Immutable-cpp: persistent immutable data structures for C++ | Hacker News Reader

Immutable-cpp: persistent immutable data structures for C++

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42 pointsjiyinyiyong16 comments

Nice to see more immutable data structures for C++. I recently implement a persistent list inside a personal project, it's quite easy using shared_ptr (a set of persistent list forms a DAG, so no loop).

It would be nice to have a persistent version of all standard containers. It seems that implementing "Finger Tree" would be a good start as a basic building block. But I didn't find any c++ implementation.

"Practically O(1)" is meaningless. If it's not actually constant time, then how does it grow?

Every operation that is not actually constant time grows by log32. Or in other words, by no more than the constant factor 5.

In practice, these data structures live and die by your memory allocator. They do so many allocations that unless you are using a world-class GC, the memory allocation costs completely overshadow any other time costs. Using the typical platform malloc, or even jemalloc or it's ilk, this will be very slow indeed.

Yes. This begs for a port to Rust, to see if the fine-grained memory support there pays off. While Rust can do anything, it wants to be bare-metal Haskell.

C has just as much fine-grained memory support as Rust. Rust does however have better memory safety with its borrow checker, but I'm not sure if you could implement something like this without unsafe.

It seems I was a bit too fast. I meant to write C++ and not C.

Yes. Copying GC with bump pointer allocation is critical for pretty much any persistent data structure. In C++ and Rust one should use memory pools/arenas for them instead.

"Practically O(1)" == amortized constant time?

Sorry, I am lost: why is this a big deal? I use std::array and it's fine.

Immutability makes reasoning with state and concurrency much easier. Persistent data structures help make immutability practical in terms of performance and memory overhead.

Their Array class is perhaps unfortunately named, as it behaves as a vector or list (i.e. you can push things to the collection) rather than fixed size memory block.

The important thing to notice in their example is the assignment to 'a' when pushing items (`a = a->push(1);`), and the ability to reassign the new change to a new variable (`auto b = a->push(4);`). That last command keeps 'a' the same (hence "immutable" data structure), but creates a new variable 'b' to hold the new data.

This technique allows you to, for example, pass the same data structure to multiple threads without worrying that they will simultaneously change data (race conditions).

More information can be found here https://en.wikipedia.org/wiki/Persistent_data_structure (check out the linked list and tree diagrams in examples).

For this to work properly in C++, I think you'd need to const all types for it to be effective (immutable::Array<const MyStruct>).

I'd really like to see benchmarks comparing these structures implemented in C++ vs those in functional languages; I don't know if the compiler optimization algorithms produce similar code.

Naive question: even so, what's the advantage over using, say, a const std::array<MyStruct>?

Well, when you pass a variable around, it doesn't and cannot change. This means that different threads can't get in eachothers' way anymore, for instance, but also that you can't make a big chunk of mistakes at all.

They also have serious disadvantages : they can't be memory managed in the traditional way (since they tend to reuse other instances' memory in complex ways), and thus require a GC (refcounting can work, but ...). They are VERY allocation intensive, and they are worse than most non-persistent data structures. Assuming an O(1) allocator they can match non-persistent data structures in O-ness (ie. when making an invalid assumption that is quite popular in academia. In practice memory allocation is O(1) for small values, then O(n^2) once you get close to the system's memory capacity (scanning for holes in a long list) but don't go over it, and then O(oh fuck it let's just reboot this bloody BOAT ANCHOR) when crossing that line).

Clojure is famous for having good persistent data structures. Rich Hickey went touring academia touting the benefits of immutable/persistent/functional data structures : https://www.youtube.com/watch?v=dGVqrGmwOAw&feature=youtu.be...

There's also a famous book: https://www.amazon.com/Purely-Functional-Structures-Chris-Ok...

> Well, when you pass a variable around, it doesn't and cannot change

Yeah but how's that different than a const?

std::array is not really comparable since its size is fixed at compile time. std::vector or std::list are more similar.

The big advantage is creating copies. Copying a list or vector is O(n) in both time and memory, while this is O(1) in both.

An std::shared_ptr to a const list or vector is O(1) in both for copies. But almost any modifications will be O(n).

For this data structure, almost all operations are O(log(n)).

A summary of their algorithmic complexities:

  Copy:
  std::vector : O(n) time and memory
  std::shared_ptr<const std::vector> : O(1) time and memory
  immutable::Array : O(1) time and memory

  Append:
  std::vector : Amortized O(1)
  std::shared_ptr<const std::vector> : O(n)
  immutable::Array : O(log(n))

  Access at index:
  std::vector : O(1)
  std::shared_ptr<const std::vector> : O(1)
  immutable::Array : O(log(n))

  Insert at index:
  std::vector : O(n)
  std::shared_ptr<const std::vector> : O(n)
  immutable::Array : O(log(n))

  Replace at index:
  std::vector : O(1)
  std::shared_ptr<const std::vector> : O(n)
  immutable::Array : O(log(n))

Is there anything in C++11/14/17 that contains this? Could you use locks, or is that less performant? I'm very interested in using this since I do have a multithreaded Windows app that appends and processes a queue (a long poll client to a web app that does work for the web app by gathering data from the local desktop).

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