CRITICAL: Use for smart pointers and resource management. Triggers: Box, Rc, Arc, Weak, RefCell, Cell, smart pointer, heap allocation, reference counting, RAII, Drop, should I use Box or Rc, when to use Arc vs Rc, 智能指针, 引用计数, 堆分配
Layer 1: Language Mechanics
What ownership pattern does this resource need?
Before choosing a smart pointer, understand:
| 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? |
Before choosing a smart pointer:
What's the ownership model?
What's the thread context?
Are there cycles?
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? |
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>>
| 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) |
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
| 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-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 |
| When | See |
|---|---|
| Ownership errors | m01-ownership |
| Interior mutability details | m03-mutability |
| Multi-thread context | m07-concurrency |
| Resource lifecycle | m12-lifecycle |