m02-resource

name	m02-resource
description	Mastering C++ Smart Pointers: unique_ptr, shared_ptr, weak_ptr. Triggers: memory management, cycles, enable_shared_from_this, make_unique, make_shared, custom deleters.

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_ptrs from one raw pointer?
Performance	Are you copying shared_ptr unnecessarily? (Atomic ops).

Thinking Prompt

1. 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).

2. Is this a Cycle?

   • Parent -> Child (shared_ptr).
   • Child -> Parent (weak_ptr).
   • If Child holds shared_ptr to Parent, they never die.

3. 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.