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/****************************************************************************
**
** Copyright (C) 2020 The Qt Company Ltd.
** Contact: https://www.qt.io/licensing/
**
** This file is part of the documentation of the Qt Toolkit.
**
** $QT_BEGIN_LICENSE:FDL$
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** Licensees holding valid commercial Qt licenses may use this file in
** accordance with the commercial license agreement provided with the
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** and conditions see https://www.qt.io/terms-conditions. For further
** information use the contact form at https://www.qt.io/contact-us.
**
** GNU Free Documentation License Usage
** Alternatively, this file may be used under the terms of the GNU Free
** Documentation License version 1.3 as published by the Free Software
** Foundation and appearing in the file included in the packaging of
** this file. Please review the following information to ensure
** the GNU Free Documentation License version 1.3 requirements
** will be met: https://www.gnu.org/licenses/fdl-1.3.html.
** $QT_END_LICENSE$
**
****************************************************************************/
/*!
\page qtcore-changes-qt6.html
\title Porting to Qt 6 - Qt Core
\ingroup porting-guides-5-to-6
\brief Migrate Qt Core to Qt 6.
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.
\section1 Container Classes
\section2 QHash, QMultiHash, QSet
\section3 qHash() Signature
For custom types, QHash and QMultiHash rely on you providing
a \l{The qHash() hashing function} {custom qHash() function}
in the same namespace. In Qt 4 and Qt 5, the return
value and optional second argument of a \c qHash function
was of type \c uint. In Qt 6, it is \c size_t.
That is, you need to change
\code
uint qHash(MyType x, uint seed);
\endcode
to
\code
size_t qHash(MyType x, size_t seed);
\endcode
This allows QHash, QMultiHash and QSet to hold more than 2^32 items on
64 bit platforms.
\section3 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.
\section3 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.
\section2 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.
\section3 API Changes
QList's (and hence QVector's) size type is changed from \c int to \c
qsizetype. Together with the size type, all relevant methods' signatures are
updated to use \c 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:
\code
void myFunction(QList<MyType> &data) {
int size = data.size();
// ...
const int pos = getInsertPosition(size);
data.insert(pos, MyType());
// ...
}
\endcode
you would need to update it to use either \c qsizetype or an auto keyword:
\code
void myFunction(QList<MyType> &data) {
auto size = data.size();
// ...
const auto pos = getInsertPosition(size);
data.insert(pos, MyType());
// ...
}
\endcode
Alternatively, you may use type casting and cast everything to \c int or to
\c 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.
\section3 Memory Layout
QList received multiple changes related to the memory layout in Qt 6.
In Qt 5, \c{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 \c{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 \c{QList<MySmartPointer<MyLargeObject>>} as opposed to
\c{QList<MyLargeObject>} in Qt 5.
\section3 Stability of References
There are several changes made to the QVector/QList implementation. The
QVector related ones are: insertion at the beginning is optimized (similarly
to QList in Qt 5) and element removal can reallocate in order to remove the
unused capacity. 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 \l{Implicit Sharing}{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.
*/
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