Code: #[derive(Eq, PartialEq, Hash)] enum Key { String(String), Bytes(Vec<u8>), } enum KeyRef<'a> { String(&'a str), Bytes(&'a [u8]), } fn query(map: &HashMap<Key, u32>, key: KeyRef) -> Option<u32> { // how to do it without allocating? } I tried various approaches, the only o…

You can define a trait AsKeyRef, and then implement Borrow<dyn AsKeyRef> for Key: pub trait AsKeyRef { fn as_key_ref(&self)->KeyRef<'_>; } impl<'a> PartialEq for dyn AsKeyRef+'a { fn eq(&self, other:&Self)->bool { self.as_key_ref() == other.as_key_ref() } } impl<'a> Eq for dyn AsKeyRef+'…

Instead of having a key which is either a String or some bytes, what about just using a Vec<u8> as your key then calling some_string.as_bytes() when you want to do a lookup with a string?

This is just simplied example of what I actually wanted to do, in my real use case enum has not just string and bytes. However, now I think that maybe I can instead create multiple maps: struct MyMaps { string_map: HashMap<String, u32>, bytes_map: HashMap<Vec<u8>, u32>, } and query different …

Thank you very much! I tried to do something similar, my mistake was writing: impl Hash for dyn AsKeyRef { ... } instead of impl<'a> Hash for dyn AsKeyRef + 'a { ... } (I did not even know the latter is possible) and Rust complained: 72 | map.get(&key as &dyn AsKeyRef).cloned() | …

Stuck with querying a hashmap

mbrubeck August 31, 2020, 4:46am 4

Another solution is to use the indexmap crate, and impl Equivalent<Key> for KeyRef.

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Stuck with querying a hashmap

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