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<ol class="chapter"><li class="affix"><a href="README.html">Introduction</a></li><li><a href="meet-safe-and-unsafe.html"><strong aria-hidden="true">1.</strong> Meet Safe and Unsafe</a></li><li><ol class="section"><li><a href="safe-unsafe-meaning.html"><strong aria-hidden="true">1.1.</strong> How Safe and Unsafe Interact</a></li><li><a href="what-unsafe-does.html"><strong aria-hidden="true">1.2.</strong> What Unsafe Can Do</a></li><li><a href="working-with-unsafe.html"><strong aria-hidden="true">1.3.</strong> Working with Unsafe</a></li></ol></li><li><a href="data.html"><strong aria-hidden="true">2.</strong> Data Layout</a></li><li><ol class="section"><li><a href="repr-rust.html"><strong aria-hidden="true">2.1.</strong> repr(Rust)</a></li><li><a href="exotic-sizes.html"><strong aria-hidden="true">2.2.</strong> Exotically Sized Types</a></li><li><a href="other-reprs.html"><strong aria-hidden="true">2.3.</strong> Other reprs</a></li></ol></li><li><a href="ownership.html"><strong aria-hidden="true">3.</strong> Ownership</a></li><li><ol class="section"><li><a href="references.html"><strong aria-hidden="true">3.1.</strong> References</a></li><li><a href="aliasing.html"><strong aria-hidden="true">3.2.</strong> Aliasing</a></li><li><a href="lifetimes.html"><strong aria-hidden="true">3.3.</strong> Lifetimes</a></li><li><a href="lifetime-mismatch.html"><strong aria-hidden="true">3.4.</strong> Limits of Lifetimes</a></li><li><a href="lifetime-elision.html"><strong aria-hidden="true">3.5.</strong> Lifetime Elision</a></li><li><a href="unbounded-lifetimes.html"><strong aria-hidden="true">3.6.</strong> Unbounded Lifetimes</a></li><li><a href="hrtb.html"><strong aria-hidden="true">3.7.</strong> Higher-Rank Trait Bounds</a></li><li><a href="subtyping.html"><strong aria-hidden="true">3.8.</strong> Subtyping and Variance</a></li><li><a href="dropck.html"><strong aria-hidden="true">3.9.</strong> Drop Check</a></li><li><a href="phantom-data.html"><strong aria-hidden="true">3.10.</strong> PhantomData</a></li><li><a href="borrow-splitting.html"><strong aria-hidden="true">3.11.</strong> Splitting Borrows</a></li></ol></li><li><a href="conversions.html"><strong aria-hidden="true">4.</strong> Type Conversions</a></li><li><ol class="section"><li><a href="coercions.html"><strong aria-hidden="true">4.1.</strong> Coercions</a></li><li><a href="dot-operator.html"><strong aria-hidden="true">4.2.</strong> The Dot Operator</a></li><li><a href="casts.html"><strong aria-hidden="true">4.3.</strong> Casts</a></li><li><a href="transmutes.html"><strong aria-hidden="true">4.4.</strong> Transmutes</a></li></ol></li><li><a href="uninitialized.html"><strong aria-hidden="true">5.</strong> Uninitialized Memory</a></li><li><ol class="section"><li><a href="checked-uninit.html"><strong aria-hidden="true">5.1.</strong> Checked</a></li><li><a href="drop-flags.html"><strong aria-hidden="true">5.2.</strong> Drop Flags</a></li><li><a href="unchecked-uninit.html"><strong aria-hidden="true">5.3.</strong> Unchecked</a></li></ol></li><li><a href="obrm.html"><strong aria-hidden="true">6.</strong> Ownership Based Resource Management</a></li><li><ol class="section"><li><a href="constructors.html"><strong aria-hidden="true">6.1.</strong> Constructors</a></li><li><a href="destructors.html"><strong aria-hidden="true">6.2.</strong> Destructors</a></li><li><a href="leaking.html"><strong aria-hidden="true">6.3.</strong> Leaking</a></li></ol></li><li><a href="unwinding.html"><strong aria-hidden="true">7.</strong> Unwinding</a></li><li><ol class="section"><li><a href="exception-safety.html"><strong aria-hidden="true">7.1.</strong> Exception Safety</a></li><li><a href="poisoning.html"><strong aria-hidden="true">7.2.</strong> Poisoning</a></li></ol></li><li><a href="concurrency.html"><strong aria-hidden="true">8.</strong> Concurrency</a></li><li><ol class="section"><li><a href="races.html"><strong aria-hidden="true">8.1.</strong> Races</a></li><li><a href="send-and-sync.html"><strong aria-hidden="true">8.2.</strong> Send and Sync</a></li><li><a href="atomics.html"><strong aria-hidden="true">8.3.</strong> Atomics</a></li></ol></li><li><a href="vec.html"><strong aria-hidden="true">9.</strong> Implementing Vec</a></li><li><ol class="section"><li><a href="vec-layout.html"><strong aria-hidden="true">9.1.</strong> Layout</a></li><li><a href="vec-alloc.html"><strong aria-hidden="true">9.2.</strong> Allocating</a></li><li><a href="vec-push-pop.html"><strong aria-hidden="true">9.3.</strong> Push and Pop</a></li><li><a href="vec-dealloc.html"><strong aria-hidden="true">9.4.</strong> Deallocating</a></li><li><a href="vec-deref.html"><strong aria-hidden="true">9.5.</strong> Deref</a></li><li><a href="vec-insert-remove.html"><strong aria-hidden="true">9.6.</strong> Insert and Remove</a></li><li><a href="vec-into-iter.html"><strong aria-hidden="true">9.7.</strong> IntoIter</a></li><li><a href="vec-raw.html"><strong aria-hidden="true">9.8.</strong> RawVec</a></li><li><a href="vec-drain.html" class="active"><strong aria-hidden="true">9.9.</strong> Drain</a></li><li><a href="vec-zsts.html"><strong aria-hidden="true">9.10.</strong> Handling Zero-Sized Types</a></li><li><a href="vec-final.html"><strong aria-hidden="true">9.11.</strong> Final Code</a></li></ol></li><li><a href="arc-and-mutex.html"><strong aria-hidden="true">10.</strong> Implementing Arc and Mutex</a></li><li><a href="ffi.html"><strong aria-hidden="true">11.</strong> FFI</a></li></ol>
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<a class="header" href="vec-drain.html#drain" id="drain"><h1>Drain</h1></a>
<p>Let's move on to Drain. Drain is largely the same as IntoIter, except that
instead of consuming the Vec, it borrows the Vec and leaves its allocation
untouched. For now we'll only implement the "basic" full-range version.</p>
<pre><code class="language-rust ignore">use std::marker::PhantomData;
struct Drain<'a, T: 'a> {
// Need to bound the lifetime here, so we do it with `&'a mut Vec<T>`
// because that's semantically what we contain. We're "just" calling
// `pop()` and `remove(0)`.
vec: PhantomData<&'a mut Vec<T>>,
start: *const T,
end: *const T,
}
impl<'a, T> Iterator for Drain<'a, T> {
type Item = T;
fn next(&mut self) -> Option<T> {
if self.start == self.end {
None
</code></pre>
<p>-- wait, this is seeming familiar. Let's do some more compression. Both
IntoIter and Drain have the exact same structure, let's just factor it out.</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
struct RawValIter<T> {
start: *const T,
end: *const T,
}
impl<T> RawValIter<T> {
// unsafe to construct because it has no associated lifetimes.
// This is necessary to store a RawValIter in the same struct as
// its actual allocation. OK since it's a private implementation
// detail.
unsafe fn new(slice: &[T]) -> Self {
RawValIter {
start: slice.as_ptr(),
end: if slice.len() == 0 {
// if `len = 0`, then this is not actually allocated memory.
// Need to avoid offsetting because that will give wrong
// information to LLVM via GEP.
slice.as_ptr()
} else {
slice.as_ptr().offset(slice.len() as isize)
}
}
}
}
// Iterator and DoubleEndedIterator impls identical to IntoIter.
#}</code></pre></pre>
<p>And IntoIter becomes the following:</p>
<pre><code class="language-rust ignore">pub struct IntoIter<T> {
_buf: RawVec<T>, // we don't actually care about this. Just need it to live.
iter: RawValIter<T>,
}
impl<T> Iterator for IntoIter<T> {
type Item = T;
fn next(&mut self) -> Option<T> { self.iter.next() }
fn size_hint(&self) -> (usize, Option<usize>) { self.iter.size_hint() }
}
impl<T> DoubleEndedIterator for IntoIter<T> {
fn next_back(&mut self) -> Option<T> { self.iter.next_back() }
}
impl<T> Drop for IntoIter<T> {
fn drop(&mut self) {
for _ in &mut self.iter {}
}
}
impl<T> Vec<T> {
pub fn into_iter(self) -> IntoIter<T> {
unsafe {
let iter = RawValIter::new(&self);
let buf = ptr::read(&self.buf);
mem::forget(self);
IntoIter {
iter: iter,
_buf: buf,
}
}
}
}
</code></pre>
<p>Note that I've left a few quirks in this design to make upgrading Drain to work
with arbitrary subranges a bit easier. In particular we <em>could</em> have RawValIter
drain itself on drop, but that won't work right for a more complex Drain.
We also take a slice to simplify Drain initialization.</p>
<p>Alright, now Drain is really easy:</p>
<pre><code class="language-rust ignore">use std::marker::PhantomData;
pub struct Drain<'a, T: 'a> {
vec: PhantomData<&'a mut Vec<T>>,
iter: RawValIter<T>,
}
impl<'a, T> Iterator for Drain<'a, T> {
type Item = T;
fn next(&mut self) -> Option<T> { self.iter.next() }
fn size_hint(&self) -> (usize, Option<usize>) { self.iter.size_hint() }
}
impl<'a, T> DoubleEndedIterator for Drain<'a, T> {
fn next_back(&mut self) -> Option<T> { self.iter.next_back() }
}
impl<'a, T> Drop for Drain<'a, T> {
fn drop(&mut self) {
for _ in &mut self.iter {}
}
}
impl<T> Vec<T> {
pub fn drain(&mut self) -> Drain<T> {
unsafe {
let iter = RawValIter::new(&self);
// this is a mem::forget safety thing. If Drain is forgotten, we just
// leak the whole Vec's contents. Also we need to do this *eventually*
// anyway, so why not do it now?
self.len = 0;
Drain {
iter: iter,
vec: PhantomData,
}
}
}
}
</code></pre>
<p>For more details on the <code>mem::forget</code> problem, see the
<a href="leaking.html">section on leaks</a>.</p>
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