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Mastering C++ Error Handling. Triggers: exceptions, try-catch, noexcept, std::expected, std::optional, error codes, assert, terminate.

SKILL.md

C++ Error Handling

Core Question

Is this error recoverable?

Yes (Local): std::expected or return code.
Yes (Distant): Exceptions (throw).
No (Bug): assert or std::terminate.

Error → Design Question

Issue	Design Question
Uncaught Exception	Did you forget to catch, or throw in `noexcept`?
Silent Failure	Did you ignore a return code? (Use `[[nodiscard]]`).
Destructor Throw	Never throw from destructor (`std::terminate`).

Thinking Prompt

Is absence valid?
- Yes? → std::optional<T>.
Does caller need details?
- Yes? → std::expected<T, E> or Exception.
- No? → bool or std::optional.
Is it a Logic Error (Bug)?
- Yes? → assert() or std::terminate(). Do not throw for bugs in C++ (Contract Violation).

Trace Up / Down

Trace Up:
- Issue: "Program aborted with 'terminate called recursively'."
- Cause: An exception was thrown while stack unwinding (in a destructor).
- Fix: Fix the destructor. Ensure RAII cleanup is noexcept.

Quick Reference

Mechanism	Cost (Happy)	Cost (Sad)	Use When
`std::optional`	Branch	Branch	Return may be empty.
`std::expected`	Branch	Branch	Recoverable error (Parsing).
Exception	Zero	Huge	Rare IO/Resource errors.
Assert	Zero (Release)	Abort	Logic bugs / Invariants.