[Const-me]

I never used Rust in commercial projects. But to my knowledge, data structures in Rust are not exceptionally good at composing, because the language lacks raw pointers.

In C++ I can add hashmap-based index to existing collection of values without duplicating the values, because I can only keep pointers in the map. I can add linked list-based LRU tracking to existing collection of values. I can use pointers to implement graph-based structures. While there’re workarounds in Rust, such as reference counted pointers, raw pointers in C and C++ are just faster.

[kibwen]

> the language lacks raw pointers.

This is incorrect, Rust has raw pointers. Those reference-counted pointers that you mention are all just standard library types implemented using raw pointers under the hood.

[Const-me]

I’m sure Rust smart pointers are implemented using traditional C pointers.

Regardless of how you call them, you still can’t use them to compose data structures into higher-level structures in Rust.

In C++ with raw pointers, I can combine a hashmap with an existing collection of values to speed up lookups, without duplicating values. I can combine hash map with linked list to create an efficient LRU cache, with a few lines of my own code.

C++ can do that because collections, std::unordered_map, std::list and the rest of them, expose raw pointers at their API boundaries. It’s possible to store these pointers in other collections, encapsulating one or more simple data structures into a higher-level specialized data structure, with the performance characteristics suitable for the job.

[steveklabnik]

A star-const T or star-mut T (wow I cannot figure out HN's formatting here) in Rust does absolutely everything a C pointer can. They're the same thing. You can do all the stuff you're talking about here. Exposing a safe interface does not also mean not exposing an unsafe one as well for these kinds of things.

[Const-me]

Technically, yes I can.

However, look at the Rust implementation of the LRU cache: https://github.com/contain-rs/linked-hash-map/blob/master/sr...

See how much code is there (more than 1k lines), and note most code is unsafe.

In C++, one would probably use list<pair<K,V>> for storage, together with unordered_map<K, list<pair<K,V>>::iterator> for indexing. And the implementation would be trivial, because there’s no need to re-implement a linked list, you only need to implement these get/set methods that update both collections at the same time.

In Rust however, they had to come up with their own implementation of linked list. The LinkedList from standard library doesn’t expose raw node pointers in the API, therefore it can’t be used for the job.

[steveklabnik]

Sure, but that's an oversight of the LinkedList API (which, honestly, was almost removed from std all-together, it's not exactly a shining beacon of API design) than some sort of inherent limitation of smart pointers, which seems to be what you're saying here.

[Const-me]

I don’t think that was an oversight.

I think rust safety model prevents rust containers from returning mutable pointers to the contained items. Especially pointers to item's implementation details, like nodes of that linked lists.

And without raw pointers to items, you can’t easily compose containers to make higher-level data structures from them.