Is a single atomic increment really that expensive? I mean we are not even talking about a full memory barrier here, just the atomic increment's implied acquire and release on the single variable. Other operations not dependent on a subsequent read could still be re-ordered in both directions.

And also keep in mind that the alternative was copying the whole string instead. Which means both heap memory allocation (which is often pretty expensive, even with per-core heaps), plus the actual copying. Unless a platform has a terrible implementation of atomic increment, or you have a std::string that is frequently getting copied on multiple cores (so as to have meaningful contention), I would have expected the actual copying implementation to be slower. But I'm not super familiar with the timings of these things, so i certainly could be mistaken.

My understanding was that the change was more for about being able to set proper bounds on some operations, ensuring .c_str() is O(1), and not O(n) sometimes, and similarly with string writes, etc.