# How to Zero a StringHashMap?

**URL:** <https://ziggit.dev/t/how-to-zero-a-stringhashmap/17734>\
**Category:** Help\
**Tags:** language, memory-management, standard-library\
**Created:** [September 27, 2026, 5:19pm UTC](https://ziggit.dev/t/how-to-zero-a-stringhashmap/17734 "2026-09-27T17:19:11Z")\
**Posts on this page:** 6\
**Page:** 1

<div class="post-metadata">

**Author:** ![Vexo413](https://ziggit.dev/user_avatar/ziggit.dev/vexo413/32/9941_2.png) [@Vexo413](https://ziggit.dev/u/Vexo413)\
**Post date:** [September 27, 2026, 5:19pm UTC](https://ziggit.dev/t/how-to-zero-a-stringhashmap/17734/1 "2026-09-27T17:19:11Z")

</div>

I have a `StringHashMap`, with sensitive data, and I need to zero it.  
There are two problems when using an `Iterator`:

1. Zeroing the value or key mutates the hash map, and invalidates the iterator.
2. The key is `[]const u8`, so it can’t be mutated.

```zig
pub fn deinit(self: *Self, allocator: std.mem.Allocator) void {
    var iterator = self.hash_map.iterator();

    while (iterator.next()) |entry| {
        
        // Problems 1 and 2.
        std.crypto.secureZero(u8, entry.key_ptr.*);
        allocator.free(entry.key_ptr.*);

        // Just problem 1.
        entry.value_ptr.deinit(allocator);
    }

    self.fields.deinit();
}

```

This is a `deinit` function so it doesn’t matter if the zeroing destroys the `StringHashMap`’s ability to do anything after.

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<div class="post-metadata">

**Author:** ![alanza](https://ziggit.dev/user_avatar/ziggit.dev/alanza/32/6006_2.png) [@alanza](https://ziggit.dev/u/alanza)\
**Post date:** [September 27, 2026, 5:37pm UTC](https://ziggit.dev/t/how-to-zero-a-stringhashmap/17734/2 "2026-09-27T17:37:06Z")

</div>

`self.* = undefined` is typical.

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<div class="post-metadata">

**Author:** ![Vexo413](https://ziggit.dev/user_avatar/ziggit.dev/vexo413/32/9941_2.png) [@Vexo413](https://ziggit.dev/u/Vexo413)\
**Post date:** [September 27, 2026, 5:57pm UTC](https://ziggit.dev/t/how-to-zero-a-stringhashmap/17734/3 "2026-09-27T17:57:32Z")

</div>

Isn’t undefined mean just whatever memory is already there, so wouldn’t this just not do anything?

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<div class="post-metadata">

**Author:** ![npc1054657282](https://ziggit.dev/user_avatar/ziggit.dev/npc1054657282/32/5790_2.png) [@npc1054657282](https://ziggit.dev/u/npc1054657282)\
**Post date:** [September 27, 2026, 6:15pm UTC](https://ziggit.dev/t/how-to-zero-a-stringhashmap/17734/4 "2026-09-27T18:15:38Z")

</div>

> [@Vexo413](#):
>
> Zeroing the value or key mutates the hash map, and invalidates the iterator.

I think there’s some misunderstanding here. I believe that neither using `secureZero` to zero out, nor freeing the key or value pointers, would actually modify the hash map. Only actually adding or removing entries from the hash map will really invalidate the iterator.

> [@Vexo413](#):
>
> The key is `[]const u8`, so it can’t be mutated.

Yes, because `StringHashMap` assumes that the ownership of the keys is managed by the caller. That means you probably have other places managing the pointers to all the keys (for example, an arena). In that case, the problem of zeroing out shifts from how to iterate over the hash map to zeroing to how to zero the arena. If you want the HashMap to be the ultimate owner of the keys, you might want to consider something like `HashMap([]u8, V, StringContext, default_max_load_percentage)`, but I still recommend using an arena to store the keys’ memory and try zeroing them that way.

---

<div class="post-metadata">

**Author:** ![Vexo413](https://ziggit.dev/user_avatar/ziggit.dev/vexo413/32/9941_2.png) [@Vexo413](https://ziggit.dev/u/Vexo413)\
**Post date:** [September 27, 2026, 6:24pm UTC](https://ziggit.dev/t/how-to-zero-a-stringhashmap/17734/5 "2026-09-27T18:24:46Z")

</div>

Thanks!

So I could use an arena only for the `StringHashMap` and zero all memory from there, or I could use `AutoHashMap` that owns the string.

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<div class="post-metadata">

**Author:** ![AndrewKraevskii](https://ziggit.dev/user_avatar/ziggit.dev/andrewkraevskii/32/2564_2.png) [@AndrewKraevskii](https://ziggit.dev/u/AndrewKraevskii)\
**Post date:** [September 28, 2026, 5:39am UTC](https://ziggit.dev/t/how-to-zero-a-stringhashmap/17734/7 "2026-09-28T05:39:10Z")

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1. Iterators will still work even if you change the key because it doesn’t use key values for iteration. So as long as you don’t do anything else with hashmap after that it will still work.

2. If you know values in hashmap are definitely mutable (not string literals) you can just `@constCast()` them so you can zero them out. Or alternatively you can use underling `HashMap` but with `[]u8` instead of `[]const u8`. If you never modify the keys outside of zeroing them out it might actually be more appropriate to use `[]const u8` with `@constCast()` because it will catch potential mutations in all other parts of the code.

3. You can also consider using `StringArrayHashMapUnmanaged` which provides access to keys and values without iterator. It just gives you slice of keys and slice of values. Since no iterator is involved you can be sure its impossible for logic inside to break.

4. Just using `ArenaAllocator` doesn’t mean keys will be zeroed out. It writes `undefined` to keys on `deinit` which doesn’t do anything in `ReleaseFast`.

5. Also hashmaps store hashes of values inside and if data is really sensitive you might want to zero out hashes as well. Not sure how to do that tho if not just dig into hashmap implementation. I think it really depends on how secure hash function you use in hashmap is. Default one is unsalted wyhash which is probably not super secure.