Migrating the driver terminal of a sugar beet harvester from Qt 5 to Qt 6

Migrating a Harvester HMI from Qt 5.12 to Qt 6.2

September 2021 saw the launch of Qt 6.2 – the first Qt 6 version with feature parity to Qt 5.15. I wanted to find out whether Qt 6.2 is fit for products. So, I migrated the HMI application of the ROPA sugar beet harvesters from Qt 5.12 over Qt 5.15 to Qt 6.2. The migration went very smooth and quick. And yes, Qt 6.2 is fit for products!

Introduction

The migration to Qt 6 is my 5th Qt migration. The migrations from Qt 1 to Qt 2, from Qt 2 to Qt 3 and from Qt 4 to Qt 5 were fairly smooth. The migration of an IDE for formal verification and of the driver terminal of a forage harvester were a matter of 2-3 days. The migration from Qt 3 to Qt 4 was extremely painful. Migrating an IDE for Bluetooth development took me 6 months.

I was curious how long I would need to migrate the driver terminal of a sugar beet harvester from Qt 5.12 to Qt 6.2. The driver terminal has 350 source files (.cpp, .h, .qml) with 50K lines of hand-written code. It also has 45 source files with 74K lines of generated code for the terminal-machine communication.

The guide Porting to Qt 6 from the official Qt documentation is the starting point for the migration. It suggests to port the application to Qt 5.15 in a first step and to Qt 6.2 in a second step. The Qt developers made the APIs of Qt 5.15 as similar as possible to the APIs of Qt 6.2 to reduce the migration efforts.

I’ll describe step by step how to change the CMake files, how to fix the warnings and errors flagged by the C++ compiler, how to find the QML incompatibilities and how to set up the development environment for Qt 6.2.

Originally, I migrated the terminal application to Qt 6.0 in December 2020 – just after Qt 6.0 came out. Qt 6.0 and Qt 6.1 were not ready for product releases, because they lacked important modules like SerialBus, Multimedia, Charts, RemoteObjects and WebEngine. I had to to exclude some parts from the migration.

The Qt Project promised to fix these shortcomings with Qt 6.2 in autumn 2021 and they delivered. Excellent work! I decided to update my original post as well. The post now covers the migration from Qt 5.12 all the way to Qt 6.2. With Qt 6.2, I could include SerialBus and Multimedia. SerialBus with CAN worked right ouf the box. Multimedia required a straightforward rewrite for playing back MP3 files, because QMediaPlaylist was removed from Multimedia.

The migration of the harvester terminal from Qt 5.12 to Qt 6.2 took me two days. Writing this post took me longer. In October, I started to implement the driver terminal of a construction machine with Qt 6.2. I use Qt 6.2 for my FOSS compliance checker. Hence, I can recommend the use of Qt 6 in products!

Migrating from Qt 5.12 to Qt 5.15

The harvester application runs with Qt 5.12. We install Qt 5.15.2 and QtCreator 4.13 on our development PC. The PC should run Ubuntu 16.04 or newer.

C++ Compiler Warnings and Errors

The harvester application consists of three CMake projects: the executable Main, the library Hmi and the library Can. We switch on the deprecation warnings by adding the line

target_compile_definitions(${PROJECT_NAME} PUBLIC 
    "QT_DISABLE_DEPRECATED_BEFORE=0x050F00")

to the CMakeLists.txt of each project. This macro triggers a warning for every function that is deprecated in Qt 5.15 or older. Sometimes the use of a deprecated function causes an error. The next subsections list the C++ compiler warnings and errors I encountered while migrating the harvester application from Qt 5.12 to Qt 5.15.

The documentation page Obsolete Classes lists all the classes that may be removed in future releases and all the classes with functions that may be removed in future releases. The documentation often gives a hint how to replace an obsolete class or function.

Error/Warning: ‘endl’ is deprecated: Use Qt::endl

Problem:

QTextStream os(&dbFile);
os << QStringLiteral("[access]") << endl;

Most occurrences of endl triggered the warning: 'endl' is deprecated: Use Qt::endl. Some occurrences triggered an error: ‘endl’ was not declared in this scope. The compiler couldn’t distinguish between std::endl and Qt::endl.

Fix:

QTextStream os(&dbFile);
os << QStringLiteral("[access]") << Qt::endl;

We replace each occurrence of endl by Qt::endl. All stream manipulators are now prefixed with the Qt namespace: for example, Qt::hex, Qt::fixed, Qt::left and Qt::ws.

Error: ‘longMonthName’ is not a member of ‘QDate’

Problem:

return QDate::longMonthName(i, QDate::StandaloneFormat);

The static function QDate::longMonthName was removed from QDate.

Fix:

return QLocale().monthName(i);

The documentation of QDate::longMonthName hints at QLocale for a replacement. QLocale::monthName returns the long month name by default.

Error: no matching function for call to ‘QProcess::execute(const char [10])’

Problem:

QProcess::execute("/bin/sync");

The variant of QProcess::execute with the command as its only argument was removed.

Fix:

QProcess::execute("/bin/sync", {});

We must always use the two-argument variant, where the second argument is a QStringList with the options and arguments of the command. As sync doesn’t have any arguments or options, the second argument is the empty list.

Error: ‘mapped’ is not a member of ‘QSignalMapper’

Problem:

using MappedSignal = void(QSignalMapper::*)(int);
connect(&m_impl->m_signalMapper, 
        static_cast<MappedSignal>(&QSignalMapper::mapped),
        m_impl.data(),
        &DriverModel::Impl::onDriverChanged);

Before Qt 5.15, QSignalMapper::mapped had an overload for each of the four types of the single argument: QObject*, QWidget*, const QString& and int. We had to tell the compiler with a static_cast which overload to use in the connect statements. The overloads are obsolete in Qt 5.15.

Fix:

connect(&m_impl->m_signalMapper, &QSignalMapper::mappedInt,
        m_impl.data(), &DriverModel::Impl::onDriverChanged);

From Qt 5.15, the four overloads have different names mappedInt, mappedObject, mappedString and mappedWidget. This eliminates the cast and makes the code simpler.

Error: no member named ‘insertMulti’ in ‘QMap >’

Problem:

QMap<int, std::function<void()>> importCalls;
importCalls.insertMulti(0, [this, path](
    {m_customerModel.importCsvFile(path);});

Before Qt 5.15, QMap distinguished between maps and multi-maps by insert and insertMulti.

Fix:

QMultiMap<int, std::function<void()>> importCalls;
importCalls.insert(0, [this, path](
    {m_customerModel.importCsvFile(path);});

Qt 5.15 introduces a new class QMultiMap, which inherits from QMap. We insert elements into a QMultiMap with insert now.

Error/Warning: ‘QString::SplitBehavior’ has not been declared

Problem:

auto nameParts = customer->name()
    .split(' ', QString::SkipEmptyParts);

Like many other enum constants, QString::SkipEmptyParts was moved into the namespace Qt. The problem often occurs as a warning:

‘... QString::split ...’ is deprecated: Use Qt::SplitBehavior variant instead [-Wdeprecated-declarations]

Fix:

auto nameParts = customer->name()
    .split(' ', Qt::SkipEmptyParts);

The warning tells us what to do. We replace QString::SplitBehavior by Qt::SplitBehavior.

Error: call of overloaded ‘append()’ is ambiguous

Problem:

QVector<QPair<QString, QString>> m_counterCats;
m_counterCats.append({"Gesamt", ""});

QVector<T>::append has gained a third overload for rvalue references T&& in addition to the existing const T& and const QVector<T>&. The C++17 compiler cannot distinguish between these three overloads.

Fix:

QVector<QPair<QString, QString>> m_counterCats;
m_counterCats.append(QPair<QString, QString>{"Gesamt", ""});

We must help the C++17 compiler by spelling out the type of the appended element.

QML Runtime Errors

Unfortunately, we don’t have a friendly compiler telling us which lines of our QML code are deprecated. The documentation page Obsolete QML Types lists the known obsolete types, properties and methods. It’s worth going through these lists and checking our QML code for problems. I didn’t find any problems in the harvester QML code.

Before we test the most common usage scenarios (hopefully in an automated way), we update the versions in the import statements. I had to perform the following replacements in the QML files.

Qt 5.12                      ->  Qt 5.15
import QtQuick 2.10          ->  import QtQuick 2.15
import QtQuick.Controls 2.3  ->  import QtQuick.Controls 2.15
import QtMultimedia 5.8      ->  import QtMultimedia 5.15

In my case, testing the most common usage scenarios unearthed a single problem about signal handlers or slots in Connections types.

Warning: QML Connections: Implicitly defined onFoo properties in Connections are deprecated

Problem:

Connections {
    target: csvExportModel
    onMessageOccurred: messagePane.text = message
}

The signal handler onMessageOccurred is implicitly defined as a function.

Fix:

Connections {
    target: csvExportModel
    function onMessageOccurred(message)
    {
        messagePane.text = message
    }
}

We must define onMessageOccurred as a function explicitly with an argument list.

Migrating from Qt 5.15 to Qt 6.2

It’s best to install QtCreator 4.14 or newer, as these versions fully support Qt 6. QtCreator 4.14 adds proper syntax highlighting for Qt 6 code and some improvements for CMake. My development PC ran Ubuntu 18.04.

When I installed Qt 6 with the online installer, I encountered this error:

My Internet search lead me to QTBUG-89218. The comment section reveals that Ubuntu 18.04 is not a supported development platform any more. We have two options. We can upgrade to Ubuntu 20.04 or newer or we can build Qt 6.2 from sources. I decided to upgrade my Ubuntu version.

Building with CMake

Updating CMakeLists.txt Files

The CMakeLists.txt files refer to Qt 5 explicitly in find_package and target_link_libraries commands.

find_package(Qt5 COMPONENTS Core Gui Qml REQUIRED)
target_link_libraries(${PROJECT_NAME}
  Ag::Can Ag::Hmi Qt5::Core Qt5::Gui Qt5::Qml
)

We must change Qt 5 to Qt 6.

find_package(Qt6 COMPONENTS Core Gui Qml REQUIRED)
target_link_libraries(${PROJECT_NAME}
  Ag::Can Ag::Hmi Qt6::Core Qt6::Gui Qt6::Qml
)

Qt5 was built on C++11. The top-level CMakeLists.txt file contained the following lines.

set(CMAKE_CXX_STANDARD 11)
set(CMAKE_CXX_STANDARD_REQUIRED ON)

Qt6 requires C++17. We must change the top-level CMakeLists.txt file accordingly.

set(CMAKE_CXX_STANDARD 17)
set(CMAKE_CXX_STANDARD_REQUIRED ON)

Setting Up the QtCreator Kit

When I started QtCreator the first time after installing Qt 6, the auto-detected kit for Qt 6 was messed up. It referred to outdated C and C++ compilers from a Qt 5.14 installation and the system CMake 3.16 from Ubuntu 20.04. We can fix this mess in the QtCreator dialog Tools | Options | Kits.

  • We clone the auto-detected read-only kit for Qt 6.2.
  • We define /usr/bin/gcc and /usr/bin/g++ from Ubuntu 20.04 as the new C and C++ Compilers. Both compilers are version 9.3. GCC9 supports C++17. If we must work with different compiler versions, we set the Compilers to /usr/bin/gcc-9 and /usr/bin/g++-9.
  • We define the CMake coming with Qt6 – /path/to/Qt-6.2.0/Tools/CMake/bin/cmake (version 3.21) – as the CMake Tool of the kit. Qt 6 requires at least CMake 3.18.
  • The CMake generator is Ninja and the extra generator is <none>.

When we run CMake to configure the project, the CMake command should look similar to this:

/path/to/Qt6.2.0/Tools/CMake/bin/cmake 
    -S /private/Projects/Ag/terminal
    -B /tmp/QtCreator-UDwQep/qtc-cmake-sOofIBPC 
    -GNinja 
    -DCMAKE_BUILD_TYPE:String=Debug
    -DQT_QMAKE_EXECUTABLE:STRING=/path/to/Qt-6.2.0/6.2.0/gcc_64/bin/qmake
    -DCMAKE_PREFIX_PATH:STRING=/path/to/Qt/Qt-6.2.0/6.2.0/gcc_64
    -DCMAKE_C_COMPILER:STRING=/usr/bin/gcc-9
    -DCMAKE_CXX_COMPILER:STRING=/usr/bin/g++-9

We can force a re-configuration of the project by removing the build directory recursively and by running Build | Run CMake, or by pressing the button Re-configure with Initial Parameters on the page Projects | Build & Run | Desktop Qt 6.2.0 GCC 64bit | Build Settings.

Core5Compat Library

The Core5Compat library adds the obsolete Qt 5 classes and functions that didn’t make it into Qt 6. We can add Core5Compat to the find_package commands and Qt6::Core5Compat to the target_link_libraries commands. If we do, the compiler will flag occurrences of QRegExp, QTextCodec and some other classes from Qt6::Core as warnings instead of errors (see this post for a mapping between obsolete Qt 5 classes and their Qt 6 replacements).

Core5Compat makes the transition from Qt 5 to Qt 6 smoother. We must fix fewer errors before our application builds and runs again. I didn’t use Core5Compat, because I learned about it too late. I could fix the resulting errors in a mechanical way like replacing QRegExp functions by their counterparts in QRegularExpression.

C++ Compiler Errors and Warnings

Modules Missing in Qt 6.0 and 6.1 Now Available in Qt 6.2

Qt 5 modules like Multimedia, SerialBus, SerialPort, RemoteObjects, Virtual Keyboard, Charts, Bluetooth, WebEngine and WebView didn’t make it into Qt 6.0 (see this post for a complete list). A few became available in Qt 6.1 (April 2021). Qt 6.2, which was released in September 2021, contains all modules from Qt 5.15 and has long-term support.

As the harvester application depends on SerialBus and Multimedia, Qt 6.2 is the first version fit for product releases. With Qt 6.0 and 6.1, I commented out the parts using Multimedia and performed a very quick and very dirty migration of SerialBus. The harvester application uses QtCanBus from SerialBus.

New Qt Module StateMachine

The state machine framework with classes like QStateMachine, QState and QFinalState was moved from the module Core to the new module StateMachine. We add StateMachine to the find_package call and Qt6::StateMachine to the target_link_libraries call in a CMakeLists.txt file.

Error: no match for ‘operator<’ (operand types are ‘const QVariant’ and ‘const QVariant’)

Problem:

QMap<std::pair<QString, QVariant>, QObject *> m_entities;
m_impl->m_entities.insert(it.key(), it.value());

The function it.key() returns an object of type std::pair<QString, QVariant>, which is the same as the key type of the QMap. QMap requires the definition of operator< on the key type to know where to insert new values.

QMap compares whether the pair p1 is less than p2, where both p1 and p2 have the type std::pair<QString, QVariant>. If p1.first is equal to p2.first, the comparison must evaluate p1.second < p2.second. G++-9 rightly complains that operator< is not defined for QVariants. G++-7 overlooked the missing operator< for QVariants.

Fix:

QMap<std::pair<QString, QString>, QObject *> m_entities;
m_impl->m_entities.insert(it.key(), it.value());

Converting a QVariant into a QString with QVariant::toString() works for most types supported by QVariant. As m_entities only uses supported types for the second element of its key, we can safely replace QVariant by QString in the key type.

Error: QTextCodec: No such file or directory

Problem:

#include <QTextCodec>
...
QTextStream is{&csvFile};
is.setCodec(QTextCodec::codecForName("ISO-8859-1"));

The class QTextCodec was removed from Qt 6 and was replaced by the new class QStringConverter. Similarly, the classes QTextEncoder and QTextDecoder were replaced by QStringEncoder and QStringDecoder (see this post). Code using any of the old classes doesn’t compile.

Fix:

#include <QStringConverter>
...
QTextStream is{&csvFile};
is.setEncoding(QStringConverter::Latin1);

We include the header of the replacement class QStringConverter. A search through the functions of QStringConverter makes setEncoding the most likely replacement for setCodec. Understanding that ISO-8859-1 is the formal name for Latin1 helps us call setEncoding with the right constant QStringConverter::Latin1.

Error: QRegExp: No such file or directory

Problem:

#include <QRegExp>

// In anonymous namespace
QRegExp &rxHexFilename()
{
    static QRegExp rx("_(A\\d\\d)_V_(\\d\\d)_(\\d\\d)\\.hex$");
    return rx;
}

// In constructor
auto pos = rxHexFilename().indexIn(fileName);
auto ecuName = rxHexFilename().cap(1);
if (pos == -1 || ...) {
    return;
}
m_version = QString("v%1.%2")
    .arg(rxHexFilename().cap(2).toUInt())
    .arg(rxHexFilename().cap(3).toUInt());
m_ecuType = fileName.left(fileName.size() - 
    rxHexFilename().matchedLength());

Replacing QRegExp – a relic of Qt 4 – by QRegularExpression has been in the making since Qt 5.0. QRegularExpression is the only way to work with regular expressions in Qt 6. The interface has changed considerably.

Fix:

#include <QRegularExpression>

// In anonymous namespace
const QRegularExpression rxHexFilename("_(A\\d\\d)_V_(\\d\\d)_(\\d\\d)\\.hex$");

// In constructor
auto match = rxHexFilename.match(fileName);
auto ecuName = match.captured(1);
if (!match.hasMatch() || ...) {
    return;
}
m_version = QString("v%1.%2")
    .arg(match.captured(2).toUInt())
    .arg(match.captured(3).toUInt());
m_ecuType = fileName.left(fileName.size() - 
    match.capturedLength(0));

The function for matching a string against a regular expression has got its natural name: match (instead of indexIn). It returns a QRegularExpressionMatch object match instead of an index. We retrieve the nth captured substring by calling match.captured(nth). match.hasMatch() tells us whether the string matched the regular expression.

QRegExp::matchedLength returns the length of the substring matching the complete regular expression. This substring is the same as the 0th captured substring match.captured(0), which has the length match.capturedLength(0).

Error: ‘class QString’ has no member named ‘midRef’

Problem:

subDev = m_deviceName.midRef(splitPos + 1).toLatin1();

The function QString::midRef is obsolete in Qt 6. The same goes for the function QString::leftRef.

Fix:

subDev = m_deviceName.mid(splitPos + 1).toLatin1();

We replace QString::midRef and QString::leftRef by QString::mid and QString::left, respectively.

Error: cannot convert ‘QString’ to ‘const QFileInfo&’

Problem:

QString extractVersion(const QFileInfo &info) const;

QString fileName(...);
auto version = extractVersion(fileName);

In Qt 5, the function extractVersion implicitly converts the QString filename into a QFileInfo object with the constructor QFileInfo(const QString &). In Qt 6, this constructor is marked explicit, which blocks the implicit conversion.

Fix:

QString extractVersion(const QFileInfo &info) const;

QString fileName(...);
auto version = extractVersion(QFileInfo(fileName));

We get rid of the error by calling the constructor QFileInfo(const QString &) explicitly.

Error: invalid application of ‘sizeof’ to incomplete type ‘IntegerRangeModel’

Problem:

class IntegerRangeModel;

class DateTimeModel : public QObject
{
    Q_OBJECT
    Q_PROPERTY(IntegerRangeModel* days READ days CONSTANT)

The forward declaration of IntegerRangeModel was good enough for the Q_PROPERTY definition in Qt 5, because days is declared as a pointer. Qt 6 added a static assertion. A further error message documents the violation of the static assertion:

static_assert(sizeof(T), "Type argument of Q_PROPERTY or Q_DECLARE_METATYPE(T*) must be fully defined");

Fix:

#include "IntegerRangeModel.h";

class DateTimeModel : public QObject
{
    Q_OBJECT
    Q_PROPERTY(IntegerRangeModel* days READ days CONSTANT)

We eliminate the error message by including the header file for IntegerRangeModel instead of forward declaring IntegerRangeModel.

Warning: ‘Type’ is deprecated: Use QMetaType::Type instead. [-Wdeprecated-declarations]

Problem:

EntityColumn(const QString &name, QVariant::Type type, ...)

The enumeration QVariant::Type lists the types that can be stored in a QVariant. It is roughly a subset of the enumeration QMetaType::Type, which lists the types that are known to Qt’s meta object system. Replacing QVariant::Type by QMetaType::Type suggested itself. QVariant::Type is not contained in Qt 6 any more.

Fix:

EntityColumn(const QString &name, QMetaType::Type type, ...)

In Qt 6, we use QMetaType::Type instead of QVariant::Type. We must also replace obsolete enum constants like QVariant::Invalid, QVariant::UInt and QVariant::Bool by QMetaType::UnknownType, QMetaType::UInt and QMetaType::Bool, respectively.

Error: cannot convert ‘QVariant::Type’ to ‘QMetaType::Type’

Problem:

bool typesAreAffine(QMetaType::Type sqlType, ...) const;

QSqlField tableColumn = ...;
if (!typesAreAffine(tableColumn.type(), ...)) ...

Originally, the first argument of typesAreAffine had the type QVariant::Type, which was replaced by QMetaType::Type by the previous migration step. The function QSqlField::type with the return type QVariant::Type doesn’t exist in Qt 6.

Fix:

bool typesAreAffine(QMetaType::Type sqlType, ...) const;

QSqlField tableColumn = ...;
if (!typesAreAffine(QMetaType::Type(tableColumn.metaType().id()), ...)) ...

We substitute the Qt 6 function QSqlField::metaType() for the now obsolete Qt 5 function QSqlField::type(). QSqlField::metaType() returns a QMetaType object. We retrieve the type ID of the QMetaType object with id(), which returns an int instead of a QMetaType::Type to be open for custom meta types. QMetaType::Type(typeID) creates a enum constant from the integer typeID.

Error: warning: conversion from ‘qsizetype’ {aka ‘long long int’} to ‘int’ may change value [-Wconversion]

Problem:

QList<QFileInfo> m_dirEntries.

int DirectoryModel::rowCount(const QModelIndex &parent) const
{
    Q_UNUSED(parent)
    return m_impl->m_dirEntries.size();
}

Starting with Qt 6.0, QList<T>::size() has the return type qsizetype, which is equivalent to long long int. The class DirectoryModel indirectly derives from QAbstractItemModel, which declares the pure virtual function rowCount like this:

virtual int rowCount(const QModelIndex &parent = QModelIndex()) const = 0

The return statement may narrow a long long int value to an int value, which may change the value. It seems that the Qt developers forgot to change the return type of rowCount from int to qsizetype.

Functions like beginInsertRows, beginRemoveRows, beginMoveRows and beginInsertColumns also take int-type arguments. We may have to watch out for narrowing conversions as well.

Fix:

QList<QFileInfo> m_dirEntries.

int DirectoryModel::rowCount(const QModelIndex &parent) const
{
    Q_UNUSED(parent)
    return static_cast<int>(m_impl->m_dirEntries.size());
}

Sometimes there is no way around a static cast. This is one of those times. We force the qsizetype value into an int value and silence the error.

Warning: conversion from ‘size_t’ {aka ‘long unsigned int’} to ‘uint’ {aka ‘unsigned int’} may change value [-Wconversion]

Problem:

enum class UnitId : quint16 { ... }

inline uint qHash(UnitId unit)
{
    return qHash(static_cast<quint16>(unit));
}

qHash functions return uint values in Qt 5, where uint is unsigned int. In Qt 6, they return size_t values, where size_t is long unsigned int. The qHash function in the return statement is a built-in hash function. It returns a long unsigned int value, which may be narrowed to a an unsigned int value by the custom-defined hash function. As this may change the value, the compiler emits a warning.

Fix:

enum class UnitId : quint16 { ... }

inline size_t qHash(UnitId unit)
{
    return qHash(static_cast<quint16>(unit));
}

We change the return type of the custom-defined hash function from uint to size_t.

Warning: ‘static QQmlFileSelector* QQmlFileSelector::get(QQmlEngine*)’ is deprecated [-Wdeprecated-declarations]

Problem:

auto fs = QQmlFileSelector::get(engine);
fs->setExtraSelectors({"left"});

The static function QQmlFileSelector::get returns the file selector currently active on the QML engine. It is obsolete in Qt 6, because it duplicates the functionality provided by the QQmlFileSelector constructor.

Fix:

auto fs = new QQmlFileSelector(engine);
fs->setExtraSelectors({"left"});

Creating a QQmlFileSelector object on the heap does the same as calling QQmlFileSelector::get. We must create the object on the heap, because the constructor argument engine takes ownership of the file selector.

Error: using typedef-name ‘using QStringList = class QList<QString>’ after ‘class’

Problem:

class QStringList;

An included Qt header file introduces the type definition using QStringList = class QList<QString>. The problematic line forward declares this type definition, which the compiler doesn’t like.

Fix:

#include <QStringList>

We replace the forward declaration by the inclusion of the header file.

Error: ‘QLatin1Literal’ was not declared in this scope; did you mean ‘QStringLiteral’?

Problem:

QStringList errors;
Q_ASSERT_X(false, __PRETTY_FUNCTION__, 
    errors.join(QLatin1Literal("; ")).toUtf8());

QLatin1Literal doesn’t exist in Qt 6 any more.

Fix:

QStringList errors;
Q_ASSERT_X(false, __PRETTY_FUNCTION__, 
    errors.join(QLatin1String("; ")).toUtf8());

We replace QLatin1Literal by QLatin1String.

Error: No type name ‘State’ in QMediaPlayer

Problem:

void onStateChanged(QMediaPlayer::State state);

connect(&m_impl->m_player, &QMediaPlayer::stateChanged,
        m_impl, &Impl::onStateChanged);

if (m_player.state() != QMediaPlayer::PlayingState) { ... }

We replace QMediaPlayer::State by QMediaPlayer::PlaybackState and connect the slot onStateChanged with the signal QMediaPlayer::playbackStateChanged. We also need to replace calls to QMediaPlayer::state() by QMediaPlayer::playbackState().

Fix:

void onStateChanged(QMediaPlayer::PlaybackState state);

connect(&m_impl->m_player, &QMediaPlayer::playbackStateChanged,
        m_impl, &Impl::onStateChanged);

if (m_player.playbackState() != QMediaPlayer::PlayingState) { ... }

Error: ‘QMediaPlaylist’: No such file or directory

Problem:

#include <QMediaPlaylist>

The page Changes to Qt Multimedia informs us that the class QMediaPlaylist was removed from Qt 6. The application plays different MP3 files when info, warning or error dialogs are shown. It passes a QMediaPlaylist with the correct MP3 file to QMediaPlayer. Users can also change the playback volume.

Fix:

QMediaPlaylist is replaced by QMediaPlayer::setSource. The volume cannot be changed directly with QMediaPlayer::setVolume but is changed indirectly with QAudioOutput::setVolume. The following error messages about QMediaPlayer and QMediaPlaylist tell us what to fix.

Error: No member named ‘setVolume’ in ‘QMediaPlayer’

Problem:

m_player.setVolume(80);

Qt 6 moved the functions setVolume and volume from QMediaPlayer to QAudioOutput.

Fix:

The client class using QMediaPlayer must register a QAudioOutput object with the QMediaPlayer object m_player in its constructor. We must tell the QAudioOutput object on which loudspeaker it shall play the MP3 file. We can access the default loudspeaker with QMediaDevices::defaultAudioOutput. That’s good enough for the terminal hardware, which has only one loudspeaker.

The client class sets the volume to a default value (e.g., 0.8). Note that the new QAudioOutput::setVolume takes a floating-point value between 0.0 and 1.0 as an argument, whereas the old QMediaPlayer::setVolume took an integer between 0 and 100.

auto audioOut = new QAudioOutput{};
audioOut->setDevice(QMediaDevices::defaultAudioOutput());
audioOut->setVolume(0.8f);
m_player.setAudioOutput(audioOut);

The setVolume(int vol) function of the client class changes the volume like this:

m_player.audioOutput()->setVolume(0.8f);

Error: QMediaPlayer cannot play back a QMediaPlaylist any more

Problem:

Of course, the title error message is not a real compiler error. As QMediaPlaylist was removed from Qt 6, the following playback function of an error category does not compile any more.

void AudioService::Impl::playFeedback(int category)
{
    m_playlist.addMedia(m_audioFiles[category]);
    m_player.play();
}

m_audioFiles[category] returns the QRC URL of the MP3 file for the given error category. For example, it returns QUrl("qrc:/Can/Audio/ding.mp3") for the category 2.

Fix:

The modified function for playing an MP3 file for the given category looks as follows.

void AudioService::Impl::playFeedback(int category)
{
    m_player.setSource(m_audioFiles[category]);
    m_player.play();
}

QML Warnings, Errors and Improvements

Remove versions from import statements

Problem:

import QtQuick 2.15
import QtQuick.Controls 2.15
import Ag.Models 1.0

Qt 5 requires a version for the imported modules. Qt 6 doesn’t complain about these versions, but it doesn’t require them any more.

Fix:

import QtQuick
import QtQuick.Controls
import Ag.Models

Just importing a module without a version is enough in Qt 6. We remove the versions from all the import statements.

Warning: Parameter “page” is not declared. Injection of parameters into signal handlers is deprecated.

Problem:

// File: RDirectBar.qml
Pane 
{
    signal selectedPage(url page, bool fullscreen)

// File: main.qml
RDirectBar
{
    onSelectedPage: { ... }

The signal selectedPage is connected to the signal handler onSelectedPage in the instantiation of RDirectBar. Qt 6 accepts the old-style signal handlers with a warning.

Fix:

The new way is to use arrow functions or anonymous functions (see Signal Parameters in the Qt 6.2 documentation). We would call them lambda functions in C++.

// File: RDirectBar.qml
Pane 
{
    signal selectedPage(url page, bool fullscreen)

// File: main.qml
RDirectBar
{
    // Arrow function
    onSelectedPage: (page, fullscreen) => { ... }

    // Anonymous function (alternative)
    onSelectedPage: function (page, fullscreen) { ... }

Signal handlers without parameters need not be changed from Qt 5 to Qt 6.

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