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

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: 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: 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: 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 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

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