Qt 5 to Qt 6 Migration Guide for 2026

Qt 6 is a major version release of the Qt framework, introducing significant breaking changes that require careful planning and execution when migrating from Qt 5. This guide outlines the key differences and provides a roadmap for a successful migration in 2026.

The primary differences between Qt 5 and Qt 6 include:

1. Build System: Qt 6 adopts CMake as the primary build system, replacing the older qmake system. While qmake still exists, the investment and documentation are now focused on CMake.

2. Modularization: Qt has been restructured, with some functionality moved out of core modules into separate ones. Code that relied on implicitly available classes may now need explicit module dependencies. Some pieces have been relocated to the Qt 5 Compatibility module.

3. Removed and Changed APIs: Deprecated Qt 5 APIs have been removed, and some classes, functions, and enums have been replaced or had their signatures changed. Common areas of change include containers, string handling, QRegExp (replaced by QRegularExpression), enums, and various parts of the graphics and multimedia stacks.

4. QML Improvements: The QML engine and associated tooling have received significant improvements in Qt 6. QML-heavy applications may need some adjustments to take advantage of these enhancements.

For a successful migration, consider the following steps:

1. Update to the latest Qt 5 release: Before migrating, get onto the most recent Qt 5 release and address all deprecation warnings. Qt provides compatibility helpers to help clear most deprecations before making the major version switch.

2. Convert the build system to CMake: If your project currently uses qmake, plan and execute the CMake conversion as a dedicated workstream. This is often the most significant portion of the migration effort.

3. Update module dependencies: Add explicit dependencies for any modules that have moved to new locations. For components that cannot be immediately replaced, consider using the Qt 5 Compatibility module to maintain functionality while you work toward full Qt 6 equivalents.

4. Fix removed and changed APIs: Address compiler errors related to removed APIs by replacing them with their Qt 6 equivalents. Leverage your existing test suite to help identify and resolve issues.

5. Migrate incrementally: Rather than attempting a complete switch in one go, tackle modules or components incrementally. This approach reduces risk and makes it easier to catch and address issues as they arise.

Keep in mind the following pitfalls to avoid:

- Underestimating the build-system conversion effort. The move to CMake can be more extensive than expected, so allocate sufficient time and resources.

- Ignoring the impact of third-party dependencies. Ensure any external libraries your project relies on are Qt 6-ready, or plan for potential blockers during the migration.

- Forgetting to clear deprecations in the final Qt 5 release before switching entirely. Skipping this step can make the migration process much more challenging.

In summary, migrating from Qt 5 to Qt 6 is a major version upgrade that involves more than just a recompile. By carefully addressing the changes in the build system, module structure, APIs, and QML, and by planning a gradual migration process, you can successfully transition to the more modern and actively supported Qt 6 framework.