m05-type-driven

name	m05-type-driven
description	Mastering C++ Type-Driven Design. Triggers: strong types, phantom types, type state pattern, builder pattern, invalid state unrepresentable. Use when this capability is needed.

C++ Type-Driven Design

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.

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