m09-domain

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Mastering C++ Domain Modeling (DDD). Triggers: Entity, Value Object, Aggregate, Repository, Pimpl, Class Design, Invariant.

13eholder By 13eholder schedule Updated 1/25/2026
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name: m09-domain description: "Mastering C++ Domain Modeling (DDD). Triggers: Entity, Value Object, Aggregate, Repository, Pimpl, Class Design, Invariant."

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.
Install via CLI
npx skills add https://github.com/13eholder/Modern-Cpp-Skills --skill m09-domain
Repository Details
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