Strings, something that C still doesn't do properly, not even having something like SDS into the standard library.
<map> does the work just fine, not everyone has winning microbenchmarks as part of their daily work.
> <map> does the work just fine
It has a rather weird interface, at least until C++ 17 when some of the deficiencies were patched somewhat.
Many of the big C++ projects I've worked with have custom string types since the standard one was defficient for some reason or another.