m05-type-driven

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Mastering C++ Type-Driven Design. Triggers: strong types, phantom types, type state pattern, builder pattern, invalid state unrepresentable.

13eholder By 13eholder schedule Updated 1/25/2026
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name: m05-type-driven description: "Mastering C++ Type-Driven Design. Triggers: strong types, phantom types, type state pattern, builder pattern, invalid state unrepresentable."

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.
Install via CLI
npx skills add https://github.com/13eholder/Modern-Cpp-Skills --skill m05-type-driven
Repository Details
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