what is the Macro "QT_BEGIN_NAMESPACE" mean in Qt 5?
<p>In Qt 5.6 Example "basicLayout",there is a Macro "QT_BEGIN_NAMESPACE",I try to search for it in Qt Documentation,but I still can't get the answer.So what is the meaning in Qt?</p>

<p>The Code related is here:</p>

<pre><code>#ifndef DIALOG_H
#define DIALOG_H

#include &lt;QDialog&gt;

QT_BEGIN_NAMESPACE
class QAction;
class QDialogButtonBox;
class QGroupBox;
class QLabel;
class QLineEdit;
class QMenu;
class QMenuBar;
class QPushButton;
class QTextEdit;
QT_END_NAMESPACE

//! [0]
class Dialog : public QDialog
{
    Q_OBJECT

public:
    Dialog();

private:
    void createMenu();
    void createHorizontalGroupBox();
    void createGridGroupBox();
    void createFormGroupBox();

    enum { NumGridRows = 3, NumButtons = 4 };

    QMenuBar *menuBar;
    QGroupBox *horizontalGroupBox;
    QGroupBox *gridGroupBox;
    QGroupBox *formGroupBox;
    QTextEdit *smallEditor;
    QTextEdit *bigEditor;
    QLabel *labels[NumGridRows];
    QLineEdit *lineEdits[NumGridRows];
    QPushButton *buttons[NumButtons];
    QDialogButtonBox *buttonBox;

    QMenu *fileMenu;
    QAction *exitAction;
};
//! [0]

#endif // DIALOG_H
</code></pre>