13eholder/m09-domain

C++ Domain Modeling

Core Question

Identity or Value?

• Value Object: Defined by attributes (Color, Money). Equality via comparison. Copyable.
• Entity: Defined by Identity (User, Socket). Equality via ID. Non-copyable (usually).

Error → Design Question

| Issue | Design Question | | ---------------------- | ---------------------------------------------------- | | Data Inconsistency | Are public fields allowing invalid states? | | Object Slicing | Are you passing polymorphic Entities by Value? | | Header Hell | Are you leaking implementation details? (Use Pimpl). |

Thinking Prompt

1. Is it copyable?

   • Yes? → Value Type (Rule of Zero, defaults).
   • No? → Entity (Delete copy ctor, enable move).

2. Does it have invariants?

   • Yes? → class with private data + public methods.
   • No? → struct (POD).

3. Does it own others?

   • Aggregate Root? → Owns children via std::vector / unique_ptr.

Trace Up / Down

• Trace Down:
  • Intent: "User has a Name and an Address."
  • Code: class User (Entity) holds Name (Value) and Address (Value).

Quick Reference

| Pattern | C++ Implementation | | ---------------- | ------------------------------------------- | | Value Object | struct + operator<=>. | | Entity | class + Deleted Copy + ID field. | | Repository | Pure Virtual Interface (virtual ... = 0). | | Aggregate | Parent class owning children. |