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/*!
    \page qtcore-changes-qt6.html
    \title Porting to Qt 6 - 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.

    \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.
*/