设计模式之单例模式

单例模式非常常用，客户端软件的话，主窗口只有一个通常就是采用单例模式。
主要思想，就是保证进程中只有一个实例对象。

//懒汉式
//懒汉式
class LazyMan{
private:
    LazyMan(){
        pthread_mutex_init(&mutex, NULL);
    }
    static pthread_mutex_t mutex;
public:
    static LazyMan* getInstance(){
        if(singleton == NULL){//如果已经创建了，就没有必要走下面的了
            pthread_mutex_lock(&mutex);
            if(singleton == NULL){
                singleton = new LazyMan;
            }
            pthread_mutex_unlock(&mutex);
        }
        return singleton;
    }

private:
    static LazyMan* singleton;
};
pthread_mutex_t LazyMan::mutex = NULL;
LazyMan *LazyMan::singleton = NULL;

void test_lazyman()
{
    LazyMan* lazyman01 = LazyMan::getInstance();
    LazyMan* lazyman02 = LazyMan::getInstance();
    if(lazyman01 == lazyman01){
        cout << "equal" << endl;
    }
}


// 饿汉式
class HungryMan{
private:
    HungryMan(){}
public:
    static HungryMan* getInstance(){
        return singleton;
    }

private:
    static HungryMan* singleton;
};
HungryMan *HungryMan::singleton = new HungryMan;
void test_hungryman()
{
    HungryMan* hungryman01 = HungryMan::getInstance();
    HungryMan* hungryman02 = HungryMan::getInstance();
    if(hungryman01 == hungryman02){
        cout << "equal" << endl;
    }
}

懒汉式存在多线程安全的问题. 上面代码对其进行了加锁改进。饿汉式比较简单，再运行之前就已经初始化了，多线程安全问题。是一种空间换便利的方案。