they heap allocate strings because their string_range implementation is a shared_ptr to the original string plus two indices. It might be somewhat worth it assuming the strings are large enough. But if most strings are small, passing them around by move-value would probably be an overall win. One would need to benchmark it.
In that case you'd still want a shared_string not a shared_ptr<string>. The double pointer chases add up quick.
Edit: It might help to keep in mind that string is essentially an alias for unique_ptr<char[]>. As in, string is already heap allocated. They heap allocated a pointer to a heap allocation.
In my experience, this kind of error is the cause of 90% of the "wait, I thought C++ was supposed to be fast so why is it chugging compared to my Java implementation" problems. People turn everything into two dereferences.
