# Ownership of memory with hashmaps

**URL:** <https://ziggit.dev/t/ownership-of-memory-with-hashmaps/18152>\
**Category:** Help\
**Tags:** language, memory-management\
**Created:** [October 30, 2026, 1:51pm UTC](https://ziggit.dev/t/ownership-of-memory-with-hashmaps/18152 "2026-10-30T13:51:47Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![Vexo413](https://ziggit.dev/user_avatar/ziggit.dev/vexo413/32/10418_2.png) [@Vexo413](https://ziggit.dev/u/Vexo413)\
**Post date:** [October 30, 2026, 1:51pm UTC](https://ziggit.dev/t/ownership-of-memory-with-hashmaps/18152/1 "2026-10-30T13:51:47Z")

</div>

I am making a library, when I say user I mean whoever is using this library, not me using the std library.  
I have this struct:

```zig
const Foo = struct {
    bars: std.StringHashMap(Bar),
};

```

The `Bar` struct has a `deinit` function that has an `Allocator` as a parameter.  
Most of the ways to interact with this struct is just updating `bars` in with the hashmap functions.  
`StringHashMap` does not own the memory of its keys and values.  
I could do something similar to this and make the user responsible for deallocating the memory with `Bar.deinit` or, because the type is always the same, should I provide a `Foo.deinit` function that loops through each bar and runs `Bar.deinit`.  
Then who owns the memory? If the user puts a value into the `StringHashMap`, it documentation says that they are responsible for freeing the memory, but that is done by `Foo.deinit` then, not the user.  
Should I make `put` functions for `Foo` that just call the corresponding `StringHashMap` function, that would make ownership clearer, but would add a lot of bloat.  
Does anyone know any std libary structs that might be similar to this?

---

<div class="post-metadata">

**Author:** ![Southporter](https://ziggit.dev/user_avatar/ziggit.dev/southporter/32/2500_2.png) [@Southporter](https://ziggit.dev/u/Southporter)\
**Post date:** [October 30, 2026, 2:03pm UTC](https://ziggit.dev/t/ownership-of-memory-with-hashmaps/18152/2 "2026-10-30T14:03:57Z")

</div>

For HashMaps, the map does NOT own the memory. The user is incharge of managing the memory for any heap allocated structs. Calling `deinit` on a Map only clears the memory used by the Map to store keys and values.  
For a StringHashMap, the string keys would be managed by the caller. It is up to you as the user to manage those.  
That being said, Bar here would be part of that memory. If Bar has any members that are heap allocated, you would need to handle that yourself. When you are cleaning up, you would probably want to [iterate](https://ziglang.org/documentation/0.17.0/std/#std.hash_map.HashMap.iterator) over the keys and values and clean up both.

```zig
var iter = bars.iterator()
while (iter.next()) |entry| {
    // If the string keys need to be freed
    gpa.free(entry.key_ptr.*);
    // If Bar has a deinit function, use it
    entry.value_ptr.deinit(gpa);
}

```

I would put this in the Foo.deinit function, but it depends on how you are design the API. If you want Foo to take ownership of all `Bar`s placed in Foo, then I would say having the Foo.deinit call all the Bar.deinit in a loop like above would be expected.

> [@Vexo413](#):
>
> documentation says that they are responsible for freeing the memory, but that is done by `Foo.deinit` then, not the user.

In this case, the documentation calls out the “caller” as the one responsible for managing the memory. `Foo.deinit` would be a good place to handle this, so you are following the documentation.

---

<div class="post-metadata">

**Author:** ![Vexo413](https://ziggit.dev/user_avatar/ziggit.dev/vexo413/32/10418_2.png) [@Vexo413](https://ziggit.dev/u/Vexo413)\
**Post date:** [October 30, 2026, 2:11pm UTC](https://ziggit.dev/t/ownership-of-memory-with-hashmaps/18152/3 "2026-10-30T14:11:13Z")

</div>

I made this very unclear, sorry. I am making a library with the `Foo` struct. When I say the user, I mean the user of my library containing `Foo`, not me the user using the std library.

---

<div class="post-metadata">

**Author:** ![Southporter](https://ziggit.dev/user_avatar/ziggit.dev/southporter/32/2500_2.png) [@Southporter](https://ziggit.dev/u/Southporter)\
**Post date:** [October 30, 2026, 2:34pm UTC](https://ziggit.dev/t/ownership-of-memory-with-hashmaps/18152/4 "2026-10-30T14:34:50Z")

</div>

That’s partially on me as well, probably didn’t read it closely enough.

So if I understand correctly, you are asking whether you should make the user of your library should handle ownership of Bar? The following assumes Bar is a type from your library.

My personal approach would be that I would expect Foo to have a `deinit` function and I would expect Foo to have ownership of Bar at this point. If that is not the case, I (as a hypothetical user) would expect it documented somewhere. That is my default approach in this case.

As for the `put` question, I would expect that rather than having to call `foo.bars.put` manually if this is a more complex type. If it is a simple wrapper around the StringHashMap, that may be untenable because you would need to basicaly wrap all of the `get*` or `put*` methods. Adn at that point, there may not be value in Foo if it is a simple wrapper and instead you should let the user determine how they want to keep track of Bar.