QPointer
QPointer can only point to QObject instances. It will be automatically set to nullptr if the pointed to object is destroyed. It is a weak pointer specialized for QObject.
QPointer只能指向QObject实例。如果指向的对象被销毁，它将自动设置为 nullptr。它是一个专门用于QObject的弱指针。
template <typename T>
class QPointer
QObject *obj = new QObject;
QPointer<QObject> pObj(obj);
delete obj;
Q_ASSERT(pObj.isNull()); // pObj will be nullptr now
PS：请注意，类T必须继承自QObject，否则将导致编译或链接错误。
QSharedPointer
A reference-counted pointer. The actual object will only be deleted, when all shared pointers are destroyed. Equivalent to std::shared_ptr.
引用计数指针。只有当所有共享指针都被销毁时，实际对象才会被删除。相当于 std::shared_ptr。
template <typename T>
class QSharedPointer
int *pI = new int;
QSharedPointer<int> pI1(pI);
QSharedPointer<int> pI2 = pI1;
pI1.clear();
// pI2 is still pointing to pI, so it is not deleted
pI2.clear();
// No shared pointers anymore, pI is deleted
PS：QSharedPointer 是线程安全的，因此即使有多个线程同时修改 QSharedPointer 对象也不需要加锁。虽然 QSharedPointer 是线程安全的，但是 QSharedPointer 指向的内存区域可不一定是线程安全的。所以多个线程同时修改 QSharedPointer 指向的数据时还要应该考虑加锁。
QWeakPointer
Can hold a weak reference to a shared pointer. It will not prevent the object from being destroyed, and is simply reset. Equivalent to std::weak_ptr, where lock is equivalent to toStrongRef.
可以保存对共享指针的弱引用。它不会阻止对象被破坏，而只是重置。相当于std::weak_ptr，其中lock相当于toStrongRef
int *pI = new int;
QSharedPointer<int> pI1(pI);
QWeakPointer<int> pI2 = pI1;
pI1.clear();
// No shared pointers anymore, pI is deleted
//
// To use the shared pointer, we must "lock" it for use:
QSharedPointer<int> pI2_locked = pI2.toStrongRef();
Q_ASSERT(pI2_locked.isNull());
QScopedPointer
This is just a helper class that will delete the referenced object when the pointer goes out of scope. Thus, binds a dynamically allocated object to a variable scope.
这只是一个辅助类，当指针超出范围时，它将删除引用的对象。因此，将动态分配的对象绑定到变量范围。
手动管理堆分配的对象既困难又容易出错，常见的结果是代码泄漏内存，难以维护。QScopedPointer是一个小型实用程序类，它通过将基于堆栈的内存所有权分配给堆分配（通常称为资源获取即初始化(resource acquisition is initialization：RAII）来大大简化这一过程。
QScopedPointer保证当当前作用域消失时，所指向的对象将被删除。
template <typename T, typename Cleanup>
class QScopedPointer
MyClass *foo() {
QScopedPointer<MyClass> myItem(new MyClass);
// Some logic
if (some condition) {
return nullptr; // myItem will be deleted here
}
return myItem.take(); // Release item from scoped pointer and return it
}
Example
class UISvgIcon::Pimpl
{
public:
Pimpl(const QString &svgPath);
~Pimpl();
public:
QScopedPointer<QDomDocument> mSvgDoc_;
QScopedPointer<QSvgRenderer> pSvgRender_;
QScopedPointer<QPixmap> pPixmap_;
};
UISvgIcon::Pimpl::Pimpl(const QString &svgPath)
{
mSvgDoc_.reset(new QDomDocument());
pSvgRender_.reset(new QSvgRenderer(mSvgDoc_->toByteArray()));
auto tDpi = UPGLUtil::getScreen()->logicalDotsPerInch();
pPixmap_.reset(new QPixmap(SVGProperty(mSvgDoc_->documentElement(), "svg", "width").toInt() * tDpi,
SVGProperty(mSvgDoc_->documentElement(), "svg", "height").toInt() * tDpi));
}
