actionbook/m02-resource

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?

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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? |

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Thinking Prompt

Before choosing a smart pointer:

1. What's the ownership model?

   • Single owner → Box or owned value
   • Shared ownership → Rc/Arc
   • Weak reference → Weak

2. What's the thread context?

   • Single-thread → Rc, Cell, RefCell
   • Multi-thread → Arc, Mutex, RwLock

3. Are there cycles?

   • Yes → One direction must be Weak
   • No → Regular Rc/Arc is fine

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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? |

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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>>

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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

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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 |

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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 |

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Related Skills

| When | See | |------|-----| | Ownership errors | m01-ownership | | Interior mutability details | m03-mutability | | Multi-thread context | m07-concurrency | | Resource lifecycle | m12-lifecycle |