C++ Coding Guidance

This skill adds portable C++ implementation, refactoring, and review guidance.

Adjacent Skills

This skill provides portable C++ engineering principles. Compose with:

• Workflow: thinking (ambiguous decision framing), recursive-thinking (stress-testing), security (threat modeling)
• Domain overlays: backend-guidance (server-side code), backend-systems-guidance (stronger backend architecture, reliability, and trust-boundary work), ui-guidance (graphical UI/web frontend), project-core-dev (repo-specific completion discovery and reporting when needed)
• Qt specialization: coding-guidance-qt when Qt object lifetime, eventing, widgets, models, thread affinity, or generated build steps dominate

When Not to Lean on This Skill

• non-C++ work
• legacy or bare-metal environments where modern C++ guidance must be adapted selectively
• pure architecture or process work with no C++ design or code judgment needed
• repo-specific style packs or platform policies that should be enforced by local clang-tidy, compiler, or overlay rules rather than a portable principle skill

Boundary Contract

Keep this skill focused on portable C++ engineering judgment.

• Put repo-specific exception policy, warning policy, formatter choices, and include-order rules in repo config or repo docs
• Put style-pack rules from ecosystems such as Google, LLVM, Abseil, or platform/vendor bundles in repo config or overlays, not here
• Put library- or platform-specific API policy in repo config or a domain overlay
• When a rule is analyzer-shaped but not portable, keep it in clang-tidy config or the reference note rather than adding it to the main skill

Implementation Workflow

1. Read the touched code, build shape, existing tests, and any nearby docs or shorthand notes before editing.
2. If the request is partially specified, infer the intended behavior from the existing code and tests. Ask only when multiple plausible C++ designs would change semantics.
3. Choose the narrowest change that solves the problem without hiding ownership, lifetime, or error-handling contracts.
4. Implement with simple, strongly typed interfaces and modern C++ defaults.
5. Add or update tests close to the changed behavior.
6. Run the narrowest relevant format, build, test, sanitizer, and analyzer targets the repo supports.

Refactoring Workflow

Use this instead of the default implementation workflow when the task is primarily cleanup or restructuring:

1. Capture current behavior, invariants, side effects, and risky hotspots.
2. Break the refactor into small slices that preserve behavior.
3. Remove duplication, long functions, or muddled responsibilities one step at a time.
4. Keep tests passing after each slice; add characterization coverage first when behavior is unclear.
5. Stop when the code is simpler and safer.

Review Workflow

When reviewing (not implementing), skip the implementation workflow and use this instead:

1. Read the change in full before commenting.
2. Identify findings, ordered by severity: Critical > Important > Suggestion.
3. Prioritize bugs and regressions, ownership and lifetime errors, exception or error-path holes, thread-safety issues, security risks, performance mistakes with real impact, and missing tests.
4. State findings with concrete evidence and the likely consequence.

Do not edit code or require findings to be fixed unless the user also asks for remediation.

C++ Rules

Keep the critical portable defaults in view:

• Make ownership explicit. Prefer values and RAII, use std::unique_ptr for sole ownership, and treat raw pointers and references as non-owning unless an explicit legacy or C API transfers ownership; adopt such results into the matching RAII owner promptly.
• Establish invariants during construction and prefer rule-of-zero types.
• Do not let references, views, iterators, pointers, or other borrowed state outlive their owners, especially across callbacks, coroutines, or threads.
• Prevent unchecked bounds access, silent narrowing, signed/unsigned mistakes, and representation operations that violate aliasing or lifetime rules.
• Keep error handling consistent within a path, preserve must-check failures, and use [[nodiscard]] when ignoring a result is likely a bug.
• Encode ownership, units, nullability, and easily confused argument semantics in types or named parameter objects when call sites would otherwise be ambiguous.
• Keep headers free of namespace pollution, hidden global initialization, fragile macros, and accidental transitive-include dependencies.
• Prefer structured thread ownership, explicit cancellation, and test seams over detached work or ambient process state.
• Use modern vocabulary types and library features only when the repository's C++ target supports them and they make the contract clearer.

Read references/cpp-core-rules.md when the task needs the detailed ownership, type-safety, API, header, concurrency, or modern C++ checklist.

Advanced design judgment

Load references/cpp-advanced-design-judgment.md when the task involves public APIs, error-model choices, advanced language features, template-heavy interfaces, headers with broad rebuild impact, coroutines, synchronization strategy, ABI/plugin/C interop boundaries, or abstraction design. Keep ordinary feature work in the default rules above.

Resource map

• references/cpp-core-rules.md: detailed portable rules for ownership, bounds and representation safety, API clarity, headers, concurrency, and modern vocabulary types
• references/clang-tidy-derived-guidance.md: read when analyzer findings, enabled check families, or portable versus repo-specific static-analysis policy is part of the task
• references/cpp-advanced-design-judgment.md: deeper guidance for public API design, error models, advanced features, headers, synchronization, and abstraction choices

Decision Heuristics

Use these when the right choice is not obvious:

• Scope check: if a change crosses several public interfaces or compatibility boundaries, stop and plan the contract changes before continuing.
• Ownership clarity: if ownership is not obvious from the type signature, redesign the interface or add a one-line contract comment.
• Error-model consistency: do not mix exceptions, error codes, and expected-style returns within one subsystem unless the boundary is explicit.
• Exception-safety pressure: when mutating multi-step state, decide whether the operation offers no-fail, strong, or basic exception safety and structure the code to match.
• Repo conventions: if the repo has established rules for exceptions, containers, ownership types, or naming, follow them unless they create a correctness or safety problem.
• Feature pressure: do not introduce concepts, ranges, coroutines, or metaprogramming unless they make the code simpler for this repo's likely maintainers.
• Interface pressure: if a header starts dragging in broad dependencies or exposing implementation detail, narrow the interface before adding more code.
• Build-surface pressure: if a design pushes more logic, templates, or dependencies into public headers, justify the compile-time and rebuild cost.
• Parameter pressure: when adjacent parameters have the same type or call sites cannot communicate argument meaning clearly, prefer named types or a cohesive parameter object.
• Lifetime pressure: if a non-owning type crosses async, callback, return, or storage boundaries, prefer an owning type unless the lifetime proof is obvious from the interface.
• Initialization pressure: if correct behavior depends on a later “remember to initialize” step, move that requirement into construction or the type itself.
• Call-site pressure: if two arguments are easy to swap or a call needs comments to explain literals, redesign the API before adding more call sites.
• Header pressure: if a header starts accumulating definitions, globals, unnecessary includes, or hidden initialization, push behavior back behind a source boundary.
• Testability pressure: if a design forces tests to spin threads, sleep, touch the real filesystem, or patch globals just to exercise core logic, introduce a seam before adding more behavior.
• Test setup pressure: extract a fixture when repeated setup or incidental detail obscures the behavior under test; keep one-off setup local when it is clearer there.
• Narrowness vs. quality: implement the narrowest change that solves the problem. When narrowness conflicts with correctness or safety, prefer correctness. When it conflicts with style alone, prefer narrowness unless the task is explicitly a cleanup.
• Adjacent issues: do not modify unrelated issues unless they are required for the requested change's correctness or safety; report them separately.
• Abstraction threshold: three similar code blocks or repeated API-shaping pain is a pattern; before extracting, check whether a free function, helper type, or composed object is the simpler move.
• Performance rule: optimize only after measurement, except for obvious ownership, allocation, or algorithmic mistakes on hot paths.
• UB-sensitive optimization: treat optimizations that rely on subtle lifetime, aliasing, or memory-order assumptions as high-risk until proven by evidence and tooling.

Validation

For implementation, a change is done when:

• the code compiles without new warnings, unless the repo explicitly treats a known warning set as baseline debt outside the change
• existing tests pass
• new or changed behavior has test coverage, or the lack of coverage is called out with a concrete reason
• the repo's formatter has been run
• configured static analyzers report no new findings
• available sanitizers are clean for the touched paths when the change affects memory safety, threading, or undefined-behavior risk
• performance-sensitive changes are measured instead of justified by intuition

For review, completion means Critical and Important findings are reported with concrete evidence, likely consequence, and any validation gap. Unfixed findings do not make the review incomplete.