160 lines
4.3 KiB
Markdown
160 lines
4.3 KiB
Markdown
---
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name: m02-resource
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description: "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"
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user-invocable: false
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---
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# Resource Management
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> **Layer 1: Language Mechanics**
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## Core Question
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**What ownership pattern does this resource need?**
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Before choosing a smart pointer, understand:
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- Is ownership single or shared?
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- Is access single-threaded or multi-threaded?
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- Are there potential cycles?
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---
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## Error → Design Question
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| Error | Don't Just Say | Ask Instead |
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|-------|----------------|-------------|
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| "Need heap allocation" | "Use Box" | Why can't this be on stack? |
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| Rc memory leak | "Use Weak" | Is the cycle necessary in design? |
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| RefCell panic | "Use try_borrow" | Is runtime check the right approach? |
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| Arc overhead complaint | "Accept it" | Is multi-thread access actually needed? |
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---
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## Thinking Prompt
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Before choosing a smart pointer:
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1. **What's the ownership model?**
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- Single owner → Box or owned value
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- Shared ownership → Rc/Arc
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- Weak reference → Weak
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2. **What's the thread context?**
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- Single-thread → Rc, Cell, RefCell
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- Multi-thread → Arc, Mutex, RwLock
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3. **Are there cycles?**
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- Yes → One direction must be Weak
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- No → Regular Rc/Arc is fine
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---
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## Trace Up ↑
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When pointer choice is unclear, trace to design:
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```
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"Should I use Arc or Rc?"
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↑ Ask: Is this data shared across threads?
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↑ Check: m07-concurrency (thread model)
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↑ Check: domain-* (performance constraints)
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```
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| Situation | Trace To | Question |
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|-----------|----------|----------|
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| Rc vs Arc confusion | m07-concurrency | What's the concurrency model? |
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| RefCell panics | m03-mutability | Is interior mutability right here? |
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| Memory leaks | m12-lifecycle | Where should cleanup happen? |
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---
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## Trace Down ↓
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From design to implementation:
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```
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"Need single-owner heap data"
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↓ Use: Box<T>
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"Need shared immutable data (single-thread)"
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↓ Use: Rc<T>
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"Need shared immutable data (multi-thread)"
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↓ Use: Arc<T>
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"Need to break reference cycle"
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↓ Use: Weak<T>
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"Need shared mutable data"
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↓ Single-thread: Rc<RefCell<T>>
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↓ Multi-thread: Arc<Mutex<T>> or Arc<RwLock<T>>
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```
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---
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## Quick Reference
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| Type | Ownership | Thread-Safe | Use When |
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|------|-----------|-------------|----------|
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| `Box<T>` | Single | Yes | Heap allocation, recursive types |
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| `Rc<T>` | Shared | No | Single-thread shared ownership |
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| `Arc<T>` | Shared | Yes | Multi-thread shared ownership |
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| `Weak<T>` | Weak ref | Same as Rc/Arc | Break reference cycles |
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| `Cell<T>` | Single | No | Interior mutability (Copy types) |
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| `RefCell<T>` | Single | No | Interior mutability (runtime check) |
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## Decision Flowchart
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```
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Need heap allocation?
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├─ Yes → Single owner?
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│ ├─ Yes → Box<T>
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│ └─ No → Multi-thread?
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│ ├─ Yes → Arc<T>
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│ └─ No → Rc<T>
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└─ No → Stack allocation (default)
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Have reference cycles?
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├─ Yes → Use Weak for one direction
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└─ No → Regular Rc/Arc
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Need interior mutability?
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├─ Yes → Thread-safe needed?
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│ ├─ Yes → Mutex<T> or RwLock<T>
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│ └─ No → T: Copy? → Cell<T> : RefCell<T>
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└─ No → Use &mut T
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```
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---
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## Common Errors
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| Problem | Cause | Fix |
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|---------|-------|-----|
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| Rc cycle leak | Mutual strong refs | Use Weak for one direction |
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| RefCell panic | Borrow conflict at runtime | Use try_borrow or restructure |
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| Arc overhead | Atomic ops in hot path | Consider Rc if single-threaded |
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| Box unnecessary | Data fits on stack | Remove Box |
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---
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## Anti-Patterns
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| Anti-Pattern | Why Bad | Better |
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|--------------|---------|--------|
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| Arc everywhere | Unnecessary atomic overhead | Use Rc for single-thread |
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| RefCell everywhere | Runtime panics | Design clear ownership |
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| Box for small types | Unnecessary allocation | Stack allocation |
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| Ignore Weak for cycles | Memory leaks | Design parent-child with Weak |
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---
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## Related Skills
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| When | See |
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|------|-----|
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| Ownership errors | m01-ownership |
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| Interior mutability details | m03-mutability |
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| Multi-thread context | m07-concurrency |
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| Resource lifecycle | m12-lifecycle |
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