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 |