It's a drop-in replacement for neither since it's an entirely different language. C++ went to some great pain to be a drop-in replacement for C; you can "drop in" a C++ compiler in your toolchain and still compile your C code (mostly).

This was kind of true till C99, but not so much anymore. restrict is the most obvious offender but far from the only one[1].

[1]: https://en.m.wikipedia.org/wiki/Compatibility_of_C_and_C%2B%...