This is especially true for C which supports almost nothing (it doesn't even have a sensible array type!). But is also true for C++: while it supports a lot, it doesn't support everything.
Methods in C, just have function pointers as members. Common in many codebases.
Guaranteed tail calls, all the compilers guarantee that function calls that are a return expression are tail calls.
Tagged union in C++, it's trivial as a library, see std::variant for a bad example of it, and all the various monadic/pattern-matching variants (pun intended) people have written. C is at a disadvantage here due to lack of lambdas, but I'm sure people have built stuff using some GCC extensions.
You can do the same in C by wrapping your array in a struct.
