Coding Guidance Qt

Qt Coding Guidance

This skill adds portable Qt implementation, refactoring, and review guidance for Qt C++ code with a QWidget desktop focus. Widget, dialog, model/view, Designer, and desktop-UX guidance here is specific to Widgets work; broader QObject, threading, and build guidance also applies to non-UI Qt C++ code.

Adjacent Skills

This skill provides portable Qt engineering principles. Compose with:

• Workflow: thinking (ambiguous decision framing), recursive-thinking (stress-testing), security (threat modeling)
• Domain overlays: ui-guidance (ordinary graphical UI work), ui-design-guidance (stronger design and UX work), project-core-dev (repo-specific completion discovery and reporting when needed), project-platform-diagnose (environment-sensitive diagnosis)

Use this as the default principle skill for Qt code. Reach for coding-guidance-cpp only when the task is mostly non-Qt C++ or needs deeper general C++ design judgment than Qt-specific guidance.

Quick Routing

Open bundled references only when the task actually needs them:

• references/qt-build-compatibility.md for CMake, generated-code, or Qt5/Qt6 compatibility work
• references/qt-debugging-checklist.md for diagnosis-heavy tasks
• references/qt-designer-ui-files.md for .ui files, Qt Widgets Designer, uic, or generated ui_*.h code
• references/qt-layouts-and-desktop-ux.md for layout, dialog, panel, and desktop-UX work
• references/qt-model-view-checklist.md for QAbstractItemModel, roles, resets, selections, and model/view contract work
• references/qt-review-evidence.md for Qt-specific review hotspots and the evidence needed to support findings

Stay in the main skill when the task is broad Qt implementation or review and no single failure mode dominates yet.

When Not to Lean on This Skill

• non-Qt work
• pure backend or library code written in C++ that does not use Qt types, eventing, object lifetime, or Qt build tooling
• Qt Quick or QML-first work; use only the QObject, threading, and build guidance here when it genuinely applies, and rely on a QML/Qt Quick-specific skill or repo guidance for declarative UI architecture, bindings, and scene graph concerns
• design-only work where no Qt API, object model, build, or code judgment is needed

Implementation Workflow

1. Read the touched widgets, models, controllers, dialogs, .ui forms, tests, and build files before editing.
2. Infer intended behavior from current signals and slots, ownership, thread affinity, layouts, and tests when the request is partially specified. Ask only when multiple plausible Qt designs would change semantics.
3. Choose the narrowest change that keeps QObject ownership, event ordering, layout behavior, model contracts, and UI-thread rules explicit.
4. Implement with one clear application bootstrap, small widget responsibilities, modern typed connect usage, minimal GUI-thread work, and narrow seams between widgets and domain logic.
5. Add meaningful deterministic checks for the changed behavior using the repo's existing test framework. Qt Test and QSignalSpy fit Qt signal and model contracts; preserve the existing framework for plain C++ logic. Do not add or migrate a framework merely to follow a portable preference.
6. Run the narrowest relevant formatter, build, test, and affected-platform smoke path the repo supports. If the change touches Qt5/Qt6 compatibility, validate each supported build variant.

Refactoring Workflow

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

1. Capture current ownership, signal flow, event-loop assumptions, thread affinity, layout behavior, and model or binding contracts.
2. Break the refactor into small slices that preserve visible behavior.
3. Remove long widget constructors, tangled signal chains, view-model leakage, duplicated object wiring, and layout hacks one step at a time.
4. Keep tests or smoke paths passing after each slice; add characterization coverage first when behavior is unclear.
5. Stop when the code is simpler, safer around lifetime and threading, and easier to reason about.

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 ownership and deletion bugs, cross-thread QObject misuse, connection lifetime mistakes, GUI-thread blocking, model/view contract violations, layout breakage, Qt5/Qt6 build regressions, resource-path and platform assumptions, and missing tests.
4. State findings with concrete evidence and the likely consequence.
5. For Qt-specific claims, name the proof: sender/receiver types and lifetimes for signal issues, thread affinity for cross-thread issues, begin/end and reset boundaries for model issues, and the visible interaction path for layout or .ui regressions.

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

Qt Rules

First tier - causes bugs

• These rules apply to Qt C++ broadly; Widget-specific rules appear in later sections.
• Treat QObject ownership, parent-child lifetime, and thread affinity as core contracts, not cleanup details
• Every Widgets application gets exactly one QApplication; create it before any widget, QPixmap, QIcon, or other GUI object
• Do not copy QObject subclasses; keep lifetime and ownership explicit
• Use deleteLater() when direct deletion could occur during event handling, from the wrong thread, or before queued work is safely drained. Direct destruction is appropriate when ownership, thread affinity, and event-loop state make it safe. Verify that the owning event loop will actually process a deferred deletion.
• Do not touch GUI objects from non-GUI threads
• Bind connection lifetime to a context object when possible so queued work does not outlive the receiver
• Prefer the typed connect syntax over string-based SIGNAL and SLOT macros unless the repo is constrained by old APIs
• Do not block the GUI thread with filesystem, network, database, or heavy CPU work
• When model data changes, emit the correct notifications and bracket structural changes with the correct begin/end calls
• Do not hand-place widgets with setGeometry() when a real layout should own sizing and positioning

Second tier - prevents mistakes

• Keep widgets, models, delegates, dialogs, and business logic separated enough to test behavior without full UI setup
• Prefer explicit ownership and small helper objects over hidden globals or wide controller classes
• Keep slot bodies short; move non-trivial work into named helpers, presenters, or model logic
• Prefer Qt containers, strings, and utilities where the repo already uses them; do not introduce needless conversions at every boundary
• Follow the repo's Qt version and idioms before introducing newer Qt APIs
• Introduce no new compiler, clazy, or Qt-specific warnings; keep existing debt outside the change separate from regressions

Ownership, eventing, and threads

• Parent a QObject only when the parent truly owns it for the same lifetime. A stack child is valid when its lifetime ends before its parent, as when the parent is constructed first in the same scope. Reject destruction orders that let a parent delete a still-live stack child. Do not add a second owning smart pointer to an object already owned by a QObject parent.
• Use QPointer or a clearer ownership seam when a non-owning QObject reference can be invalidated by queued work or callbacks.
• Tie lambda connections to a context object unless every capture demonstrably outlives the connection.
• Treat signals as contracts. Keep connection types, reentrancy, thread hops, recursive updates, and signal-chain length explicit when they affect behavior.
• Do not create children for a parent in another thread or move an object while its ownership, children, or event handling make the move invalid.
• Own cancellation and teardown for timers, workers, asynchronous replies, and queued callbacks, including application quit and window-close paths.

UI, model, build, and test boundaries

• Keep domain rules in plain C++ where practical, translate through Qt-facing models or adapters, and keep widgets focused on presentation and local interaction.
• Treat item-model indexes, roles, begin/end notifications, resets, selections, and persistent indexes as formal compatibility contracts. Load the model/view reference for contract-heavy work.
• Let layouts own geometry and keep accessibility, keyboard flow, size policy, margins, and platform-native widget semantics deliberate. Load the layout reference for detailed desktop-UX work.
• Treat moc, uic, qrc, .ui files, resources, translations, and claimed Qt5 or Qt6 variants as source and build contracts. Never edit generated ui_*.h files; load the Designer and build references when those seams change.
• Test visible behavior, model transitions, emitted signals, and lifecycle edges. Prefer deterministic Qt Test or offscreen checks, and use the debugging reference plus sanitizers for diagnosis rather than guessing.

The Quick Routing references contain the detailed model/view, layout, Designer, build, review, and diagnosis checklists. Load only the reference matching the dominant failure mode.

Decision Heuristics

Use these when the right choice is not obvious:

• Lifetime pressure: if it is not obvious who owns a QObject or when it can die, redesign the boundary before adding more behavior.
• Thread-affinity pressure: if a callback, timer, or reply may arrive on a different thread, make that hop explicit and local.
• Model-contract pressure: if a change touches model indexes, role names, or reset behavior, treat it as a compatibility boundary rather than a local cleanup.
• Layout pressure: if you are about to use manual geometry, spacer hacks, or magic sizes, stop and check whether the right layout type, size policy, or stretch factor would solve it cleanly.
• Build-compatibility pressure: if a repo claims both Qt5 and Qt6 support, do not hard-code one target family or test only one build.
• Architecture pressure: if widget code starts owning business rules, persistence, and orchestration together, introduce a presenter/controller seam before adding more slots.
• UI responsiveness: if the feature can stall input, painting, or startup, rethink the design before polishing the code.
• Repo conventions: if the repo has established patterns for ownership, signals and slots, widgets vs. QML, testing, or Qt version support, follow them unless they create a correctness problem.
• Narrowness vs. quality: implement the narrowest change that solves the problem. When narrowness conflicts with correctness or lifecycle 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 lifecycle safety; report them separately.
• Abstraction threshold: three similar widget handlers, signal-wiring paths, model-shaping steps, or dialog flows is a pattern; before extracting, check whether a small helper, adapter, presenter, or model object is the simpler move.
• Qt vs. plain C++: if logic does not need the event loop, QObject identity, or Qt containers, keep it as plain testable C++ instead of forcing Qt into the core domain.

Validation

For implementation, a change is done when:

• the code builds without new compiler, moc, or Qt-specific warnings
• existing tests pass
• new or changed behavior has test coverage, or the lack of coverage is called out with a concrete reason
• changed signal, layout, model, or thread-hop behavior has a deterministic validation path
• UI-facing changes have at least a narrow smoke path on the affected platform
• Qt5/Qt6 compatibility changes were validated on each claimed build variant
• resource paths, generated-code inputs, and translation-sensitive changes were verified when touched

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