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m02-resource

Skill14 starsChanged 9 months agoMITView on GitHub

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

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The file

We split this skill into the rules it gives an agent

2 rules. 0 are common, 1 is distinctive, and 1 is very specific to this project.

  • 0

    Common

    Most of the 7 edited versions we read keep these.

  • 1

    Distinctive

    This author's own choices. Worth a look.

  • 1

    Very specific

    Tied to their project's names, files or tools. Probably skip.

  • Common
  • Distinctive
  • Very specific
  1. Distinctive:

    Always start here. It has zero overhead (size = raw pointer).

    line 28
  2. Very specific:

    If Child holds shared_ptr to Parent, they never die.

    line 34

“Common” is measured: kept by at least 6 in 10 of the 7 edited versions of this file we could read. “Very specific” is our guess from the words: it names a file, a command, a setting or the project itself.

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Files
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

IssueDesign Question
Memory Leak (Cycles)Do you have shared_ptr pointing to each other? Cycle = Leak.
Dangling PointerDid you store a weak_ptr or raw pointer but check it too late?
Double FreeDid you make two unique_ptrs from one raw pointer?
PerformanceAre 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

PatternCostUse When
make_uniqueZeroCreating new heap objects.
make_shared1 AllocCreating access-controlled shared objects.
weak_ptrControl BlockBreaking cycles, Caching.
enable_shared_from_thisZeroNeed shared_from_this() inside member function.

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