在 C++ 中类名后使用冒号

This is a header file extracted from a blackberry 10 helloworld program.

#ifndef ApplicationUI_HPP_
#define ApplicationUI_HPP_

#include <QObject>

namespace bb
{
    namespace cascades
    {
        class Application;
        class LocaleHandler;
    }
}

class QTranslator;

/*!
 * @brief Application object
 *
 *
 */

class ApplicationUI : public QObject
{
    Q_OBJECT
public:
    ApplicationUI(bb::cascades::Application *app);
    virtual ~ApplicationUI() { }
private slots:
    void onSystemLanguageChanged();
private:
    QTranslator* m_pTranslator;
    bb::cascades::LocaleHandler* m_pLocaleHandler;
};

#endif /* ApplicationUI_HPP_ */

我对类名声明后的冒号运算符感到困惑.

class ApplicationUI : public QObject