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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 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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 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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"><strong aria-hidden="true">9.9.</strong> Drain</a></li><li><a href="vec-zsts.html" class="active"><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-zsts.html#handling-zero-sized-types" id="handling-zero-sized-types"><h1>Handling Zero-Sized Types</h1></a>
<p>It's time. We're going to fight the specter that is zero-sized types. Safe Rust
<em>never</em> needs to care about this, but Vec is very intensive on raw pointers and
raw allocations, which are exactly the two things that care about
zero-sized types. We need to be careful of two things:</p>
<ul>
<li>The raw allocator API has undefined behavior if you pass in 0 for an
allocation size.</li>
<li>raw pointer offsets are no-ops for zero-sized types, which will break our
C-style pointer iterator.</li>
</ul>
<p>Thankfully we abstracted out pointer-iterators and allocating handling into
RawValIter and RawVec respectively. How mysteriously convenient.</p>
<a class="header" href="vec-zsts.html#allocating-zero-sized-types" id="allocating-zero-sized-types"><h2>Allocating Zero-Sized Types</h2></a>
<p>So if the allocator API doesn't support zero-sized allocations, what on earth
do we store as our allocation? <code>Unique::empty()</code> of course! Almost every operation
with a ZST is a no-op since ZSTs have exactly one value, and therefore no state needs
to be considered to store or load them. This actually extends to <code>ptr::read</code> and
<code>ptr::write</code>: they won't actually look at the pointer at all. As such we never need
to change the pointer.</p>
<p>Note however that our previous reliance on running out of memory before overflow is
no longer valid with zero-sized types. We must explicitly guard against capacity
overflow for zero-sized types.</p>
<p>Due to our current architecture, all this means is writing 3 guards, one in each
method of RawVec.</p>
<pre><code class="language-rust ignore">impl<T> RawVec<T> {
fn new() -> Self {
// !0 is usize::MAX. This branch should be stripped at compile time.
let cap = if mem::size_of::<T>() == 0 { !0 } else { 0 };
// Unique::empty() doubles as "unallocated" and "zero-sized allocation"
RawVec { ptr: Unique::empty(), cap: cap }
}
fn grow(&mut self) {
unsafe {
let elem_size = mem::size_of::<T>();
// since we set the capacity to usize::MAX when elem_size is
// 0, getting to here necessarily means the Vec is overfull.
assert!(elem_size != 0, "capacity overflow");
let align = mem::align_of::<T>();
let (new_cap, ptr) = if self.cap == 0 {
let ptr = heap::allocate(elem_size, align);
(1, ptr)
} else {
let new_cap = 2 * self.cap;
let ptr = heap::reallocate(self.ptr.as_ptr() as *mut _,
self.cap * elem_size,
new_cap * elem_size,
align);
(new_cap, ptr)
};
// If allocate or reallocate fail, we'll get `null` back
if ptr.is_null() { oom() }
self.ptr = Unique::new(ptr as *mut _);
self.cap = new_cap;
}
}
}
impl<T> Drop for RawVec<T> {
fn drop(&mut self) {
let elem_size = mem::size_of::<T>();
// don't free zero-sized allocations, as they were never allocated.
if self.cap != 0 && elem_size != 0 {
let align = mem::align_of::<T>();
let num_bytes = elem_size * self.cap;
unsafe {
heap::deallocate(self.ptr.as_ptr() as *mut _, num_bytes, align);
}
}
}
}
</code></pre>
<p>That's it. We support pushing and popping zero-sized types now. Our iterators
(that aren't provided by slice Deref) are still busted, though.</p>
<a class="header" href="vec-zsts.html#iterating-zero-sized-types" id="iterating-zero-sized-types"><h2>Iterating Zero-Sized Types</h2></a>
<p>Zero-sized offsets are no-ops. This means that our current design will always
initialize <code>start</code> and <code>end</code> as the same value, and our iterators will yield
nothing. The current solution to this is to cast the pointers to integers,
increment, and then cast them back:</p>
<pre><code class="language-rust ignore">impl<T> RawValIter<T> {
unsafe fn new(slice: &[T]) -> Self {
RawValIter {
start: slice.as_ptr(),
end: if mem::size_of::<T>() == 0 {
((slice.as_ptr() as usize) + slice.len()) as *const _
} else if slice.len() == 0 {
slice.as_ptr()
} else {
slice.as_ptr().offset(slice.len() as isize)
}
}
}
}
</code></pre>
<p>Now we have a different bug. Instead of our iterators not running at all, our
iterators now run <em>forever</em>. We need to do the same trick in our iterator impls.
Also, our size_hint computation code will divide by 0 for ZSTs. Since we'll
basically be treating the two pointers as if they point to bytes, we'll just
map size 0 to divide by 1.</p>
<pre><code class="language-rust ignore">impl<T> Iterator for RawValIter<T> {
type Item = T;
fn next(&mut self) -> Option<T> {
if self.start == self.end {
None
} else {
unsafe {
let result = ptr::read(self.start);
self.start = if mem::size_of::<T>() == 0 {
(self.start as usize + 1) as *const _
} else {
self.start.offset(1)
};
Some(result)
}
}
}
fn size_hint(&self) -> (usize, Option<usize>) {
let elem_size = mem::size_of::<T>();
let len = (self.end as usize - self.start as usize)
/ if elem_size == 0 { 1 } else { elem_size };
(len, Some(len))
}
}
impl<T> DoubleEndedIterator for RawValIter<T> {
fn next_back(&mut self) -> Option<T> {
if self.start == self.end {
None
} else {
unsafe {
self.end = if mem::size_of::<T>() == 0 {
(self.end as usize - 1) as *const _
} else {
self.end.offset(-1)
};
Some(ptr::read(self.end))
}
}
}
}
</code></pre>
<p>And that's it. Iteration works!</p>
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