This is only a first cleanup step to improve compile time and memory related to the HistogramRegistry.
More changes will be needed that directly affect the registry code, but I'd prefer to do this incrementally.

Thank you for this. You can check how long a given file takes to compile by using:

scripts/profile-compilation ./Analysis/Tasks/PWGHF/taskXic.cxx

There is also a script which is called scripts/profile-compilation-merge which allows you to combine more then one compilation unit. You can look at the results with https://speedscope.app.

That said when I try to do a full compilation with clang and -ftime-trace, using https://github.com/aras-p/ClangBuildAnalyzer to do a full report, I still see roughly the same compilation time.

Thanks for the hints! That will be very helpful.
You are right, I also just realised these changes will not have much effect on the compilation since the templated function (createHist<>) which I extern'ed here was instanciated only in the HistogramRegistry.cxx itself and in the CorrelationContainer, but never in the user tasks. However, I still think these are useful changes if in the future people want to create histograms in the same manner as Jan Fiete does it in the CorrelationContainer.
The next modifications to the registry will probably have a larger impact.

Yes, agreed.

Errors unrelated. macOS tested on my own box.