C++ copy-paste compatibility with C is based on C89, since then C has introduced many features that won't compile under a C++ compiler.

ISO C++ has kept upgrading the support regarding ISO C libraries, as long as new grammar features aren't required.

Even ISO C++20 has finally added support for structure member initialisation, it is just a subset of what is possible in C.

So while UNIX vendors are free to use all C features, due to their symbiotic relationship, ISO C++ applications can only safely compile C89 code.

No full structure member initialisation, restrict, VLA,...