I stumbled across this CppCon talk and found it interesting. The Ewig text editor example with concurrent loading and editing is impressive. The code seems clean.
There is a lucid exposition of Relaxed Radix Balanced Trees (RRB trees) in the video.
Although I lost the plot with C++ around C++ 11, it is impressive how the language has fully delegated its data structures to library code.
https://github.com/arximboldi/immer