1、Java

public class Singleton {
    private volatile static Singleton instance;
    public static Singleton getInstance () {
        if (instance == null) {
            synchronized (Singleton.class) {
                if (instance == null) {
                    instance = new Singleton();
                }
            }
        }
        return instance;
    }
}

2、C++

class Singleton {
    private:
        volatile Singleton* pInst = 0;
    public:
        static Singleton* GetInstance() {
            if (pInst == 0) {
                lock();
                if (pInst == 0) {
                    pInst = new Singleton();
                }
                unlock();
            }
            return pInst;
        }
}

3、总结

同步机制等价于锁机制