript><link rel="stylesheet" href="../noscript.css"></noscript><link rel="shortcut icon" href="../favicon.ico"><style type="text/css">#crate-search{background-image:url("../down-arrow.svg");}</style></head><body class="rustdoc mod"><!--[if lte IE 8]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="sidebar"><div class="sidebar-menu">☰</div><a href='../downcast_rs/index.html'><div class='logo-container'><img src='../rust-logo.png' alt='logo'></div></a><p class='location'>Crate downcast_rs</p><div class="sidebar-elems"><a id='all-types' href='all.html'><p>See all downcast_rs's items</p></a><div class="block items"><ul><li><a href="#macros">Macros</a></li><li><a href="#traits">Traits</a></li></ul></div><p class='location'></p><script>window.sidebarCurrent = {name: 'downcast_rs', ty: 'mod', relpath: '../'};</script></div></nav><div class="theme-picker"><button id="theme-picker" aria-label="Pick another theme!"><img src="../brush.svg" width="18" alt="Pick another theme!"></button><div id="theme-choices"></div></div><script src="../theme.js"></script><nav class="sub"><form class="search-form js-only"><div class="search-container"><div><select id="crate-search"><option value="All crates">All crates</option></select><input class="search-input" name="search" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"></div><a id="settings-menu" href="../settings.html"><img src="../wheel.svg" width="18" alt="Change settings"></a></div></form></nav><section id="main" class="content"><h1 class='fqn'><span class='out-of-band'><span id='render-detail'><a id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs">[<span class='inner'>−</span>]</a></span><a class='srclink' href='../src/downcast_rs/lib.rs.html#1-402' title='goto source code'>[src]</a></span><span class='in-band'>Crate <a class="mod" href=''>downcast_rs</a></span></h1><div class='docblock'><p>Rust enums are great for types where all variations are known beforehand. But in
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the case where you want to implement a container of user-defined types, an
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open-ended type like a <strong>trait object</strong> is needed. In some cases, it is useful to
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cast the trait object back into its original concrete type to access additional
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functionality and performant inlined implementations.</p>
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<p><code>downcast-rs</code> adds downcasting support to trait objects using only safe Rust. It
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supports <strong>type parameters</strong>, <strong>associated types</strong>, and <strong>constraints</strong>.</p>
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<p>To make a trait downcastable, make it extend the <code>downcast::Downcast</code> trait and
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invoke <code>impl_downcast!</code> on it as follows:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="kw">trait</span> <span class="ident">Trait</span>: <span class="ident">Downcast</span> {}
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<span class="macro">impl_downcast</span><span class="macro">!</span>(<span class="ident">Trait</span>);
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<span class="comment">// With type parameters.</span>
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<span class="kw">trait</span> <span class="ident">TraitGeneric1</span><span class="op"><</span><span class="ident">T</span><span class="op">></span>: <span class="ident">Downcast</span> {}
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<span class="macro">impl_downcast</span><span class="macro">!</span>(<span class="ident">TraitGeneric1</span><span class="op"><</span><span class="ident">T</span><span class="op">></span>);
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<span class="comment">// With associated types.</span>
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<span class="kw">trait</span> <span class="ident">TraitGeneric2</span>: <span class="ident">Downcast</span> { <span class="kw">type</span> <span class="ident">G</span>; <span class="kw">type</span> <span class="ident">H</span>; }
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<span class="macro">impl_downcast</span><span class="macro">!</span>(<span class="ident">TraitGeneric2</span> <span class="ident">assoc</span> <span class="ident">G</span>, <span class="ident">H</span>);
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<span class="comment">// With constraints on types.</span>
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<span class="kw">trait</span> <span class="ident">TraitGeneric3</span><span class="op"><</span><span class="ident">T</span>: <span class="ident">Copy</span><span class="op">></span>: <span class="ident">Downcast</span> {
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<span class="kw">type</span> <span class="ident">H</span>: <span class="ident">Clone</span>;
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}
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<span class="macro">impl_downcast</span><span class="macro">!</span>(<span class="ident">TraitGeneric3</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> <span class="ident">assoc</span> <span class="ident">H</span> <span class="kw">where</span> <span class="ident">T</span>: <span class="ident">Copy</span>, <span class="ident">H</span>: <span class="ident">Clone</span>);
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<span class="comment">// With concrete types.</span>
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<span class="kw">trait</span> <span class="ident">TraitConcrete1</span><span class="op"><</span><span class="ident">T</span>: <span class="ident">Copy</span><span class="op">></span>: <span class="ident">Downcast</span> {}
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<span class="macro">impl_downcast</span><span class="macro">!</span>(<span class="ident">concrete</span> <span class="ident">TraitConcrete1</span><span class="op"><</span><span class="ident">u32</span><span class="op">></span>);
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<span class="kw">trait</span> <span class="ident">TraitConcrete2</span><span class="op"><</span><span class="ident">T</span>: <span class="ident">Copy</span><span class="op">></span>: <span class="ident">Downcast</span> { <span class="kw">type</span> <span class="ident">H</span>; }
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<span class="macro">impl_downcast</span><span class="macro">!</span>(<span class="ident">concrete</span> <span class="ident">TraitConcrete2</span><span class="op"><</span><span class="ident">u32</span><span class="op">></span> <span class="ident">assoc</span> <span class="ident">H</span><span class="op">=</span><span class="ident">f64</span>);</pre></div>
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<h1 id="example-without-generics" class="section-header"><a href="#example-without-generics">Example without generics</a></h1>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="comment">// Import macro via `macro_use` pre-1.30.</span>
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<span class="attribute">#[<span class="ident">macro_use</span>]</span>
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<span class="kw">extern</span> <span class="kw">crate</span> <span class="ident">downcast_rs</span>;
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<span class="kw">use</span> <span class="ident">downcast_rs</span>::<span class="ident">Downcast</span>;
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<span class="comment">// To create a trait with downcasting methods, extend `Downcast` and run</span>
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<span class="comment">// `impl_downcast!()` on the trait.</span>
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<span class="kw">trait</span> <span class="ident">Base</span>: <span class="ident">Downcast</span> {}
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<span class="macro">impl_downcast</span><span class="macro">!</span>(<span class="ident">Base</span>);
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<span class="comment">// Concrete types implementing Base.</span>
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<span class="attribute">#[<span class="ident">derive</span>(<span class="ident">Debug</span>)]</span>
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<span class="kw">struct</span> <span class="ident">Foo</span>(<span class="ident">u32</span>);
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<span class="kw">impl</span> <span class="ident">Base</span> <span class="kw">for</span> <span class="ident">Foo</span> {}
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<span class="attribute">#[<span class="ident">derive</span>(<span class="ident">Debug</span>)]</span>
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<span class="kw">struct</span> <span class="ident">Bar</span>(<span class="ident">f64</span>);
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<span class="kw">impl</span> <span class="ident">Base</span> <span class="kw">for</span> <span class="ident">Bar</span> {}
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<span class="kw">fn</span> <span class="ident">main</span>() {
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<span class="comment">// Create a trait object.</span>
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<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">base</span>: <span class="ident">Box</span><span class="op"><</span><span class="ident">Base</span><span class="op">></span> <span class="op">=</span> <span class="ident">Box</span>::<span class="ident">new</span>(<span class="ident">Foo</span>(<span class="number">42</span>));
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<span class="comment">// Try sequential downcasts.</span>
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<span class="kw">if</span> <span class="kw">let</span> <span class="prelude-val">Some</span>(<span class="ident">foo</span>) <span class="op">=</span> <span class="ident">base</span>.<span class="ident">downcast_ref</span>::<span class="op"><</span><span class="ident">Foo</span><span class="op">></span>() {
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<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">foo</span>.<span class="number">0</span>, <span class="number">42</span>);
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} <span class="kw">else</span> <span class="kw">if</span> <span class="kw">let</span> <span class="prelude-val">Some</span>(<span class="ident">bar</span>) <span class="op">=</span> <span class="ident">base</span>.<span class="ident">downcast_ref</span>::<span class="op"><</span><span class="ident">Bar</span><span class="op">></span>() {
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<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">bar</span>.<span class="number">0</span>, <span class="number">42.0</span>);
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}
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<span class="macro">assert</span><span class="macro">!</span>(<span class="ident">base</span>.<span class="ident">is</span>::<span class="op"><</span><span class="ident">Foo</span><span class="op">></span>());
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<span class="comment">// Fail to convert `Box<Base>` into `Box<Bar>`.</span>
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<span class="kw">let</span> <span class="ident">res</span> <span class="op">=</span> <span class="ident">base</span>.<span class="ident">downcast</span>::<span class="op"><</span><span class="ident">Bar</span><span class="op">></span>();
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<span class="macro">assert</span><span class="macro">!</span>(<span class="ident">res</span>.<span class="ident">is_err</span>());
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<span class="kw">let</span> <span class="ident">base</span> <span class="op">=</span> <span class="ident">res</span>.<span class="ident">unwrap_err</span>();
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<span class="comment">// Convert `Box<Base>` into `Box<Foo>`.</span>
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<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="number">42</span>, <span class="ident">base</span>.<span class="ident">downcast</span>::<span class="op"><</span><span class="ident">Foo</span><span class="op">></span>().<span class="ident">map_err</span>(<span class="op">|</span><span class="kw">_</span><span class="op">|</span> <span class="string">"Shouldn't happen."</span>).<span class="ident">unwrap</span>().<span class="number">0</span>);
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}</pre></div>
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<h1 id="example-with-a-generic-trait-with-associated-types-and-constraints" class="section-header"><a href="#example-with-a-generic-trait-with-associated-types-and-constraints">Example with a generic trait with associated types and constraints</a></h1>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="comment">// Can call macro via namespace since rust 1.30.</span>
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<span class="kw">extern</span> <span class="kw">crate</span> <span class="ident">downcast_rs</span>;
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<span class="kw">use</span> <span class="ident">downcast_rs</span>::<span class="ident">Downcast</span>;
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<span class="comment">// To create a trait with downcasting methods, extend `Downcast` and run</span>
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<span class="comment">// `impl_downcast!()` on the trait.</span>
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<span class="kw">trait</span> <span class="ident">Base</span><span class="op"><</span><span class="ident">T</span>: <span class="ident">Clone</span><span class="op">></span>: <span class="ident">Downcast</span> { <span class="kw">type</span> <span class="ident">H</span>: <span class="ident">Copy</span>; }
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<span class="ident">downcast_rs</span>::<span class="macro">impl_downcast</span><span class="macro">!</span>(<span class="ident">Base</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> <span class="ident">assoc</span> <span class="ident">H</span> <span class="kw">where</span> <span class="ident">T</span>: <span class="ident">Clone</span>, <span class="ident">H</span>: <span class="ident">Copy</span>);
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<span class="comment">// or: impl_downcast!(concrete Base<u32> assoc H=f32)</span>
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<span class="comment">// Concrete types implementing Base.</span>
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<span class="kw">struct</span> <span class="ident">Foo</span>(<span class="ident">u32</span>);
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<span class="kw">impl</span> <span class="ident">Base</span><span class="op"><</span><span class="ident">u32</span><span class="op">></span> <span class="kw">for</span> <span class="ident">Foo</span> { <span class="kw">type</span> <span class="ident">H</span> <span class="op">=</span> <span class="ident">f32</span>; }
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<span class="kw">struct</span> <span class="ident">Bar</span>(<span class="ident">f64</span>);
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<span class="kw">impl</span> <span class="ident">Base</span><span class="op"><</span><span class="ident">u32</span><span class="op">></span> <span class="kw">for</span> <span class="ident">Bar</span> { <span class="kw">type</span> <span class="ident">H</span> <span class="op">=</span> <span class="ident">f32</span>; }
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<span class="kw">fn</span> <span class="ident">main</span>() {
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<span class="comment">// Create a trait object.</span>
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<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">base</span>: <span class="ident">Box</span><span class="op"><</span><span class="ident">Base</span><span class="op"><</span><span class="ident">u32</span>, <span class="ident">H</span><span class="op">=</span><span class="ident">f32</span><span class="op">></span><span class="op">></span> <span class="op">=</span> <span class="ident">Box</span>::<span class="ident">new</span>(<span class="ident">Bar</span>(<span class="number">42.0</span>));
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<span class="comment">// Try sequential downcasts.</span>
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<span class="kw">if</span> <span class="kw">let</span> <span class="prelude-val">Some</span>(<span class="ident">foo</span>) <span class="op">=</span> <span class="ident">base</span>.<span class="ident">downcast_ref</span>::<span class="op"><</span><span class="ident">Foo</span><span class="op">></span>() {
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<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">foo</span>.<span class="number">0</span>, <span class="number">42</span>);
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} <span class="kw">else</span> <span class="kw">if</span> <span class="kw">let</span> <span class="prelude-val">Some</span>(<span class="ident">bar</span>) <span class="op">=</span> <span class="ident">base</span>.<span class="ident">downcast_ref</span>::<span class="op"><</span><span class="ident">Bar</span><span class="op">></span>() {
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<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">bar</span>.<span class="number">0</span>, <span class="number">42.0</span>);
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}
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<span class="macro">assert</span><span class="macro">!</span>(<span class="ident">base</span>.<span class="ident">is</span>::<span class="op"><</span><span class="ident">Bar</span><span class="op">></span>());
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}</pre></div>
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</div><h2 id='macros' class='section-header'><a href="#macros">Macros</a></h2>
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<table><tr class='module-item'><td><a class="macro" href="macro.impl_downcast.html" title='downcast_rs::impl_downcast macro'>impl_downcast</a></td><td class='docblock-short'><p>Adds downcasting support to traits that extend <code>downcast::Downcast</code> by defining forwarding
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methods to the corresponding implementations on <code>std::any::Any</code> in the standard library.</p>
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</td></tr></table><h2 id='traits' class='section-header'><a href="#traits">Traits</a></h2>
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<table><tr class='module-item'><td><a class="trait" href="trait.Downcast.html" title='downcast_rs::Downcast trait'>Downcast</a></td><td class='docblock-short'><p>Supports conversion to <code>Any</code>. Traits to be extended by <code>impl_downcast!</code> must extend <code>Downcast</code>.</p>
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