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m02-resource

Skill1 starsChanged 4 months agoMITView on GitHub

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

We split this skill into the rules it gives an agent

3 rules. About 0 are common, 3 are distinctive, and 0 are very specific to this project.

  • ≈0

    Common · estimate

    Many files like this have a rule of this kind.

  • ≈3

    Distinctive · estimate

    This author's own choices. Worth a look.

  • 0

    Very specific

    Tied to their project's names, files or tools. Probably skip.

  • Common
  • Distinctive
  • Very specific
  1. Distinctive:

    Before choosing a smart pointer, understand:

    line 15
  2. Distinctive:

    Before choosing a smart pointer:

    line 35
  3. Distinctive:

    When pointer choice is unclear, trace to design:

    line 54

≈ “Common” is an estimate: at least 1 in 5 of the 308,232 skill files we read have a rule of the same kind, not the same words. “Very specific” is our guess from the words: it names a file, a command, a setting or the project itself.

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Files
SKILL.md

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

ErrorDon't Just SayAsk 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:

  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

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)
SituationTrace ToQuestion
Rc vs Arc confusionm07-concurrencyWhat's the concurrency model?
RefCell panicsm03-mutabilityIs interior mutability right here?
Memory leaksm12-lifecycleWhere 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

TypeOwnershipThread-SafeUse When
Box<T>SingleYesHeap allocation, recursive types
Rc<T>SharedNoSingle-thread shared ownership
Arc<T>SharedYesMulti-thread shared ownership
Weak<T>Weak refSame as Rc/ArcBreak reference cycles
Cell<T>SingleNoInterior mutability (Copy types)
RefCell<T>SingleNoInterior 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

ProblemCauseFix
Rc cycle leakMutual strong refsUse Weak for one direction
RefCell panicBorrow conflict at runtimeUse try_borrow or restructure
Arc overheadAtomic ops in hot pathConsider Rc if single-threaded
Box unnecessaryData fits on stackRemove Box

Anti-Patterns

Anti-PatternWhy BadBetter
Arc everywhereUnnecessary atomic overheadUse Rc for single-thread
RefCell everywhereRuntime panicsDesign clear ownership
Box for small typesUnnecessary allocationStack allocation
Ignore Weak for cyclesMemory leaksDesign parent-child with Weak

Related Skills

WhenSee
Ownership errorsm01-ownership
Interior mutability detailsm03-mutability
Multi-thread contextm07-concurrency
Resource lifecyclem12-lifecycle

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