And another one dies every time you need to step through a call to std::function. Whatever you do, the kittens are never going to escape.

To save the kitten, one could at least write classes that override the call operator::(). No need for void* or any ptrs at all and std::function is avoided as well. That will at least get you to 2010 C++ code.

Unless you mutter the magic incantation "C compatibility" while doing it

At this point, C++ has long abandoned C at the language level. If you are going to write C code, write C code -- don't hamstring modern C++.

did nullptr exist in c++ back in 1995 - i can't remember

Nope, it was introduced in C++11, along with the type std::nullptr_t. Before that, you either used 0 or NULL, which was a macro constant defined to be 0.

That is the only thing "modern" in the above code and it does not save the kitten.