That's author's words, I'd say it's stuff like narrowing conversions, arrays and memcpy. Though the author doesn't claim that memcpy is a C idiom, and indeed it's used broadly in C++.
A properly formed modern C++ program doesn’t really use memcpy. Most idioms from the C language have a much stricter counterpart that is part of the C++ language.
You’ve made a few comments now about a language that you don’t seem to actually use or know.
