qt 實現單例

標頭檔：

class ConnectionManager
{
public:
    static ConnectionManager *instance();

};

實現檔案：

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、先通過Q_GLOBAL_STATIC_INIT聲明一個靜態全域變數（this_##NAME ），並初始化為0

2、聲明一個名字叫做全域變數（NAME）的static的函數，用來取得這個全域變數

3、如果這個變數沒初始化且沒刪除，就通過testAndSetOrdered以原子方式建立

4、建立成功，再定義一個函數內的QGlobalStaticDeleter來清理這個全域變數

5、函數返回最後結果

 

qt在每個arch上面都實現了原子操作，移植時候修改src/corelib/arch/qatomic_ARCH.h

參考文檔： Implementing Atomic Operations