Writing my own object to a binary file.

[ChrisW67]

You need to provide the operator<<() and operator>>() implementation for your User class. Its prototype looks like:

QDataStream &operator<<(QDataStream &, const User &);
QDataStream &operator>>(QDataStream &, User &);

QDataStream &operator<<(QDataStream &, const User &);
QDataStream &operator>>(QDataStream &, User &);

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Each should do whatever is necessary to save/restore the internal state of the User object.