QWeakPointer Class

The QWeakPointer class holds a weak reference to a shared pointer. More...

Header:	#include <QWeakPointer>
qmake:	QT += core
Since:	Qt 4.5

This class was introduced in Qt 4.5.

• List of all members, including inherited members
• Obsolete members

Note: All functions in this class are reentrant.

Public Functions

	QWeakPointer(const QSharedPointer<T> &other)
	QWeakPointer(const QWeakPointer<T> &other)
	QWeakPointer()
QWeakPointer<T> &	operator=(const QSharedPointer<T> &other)
QWeakPointer<T> &	operator=(const QWeakPointer<T> &other)
	~QWeakPointer()
void	clear()
T *	data() const
bool	isNull() const
QSharedPointer<T>	lock() const
void	swap(QWeakPointer<T> &other)
QSharedPointer<T>	toStrongRef() const
bool	operator bool() const
bool	operator!() const

Related Non-Members

QWeakPointer<X>	qWeakPointerCast(const QWeakPointer<T> &src)
bool	operator!=(const QSharedPointer<T> &ptr1, const QWeakPointer<X> &ptr2)
bool	operator!=(const QWeakPointer<T> &ptr1, const QSharedPointer<X> &ptr2)
bool	operator!=(const QWeakPointer<T> &lhs, std::nullptr_t)
bool	operator!=(std::nullptr_t, const QWeakPointer<T> &rhs)
bool	operator==(const QSharedPointer<T> &ptr1, const QWeakPointer<X> &ptr2)
bool	operator==(const QWeakPointer<T> &lhs, std::nullptr_t)
bool	operator==(const QWeakPointer<T> &ptr1, const QSharedPointer<X> &ptr2)
bool	operator==(std::nullptr_t, const QWeakPointer<T> &rhs)

Detailed Description

The QWeakPointer is an automatic weak reference to a pointer in C++. It cannot be used to dereference the pointer directly, but it can be used to verify if the pointer has been deleted or not in another context.

QWeakPointer objects can only be created by assignment from a QSharedPointer.

It's important to note that QWeakPointer provides no automatic casting operators to prevent mistakes from happening. Even though QWeakPointer tracks a pointer, it should not be considered a pointer itself, since it doesn't guarantee that the pointed object remains valid.

Therefore, to access the pointer that QWeakPointer is tracking, you must first promote it to QSharedPointer and verify if the resulting object is null or not. QSharedPointer guarantees that the object isn't deleted, so if you obtain a non-null object, you may use the pointer. See QWeakPointer::toStrongRef() for an example.

QWeakPointer also provides the QWeakPointer::data() method that returns the tracked pointer without ensuring that it remains valid. This function is provided if you can guarantee by external means that the object will not get deleted (or if you only need the pointer value) and the cost of creating a QSharedPointer using toStrongRef() is too high.

See also QSharedPointer and QScopedPointer.

Member Function Documentation

QWeakPointer::QWeakPointer(const QSharedPointer<T> &other)

Creates a QWeakPointer that holds a weak reference to the pointer referenced by other.

If T is a derived type of the template parameter of this class, QWeakPointer will perform an automatic cast. Otherwise, you will get a compiler error.

QWeakPointer::QWeakPointer(const QWeakPointer<T> &other)

Creates a QWeakPointer that holds a weak reference to the pointer referenced by other.

If T is a derived type of the template parameter of this class, QWeakPointer will perform an automatic cast. Otherwise, you will get a compiler error.

QWeakPointer::QWeakPointer()

Creates a QWeakPointer that points to nothing.

QWeakPointer<T> &QWeakPointer::operator=(const QSharedPointer<T> &other)

Makes this object share other's pointer. The current pointer reference is discarded but is not deleted.

If T is a derived type of the template parameter of this class, QWeakPointer will perform an automatic cast. Otherwise, you will get a compiler error.

QWeakPointer<T> &QWeakPointer::operator=(const QWeakPointer<T> &other)

Makes this object share other's pointer. The current pointer reference is discarded but is not deleted.

If T is a derived type of the template parameter of this class, QWeakPointer will perform an automatic cast. Otherwise, you will get a compiler error.

QWeakPointer::~QWeakPointer()

Destroys this QWeakPointer object. The pointer referenced by this object will not be deleted.

void QWeakPointer::clear()

Clears this QWeakPointer object, dropping the reference that it may have had to the pointer.

T *QWeakPointer::data() const

Returns the value of the pointer being tracked by this QWeakPointer, without ensuring that it cannot get deleted. To have that guarantee, use toStrongRef(), which returns a QSharedPointer object. If this function can determine that the pointer has already been deleted, it returns nullptr.

It is ok to obtain the value of the pointer and using that value itself, like for example in debugging statements:

    qDebug("Tracking %p", weakref.data());

However, dereferencing the pointer is only allowed if you can guarantee by external means that the pointer does not get deleted. For example, if you can be certain that no other thread can delete it, nor the functions that you may call.

If that is the case, then the following code is valid:

    // this pointer cannot be used in another thread
    // so other threads cannot delete it
    QWeakPointer<int> weakref = obtainReference();

    Object *obj = weakref.data();
    if (obj) {
        // if the pointer wasn't deleted yet, we know it can't get
        // deleted by our own code here nor the functions we call
        otherFunction(obj);
    }

Use this function with care.

This function was introduced in Qt 4.6.

See also isNull() and toStrongRef().

bool QWeakPointer::isNull() const

Returns true if this object refers to nullptr.

Note that, due to the nature of weak references, the pointer that QWeakPointer references can become nullptr at any moment, so the value returned from this function can change from false to true from one call to the next.

QSharedPointer<T> QWeakPointer::lock() const

Same as toStrongRef().

This function is provided for API compatibility with std::weak_ptr.

This function was introduced in Qt 5.4.

void QWeakPointer::swap(QWeakPointer<T> &other)

Swaps this weak pointer instance with other. This function is very fast and never fails.

This function was introduced in Qt 5.4.

QSharedPointer<T> QWeakPointer::toStrongRef() const

Promotes this weak reference to a strong one and returns a QSharedPointer object holding that reference. When promoting to QSharedPointer, this function verifies if the object has been deleted already or not. If it hasn't, this function increases the reference count to the shared object, thus ensuring that it will not get deleted.

Since this function can fail to obtain a valid strong reference to the shared object, you should always verify if the conversion succeeded, by calling QSharedPointer::isNull() on the returned object.

For example, the following code promotes a QWeakPointer that was held to a strong reference and, if it succeeded, it prints the value of the integer that was held:

    QWeakPointer<int> weakref;

    // ...

    QSharedPointer<int> strong = weakref.toStrongRef();
    if (strong)
        qDebug() << "The value is:" << *strong;
    else
        qDebug() << "The value has already been deleted";

See also QSharedPointer::QSharedPointer().

bool QWeakPointer::operator bool() const

Returns true if the contained pointer is not nullptr. This function is suitable for use in if-constructs, like:

    if (weakref) { ... }

Note that, due to the nature of weak references, the pointer that QWeakPointer references can become nullptr at any moment, so the value returned from this function can change from true to false from one call to the next.

See also isNull().

bool QWeakPointer::operator!() const

Returns true if this object refers to nullptr. This function is suitable for use in if-constructs, like:

    if (!weakref) { ... }

Note that, due to the nature of weak references, the pointer that QWeakPointer references can become nullptr at any moment, so the value returned from this function can change from false to true from one call to the next.

See also isNull().