Question

I am reading through the examples on the Qt page and am wondering why they add references to already existing classes in their code example:

#ifndef HTTPWINDOW_H
#define HTTPWINDOW_H

#include <QDialog>

class QFile;
class QHttp;
class QHttpResponseHeader;
class QLabel;
class QLineEdit;
class QProgressDialog;
class QPushButton;

class HttpWindow : public QDialog
{
...

Answer 1

Those are forward declarations. Using them can (in some cases) obviate the need to #include the relevant header files, thus speeding up compilation. The Standard C++ library does something similar with the <iosfwd> header.