[taspeotis]

> Yes, those are the cases where it isn't possible to avoid forward declarations, so it's fine to use them?

The author explicitly mentions how it is possible to avoid forward declarations in those cases: "both of these forward declarations are possible to avoid by type-punning through a void* but it is not a good pattern."

[kazinator]

By definition, if avoiding something isn't possible, then there cannot be a "how to avoid".

Why even mention a non-starter choice, like using type punning through a void pointer, as an alternative to a forward declaration.

If a construct is "not possible to avoid" that generally means "not possible to avoid without changing the structure of code and run-time data (let alone changing them to something unsafe)".

Gratuitous forward declarations of C++ classes are "bad" for various reasons. Such as: people sometimes write them just to shut up the compiler, instead of including the right header to provide that declaration. "I'm just using a pointer to this; what's the harm."

It makes perfect sense for a coding standard to advise against this.