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003-qt6-migration.md

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ADR-003: Migrate from Qt5 to Qt6

Status

Accepted (implemented in Phase 3)

Context

The original NIDS used Qt5 for its GUI. Qt5 reached end-of-life for open-source users and is no longer receiving feature updates. Qt6 is the current long-term supported version with improved performance, better HiDPI support, and modernized APIs.

Issues with staying on Qt5:

  • No new features or security patches after Qt 5.15 LTS
  • Ubuntu 24.04 ships Qt6 as default; Qt5 packages becoming harder to find
  • Cannot use Qt6-only features (e.g., improved property system, better CMake integration)
  • vcpkg is transitioning to Qt6 as the primary Qt version

Decision

Migrate from Qt5 to Qt6 with versionless CMake targets.

CMake Changes

# Before (Qt5)
find_package(Qt5 COMPONENTS Core Gui Widgets REQUIRED)
target_link_libraries(NIDS PRIVATE Qt5::Widgets Qt5::Core Qt5::Gui)

# After (Qt6, versionless targets)
find_package(Qt6 COMPONENTS Core Gui Widgets REQUIRED)
target_link_libraries(NIDS PRIVATE Qt::Widgets Qt::Core Qt::Gui)

Source Changes

  • Removed QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling) — this is the default behavior in Qt6.
  • Verified all signal/slot connections use the new function pointer syntax (no string-based SIGNAL()/SLOT() macros).
  • No API-breaking changes found in the widgets used by NIDS.

Consequences

Positive

  • Long-term support: Qt6 LTS releases will continue to receive updates.
  • Better HiDPI: Qt6 handles high-DPI displays natively without application-level opt-in.
  • Improved CMake: Versionless targets (Qt::Core) make future major version transitions smoother.
  • Consistent with CI and Docker environments (Ubuntu 24.04 ships qt6-base-dev).

Negative

  • Minimum system requirement now Ubuntu 22.04+ (Qt6 not available on older distros).
  • Some third-party Qt5 widgets or plugins may not be compatible.
  • Qt6 requires CMake 3.16+ (we already require 3.20).

Simultaneously: C++20 Upgrade

The Qt6 migration was bundled with the C++17 -> C++20 upgrade:

  • CMAKE_CXX_STANDARD set to 20
  • Enabled: std::span, std::ranges, [[likely]]/[[unlikely]], concepts, std::jthread, consteval, designated initializers
  • Updated .clang-tidy with C++20 modernize checks