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m05-type-driven

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更新日2026年1月25日 07:37

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

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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

Is this int unique?
- Yes? → Wrap in struct.
- struct UserId { int val; }; prevents process(OrderId).
Does valid usage depend on order?
- Yes? → Encode state in type.
- Builder::port() returns BuilderWithPort.
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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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

Is this int unique?
- Yes? → Wrap in struct.
- struct UserId { int val; }; prevents process(OrderId).
Does valid usage depend on order?
- Yes? → Encode state in type.
- Builder::port() returns BuilderWithPort.
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`.