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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 `ctx` mod in crate `scroll`."><meta name="keywords" content="rust, rustlang, rust-lang, ctx"><title>scroll::ctx - 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='../../scroll/index.html'><div class='logo-container'><img src='../../rust-logo.png' alt='logo'></div></a><p class='location'>Module ctx</p><div class="sidebar-elems"><div class="block items"><ul><li><a href="#enums">Enums</a></li><li><a href="#constants">Constants</a></li><li><a href="#traits">Traits</a></li></ul></div><p class='location'><a href='../index.html'>scroll</a></p><script>window.sidebarCurrent = {name: 'ctx', ty: 'mod', relpath: '../'};</script><script defer src="../sidebar-items.js"></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/scroll/ctx.rs.html#1-668' title='goto source code'>[src]</a></span><span class='in-band'>Module <a href='../index.html'>scroll</a>::<wbr><a class="mod" href=''>ctx</a></span></h1><div class='docblock'><p>Generic context-aware conversion traits, for automatic <em>downstream</em> extension of <code>Pread</code>, et. al</p>
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<h1 id="discussion" class="section-header"><a href="#discussion">Discussion</a></h1>
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<p>Implementors of <code>TryFromCtx</code> automatically grant any client user of <code>pread, pwrite, gread, gwrite</code> the ability to parse their structure out of the source it has been implemented for, typically <code>&[u8]</code>.</p>
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<p>The implementor only needs to specify the error type, and the type of their size, and then implement the parsing/marshalling logic given a byte sequence, starting at the offset <code>pread</code>, et. al was called at, with the context you have implemented it for.</p>
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<p>Returning the size allows dynamic content (e.g., <code>&str</code>s) to be parsed alongside fixed size content (e.g., <code>u64</code>). The parsing context is any information you the implementor need to correctly parse out your datatype - this could be the endianness of the type, more offsets, or other complex data. The only requirement is that your <code>Ctx</code> be <code>Copy</code>, and hence encourages lightweight contexts (but this isn't required of course).</p>
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<h1 id="example" class="section-header"><a href="#example">Example</a></h1>
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<p>Suppose we have a datatype and we want to specify how to parse or serialize this datatype out of some arbitrary
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byte buffer. In order to do this, we need to provide a <code>TryFromCtx</code> impl for our datatype. In particular, if we
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do this for the <code>[u8]</code> target, with a "parsing contex", <code>YourCtx</code>, you will automatically get access to
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calling <code>pread_with::<YourDatatype>(offset, your_ctx)</code> on arrays of bytes.</p>
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<p>In the example below, we implement <code>TryFromCtx</code> using the <code>Endian</code> parsing context provided by <code>scroll</code>, which is used to specifying the endianness at which numbers should be parsed, but you could provide anything, as long as it implements <code>Copy</code>.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="kw">use</span> <span class="ident">scroll</span>::{<span class="self">self</span>, <span class="ident">ctx</span>, <span class="ident">Endian</span>, <span class="ident">Pread</span>, <span class="ident">BE</span>};
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<span class="kw">struct</span> <span class="ident">Data</span><span class="op"><</span><span class="lifetime">'a</span><span class="op">></span> {
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<span class="ident">name</span>: <span class="kw-2">&</span><span class="lifetime">'a</span> <span class="ident">str</span>,
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<span class="ident">id</span>: <span class="ident">u32</span>,
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}
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<span class="kw">impl</span><span class="op"><</span><span class="lifetime">'a</span><span class="op">></span> <span class="ident">ctx</span>::<span class="ident">TryFromCtx</span><span class="op"><</span><span class="lifetime">'a</span>, <span class="ident">Endian</span><span class="op">></span> <span class="kw">for</span> <span class="ident">Data</span><span class="op"><</span><span class="lifetime">'a</span><span class="op">></span> {
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<span class="kw">type</span> <span class="ident">Error</span> <span class="op">=</span> <span class="ident">scroll</span>::<span class="ident">Error</span>;
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<span class="kw">type</span> <span class="ident">Size</span> <span class="op">=</span> <span class="ident">usize</span>;
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<span class="kw">fn</span> <span class="ident">try_from_ctx</span> (<span class="ident">src</span>: <span class="kw-2">&</span><span class="lifetime">'a</span> [<span class="ident">u8</span>], <span class="ident">ctx</span>: <span class="ident">Endian</span>)
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<span class="op">-</span><span class="op">></span> <span class="prelude-ty">Result</span><span class="op"><</span>(<span class="self">Self</span>, <span class="self">Self</span>::<span class="ident">Size</span>), <span class="self">Self</span>::<span class="ident">Error</span><span class="op">></span> {
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<span class="kw">let</span> <span class="ident">name</span> <span class="op">=</span> <span class="ident">src</span>.<span class="ident">pread</span>::<span class="op"><</span><span class="kw-2">&</span><span class="ident">str</span><span class="op">></span>(<span class="number">0</span>)<span class="question-mark">?</span>;
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<span class="kw">let</span> <span class="ident">id</span> <span class="op">=</span> <span class="ident">src</span>.<span class="ident">pread_with</span>(<span class="ident">name</span>.<span class="ident">len</span>() <span class="op">+</span> <span class="number">1</span>, <span class="ident">ctx</span>)<span class="question-mark">?</span>;
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<span class="prelude-val">Ok</span>((<span class="ident">Data</span> { <span class="ident">name</span>: <span class="ident">name</span>, <span class="ident">id</span>: <span class="ident">id</span> }, <span class="ident">name</span>.<span class="ident">len</span>() <span class="op">+</span> <span class="number">1</span> <span class="op">+</span> <span class="number">4</span>))
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}
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}
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<span class="kw">let</span> <span class="ident">bytes</span> <span class="op">=</span> <span class="string">b"UserName\x00\x01\x02\x03\x04"</span>;
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<span class="kw">let</span> <span class="ident">data</span> <span class="op">=</span> <span class="ident">bytes</span>.<span class="ident">pread_with</span>::<span class="op"><</span><span class="ident">Data</span><span class="op">></span>(<span class="number">0</span>, <span class="ident">BE</span>).<span class="ident">unwrap</span>();
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<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">data</span>.<span class="ident">id</span>, <span class="number">0x01020304</span>);
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<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">data</span>.<span class="ident">name</span>.<span class="ident">to_string</span>(), <span class="string">"UserName"</span>.<span class="ident">to_string</span>());
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</pre></div>
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</div><h2 id='enums' class='section-header'><a href="#enums">Enums</a></h2>
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<table><tr class='module-item'><td><a class="enum" href="enum.StrCtx.html" title='scroll::ctx::StrCtx enum'>StrCtx</a></td><td class='docblock-short'><p>The parsing context for converting a byte sequence to a <code>&str</code></p>
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</td></tr></table><h2 id='constants' class='section-header'><a href="#constants">Constants</a></h2>
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<table><tr class='module-item'><td><a class="constant" href="constant.NULL.html" title='scroll::ctx::NULL constant'>NULL</a></td><td class='docblock-short'><p>A C-style, null terminator based delimiter</p>
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</td></tr><tr class='module-item'><td><a class="constant" href="constant.RET.html" title='scroll::ctx::RET constant'>RET</a></td><td class='docblock-short'><p>A newline-based delimiter</p>
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</td></tr><tr class='module-item'><td><a class="constant" href="constant.SPACE.html" title='scroll::ctx::SPACE constant'>SPACE</a></td><td class='docblock-short'><p>A space-based delimiter</p>
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</td></tr><tr class='module-item'><td><a class="constant" href="constant.TAB.html" title='scroll::ctx::TAB constant'>TAB</a></td><td class='docblock-short'><p>A tab-based delimiter</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.FromCtx.html" title='scroll::ctx::FromCtx trait'>FromCtx</a></td><td class='docblock-short'><p>Reads <code>Self</code> from <code>This</code> using the context <code>Ctx</code>; must <em>not</em> fail</p>
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</td></tr><tr class='module-item'><td><a class="trait" href="trait.IntoCtx.html" title='scroll::ctx::IntoCtx trait'>IntoCtx</a></td><td class='docblock-short'><p>Writes <code>Self</code> into <code>This</code> using the context <code>Ctx</code></p>
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</td></tr><tr class='module-item'><td><a class="trait" href="trait.MeasureWith.html" title='scroll::ctx::MeasureWith trait'>MeasureWith</a></td><td class='docblock-short'><p>A trait for measuring how large something is; for a byte sequence, it will be its length.</p>
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</td></tr><tr class='module-item'><td><a class="trait" href="trait.SizeWith.html" title='scroll::ctx::SizeWith trait'>SizeWith</a></td><td class='docblock-short'><p>Gets the size of <code>Self</code> with a <code>Ctx</code>, and in <code>Self::Units</code>. Implementors can then call <code>Gread</code> related functions</p>
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</td></tr><tr class='module-item'><td><a class="trait" href="trait.TryFromCtx.html" title='scroll::ctx::TryFromCtx trait'>TryFromCtx</a></td><td class='docblock-short'><p>Tries to read <code>Self</code> from <code>This</code> using the context <code>Ctx</code></p>
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</td></tr><tr class='module-item'><td><a class="trait" href="trait.TryIntoCtx.html" title='scroll::ctx::TryIntoCtx trait'>TryIntoCtx</a></td><td class='docblock-short'><p>Tries to write <code>Self</code> into <code>This</code> using the context <code>Ctx</code></p>
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