Are type fields pure evil?

Question

As discusses in The c++ Programming Language 3rd Edition in section 12.2.5, type fields tend to create code that is less versatile, error-prone, less intuitive, and less maintai

Answer 1

When you "know" you have a very specific, small, constant set of types, it can be easier to hardcode them like this. Of course, constants aren't and variables don't, so at some point you might have to rewrite the whole thing anyway.

This is, more or less, the technique used for discriminated unions in several of Alexandrescu's articles.

For example, if I was implementing a JSON library, I'd know each Value can only be an Object, Array, String, Integer, Boolean, or Null—the spec doesn't allow any others.