RawVec

We’ve actually reached an interesting situation here: we’ve duplicated the logicfor specifying a buffer and freeing its memory in Vec and IntoIter. Now thatwe’ve implemented it and identified actual logic duplication, this is a goodtime to perform some logic compression.

We’re going to abstract out the (ptr, cap) pair and give them the logic forallocating, growing, and freeing:

  1. struct RawVec<T> {
  2.     ptr: Unique<T>,
  3.     cap: usize,
  4. }
  6. impl<T> RawVec<T> {
  7.     fn new() -> Self {
  8.         assert!(mem::size_of::<T>() != 0, "TODO: implement ZST support");
  9.         RawVec { ptr: Unique::empty(), cap: 0 }
  10.     }
  12.     // unchanged from Vec
  13.     fn grow(&mut self) {
  14.         unsafe {
  15.             let align = mem::align_of::<T>();
  16.             let elem_size = mem::size_of::<T>();
  18.             let (new_cap, ptr) = if self.cap == 0 {
  19.                 let ptr = heap::allocate(elem_size, align);
  20.                 (1, ptr)
  21.             } else {
  22.                 let new_cap = 2 * self.cap;
  23.                 let ptr = heap::reallocate(self.ptr.as_ptr() as *mut _,
  24.                                             self.cap * elem_size,
  25.                                             new_cap * elem_size,
  26.                                             align);
  27.                 (new_cap, ptr)
  28.             };
  30.             // If allocate or reallocate fail, we'll get `null` back
  31.             if ptr.is_null() { oom() }
  33.             self.ptr = Unique::new(ptr as *mut _);
  34.             self.cap = new_cap;
  35.         }
  36.     }
  37. }
  40. impl<T> Drop for RawVec<T> {
  41.     fn drop(&mut self) {
  42.         if self.cap != 0 {
  43.             let align = mem::align_of::<T>();
  44.             let elem_size = mem::size_of::<T>();
  45.             let num_bytes = elem_size * self.cap;
  46.             unsafe {
  47.                 heap::deallocate(self.ptr.as_mut() as *mut _, num_bytes, align);
  48.             }
  49.         }
  50.     }
  51. }

And change Vec as follows:

  1. pub struct Vec<T> {
  2.     buf: RawVec<T>,
  3.     len: usize,
  4. }
  6. impl<T> Vec<T> {
  7.     fn ptr(&self) -> *mut T { self.buf.ptr.as_ptr() }
  9.     fn cap(&self) -> usize { self.buf.cap }
  11.     pub fn new() -> Self {
  12.         Vec { buf: RawVec::new(), len: 0 }
  13.     }
  15.     // push/pop/insert/remove largely unchanged:
  16.     // * `self.ptr -> self.ptr()`
  17.     // * `self.cap -> self.cap()`
  18.     // * `self.grow -> self.buf.grow()`
  19. }
  21. impl<T> Drop for Vec<T> {
  22.     fn drop(&mut self) {
  23.         while let Some(_) = self.pop() {}
  24.         // deallocation is handled by RawVec
  25.     }
  26. }

And finally we can really simplify IntoIter:

  1. struct IntoIter<T> {
  2.     _buf: RawVec<T>, // we don't actually care about this. Just need it to live.
  3.     start: *const T,
  4.     end: *const T,
  5. }
  7. // next and next_back literally unchanged since they never referred to the buf
  9. impl<T> Drop for IntoIter<T> {
  10.     fn drop(&mut self) {
  11.         // only need to ensure all our elements are read;
  12.         // buffer will clean itself up afterwards.
  13.         for _ in &mut *self {}
  14.     }
  15. }
  17. impl<T> Vec<T> {
  18.     pub fn into_iter(self) -> IntoIter<T> {
  19.         unsafe {
  20.             // need to use ptr::read to unsafely move the buf out since it's
  21.             // not Copy, and Vec implements Drop (so we can't destructure it).
  22.             let buf = ptr::read(&self.buf);
  23.             let len = self.len;
  24.             mem::forget(self);
  26.             IntoIter {
  27.                 start: *buf.ptr,
  28.                 end: buf.ptr.offset(len as isize),
  29.                 _buf: buf,
  30.             }
  31.         }
  32.     }
  33. }

Much better.