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? 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? Thinking Prompt

Before choosing a smart pointer:

What's the ownership model?

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

What's the thread context?

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

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)

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? Trace Down ↓

From design to implementation:

"Need single-owner heap data" ↓ Use: Box

"Need shared immutable data (single-thread)" ↓ Use: Rc

"Need shared immutable data (multi-thread)" ↓ Use: Arc

"Need to break reference cycle" ↓ Use: Weak

"Need shared mutable data" ↓ Single-thread: Rc> ↓ Multi-thread: Arc> or Arc>

Quick Reference Type Ownership Thread-Safe Use When Box Single Yes Heap allocation, recursive types Rc Shared No Single-thread shared ownership Arc Shared Yes Multi-thread shared ownership Weak Weak ref Same as Rc/Arc Break reference cycles Cell Single No Interior mutability (Copy types) RefCell Single No Interior mutability (runtime check) Decision Flowchart Need heap allocation? ├─ Yes → Single owner? │ ├─ Yes → Box │ └─ No → Multi-thread? │ ├─ Yes → Arc │ └─ No → Rc └─ 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 or RwLock │ └─ No → T: Copy? → Cell : RefCell └─ No → Use &mut T

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