mentat/memoffset/macro.offset_of.html

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2018-08-22 17:04:13 +00:00
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<h1 class='fqn'><span class='in-band'>Macro <a href='index.html'>memoffset</a>::<wbr><a class="macro" href=''>offset_of</a></span><span class='out-of-band'><span id='render-detail'>
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</span><a class='srclink' href='../src/memoffset/offset_of.rs.html#68-83' title='goto source code'>[src]</a></span></h1>
<pre class="rust macro">
<span class="macro">macro_rules</span><span class="macro">!</span> <span class="ident">offset_of</span> {
(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">father</span>:<span class="ident">ty</span>, $(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">field</span>:<span class="ident">tt</span>)<span class="op">+</span>) <span class="op">=&gt;</span> { ... };
}</pre>
<div class='docblock'><p>Calculates the offset of the specified field from the start of the struct.
This macro supports arbitrary amount of subscripts and recursive member-accesses.</p>
<p><em>Note</em>: This macro may not make much sense when used on structs that are not <code>#[repr(C, packed)]</code></p>
<h2 id="examples---simple" class="section-header"><a href="#examples---simple">Examples - Simple</a></h2>
<pre class="rust rust-example-rendered">
<span class="attribute">#[<span class="ident">macro_use</span>]</span>
<span class="kw">extern</span> <span class="kw">crate</span> <span class="ident">memoffset</span>;
<span class="attribute">#[<span class="ident">repr</span>(<span class="ident">C</span>, <span class="ident">packed</span>)]</span>
<span class="kw">struct</span> <span class="ident">Foo</span> {
<span class="ident">a</span>: <span class="ident">u32</span>,
<span class="ident">b</span>: <span class="ident">u64</span>,
<span class="ident">c</span>: [<span class="ident">u8</span>; <span class="number">5</span>]
}
<span class="kw">fn</span> <span class="ident">main</span>() {
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="macro">offset_of</span><span class="macro">!</span>(<span class="ident">Foo</span>, <span class="ident">a</span>), <span class="number">0</span>);
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="macro">offset_of</span><span class="macro">!</span>(<span class="ident">Foo</span>, <span class="ident">b</span>), <span class="number">4</span>);
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="macro">offset_of</span><span class="macro">!</span>(<span class="ident">Foo</span>, <span class="ident">c</span>[<span class="number">2</span>]), <span class="number">14</span>);
}</pre>
<h2 id="examples---advanced" class="section-header"><a href="#examples---advanced">Examples - Advanced</a></h2>
<pre class="rust rust-example-rendered">
<span class="attribute">#[<span class="ident">macro_use</span>]</span>
<span class="kw">extern</span> <span class="kw">crate</span> <span class="ident">memoffset</span>;
<span class="attribute">#[<span class="ident">repr</span>(<span class="ident">C</span>, <span class="ident">packed</span>)]</span>
<span class="kw">struct</span> <span class="ident">UnnecessarilyComplicatedStruct</span> {
<span class="ident">member</span>: [<span class="ident">UnnecessarilyComplexStruct</span>; <span class="number">12</span>]
}
<span class="attribute">#[<span class="ident">repr</span>(<span class="ident">C</span>, <span class="ident">packed</span>)]</span>
<span class="kw">struct</span> <span class="ident">UnnecessarilyComplexStruct</span> {
<span class="ident">a</span>: <span class="ident">u32</span>,
<span class="ident">b</span>: <span class="ident">u64</span>,
<span class="ident">c</span>: [<span class="ident">u8</span>; <span class="number">5</span>]
}
<span class="kw">fn</span> <span class="ident">main</span>() {
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="macro">offset_of</span><span class="macro">!</span>(<span class="ident">UnnecessarilyComplicatedStruct</span>, <span class="ident">member</span>[<span class="number">3</span>].<span class="ident">c</span>[<span class="number">3</span>]), <span class="number">66</span>);
}</pre>
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