nomicon/vec-into-iter.md

% IntoIter

Let's move on to writing iterators. iter and iter_mut have already been written for us thanks to The Magic of Deref. However there's two interesting iterators that Vec provides that slices can't: into_iter and drain.

IntoIter consumes the Vec by-value, and can consequently yield its elements by-value. In order to enable this, IntoIter needs to take control of Vec's allocation.

IntoIter needs to be DoubleEnded as well, to enable reading from both ends. Reading from the back could just be implemented as calling pop, but reading from the front is harder. We could call remove(0) but that would be insanely expensive. Instead we're going to just use ptr::read to copy values out of either end of the Vec without mutating the buffer at all.

To do this we're going to use a very common C idiom for array iteration. We'll make two pointers; one that points to the start of the array, and one that points to one-element past the end. When we want an element from one end, we'll read out the value pointed to at that end and move the pointer over by one. When the two pointers are equal, we know we're done.

Note that the order of read and offset are reversed for next and next_back For next_back the pointer is always after the element it wants to read next, while for next the pointer is always at the element it wants to read next. To see why this is, consider the case where every element but one has been yielded.

The array looks like this:

          S  E
[X, X, X, O, X, X, X]

If E pointed directly at the element it wanted to yield next, it would be indistinguishable from the case where there are no more elements to yield.

Although we don't actually care about it during iteration, we also need to hold onto the Vec's allocation information in order to free it once IntoIter is dropped.

So we're going to use the following struct:

struct IntoIter<T> {
    buf: Unique<T>,
    cap: usize,
    start: *const T,
    end: *const T,
}

One last subtle detail: if our Vec is empty, we want to produce an empty iterator. This will actually technically fall out doing the naive thing of:

start = ptr
end = ptr.offset(len)

However because offset is marked as a GEP inbounds instruction, this will tell LLVM that ptr is allocated and won't alias other allocated memory. This is fine for zero-sized types, as they can't alias anything. However if we're using heap::EMPTY as a sentinel for a non-allocation for a non-zero-sized type, this can cause undefined behaviour. Alas, we must therefore special case either cap or len being 0 to not do the offset.

So this is what we end up with for initialization:

impl<T> Vec<T> {
    fn into_iter(self) -> IntoIter<T> {
        // Can't destructure Vec since it's Drop
        let ptr = self.ptr;
        let cap = self.cap;
        let len = self.len;

        // Make sure not to drop Vec since that will free the buffer
        mem::forget(self);

        unsafe {
            IntoIter {
                buf: ptr,
                cap: cap,
                start: *ptr,
                end: if cap == 0 {
                    // can't offset off this pointer, it's not allocated!
                    *ptr
                } else {
                    ptr.offset(len as isize)
                }
            }
        }
    }
}

Here's iterating forward:

impl<T> Iterator for IntoIter<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 = self.start.offset(1);
                Some(result)
            }
        }
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        let len = (self.end as usize - self.start as usize)
                  / mem::size_of::<T>();
        (len, Some(len))
    }
}

And here's iterating backwards.

impl<T> DoubleEndedIterator for IntoIter<T> {
    fn next_back(&mut self) -> Option<T> {
        if self.start == self.end {
            None
        } else {
            unsafe {
                self.end = self.end.offset(-1);
                Some(ptr::read(self.end))
            }
        }
    }
}

Because IntoIter takes ownership of its allocation, it needs to implement Drop to free it. However it also wants to implement Drop to drop any elements it contains that weren't yielded.

impl<T> Drop for IntoIter<T> {
    fn drop(&mut self) {
        if self.cap != 0 {
            // drop any remaining elements
            for _ in &mut *self {}

            let align = mem::align_of::<T>();
            let elem_size = mem::size_of::<T>();
            let num_bytes = elem_size * self.cap;
            unsafe {
                heap::deallocate(*self.buf as *mut _, num_bytes, align);
            }
        }
    }
}