13eholder/m01-ownership

C++ Ownership (Move Semantics & RAII)

Core Question

Who owns this resource, and does it need to move?

In C++, ownership is a discipline, not just a compiler check. You must decide:

• Scope-bound? Use Stack (RAII).
• Exclusive? Use std::unique_ptr.
• Shared? Use std::shared_ptr.
• View? Use T* or T& (non-owning).

Error → Design Question

| Compiler/Sanitizer Error | Design Question | | ------------------------ | ---------------------------------------------------------- | | Double Free | Who owns this? Did you copy a raw pointer? | | Use After Free | Did a reference outlive its owner? | | Memory Leak | Where is the destructor called? (Did you use new?) | | Object slicing | Why are you passing by value instead of pointer/reference? | | Moved-from usage | Why are you touching a variable after std::move? |

Thinking Prompt

1. Does it need a heap allocation?

   • No? → Stack value (Rule of Zero).
   • Yes? → std::unique_ptr (Rule of Zero).
   • DO NOT write a destructor unless managing a non-RAII C handle.

2. Is it a Transfer or a Copy?

   • Transfer? → std::move.
   • Copy? → Copy Constructor (or clone() pattern if polymorphic).

3. Is this a View?

   • Yes? → std::string_view, std::span, or const T&.
   • Never pass std::shared_ptr unless transferring shared ownership.

Trace Up / Down

• Trace Up (Diagnosing):

  • Error: "Segfault / Heap Corruption"
  • Ask: Is there a raw pointer (T*) managing a resource?
  • Fix: Wrap in unique_ptr or vector.

• Trace Down (Implementing):

  • Intent: "I want to pass this big object to a function and never see it again."
  • Code: void consume(BigThing t); called with consume(std::move(thing));

Quick Reference

| Pattern | Cost | Use When | | ----------------- | -------------- | -------------------------------------- | | Value (Stack) | Zero | Default choice. Small/Medium objects. | | unique_ptr | Zero | Heap needed. Exclusive ownership. | | shared_ptr | Atomic Inc/Dec | Cyclic graph or true shared ownership. | | T& (Ref) | Zero | Parameter passing (non-null). | | T* (Ptr) | Zero | Parameter passing (nullable). | | std::move | Zero | Transferring ownership. |