call a class

[assismvla]

what is the difference between declaring a class

this way:

#include <QDialog>
 #include <QHash>

#include <QDialog>
 #include <QHash>

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or, this way:

class QDialogButtonBox;
 class QFile;
 class QFtp;
 class QLabel;
 class QLineEdit;
 class QTreeWidget;

class QDialogButtonBox;
 class QFile;
 class QFtp;
 class QLabel;
 class QLineEdit;
 class QTreeWidget;

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