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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="Add a function to enum that allows accessing variants by its discriminant"><title>FromRepr in strum - Rust</title><link rel="preload" as="font" type="font/woff2" crossorigin href="../static.files/SourceSerif4-Regular-46f98efaafac5295.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../static.files/FiraSans-Regular-018c141bf0843ffd.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../static.files/FiraSans-Medium-8f9a781e4970d388.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../static.files/SourceCodePro-Regular-562dcc5011b6de7d.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../static.files/SourceCodePro-Semibold-d899c5a5c4aeb14a.ttf.woff2"><link rel="stylesheet" href="../static.files/normalize-76eba96aa4d2e634.css"><link rel="stylesheet" href="../static.files/rustdoc-ac92e1bbe349e143.css"><meta name="rustdoc-vars" data-root-path="../" data-static-root-path="../static.files/" data-current-crate="strum" data-themes="" data-resource-suffix="" data-rustdoc-version="1.76.0 (07dca489a 2024-02-04)" data-channel="1.76.0" data-search-js="search-2b6ce74ff89ae146.js" data-settings-js="settings-4313503d2e1961c2.js" ><script src="../static.files/storage-f2adc0d6ca4d09fb.js"></script><script defer src="sidebar-items.js"></script><script defer src="../static.files/main-305769736d49e732.js"></script><noscript><link rel="stylesheet" href="../static.files/noscript-feafe1bb7466e4bd.css"></noscript><link rel="alternate icon" type="image/png" href="../static.files/favicon-16x16-8b506e7a72182f1c.png"><link rel="alternate icon" type="image/png" href="../static.files/favicon-32x32-422f7d1d52889060.png"><link rel="icon" type="image/svg+xml" href="../static.files/favicon-2c020d218678b618.svg"></head><body class="rustdoc derive"><!--[if lte IE 11]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="mobile-topbar"><button class="sidebar-menu-toggle">☰</button></nav><nav class="sidebar"><div class="sidebar-crate"><h2><a href="../strum/index.html">strum</a><span class="version">0.26.3</span></h2></div><div class="sidebar-elems"></div></nav><div class="sidebar-resizer"></div>
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<main><div class="width-limiter"><nav class="sub"><form class="search-form"><span></span><div id="sidebar-button" tabindex="-1"><a href="../strum/all.html" title="show sidebar"></a></div><input class="search-input" name="search" aria-label="Run search in the documentation" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><div id="help-button" tabindex="-1"><a href="../help.html" title="help">?</a></div><div id="settings-menu" tabindex="-1"><a href="../settings.html" title="settings"><img width="22" height="22" alt="Change settings" src="../static.files/wheel-7b819b6101059cd0.svg"></a></div></form></nav><section id="main-content" class="content"><div class="main-heading"><h1>Derive Macro <a href="index.html">strum</a>::<wbr><a class="derive" href="#">FromRepr</a><button id="copy-path" title="Copy item path to clipboard"><img src="../static.files/clipboard-7571035ce49a181d.svg" width="19" height="18" alt="Copy item path"></button></h1><span class="out-of-band"><a class="src" href="../src/strum_macros/lib.rs.html#664">source</a> · <button id="toggle-all-docs" title="collapse all docs">[<span>−</span>]</button></span></div><pre class="rust item-decl"><code>#[derive(FromRepr)]
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{
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<span class="comment">// Attributes available to this derive:</span>
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#[strum]
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}
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</code></pre><details class="toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>Add a function to enum that allows accessing variants by its discriminant</p>
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<p>This macro adds a standalone function to obtain an enum variant by its discriminant. The macro adds
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<code>from_repr(discriminant: usize) -> Option<YourEnum></code> as a standalone function on the enum. For
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variants with additional data, the returned variant will use the <code>Default</code> trait to fill the
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data. The discriminant follows the same rules as <code>rustc</code>. The first discriminant is zero and each
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successive variant has a discriminant of one greater than the previous variant, except where an
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explicit discriminant is specified. The type of the discriminant will match the <code>repr</code> type if
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it is specified.</p>
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<p>When the macro is applied using rustc >= 1.46 and when there is no additional data on any of
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the variants, the <code>from_repr</code> function is marked <code>const</code>. rustc >= 1.46 is required
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to allow <code>match</code> statements in <code>const fn</code>. The no additional data requirement is due to the
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inability to use <code>Default::default()</code> in a <code>const fn</code>.</p>
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<p>You cannot derive <code>FromRepr</code> on any type with a lifetime bound (<code><'a></code>) because the function would surely
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create <a href="https://doc.rust-lang.org/nightly/nomicon/unbounded-lifetimes.html">unbounded lifetimes</a>.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code>
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<span class="kw">use </span>strum_macros::FromRepr;
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<span class="attr">#[derive(FromRepr, Debug, PartialEq)]
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</span><span class="kw">enum </span>Color {
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Red,
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Green { range: usize },
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Blue(usize),
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Yellow,
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}
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<span class="macro">assert_eq!</span>(<span class="prelude-val">Some</span>(Color::Red), Color::from_repr(<span class="number">0</span>));
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<span class="macro">assert_eq!</span>(<span class="prelude-val">Some</span>(Color::Green {range: <span class="number">0</span>}), Color::from_repr(<span class="number">1</span>));
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<span class="macro">assert_eq!</span>(<span class="prelude-val">Some</span>(Color::Blue(<span class="number">0</span>)), Color::from_repr(<span class="number">2</span>));
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<span class="macro">assert_eq!</span>(<span class="prelude-val">Some</span>(Color::Yellow), Color::from_repr(<span class="number">3</span>));
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<span class="macro">assert_eq!</span>(<span class="prelude-val">None</span>, Color::from_repr(<span class="number">4</span>));
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<span class="comment">// Custom discriminant tests
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</span><span class="attr">#[derive(FromRepr, Debug, PartialEq)]
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#[repr(u8)]
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</span><span class="kw">enum </span>Vehicle {
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Car = <span class="number">1</span>,
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Truck = <span class="number">3</span>,
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}
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<span class="macro">assert_eq!</span>(<span class="prelude-val">None</span>, Vehicle::from_repr(<span class="number">0</span>));</code></pre></div>
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<p>On versions of rust >= 1.46, the <code>from_repr</code> function is marked <code>const</code>.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>strum_macros::FromRepr;
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<span class="attr">#[derive(FromRepr, Debug, PartialEq)]
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#[repr(u8)]
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</span><span class="kw">enum </span>Number {
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One = <span class="number">1</span>,
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Three = <span class="number">3</span>,
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}
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<span class="kw">const fn </span>number_from_repr(d: u8) -> <span class="prelude-ty">Option</span><Number> {
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Number::from_repr(d)
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}
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<span class="macro">assert_eq!</span>(<span class="prelude-val">None</span>, number_from_repr(<span class="number">0</span>));
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<span class="macro">assert_eq!</span>(<span class="prelude-val">Some</span>(Number::One), number_from_repr(<span class="number">1</span>));
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<span class="macro">assert_eq!</span>(<span class="prelude-val">None</span>, number_from_repr(<span class="number">2</span>));
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<span class="macro">assert_eq!</span>(<span class="prelude-val">Some</span>(Number::Three), number_from_repr(<span class="number">3</span>));
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<span class="macro">assert_eq!</span>(<span class="prelude-val">None</span>, number_from_repr(<span class="number">4</span>));</code></pre></div>
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</div></details></section></div></main></body></html> |