Top 3 C++ Immutable Projects

immer

7 2,177 8.6 C++

Postmodern immutable and persistent data structures for C++ — value semantics at scale (by arximboldi)

Project mention: C++ Smart Pointers | reddit.com/r/cpp | 2022-09-13

Our primary use was what's alternatively called applicative, functional, immutable, or persistent data structures; my preferred term is applicative, so that's what I'll use in this comment. There's a C++ library called immer that I have absorbed generally good impressions of from discussions here and such but never really looked into myself for this kind of data structure. (It's worth pointing out that "persistent" is in some ways a particularly bad term for these, because database folks use that term for data structures that are located on persistent storage, which is entirely different.)
ewig

0 495 5.8 C++

The eternal text editor — Didactic Ersatz Emacs to show immutable data-structures and the single-atom architecture
InfluxDB

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zef

0 40 9.8 C++

A data-oriented toolkit for graph-relational data across time (by zefhub)

NOTE: The open source projects on this list are ordered by number of github stars. The number of mentions indicates repo mentiontions in the last 12 Months or since we started tracking (Dec 2020). The latest post mention was on 2022-09-13.