Storing pointers using QListWidgetItem::setData

There is another constructor for void*: QVariant::QVariant(int typeOrUserType, const void * copy) where you should pass an unique integer to represent the pointer type.

But as stated by the documentation, you could declare your pointer type with Q_DECLARE_METATYPE(Calendar*) and use QVariant::fromValue<Calendar*>(...) and QVariant::value<Calendar*>() to store and retrieve the value.

Or instead, because you are using a QListWidget instead of a regular model, you can just subclass QListWidgetItem, and add a Calendar* member variable with the required accessors, to avoid the overhead of using QVariant.