Unions

https://en.cppreference.com/w/cpp/language/union

• A special class type that can only hold one of its non-static data members at a time.
• Union is only as big as necessary to hold its largest data member.

Variants

• std::variant is a class template that represents a type-safe union.
• That means, at a certain point in time, one type of the possibly many types in the variant/union is defined/used.
• So it can let you use a variable like you use them in dynamically typed languages.

Variants Examples

State Machines

Very nice description of how to use variants instread of enum unions to make more customised state machines: https://www.cppstories.com/2019/06/fsm-variant-game/

Godot Game Engine

https://docs.godotengine.org/en/stable/development/cpp/variant_class.html

So, variants are used everywhere in Godot. Not for storing data permenantly, but to hold them for communication, editing, serialization and generally moving them around in a generic container instead of statically typing everything.

HOWEVER, they don't use the std::variant, but use a class made up of enums.