While updating the mjmech code-base to interoperate with the moteus servos, I simultaneously was updating it to use C++17. C++ rarely removes or deprecates features, but one of those which actually was removed in C++17, after being deprecated in C++11, was auto_ptr. unique_ptr is strictly superior in all regards, and so there is no real reason not to switch.
However, mjmech depends upon a large amount of third party software. Amazingly, only one package actually was broken by this removal of auto_ptr, log4cpp. It actually saw some updates for C++17 compliance back in 2017, but otherwise hasn’t been updated since then. I went ahead and forked it, and got it compiling with clang in C++17 mode at least:
After doing that, I discovered that someone had already posted a similar patch 2 years ago to the sourceforge page, but which was never applied. Oh well, it wasn’t that much duplicated effort.