qt 实现单例

头文件：

class ConnectionManager

{

public:

static ConnectionManager *instance();

};

实现文件：

Q_GLOBAL_STATIC(ConnectionManager, connectionManager)

ConnectionManager *ConnectionManager::instance()

{

return connectionManager();

}

使用：

ConnectionManager::instance()

// POD for Q_GLOBAL_STATIC

template <typename T>

class QGlobalStatic

{

public:

QBasicAtomicPointer<T> pointer;

bool destroyed;

};

// Created as a function-local static to delete a QGlobalStatic<T>

template <typename T>

class QGlobalStaticDeleter

{

public:

QGlobalStatic<T> &globalStatic;

QGlobalStaticDeleter(QGlobalStatic<T> &_globalStatic)

: globalStatic(_globalStatic)

{ }

inline ~QGlobalStaticDeleter()

{

delete globalStatic.pointer;

globalStatic.pointer = 0;

globalStatic.destroyed = true;

}

};

#define Q_GLOBAL_STATIC_INIT(TYPE, NAME) \

static QGlobalStatic<TYPE > this_##NAME = { Q_BASIC_ATOMIC_INITIALIZER(0), false }

#define Q_GLOBAL_STATIC(TYPE, NAME) \

Q_GLOBAL_STATIC_INIT(TYPE, NAME); \

static TYPE *NAME() \

{ \

if (!this_##NAME.pointer && !this_##NAME.destroyed) { \

TYPE *x = new TYPE; \

if (!this_##NAME.pointer.testAndSetOrdered(0, x)) \

delete x; \

else \

static QGlobalStaticDeleter<TYPE > cleanup(this_##NAME); \

} \

return this_##NAME.pointer; \

}

Q_GLOBAL_STATIC：

1、先通过Q_GLOBAL_STATIC_INIT声明一个静态的全局变量（this_##NAME ），并初始化为0

2、声明一个名字叫做全局变量（NAME）的static的函数，用来取得这个全局变量

3、如果这个变量没初始化且没删除，就通过testAndSetOrdered以原子方式创建

4、创建成功，再定义一个函数内的QGlobalStaticDeleter来清理这个全局变量

5、函数返回最后结果

qt在每个arch上面都实现了原子操作，移植时候修改src/corelib/arch/qatomic_ARCH.h

参考文档： Implementing Atomic Operations