Every developer knows this feeling: a project has lived on Qt5 for years, builds are getting more complex, warnings pile up, and the thought of Qt6 is postponed “for later.” Have you ever felt that migration looks scarier than the actual problems?
This chapter carefully removes that fear. In it, you’ll discover that transitioning to Qt6 is not a chaotic jump into the unknown, but a controlled process with clear logic. Here we’ll reveal why professional teams are already starting migration today, and you’ll learn the secret of how to turn a framework update into a reason to improve architecture, speed up rendering, and reduce technical debt.
The chapter mentions mandatory C++17, replacement of deprecated containers, new signal and slot syntax, and a step-by-step 4-step plan that allows maintaining a working build even with hybrid Qt5 and Qt6 support.
Skipping this chapter is a risk of staying in the past. Reading it provides clarity, structure, and confidence in the next technical step.
This chapter includes ready-to-use code examples.
Chapter Self-Check
What is RHI in Qt6 and what key problem does it solve for developers?Answer
Correct answer: RHI (Rendering Hardware Interface) is an abstraction over graphics APIs that provides a unified interface for working with Vulkan, Metal, Direct3D, and OpenGL, freeing developers from the need to write platform-dependent graphics code.
Why does Qt6 require mandatory use of the new signal and slot syntax instead of SIGNAL/SLOT macros?Answer
Correct answer: The new syntax provides compile-time type checking and prevents runtime errors, making the code safer and more reliable.
Why was QVector merged with QList in Qt6 instead of keeping both classes?Answer
Correct answer: QList in Qt6 was optimized and now provides performance comparable to QVector from Qt5, which eliminated the need for two separate classes for similar tasks.
How do you check in code whether an application is being compiled under Qt5 or Qt6, and why might this be needed?Answer
Correct answer: Use the preprocessor check
#if QT_VERSION_MAJOR >= 6. This is necessary during the hybrid support period for conditional compilation of different code sections for different Qt versions.Why do mouse coordinates in Qt6 events use qreal type instead of int?Answer
Correct answer: Floating-point numbers provide more precise positioning on high-resolution screens, which is critical for modern displays with high pixel density.
What will happen if you try to use old syntax SIGNAL(clicked()) and SLOT(handleClick()) in Qt6?Answer
Correct answer: The code won’t compile, as the old SIGNAL/SLOT macro syntax is no longer supported in Qt6. You must use the new syntax with function pointers.
What specific code changes are needed to get screen information when migrating from QDesktopWidget to Qt6?Answer
Correct answer: Instead of QDesktopWidget, use QScreen via
QGuiApplication::primaryScreen(), which provides richer capabilities for working with multiple monitors.Your project actively uses QRegExp. What migration strategy should you choose for Qt6?Answer
Correct answer: Replace QRegExp with QRegularExpression for a long-term solution, or temporarily use the Qt5Compat module with a plan to gradually transition to QRegularExpression.
Why were Q_WS_* family macros completely removed and replaced with Q_OS_*?Answer
Correct answer: Q_WS_* macros were tied to the window system, which became outdated with Qt’s evolution. Q_OS_* more accurately reflect the target operating system, which aligns with Qt6’s modern architecture.
What is the Qt5Compat module for and why can’t you rely on it long-term?Answer
Correct answer: Qt5Compat is a temporary solution to ease migration, providing deprecated APIs. You can’t rely on it long-term because the module may be removed in future Qt versions.
Why was the QtMultimedia module completely rewritten in Qt6 instead of gradual updates?Answer
Correct answer: The new architecture separates the concepts of media source and output devices, providing greater flexibility and better matching modern multimedia application requirements.
Which stage should you start the Qt6 migration process with and why is this critical?Answer
Correct answer: With preparation: updating development tools, creating a separate branch for migration, and auditing code with deprecated API checks enabled. This minimizes risks and allows rollback if necessary.
Why did the override keyword become mandatory for virtual functions in Qt6?Answer
Correct answer: This aligns with modern C++ practices and helps the compiler detect errors early when a developer accidentally doesn’t override a function correctly.
Practical Exercises
Easy Level
Updating Signal and Slot Syntax
Take an existing Qt5 application with a button and several slots using old SIGNAL/SLOT syntax. Translate all signal connections to the new Qt6 syntax with function pointers. Make sure the application compiles and works correctly.
Hints: Replace
connect(button, SIGNAL(clicked()), this, SLOT(onClicked())) with connect(button, &QPushButton::clicked, this, &MyClass::onClicked). The new syntax checks types at compile time. Don’t forget to include the appropriate header files. Use QT_VERSION_MAJOR for conditional compilation if you plan to support both versions.
Medium Level
Regular Expressions and Containers Migration
Create an email validation application that originally uses QRegExp and QVector for storing results. Migrate the code to Qt6: replace QRegExp with QRegularExpression, QVector with QList. Add high-resolution screen support by using QScreen instead of QDesktopWidget to display the window in the center of the screen.
Hints: QRegularExpression uses PCRE2 syntax. To check for a match, use the
match() method. QList in Qt6 is optimized and doesn’t require algorithm changes. To center the window, get geometry via QGuiApplication::primaryScreen()->geometry(). Test validation on various email formats, including international domains.
Hard Level
Comprehensive Multimedia Application Migration
Migrate a full-featured Qt5 multimedia application (audio/video player) to Qt6. The application should include: video playback with display in QVideoWidget, audio output management via QAudioOutput, mouse position tracking on video using qreal coordinates, platform-dependent code (using Q_OS_* instead of Q_WS_*), and modern controls. Implement a step-by-step migration plan with the ability to build for both Qt versions via preprocessor directives.
Hints: Start by creating a migration branch. In Qt6, MediaPlayer requires explicit QAudioOutput creation. Use
setSource() instead of setMedia(). For handling mouse events, remember the transition to event->position() instead of event->x()/y(). Macros Q_OS_WIN, Q_OS_MACOS, Q_OS_LINUX replace old Q_WS_*. Enable the QT_DISABLE_DEPRECATED_UP_TO check to detect deprecated API. Use nullptr and override according to C++17. Test on different platforms and screen resolutions.
💬 Join the Migration Discussion!
Already started the transition to Qt6? Encountered unexpected difficulties when migrating QtMultimedia or graphics code?
Share your migration experience, discuss strategies for legacy projects, or help colleagues with advice—your experience is invaluable to the community!