Changes to Qt Core

Qt 6 is a result of the conscious effort to make the framework more efficient and easy to use.

We try to maintain binary and source compatibility for all the public APIs in each release. But some changes were inevitable in an effort to make Qt a better framework.

In this topic we summarize those changes in Qt Core, and provide guidance to handle them.

Container Classes

QHash, QMultiHash, QSet

qHash() Signature

For custom types, QHash and QMultiHash rely on you providing a custom qHash() function in the same namespace. In Qt 4 and Qt 5, the return value and optional second argument of a qHash function was of type uint. In Qt 6, it is size_t.

That is, you need to change

uint qHash(MyType x, uint seed);

to

size_t qHash(MyType x, size_t seed);

This allows QHash, QMultiHash and QSet to hold more than 2^32 items on 64 bit platforms.

Stability of References

The implementation of QHash and QMultiHash in Qt 6 got changed from a node based approach to a two stage lookup table. This design allows to keep the memory overhead of a hash instance very small, while at the same time giving good performance.

One behavioral change to note is that the new QHash implementation will not provide stable references to elements in the hash when the table needs to grow, or when entries are removed. Applications that rely on such stability might now run into undefined behavior.

Removal of QHash::insertMulti

In Qt 5, QHash could be used to create multi-valued hashes by using QHash::insertMulti, and QMultiHash was deriving vom QHash.

In Qt 6, both types and use cases are distinct, and QHash::insertMulti got removed.

QVector, QList

Prior to Qt 6, QVector and QList were separate classes. In Qt 6, they are unified: Qt 5 QList implementation is gone and both classes use updated QVector implementation instead. QList is the class with the actual implementation and QVector is an alias (typedef) to QList.

QList's fromVector() and toVector(), and QVector's fromList() and toList(), no longer involve data copying in Qt 6. They now return the object that they were called for.

API Changes

QList's (and hence QVector's) size type is changed from int to qsizetype. Together with the size type, all relevant methods' signatures are updated to use qsizetype. This allows QList to hold more than 2^31 items on 64 bit platforms.

When upgrading the code base to Qt 6, this API change would most likely result in compiler warnings about narrowing type conversions. Having the following example code:

void myFunction(QList<MyType> &data) {
    int size = data.size();
    // ...
    const int pos = getInsertPosition(size);
    data.insert(pos, MyType());
    // ...
}

you would need to update it to use either qsizetype or an auto keyword:

void myFunction(QList<MyType> &data) {
    auto size = data.size();
    // ...
    const auto pos = getInsertPosition(size);
    data.insert(pos, MyType());
    // ...
}

Alternatively, you may use type casting and cast everything to int or to qsizetype.

Note: If you want to build against both Qt 5 and Qt 6, the auto keyword is a good solution to cover signature differences between the versions.

Memory Layout

QList received multiple changes related to the memory layout in Qt 6.

In Qt 5, sizeof(QList<T>) was equal to a size of a pointer. Now, the extra pointer indirection is removed and QList data members are directly stored in the object. By default, expect sizeof(QList<T>) to be equal to the size of 3 pointers.

At the same time, memory layout of the elements is also updated. QList now always stores its elements directly in the allocated memory region as opposed to Qt 5, where certain objects were separately allocated on the heap and pointers to the objects were placed into the QList instead.

Note that the latter, in particular, affects large objects. To have Qt 5 behavior, you could wrap your objects into smart pointers and store these smart pointers in QList directly. In this case, the type of your QList would be QList<MySmartPointer<MyLargeObject>> as opposed to QList<MyLargeObject> in Qt 5.

Stability of References

There are several changes made to the QVector/QList implementation. The QVector related one is: insertion at the beginning is optimized (similarly to QList in Qt 5). The QList related one is: memory layout for the elements is simplified.

Important: These changes impact the stability of references. In Qt 6, you should consider any size or capacity modifying method to invalidate all references, even when QList is not implicitly shared. Exceptions to this rule are documented explicitly.

Applications that rely on certain reference stability might run into undefined behavior when upgraded to use Qt 6. You should pay extra attention to cases where QVector or QList with a non C-compatible array layout were used originally.

View classes in Qt6

General Overview

There are several new View classes coming with Qt6. There is the already existing QStringView, now accompanied by QByteArrayView and followed by a specialized QUtf8StringView and a more universal QAnyStringView.

Introduction to view classes on the example of QStringView

The QStringView class provides a unified view on UTF-16 strings with a read-only subset of the QString API. Unlike QString, which keeps its own copy of the string (possibly ref-counted), QStringView provides a view of a string that is stored elsewhere.

char hello[]{ "Hello." };   // narrow multi-byte string literal
QString str{hello};         // needs to make a copy of the string literal
QString strToStr(str);      // atomic increment involved to not create a copy of hello again

// The above code can be re-written to avoid copying and atomic increment.

QStringView view{ u"Hello." };  // view to UTF-16 encoded string literal
QStringView viewToView{ view }; // view of the same UTF-16 encoded string literal

The string "Hello." is stored in the binary and is not allocated at run-time. view is only a view onto the string "Hello.", therefore no copy has to be created. When we copy a QStringView, the viewToView observes the same string as the copied-from view is observing. This means that viewToView does not need to create a copy or an atomic increment. They are views onto the existing string "Hello.".

Views as function argument

Views should be passed by value, not by reference-to-const.

void myfun1(QStringView sv);        // preferred
void myfun2(const QStringView &sv); // compiles and works, but slower

View manipulation functions

QStringView supports functions that let us manipulate the view of the string. This allows us to change the view without creating a partial copy of the viewed string.

QString pineapple = "Pineapple";
QString pine = pineapple.left(4);

// The above code can be re-written to avoid creating a partial copy.

QStringView pineappleView{ pineapple };
QStringView pineView = pineappleView.left(4);

Non null-terminated strings and strings containing '\0'

QStringView supports both null-terminated and non null-terminated strings. The difference comes from the way you initialize the QStringView:

QChar aToE[]{ 'a', 'b', 'c', 'd', 'e' };

QStringView nonNull{ aToE, std::size(aToE) }; // with length given
QStringView nonNull{ aToE }; // automatically determines the length

QChar fToJ[]{ 'f', 'g', 'h', '\0', 'j' };

// uses given length, doesn't search for '\0', so '\0' at position 3
// is considered to be a part of the string similarly to 'h' and 'j
QStringView nonNull{ fToJ, std::size(fToJ) };
QStringView part{ fToJ }; //stops on the first encounter of '\0'

Ownership model of views

As views do not own the memory they reference, care must be taken to ensure that the referenced data (for example, owned by a QString) outlives the view on all code paths.

QStringView sayHello()
{
    QString hello("Hello.");
    return QStringView{ hello }; // hello gets out of scope and destroyed
}

void main()
{
    QStringView hello{ sayHello() };
    qDebug() << hello; // undefined behavior
}

Converting an QStringView to QString

QStringView will not implicitly or explicitly convert to a QString, but can create a deep copy of its data:

void print(const QString &s) { qDebug() << s; }

void main()
{
    QStringView string{ u"string"};

    // print(string); // invalid, no implicit conversion
    // QString str{ string }; // invalid, no explicit conversion

    print(string.toString());
    QString str = string.toString(); // create QString from view
}

Important notes

By leveraging the new view classes, one can achieve a lot of performance boost in many use cases. However, it is important to know that there might be some caveats. Therefore it is important to remember:

  • Views should be passed by value, not by reference-to-const.
  • Constructing a view with a negative length is undefined behavior.
  • Care must be taken to ensure that the referenced data (for example, owned by a QString) outlives the view on all code paths.

The QStringView class

Starting with Qt6 it is generally recommended to use QStringView over QStringRef. QStringView references a contiguous portion of a UTF-16 string it does not own. It acts as an interface type to all kinds of UTF-16 strings, without the need to construct a QString first. The QStringView class exposes almost all read-only methods of QString and the previously existing QStringRef class.

Note: Care must be taken to ensure that the referenced string data (for example, owned by a QString) outlives the QStringView on all code paths.

Note: If a QStringView wraps a QString, care needs to be taken since unlike QStringRef QStringView will not update the internal data pointer once the QString data relocates.

QString string = ...;
QStringView view{string};

// Appending something very long might cause a relocation and will
// ultimately result in a garbled QStringView.
string += ...;

The QStringRef class

In Qt6 QStringRef got removed from Qt Core. To ease porting of existing applications without touching the whole code-base, the QStringRef class did not vanish completely and instead it got moved into the Qt5Compat module.

If you want to use QStringRef further, you need to link against the new Qt5Compat module and add this line to your qmake .pro file:

QT += core5compat

In case you already ported your application or library to the cmake build system, add the following to your CMakeList.txt:

PUBLIC_LIBRARIES
    Qt::Core5Compat

Unfortunately, some methods exposed by QString returning a QStringRef, could not be moved to Qt5Compat. Therefore some manually porting may be needed. If your code uses one or more of the following functions you need to port them to use QStringView or QStringTokenizer. It is also recommended to use QStringView::tokenize over QStringView::split for performance critical code.

Change code using QStringRef:

QString string = ...;
QStringRef left = string.leftRef(n);
QStringRef mid = string.midRef(n);
QStringRef right = string.rightRef(n);

QString value = ...;
const QVector<QStringRef> refs = string.splitRef(' ');
if (refs.contains(value))
    return true;

to:

QString string = ...;
QStringView left = QStringView{string}.left(n);
QStringView mid = QStringView{string}.mid(n);
QStringView right = QStringView{string}.right(n);

QString value = ...;
const QList<QStringView> refs = QStringView{string}.split(u' ');
if (refs.contains(QStringView{value}))
    return true;
// or
const auto refs = QStringView{string}.tokenize(u' ');
for (auto ref : refs) {
    if (ref == value)
        return true;
}

In Qt 6, QRecursiveMutex does not inherit from QMutex anymore. This change was done to improve the performance of both QMutex and QRecursiveMutex.

Due to those changes, the QMutex::RecursionMode enum has been removed, and QMutexLocker is now a templated class that can operate on both QMutex and QRecursiveMutex.

The QFuture class

To avoid unintended usage of QFuture, there were some changes to QFuture API in Qt 6, which may introduce source compatibility breaks.

Implicit conversions between QFuture and other types

Conversion of QFuture<T> to T has been disabled. The casting operator was calling QFuture::result(), which may lead to undefined behavior if the user has moved the results from QFuture via QFuture::takeResult() before trying to do the conversion. Use QFuture::result() or QFuture::takeResult() methods explicitly, where you need to convert QFuture<T> to T.

The implicit conversion from QFuture<T> to QFuture<void> has been also disabled. If you really intend to do the conversion, use the explicit QFuture<void>(const QFuture<T> &) constructor:

QFuture<int> future = ...
QFuture<void> voidFuture = QFuture<void>(future);

Equality operators

The equality operators of QFuture have been removed. They were comparing the underlying d-pointers instead of comparing the results, which is not what users might expect. If you need to compare QFuture objects, use QFuture::result() or QFuture::takeResult() methods. For example:

QFuture<int> future1 = ...;
QFuture<int> future2 = ...;
if (future1.result() == future2.result())
    // ...

Behavioral Changes to QFuture and QFutureWatcher

In Qt 6, there were some improvements to QFuture and QFutureWatcher which caused the following behavioral changes:

  • After pausing QFuture or QFutureWatcher (by calling pause() or setPaused(true)), QFutureWatcher will not immediately stop delivering progress and result ready signals. At the moment of pausing there may be still computations that are in progress and cannot be stopped. Signals for such computations may be still delivered after pause, instead of being postponed and reported only after next resume. To get notified when pause actually took effect, QFutureWatcher::suspended() signal can be used. In addition, there are new isSuspending() and isSuspended() methods, to check if the QFuture is in the process of suspending or it's already in the suspended state. Note that for consistency reasons, for both QFuture and QFutureWatcher the pause-related APIs were deprecated and replaced by similar methods having "suspend" in the name instead.
  • QFuture::waitForFinished() will now wait until QFuture is actually in the finished state, instead of exiting as soon as it is not in the running state. This prevents waitForFinished() from exiting immediately, if at the moment of calling it the future is not started yet. The same applies to QFutureWatcher::waitForFinished(). This change won't affect the behavior of code that was using QFuture with QtConcurrent. Only the code that was using it with the undocumented QFutureInterface may be affected.

The QPromise class

In Qt 6, the new QPromise class should be used instead of unofficial QFutureInterface as a "setter" counterpart of QFuture.

IO Classes

The QProcess class

In Qt 6, the QProcess::start() overload that interprets a single command string by splitting it into program name and arguments is renamed to QProcess::startCommand(). However, a QProcess::start() overload that takes a single string, as well as a QStringList for arguments exists. Since the QStringList parameter defaults to the empty list, existing code only passing a string will still compile, but will fail to execute the process if it is a complete command string that includes arguments.

Qt 5.15 introduced deprecation warnings for the respective overload to make it easy to discover and update existing code:

QProcess process;

// compiles with warnings in 5.15, compiles but fails with Qt 6
process.start("dir \"My Documents\"");

// works with both Qt 5 and Qt 6; also see QProcess::splitCommand()
process.start("dir", QStringList({"My Documents"});

// works with Qt 6
process.startCommand("dir \"My Documents\"");

QProcess::pid() and the Q_PID type have been removed; use QProcess::processId() instead to get the native process identifier. Code using native Win32 APIs to access the data in the Q_PID as a Win32 PROCESS_INFORMATION struct is no longer supported.

Meta-Type system

The QVariant class

QVariant has been rewritten to use QMetaType for all of its operations. This implies behavior changes in a few methods:

  • QVariant::isNull() now only returns true if the QVariant is empty or contains a nullptr. In Qt 5, it also returned true for classes in qtbase which had an isNull method themselves if that one returned true. Code relying on the old behavior needs to check whether the contained value returs isNull – however such code is unlikely to occur in practice, as isNull() is rarely the property one is interested in (compare QString::isEmpty() / isNull() and QTime::isValid / isNull).
  • QVariant::operator== uses QMetaType::equals in Qt 6. Therefore, some graphical types like QPixmap, QImage and QIcon will never compare equal. Moreover, floating point numbers stored in QVariant are no longer compared with qFuzzyCompare, but instead use exact comparisons.

Furthermore, QVariant::operator<, QVariant::operator<=, QVariant::operator> and QVariant::operator>= were removed, because different variants are not always orderable. This also means that QVariant cannot be used anymore as a key in a QMap.

The QMetaType class

In Qt 6, registration of comparators, and cQDebug and QDataStream streaming operators is done automatically. Consequently, QMetaType::registerEqualsComparator(), QMetaType::registerComparators(), qRegisterMetaTypeStreamOperators() and QMetaType::registerDebugStreamOperator() do no longer exist. Calls to those methods have to be removed when porting to Qt 6.

Type registration

Types used in Q_PROPERTY have their meta-type stored in the class' QMetaObject. This requires the types to be complete when moc sees them, which can lead to compilation errors in code that worked in Qt 5. There are three ways to fix this issue:

  • Include the header which defines the type.
  • Instead of using an include, use the Q_MOC_INCLUDE macro. This helps if including the header would cause a cyclic dependency, or when it would slow down compilation.
  • If the header is present in the cpp file which implements the class, it is also possible to include the moc generated file there.

Regular expression classes

The QRegularExpression class

In Qt6, all methods taking the QRegExp got removed from our code-base. Therefore it is very likely that you will have to port your application or library to QRegularExpression.

QRegularExpression implements Perl-compatible regular expressions. It fully supports Unicode. For an overview of the regular expression syntax supported by QRegularExpression, please refer to the aforementioned pcrepattern(3) man page. A regular expression is made up of two things: a pattern string and a set of pattern options that change the meaning of the pattern string.

There are some subtle differences between QRegularExpression and QRegExp that will be explained by this document to ease the porting effort.

QRegularExpression is more strict when it comes to the syntax of the regular expression. Therefore it is always good to check the expression for validity.

QRegularExpression can almost always be declared const (except when the pattern changes), while QRegExp almost never could be.

There is no replacement for the CaretMode enumeration. The QRegularExpression::AnchoredMatchOption match option can be used to emulate the QRegExp::CaretAtOffset behavior. There is no equivalent for the other QRegExp::CaretMode modes.

QRegularExpression supports only Perl-compatible regular expressions. Still, it does not support all the features available in Perl-compatible regular expressions. The most notable one is the fact that duplicated names for capturing groups are not supported, and using them can lead to undefined behavior. This may change in a future version of Qt.

Wildcard matching

There is no direct way to do wildcard matching in QRegularExpression. However, the QRegularExpression::wildcardToRegularExpression method is provided to translate glob patterns into a Perl-compatible regular expression that can be used for that purpose.

For example, if you have code like

QRegExp wildcard("*.txt");
wildcard.setPatternSyntax(QRegExp::Wildcard);

you can rewrite it as

auto wildcard = QRegularExpression(QRegularExpression::wildcardToRegularExpression("*.txt"));

Please note though that not all shell like wildcard pattern might be translated in a way you would expect it. The following example code will silently break if simply converted using the above mentioned function:

const QString fp1("C:/Users/dummy/files/content.txt");
const QString fp2("/home/dummy/files/content.txt");

QRegExp re1("*/files/*");
re1.setPatternSyntax(QRegExp::Wildcard);
... = re1.exactMatch(fp1);  // returns true
... = re1.exactMatch(fp2);  // returns true

// but converted with QRegularExpression::wildcardToRegularExpression()

QRegularExpression re2(QRegularExpression::wildcardToRegularExpression("*/files/*"));
... = re2.match(fp1).hasMatch();    // returns false
... = re2.match(fp2).hasMatch();    // returns false

Searching forward

Forward searching inside a string was usually implemented with a loop using QRegExp::indexIn and a growing offset, but can now be easily implemented with QRegularExpressionMatchIterator or QString::indexOf.

For example, if you have code like

QString subject("the quick fox");

int offset = 0;
QRegExp re("(\\w+)");
while ((offset = re.indexIn(subject, offset)) != -1) {
    offset += re.matchedLength();
    // ...
}

you can rewrite it as

QRegularExpression re("(\\w+)");
QString subject("the quick fox");

QRegularExpressionMatchIterator i = re.globalMatch(subject);
while (i.hasNext()) {
    QRegularExpressionMatch match = i.next();
    // ...
}

 // or alternatively using QString::indexOf

qsizetype from = 0;
QRegularExpressionMatch match;
while ((from = subject.indexOf(re, from, &match)) != -1) {
    from += match.capturedLength();
    // ...
}

Searching backwards

Backwards searching inside a string was usually often implemented as a loop over QRegExp::lastIndexIn, but can now be easily implemented using QString::lastIndexOf and QRegularExpressionMatch.

Note: QRegularExpressionMatchIterator is not capable of performing a backwards search.

For example, if you have code like

int offset = -1;
QString subject("Lorem ipsum dolor sit amet, consetetur sadipscing.");

QRegExp re("\\s+([ids]\\w+)");
while ((offset = re.lastIndexIn(subject, offset)) != -1) {
    --offset;
    // ...
}

you can rewrite it as

qsizetype from = -1;
QString subject("Lorem ipsum dolor sit amet, consetetur sadipscing.");

QRegularExpressionMatch match;
QRegularExpression re("\\s+([ids]\\w+)");
while ((from = subject.lastIndexOf(re, from, &match)) != -1) {
    --from;
    // ...
}

exactMatch vs. match.hasMatch

QRegExp::exactMatch served two purposes: it exactly matched a regular expression against a subject string, and it implemented partial matching. Exact matching indicates whether the regular expression matches the entire subject string. For example:

QString source("abc123");

QRegExp("\\d+").exactMatch(source);         // returns false
QRegExp("[a-z]+\\d+").exactMatch(source);   // returns true

QRegularExpression("\\d+").match(source).hasMatch();        // returns true
QRegularExpression("[a-z]+\\d+").match(source).hasMatch();  // returns true

Exact matching is not reflected in QRegularExpression. If you want to be sure that the subject string matches the regular expression exactly, you can wrap the pattern using the QRegularExpression::anchoredPattern function:

QString source("abc123");

QString pattern("\\d+");
QRegularExpression(pattern).match(source).hasMatch();  // returns true

pattern = QRegularExpression::anchoredPattern(pattern);
QRegularExpression(pattern).match(source).hasMatch();  // returns false

Minimal matching

QRegExp::setMinimal() implemented minimal matching by simply reversing the greediness of the quantifiers (QRegExp did not support lazy quantifiers, like *?, +?, etc.). QRegularExpression instead does support greedy, lazy and possessive quantifiers. The QRegularExpression::InvertedGreedinessOption pattern option can be useful to emulate the effects of QRegExp::setMinimal(): if enabled, it inverts the greediness of quantifiers (greedy ones become lazy and vice versa).

Different pattern syntax

Porting a regular expression from QRegExp to QRegularExpression may require changes to the pattern itself. Therefore it is recommended to check the pattern used with the QRegularExpression::isValid method. This is especially important for user provided pattern or pattern not controlled by the developer.

In other cases, a pattern ported from QRegExp to QRegularExpression may silently change semantics. Therefore, it is necessary to review the patterns used. The most notable cases of silent incompatibility are:

  • Curly braces are needed in order to use a hexadecimal escape like \xHHHH with more than 2 digits. A pattern like \x2022 needs to be ported to \x{2022}, or it will match a space (0x20) followed by the string "22". In general, it is highly recommended to always use curly braces with the \x escape, no matter the amount of digits specified.
  • A 0-to-n quantification like {,n} needs to be ported to {0,n} to preserve semantics. Otherwise, a pattern such as \d{,3} would actually match a digit followed by the exact string "{,3}".

Partial Matching

When using QRegExp::exactMatch(), if an exact match was not found, one could still find out how much of the subject string was matched by the regular expression by calling QRegExp::matchedLength(). If the returned length was equal to the subject string's length, then one could conclude that a partial match was found. QRegularExpression supports partial matching explicitly by means of the appropriate QRegularExpression::MatchType.

Global matching

Due to limitations of the QRegExp API it was impossible to implement global matching correctly (that is, like Perl does). In particular, patterns that can match zero characters (like "a*") are problematic. QRegularExpression::wildcardToRegularExpression implements Perl global match correctly, and the returned iterator can be used to examine each result.

Unicode properties support

When using QRegExp, character classes such as \w, \d, etc. match characters with the corresponding Unicode property: for instance, \d matches any character with the Unicode Nd (decimal digit) property. Those character classes only match ASCII characters by default. When using QRegularExpression: for instance, \d matches exactly a character in the 0-9 ASCII range. It is possible to change this behavior by using the QRegularExpression::UseUnicodePropertiesOption pattern option.

The QRegExp class

In Qt6 QRegExp got removed from Qt Core. If your application cannot be ported right now, QRegExp still exists in Qt5Compat to keep these code-bases working. If you want to use QRegExp further, you need to link against the new Qt5Compat module and add this line to your qmake .pro file:

QT += core5compat

In case you already ported your application or library to the cmake build system, add the following to your CMakeList.txt:

PUBLIC_LIBRARIES
    Qt::Core5Compat

QEvent and subclasses

The QEvent class defined a copy constructor and an assignment operator, in spite of being a polymorphic class. Copying classes with virtual methods can result in slicing when assigning objects from different classes to each other. Since copying and assigning often happens implicilty, this could lead to hard-to-debug problems.

In Qt 6, the copy constructor and assignment operator for QEvent subclasses have been made protected to prevent implicit copying. If you need to copy events, use the clone method, which will return a heap-allocated copy of the QEvent object. Make sure you delete the clone, perhaps using std::unique_ptr, unless you post it (in which case Qt will delete it once it has been delivered).

In your QEvent subclasses, override clone(), and declare the protected and default-implemented copy constructor and assignment operator like this:

class MyEvent : public QEvent
{
public:
    // ...

    MyEvent *clone() const override { return new MyEvent(*this); }

protected:
    MyEvent(const MyEvent &other) = default;
    MyEvent &operator=(const MyEvent &other) = default;
    MyEvent(MyEvent &&) = delete;
    MyEvent &operator=(MyEvent &&) = delete;
    // member data
};

Note that if your MyEvent class allocates memory (e.g. through a pointer-to-implementation pattern), then you will have to implement custom copy semantics.

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