C++中有关单例的实现和用法

#ifndef A_H
#define A_H
#include <QObject>
class A : public QObject{
    Q_OBJECT
    public:
        static A* instance(){
            static A a;
            return &a;
        }
    public:
        void emitSignalA(){
            signalA();
        }
    signals:
        void signalA();
};
#define PA (A::instance())
#endif // A_H