> Currently, memory is deallocated by an instance method on
> UnsafeMutablePointer, deallocate(count:). Like much of the Swift pointer API,
> performing this operation on a buffer pointer requires extracting
> baseAddress! and count. It is very common for the allocation code above to be
> immediately followed by:
>
> defer
>
> {
> buffer.
> baseAddress?.deallocate(capacity: buffer.count
> )
> }
>
> This method is extremely problematic because nearly all users, on first
> seeing the signature of deallocate(capacity:), will naturally conclude from
> the capacity label that deallocate(capacity:) is equivalent to some kind of
> realloc()that can only shrink the buffer. However this is not the actual
> behavior — deallocate(capacity:) actually ignores the capacity argument and
> just calls free() on self. The current API is not only awkward and
> suboptimal, it is misleading. You can write perfectly legal Swift code that
> shouldn’t segfault, but still can, for example
>
> var ptr = UnsafeMutablePointer<UInt8>.allocate(capacity: 1000000
> )
> ptr.
> initialize(to: 13, count: 1000000
> )
> ptr.
> deallocate(capacity: 500000) // deallocate the second half of the memory block
> ptr[0] // segmentation fault
> where the first 500000 addresses should still be valid if the documentation
> is to be read literally.
The fact that the Swift runtime currently uses malloc/free is an implementation
detail. Tracking deallocation size is a very useful optimization for better
allocator backends, and C++ underwent an ABI break to make it so that sized
delete can be supported. Maybe we can change the name to
`deallocate(allocatedCapacity:)` to make it clear that it isn't resizing the
memory, and/or make the capacity argument optional so that you can pay for the
overhead of the allocator deriving the size if it's inconvenient for the
calling code to carry the size around, but we shouldn't remove the
functionality altogether.
-Joe
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