QPointer是一个指针封装类,其作用类似于智能指针,但是它最大的特点应该是在指针的控制上,它希望一个Qt的指针(当然是从QObject派生的)可以同时被多个类拥有,这在
界面编程中当然是很常见的事情了,但是当这个指针被删除时,我们不希望再找到那两个界面类然后通知它们,相反我们希望这两个界面类可以直接判断QPointer中的isNull方法
很自然的知道原始指针已经不存在了
1.试验代码：
#include <QApplication>
#include <QPushButton>
#include <QPointer>
int main(int argc, char *argv[])
{
QApplication app(argc, argv);
QPushButton* pButton = new QPushButton("wgs");
QPointer<QPushButton> button = pButton;
delete pButton;
if (!button.isNull())
{
button->setText("www");
}
return app.exec();
}
一段很短的代码，这里需要的注意的是QPointer指针的使用
template <class T>
class QPointer
{
QObject *o;
public:
inline QPointer() : o(0) {}
inline QPointer(T *p) : o(p)
{ QMetaObject::addGuard(&o); }
inline QPointer(const QPointer<T> &p) : o(p.o)
{ QMetaObject::addGuard(&o); }
inline ~QPointer()
{ QMetaObject::removeGuard(&o); }
inline QPointer<T> &operator=(const QPointer<T> &p)
{ if (this != &p) QMetaObject::changeGuard(&o, p.o); return *this; }
inline QPointer<T> &operator=(T* p)
{ if (o != p) QMetaObject::changeGuard(&o, p); return *this; }
inline bool isNull() const
{ return !o; }
inline T* operator->() const
{ return static_cast<T*>(const_cast<QObject*>(o)); }
inline T& operator*() const
{ return *static_cast<T*>(const_cast<QObject*>(o)); }
inline operator T*() const
{ return static_cast<T*>(const_cast<QObject*>(o)); }
};
QPointer只是一个简单的模板，对QMetaObject的相关操作做了简单的封装，这里的基本思想是
在QPointer构造的时候调用QMetaObject::addGuard(&o)，把T的指针加入QMetaObject内的一个哈希表中，
在QPointer析构的时候调用QMetaObject::removeGuard(&o)，把T的指针从哈希表中删除
这是一个QMetaObject中静态成员，该哈希表的定义如下：
typedef QMultiHash<QObject *, QObject **> GuardHash;
这个哈希表存储的是指针的值和指针的地址，因此加入的代码如下：
void QMetaObject::addGuard(QObject **ptr)
{
if (!*ptr)
return;
GuardHash *hash = guardHash();
if (!hash) {
*ptr = 0;
return;
}
QWriteLocker locker(guardHashLock());
hash->insert(*ptr, ptr);
}
为什么不是只保存一个指针呢，原来是为了防止误删除，看看删除的代码：
void QMetaObject::removeGuard(QObject **ptr)
{
if (!*ptr)
return;
GuardHash *hash = guardHash();
if (!hash)
return;
QWriteLocker locker(guardHashLock());
GuardHash::iterator it = hash->find(*ptr);
const GuardHash::iterator end = hash->end();
for (; it.key() == *ptr && it != end; ++it) {
if (it.value() == ptr) {
(void) hash->erase(it);
break;
}
}
}
只有在it.value() == ptr的时候才会删除
但是等等，当删除普通指针时又如何更新这个哈希表呢？
delete pButton;如何通知QMetaObject中的哈希表更新？
答案是在QObject中，请注意QObject的析构函数
QObject::~QObject()
{
Q_D(QObject);
if (d->wasDeleted) {
#if defined(QT_DEBUG)
qWarning("Double QObject deletion detected");
#endif
return;
}
d->wasDeleted = true;
d->blockSig = 0; // unblock signals so we always emit destroyed()
// set all QPointers for this object to zero
GuardHash *hash = ::guardHash();
if (hash) {
QWriteLocker locker(guardHashLock());
GuardHash::iterator it = hash->find(this);
const GuardHash::iterator end = hash->end();
while (it.key() == this && it != end) {
*it.value() = 0;
it = hash->erase(it);
}
}
emit destroyed(this);
QConnectionList *list = ::connectionList();
if (list) {
QWriteLocker locker(&list->lock);
list->remove(this);
}
if (d->pendTimer) {
// have pending timers
QThread *thr = thread();
if (thr || d->thread == 0) {
// don't unregister timers in the wrong thread
QAbstractEventDispatcher *eventDispatcher = QAbstractEventDispatcher::instance(thr);
if (eventDispatcher)
eventDispatcher->unregisterTimers(this);
}
}
d->eventFilters.clear();
// delete children objects
if (!d->children.isEmpty()) {
qDeleteAll(d->children);
d->children.clear();
}
{
QWriteLocker locker(QObjectPrivate::readWriteLock());
::qt_removeObject(this);
/*
theoretically, we cannot check d->postedEvents without
holding the postEventList.mutex for the object's thread,
but since we hold the QObjectPrivate::readWriteLock(),
nothing can Go into QCoreApplication::postEvent(), which
effectively means noone can post new events, which is what
we are trying to prevent. this means we can safely check
d->postedEvents, since we are fairly sure it will not
change (it could, but only by decreasing, i.e. removing
posted events from a differebnt thread)
*/
if (d->postedEvents > 0)
QCoreApplication::removePostedEvents(this);
}
if (d->parent) // remove it from parent object
d->setParent_helper(0);
delete d;
d_ptr = 0;
}
这里做了很多很多的事情，其中的代码
// set all QPointers for this object to zero
GuardHash *hash = ::guardHash();
if (hash) {
QWriteLocker locker(guardHashLock());
GuardHash::iterator it = hash->find(this);
const GuardHash::iterator end = hash->end();
while (it.key() == this && it != end) {
*it.value() = 0;
it = hash->erase(it);
}
}
就是更新QMetaObject中哈希表的
这也就是单基类的好处，可以在这里控制很多事情
http://blog.csdn.net/oowgsoo/article/details/1529424