| 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. |
