Ownership & Lifetimes

Ownership & Lifetimes

Layer 1: Language Mechanics

Core Question

Who should own this data, and for how long?

Before fixing ownership errors, understand the data's role:

• Is it shared or exclusive?
• Is it short-lived or long-lived?
• Is it transformed or just read?

---

Error → Design Question

---

Thinking Prompt

Before fixing an ownership error, ask:

1. What is this data's domain role?

   • Entity (unique identity) → owned
   • Value Object (interchangeable) → clone/​copy OK
   • Temporary (computation result) → maybe restructure

2. Is the ownership design intentional?

   • By design → work within constraints
   • Accidental → consider redesign

3. Fix symptom or redesign?

   • If Strike 3 (3rd attempt) → escalate to Layer 2

---

Trace Up ↑

When errors persist, trace to design layer:

E0382 (moved value)
    ↑ Ask: What design choice led to this ownership pattern?
    ↑ Check: m09-domain (is this Entity or Value Object?)
    ↑ Check: domain-* (what constraints apply?)

---

Trace Down ↓

From design decisions to implementation:

"Data needs to be shared immutably"
    ↓ Use: Arc<T> (multi-thread) or Rc<T> (single-thread)

"Data needs exclusive ownership"
    ↓ Use: move semantics, take ownership

"Data is read-only view"
    ↓ Use: &T (immutable borrow)

---

Quick Reference

Error Code Reference

---

Anti-Patterns

---

Related Skills