m05-type-driven

Core Question

Can I make this bug a compile error?

• Primitive Obsession: Using int for IDs, double for Money. Bad.
• Strong Types: struct UserId, struct Money. Good.
• Type State: Connection<OFF> vs Connection<ON>.

Error → Design Question

| Issue | Design Question | | --------------------- | ------------------------------------------------------------- | | Swapped arguments | Did you pass width to height? (Use Strong Types). | | Invalid State | Did you call read() on closed file? (Use Type State). | | Unit confusion | Did you mix Meters and Feet? (Use std::chrono style units). |

Thinking Prompt

1. Is this int unique?

   • Yes? → Wrap in struct.
   • struct UserId { int val; }; prevents process(OrderId).

2. Does valid usage depend on order?

   • Yes? → Encode state in type.
   • Builder::port() returns BuilderWithPort.

3. Are units compatible?

   • No? → Template tag. Dist<Meters> + Dist<Feet>.

Trace Up / Down

• Trace Up:

  • Issue: "Rocket crashed because of Metric vs Imperial confusion."
  • Cause: double calculate_trajectory(double dist) accepted any number.
  • Fix: Dist<Meters> calculate(Dist<Meters> d). Compilation fails if you pass Feet.

• Trace Down:

  • Intent: "Ensure file is open before reading."
  • Code: File<Open> f = File<Closed>().open(); f.read();

Quick Reference

| Pattern | Cost | Use When | | ------------------ | ---- | --------------------------------------- | | Struct Wrapper | Zero | Distinct IDs, coordinates. | | Enum Class | Zero | Type-safe flags (no implicit int conv). | | Phantom Type | Zero | Tracking state without storage. | | User Literal | Zero | 10_m, 50_s. |