[swift-evolution] Pitch: Improved Swift pointers

Gor Gyolchanyan gor at gyolchanyan.com
Wed Jul 12 15:16:50 CDT 2017

In my understanding, the address manipulation abstractions are as follows:
An address is an integer denoting an index into the current address space (which, in case of a user-space process, is a virtual address space, uniquely allocated by the operating system for that process). The minimal addressable unit (also known as byte, which is not to be confused with an octet) of the address space is the number of contiguous binary digits (also known as bits) that are uniquely identified by a single address (in most modern architectures that number is 8).
A buffer is a range of addresses, denoting a fragment of the address space, where the delimited contiguous sequence of addresses share common semantics.
A pointer is a buffer with some metadata. In statically typed languages the length of the buffer is a compile-time constant and metadata contains type information of the buffer’s contents.
An array is a buffer, representing a contiguous sequence of pointers.

Notice, that in above terms the address space is essentially a buffer denoting all available addresses.

In Swift, the aforementioned concepts correspond to the following types:
address: UnsafeRawPointer, UnsafeRawMutablePointer, OpaquePointer.
buffer: UnsafeRawBufferPointer, UnsafeMutableRawBufferPointer.
pointer: UnsafePointer, UnsafeMutablePointer, AutoreleasingUnsafeMutablePointer.
array: UnsafeBufferPointer, UnsafeMutableBufferPointer.

If the unsafe tools of Swift would be redesigned, I’d suggest defining them in the following manner:
A comparable and hashable address that can be added an address offset, and subtracted another address to get an address offset. An address would also expose a mutable/immutable property that is the byte referred to by that address.
A buffer, that is a random access mutable/immutable collection of addresses, represented as a range.
A pointer, that is a generic type that is represented by an address and the inherent type information of its generic parameter. The pointer would only expose the address in the form of a buffer and would expose the referred object in the form of a mutable/immutable property of the respective type.
An array, that is a random access mutable/immutable collection of pointers, represented as a pointer coupled by a count.

So, in the end, we’d have the following set of types: UnsafeAddress, UnsafeBuffer, UnsafePointer, UnsafeArray (as well as their mutable counter-parts).
The OpaquePointer would then be a thin wrapper around UnsafeAddress and would probably better off renamed to CStructPointer, due to the fact that all non-C usages of this type should be replaced by usage of UnsafeAddress.
And the AutoreleasingUnsafeMutablePointer would also be a thing wrapper around UnsafeAddress with a possible renaming to NSObjectPointer.

> On Jul 12, 2017, at 10:26 PM, Taylor Swift via swift-evolution <swift-evolution at swift.org> wrote:
> Hi all, I’ve written up a proposal to modify the unsafe pointer API for greater consistency, safety, and ease of use.
> ~~~
> Swift currently offers two sets of pointer types — singular pointers such as UnsafeMutablePointer, and vector (buffer) pointers such as UnsafeMutableBufferPointer. This implies a natural separation of tasks the two kinds of pointers are meant to do. For example, buffer pointers implement Collection conformance, while singular pointers do not.
> However, some aspects of the pointer design contradict these implied roles. It is possible to allocate an arbitrary number of instances from a type method on a singular pointer, but not from a buffer pointer. The result of such an operation returns a singular pointer, even though a buffer pointer would be more appropriate to capture the information about the number of instances allocated. It’s possible to subscript into a singular pointer, even though they are not real Collections. Some parts of the current design turn UnsafePointers into downright DangerousPointers, leading users to believe that they have allocated or freed memory when in fact, they have not.
> This proposal seeks to iron out these inconsistencies, and offer a more convenient, more sensible, and less bug-prone API for Swift pointers.
> <https://gist.github.com/kelvin13/a9c033193a28b1d4960a89b25fbffb06 <https://gist.github.com/kelvin13/a9c033193a28b1d4960a89b25fbffb06>>
> ~~~
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