Zig: Pointer Stability for ArrayLists

amluto•20 minutes ago

This seems weak.

In a language like Rust, the compiler will “lock” the pointers for you, and you can’t forget.

In a language like C++ (and presumably Zig), one could, in theory at least, have the iterators and slices that reference the storage of a dynamic array hold some sort of lock that pins the storage.

But this API requires the programmer to remember to lock the pointers and also requires the programmer to keep the lock alive for the correct region of code. And it looks to me like even the example in the blog post has the lock taken completely outside the function that requires stability, so there is nothing whatsoever that gets the lock scoping right. Even the type system can’t help — the offending parse function can’t declare that it wants a pointer-locked ArrayList parameter.

_bohm•about 2 hours ago

It's a nice feature but I can't help feeling like, if you need a stable pointer to an item in a collection, ArrayList is the wrong data structure to use? Maybe someone can chime in and give me an example of when you'd do this instead of, e.g., just storing an index. Alternatively, you could use an Unrolled Linked List (FKA SegmentedList in Zig before it was removed in 0.16, not sure why).

hansvm•5 minutes ago

I use it in a lot of places where I know the max capacity ahead of time -- ensureCapacity() followed by a lot of *AssumeCapacity()-styled commands. It's convenient for all of the ... convenience ... methods (append() requires some bookkeeping somewhere, appendSlice() requires more, and so on). In those usages, it's basically syntactic sugar over a slice. That's not a perfect solution, but it's reasonably good often enough that I keep doing it.

The proposed change doesn't do much for me personally (memory safety is ensured in other ways, and if it weren't I wouldn't be annoyed debugging the allocator-observed errors), but I could see myself using it at some other point in time for the same class of usages, or I could see other people relying on it when they choose that class of coding.

bvrmn•14 minutes ago

ArrayList is a very generic (pun not intended) structure and could be stretched quite freely in any direction with useful property of owning underlying slice. Like readonly preallocated ArrayList is a thing.

_bohm•5 minutes ago

That's a good point. Using lockPointers would be a good way to enforce at runtime that your ArrayList is truly read-only.

the__alchemist•22 minutes ago

My mental model of Zig is that it is explicitly the language for developers who prefer using pointers in business logic (instead of just in MMIMO, and are looking for something with improvements over C); i.e. exactly this class of abstraction.

_bohm•8 minutes ago

Having written a bunch of Zig, I wouldn't say that the language design or culture explicitly encourages the use of pointers over indices in such situations. I would say it's more a language which trusts the programmer to make correct decisions about which constructs are appropriate in any given circumstance.

nvme0n1p1•about 1 hour ago

SegmentedList had a weird API, especially the way you control the list growth factor by the size of an inline array. And it hadn't kept up with stdlib norms in recent versions. I do hope it comes back eventually with an improved API.

At least we got Deque in exchange. I use that far more often than I used SegmentedList.

_bohm•about 1 hour ago

Aha, gotcha. To tell you the truth, I don't think I ever used it. I have a custom implementation I wrote because I didn't realize at the time that SegmentedList was an unrolled linked list :p

beepbooptheory•32 minutes ago

Maybe one use case is if you are interfacing with external C library and you're stuck with pointers?

Zig: Pointer Stability for ArrayLists

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