Planning: inheritance, mixins, and bases

[LB--]

Inheritance

• Models an is-a relationship, used for polymorphism
• Classes can be optionally declared final so as to disallow being inherited from

  Mixins

• Mixin types must be explicitly noted as such
• Mixins can override virtual functions from inherited classes
• The mixing class can override the mixin's overrides
• Can be used for C++-style CRTP

  Bases

• A class can base itself on types which are marked final
• Cannot upcast to the bases, because the relationship is not is-a
• Built-in types can be bases

In all cases, multiple inheritance/mixins/bases is/are allowed.

Inheritance is like traditional C++ inheritance, except that CRTP isn't possible. Mixins are designed for CRTP and are aware of the type they are being mixed into. Bases don't know they're being used as a base, and they can't stop it from happening.

[LB--]

You can alter inherited/mixed-in/based members in compatible ways without having to give entirely new definitions (e.g. just change the privacy, mark final, add or remove virtual, etc)