Boost-histogram implements the UHI indexing protocol. You can read more about it on the UHI Indexing page.

Boost-histogram specific details

Boost-histogram implements bh.loc, builtins.sum, bh.rebin, bh.underflow, and bh.overflow from the UHI spec. A locator is provided as well, which simulates the Boost.Histogram C++ .at() indexing using the UHI locator protocol.