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<ul class="chapter"><li class="affix"><a href="README.html">Introduction</a></li><li><a href="meet-safe-and-unsafe.html"><strong>1.</strong> Meet Safe and Unsafe</a></li><li><ul class="section"><li><a href="safe-unsafe-meaning.html"><strong>1.1.</strong> How Safe and Unsafe Interact</a></li><li><a href="working-with-unsafe.html"><strong>1.2.</strong> Working with Unsafe</a></li></ul></li><li><a href="data.html"><strong>2.</strong> Data Layout</a></li><li><ul class="section"><li><a href="repr-rust.html"><strong>2.1.</strong> repr(Rust)</a></li><li><a href="exotic-sizes.html"><strong>2.2.</strong> Exotically Sized Types</a></li><li><a href="other-reprs.html"><strong>2.3.</strong> Other reprs</a></li></ul></li><li><a href="ownership.html"><strong>3.</strong> Ownership</a></li><li><ul class="section"><li><a href="references.html"><strong>3.1.</strong> References</a></li><li><a href="lifetimes.html"><strong>3.2.</strong> Lifetimes</a></li><li><a href="lifetime-mismatch.html"><strong>3.3.</strong> Limits of Lifetimes</a></li><li><a href="lifetime-elision.html"><strong>3.4.</strong> Lifetime Elision</a></li><li><a href="unbounded-lifetimes.html"><strong>3.5.</strong> Unbounded Lifetimes</a></li><li><a href="hrtb.html"><strong>3.6.</strong> Higher-Rank Trait Bounds</a></li><li><a href="subtyping.html"><strong>3.7.</strong> Subtyping and Variance</a></li><li><a href="dropck.html"><strong>3.8.</strong> Drop Check</a></li><li><a href="phantom-data.html"><strong>3.9.</strong> PhantomData</a></li><li><a href="borrow-splitting.html"><strong>3.10.</strong> Splitting Borrows</a></li></ul></li><li><a href="conversions.html"><strong>4.</strong> Type Conversions</a></li><li><ul class="section"><li><a href="coercions.html"><strong>4.1.</strong> Coercions</a></li><li><a href="dot-operator.html"><strong>4.2.</strong> The Dot Operator</a></li><li><a href="casts.html"><strong>4.3.</strong> Casts</a></li><li><a href="transmutes.html"><strong>4.4.</strong> Transmutes</a></li></ul></li><li><a href="uninitialized.html"><strong>5.</strong> Uninitialized Memory</a></li><li><ul class="section"><li><a href="checked-uninit.html"><strong>5.1.</strong> Checked</a></li><li><a href="drop-flags.html"><strong>5.2.</strong> Drop Flags</a></li><li><a href="unchecked-uninit.html"><strong>5.3.</strong> Unchecked</a></li></ul></li><li><a href="obrm.html"><strong>6.</strong> Ownership Based Resource Management</a></li><li><ul class="section"><li><a href="constructors.html"><strong>6.1.</strong> Constructors</a></li><li><a href="destructors.html"><strong>6.2.</strong> Destructors</a></li><li><a href="leaking.html"><strong>6.3.</strong> Leaking</a></li></ul></li><li><a href="unwinding.html"><strong>7.</strong> Unwinding</a></li><li><ul class="section"><li><a href="exception-safety.html"><strong>7.1.</strong> Exception Safety</a></li><li><a href="poisoning.html"><strong>7.2.</strong> Poisoning</a></li></ul></li><li><a href="concurrency.html"><strong>8.</strong> Concurrency</a></li><li><ul class="section"><li><a href="races.html"><strong>8.1.</strong> Races</a></li><li><a href="send-and-sync.html"><strong>8.2.</strong> Send and Sync</a></li><li><a href="atomics.html"><strong>8.3.</strong> Atomics</a></li></ul></li><li><a href="vec.html"><strong>9.</strong> Implementing Vec</a></li><li><ul class="section"><li><a href="vec-layout.html" class="active"><strong>9.1.</strong> Layout</a></li><li><a href="vec-alloc.html"><strong>9.2.</strong> Allocating</a></li><li><a href="vec-push-pop.html"><strong>9.3.</strong> Push and Pop</a></li><li><a href="vec-dealloc.html"><strong>9.4.</strong> Deallocating</a></li><li><a href="vec-deref.html"><strong>9.5.</strong> Deref</a></li><li><a href="vec-insert-remove.html"><strong>9.6.</strong> Insert and Remove</a></li><li><a href="vec-into-iter.html"><strong>9.7.</strong> IntoIter</a></li><li><a href="vec-raw.html"><strong>9.8.</strong> RawVec</a></li><li><a href="vec-drain.html"><strong>9.9.</strong> Drain</a></li><li><a href="vec-zsts.html"><strong>9.10.</strong> Handling Zero-Sized Types</a></li><li><a href="vec-final.html"><strong>9.11.</strong> Final Code</a></li></ul></li><li><a href="arc-and-mutex.html"><strong>10.</strong> Implementing Arc and Mutex</a></li><li><a href="ffi.html"><strong>11.</strong> FFI</a></li></ul>
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<a class="header" href="vec-layout.html#layout" id="layout"><h1>Layout</h1></a>
<p>First off, we need to come up with the struct layout. A Vec has three parts:
a pointer to the allocation, the size of the allocation, and the number of
elements that have been initialized.</p>
<p>Naively, this means we just want this design:</p>
<pre><pre class="playpen"><code class="language-rust">pub struct Vec<T> {
ptr: *mut T,
cap: usize,
len: usize,
}
# fn main() {}
</code></pre></pre>
<p>And indeed this would compile. Unfortunately, it would be incorrect. First, the
compiler will give us too strict variance. So a <code>&Vec<&'static str></code>
couldn't be used where an <code>&Vec<&'a str></code> was expected. More importantly, it
will give incorrect ownership information to the drop checker, as it will
conservatively assume we don't own any values of type <code>T</code>. See <a href="ownership.html">the chapter
on ownership and lifetimes</a> for all the details on variance and
drop check.</p>
<p>As we saw in the ownership chapter, we should use <code>Unique<T></code> in place of
<code>*mut T</code> when we have a raw pointer to an allocation we own. Unique is unstable,
so we'd like to not use it if possible, though.</p>
<p>As a recap, Unique is a wrapper around a raw pointer that declares that:</p>
<ul>
<li>We are variant over <code>T</code></li>
<li>We may own a value of type <code>T</code> (for drop check)</li>
<li>We are Send/Sync if <code>T</code> is Send/Sync</li>
<li>Our pointer is never null (so <code>Option<Vec<T>></code> is null-pointer-optimized)</li>
</ul>
<p>We can implement all of the above requirements except for the last
one in stable Rust:</p>
<pre><pre class="playpen"><code class="language-rust">use std::marker::PhantomData;
use std::ops::Deref;
use std::mem;
struct Unique<T> {
ptr: *const T, // *const for variance
_marker: PhantomData<T>, // For the drop checker
}
// Deriving Send and Sync is safe because we are the Unique owners
// of this data. It's like Unique<T> is "just" T.
unsafe impl<T: Send> Send for Unique<T> {}
unsafe impl<T: Sync> Sync for Unique<T> {}
impl<T> Unique<T> {
pub fn new(ptr: *mut T) -> Self {
Unique { ptr: ptr, _marker: PhantomData }
}
pub fn as_ptr(&self) -> *mut T {
self.ptr as *mut T
}
}
# fn main() {}
</code></pre></pre>
<p>Unfortunately the mechanism for stating that your value is non-zero is
unstable and unlikely to be stabilized soon. As such we're just going to
take the hit and use std's Unique:</p>
<pre><pre class="playpen"><code class="language-rust">#![feature(unique)]
use std::ptr::{Unique, self};
pub struct Vec<T> {
ptr: Unique<T>,
cap: usize,
len: usize,
}
# fn main() {}
</code></pre></pre>
<p>If you don't care about the null-pointer optimization, then you can use the
stable code. However we will be designing the rest of the code around enabling
this optimization. It should be noted that <code>Unique::new</code> is unsafe to call, because
putting <code>null</code> inside of it is Undefined Behavior. Our stable Unique doesn't
need <code>new</code> to be unsafe because it doesn't make any interesting guarantees about
its contents.</p>
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