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Mastering C++ Smart Pointers: unique_ptr, shared_ptr, weak_ptr. Triggers: memory management, cycles, enable_shared_from_this, make_unique, make_shared, custom deleters.

SKILL.md

C++ Smart Pointers (Resource Management)

Core Question

How many owners does this resource to have?

One: std::unique_ptr (90% of cases).
Many: std::shared_ptr.
Observer: std::weak_ptr.

Error → Design Question

Issue	Design Question
Memory Leak (Cycles)	Do you have `shared_ptr` pointing to each other? Cycle = Leak.
Dangling Pointer	Did you store a `weak_ptr` or raw pointer but check it too late?
Double Free	Did you make two `unique_ptr`s from one raw pointer?
Performance	Are you copying `shared_ptr` unnecessarily? (Atomic ops).

Thinking Prompt

Can I use unique_ptr?
- Always start here. It has zero overhead (size = raw pointer).
- It forces you to think about ownership transfer (std::move).
Is this a Cycle?
- Parent -> Child (shared_ptr).
- Child -> Parent (weak_ptr).
- If Child holds shared_ptr to Parent, they never die.
Do I need a custom deleter?
- Managing C-APIs (FILE*, SDL_Surface*)?
- unique_ptr<FILE, DeclType(&fclose)> handles this perfectly.

Trace Up / Down

Trace Up:
- Issue: "Application memory usage grows endlessly but no 'leaks' reported by Valgrind."
- Cause: shared_ptr cycle. The reachable memory is technically "owned", just mutually.
- Fix: Break the cycle with weak_ptr.
Trace Down:
- Intent: "I need a list of heterogeneous objects."
- Code: std::vector<std::unique_ptr<Base>>

Quick Reference

Pattern	Cost	Use When
`make_unique`	Zero	Creating new heap objects.
`make_shared`	1 Alloc	Creating access-controlled shared objects.
`weak_ptr`	Control Block	Breaking cycles, Caching.
`enable_shared_from_this`	Zero	Need `shared_from_this()` inside member function.