Resource Management
Layer 1: Language Mechanics
Core Question
What ownership pattern does this resource need?
Before choosing a smart pointer, understand:
- Is ownership single or shared?
- Is access single-threaded or multi-threaded?
- Are there potential cycles?
Error → Design Question
| Error | Don't Just Say | Ask Instead |
|---|---|---|
| "Need heap allocation" | "Use Box" | Why can't this be on stack? |
| Rc memory leak | "Use Weak" | Is the cycle necessary in design? |
| RefCell panic | "Use try_borrow" | Is runtime check the right approach? |
| Arc overhead complaint | "Accept it" | Is multi-thread access actually needed? |
Thinking Prompt
Before choosing a smart pointer:
-
What's the ownership model?
- Single owner → Box or owned value
- Shared ownership → Rc/Arc
- Weak reference → Weak
-
What's the thread context?
- Single-thread → Rc, Cell, RefCell
- Multi-thread → Arc, Mutex, RwLock
-
Are there cycles?
- Yes → One direction must be Weak
- No → Regular Rc/Arc is fine
Trace Up ↑
When pointer choice is unclear, trace to design:
"Should I use Arc or Rc?" ↑ Ask: Is this data shared across threads? ↑ Check: m07-concurrency (thread model) ↑ Check: domain-* (performance constraints)
| Situation | Trace To | Question |
|---|---|---|
| Rc vs Arc confusion | m07-concurrency | What's the concurrency model? |
| RefCell panics | m03-mutability | Is interior mutability right here? |
| Memory leaks | m12-lifecycle | Where should cleanup happen? |
Trace Down ↓
From design to implementation:
"Need single-owner heap data" ↓ Use: Box<T> "Need shared immutable data (single-thread)" ↓ Use: Rc<T> "Need shared immutable data (multi-thread)" ↓ Use: Arc<T> "Need to break reference cycle" ↓ Use: Weak<T> "Need shared mutable data" ↓ Single-thread: Rc<RefCell<T>> ↓ Multi-thread: Arc<Mutex<T>> or Arc<RwLock<T>>
Quick Reference
| Type | Ownership | Thread-Safe | Use When |
|---|---|---|---|
Box<T> | Single | Yes | Heap allocation, recursive types |
Rc<T> | Shared | No | Single-thread shared ownership |
Arc<T> | Shared | Yes | Multi-thread shared ownership |
Weak<T> | Weak ref | Same as Rc/Arc | Break reference cycles |
Cell<T> | Single | No | Interior mutability (Copy types) |
RefCell<T> | Single | No | Interior mutability (runtime check) |
Decision Flowchart
Need heap allocation? ├─ Yes → Single owner? │ ├─ Yes → Box<T> │ └─ No → Multi-thread? │ ├─ Yes → Arc<T> │ └─ No → Rc<T> └─ No → Stack allocation (default) Have reference cycles? ├─ Yes → Use Weak for one direction └─ No → Regular Rc/Arc Need interior mutability? ├─ Yes → Thread-safe needed? │ ├─ Yes → Mutex<T> or RwLock<T> │ └─ No → T: Copy? → Cell<T> : RefCell<T> └─ No → Use &mut T
Common Errors
| Problem | Cause | Fix |
|---|---|---|
| Rc cycle leak | Mutual strong refs | Use Weak for one direction |
| RefCell panic | Borrow conflict at runtime | Use try_borrow or restructure |
| Arc overhead | Atomic ops in hot path | Consider Rc if single-threaded |
| Box unnecessary | Data fits on stack | Remove Box |
Anti-Patterns
| Anti-Pattern | Why Bad | Better |
|---|---|---|
| Arc everywhere | Unnecessary atomic overhead | Use Rc for single-thread |
| RefCell everywhere | Runtime panics | Design clear ownership |
| Box for small types | Unnecessary allocation | Stack allocation |
| Ignore Weak for cycles | Memory leaks | Design parent-child with Weak |
Related Skills
| When | See |
|---|---|
| Ownership errors | m01-ownership |
| Interior mutability details | m03-mutability |
| Multi-thread context | m07-concurrency |
| Resource lifecycle | m12-lifecycle |
