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