QWeakPointer 类

template <typename T> class QWeakPointer

QWeakPointer 类保持共享指针的弱引用。 更多...

头: #include <QWeakPointer>
CMake: find_package(Qt6 REQUIRED COMPONENTS Core)
target_link_libraries(mytarget PRIVATE Qt6::Core)
qmake: QT += core

注意: 此类的所有函数 可重入 .

公共函数

QWeakPointer ()
QWeakPointer (const QWeakPointer<T> & other )
QWeakPointer (const QSharedPointer<T> & other )
~QWeakPointer ()
void clear ()
bool isNull () const
QSharedPointer<T> lock () const
void swap (QWeakPointer<T> & other )
QSharedPointer<T> toStrongRef () const
bool operator bool () const
bool operator! () const
QWeakPointer<T> & operator= (const QWeakPointer<T> & other )
QWeakPointer<T> & operator= (const QSharedPointer<T> & other )
QWeakPointer<X> qWeakPointerCast (const QWeakPointer<T> & src )
bool operator!= (const QSharedPointer<T> & ptr1 , const QWeakPointer<X> & ptr2 )
bool operator!= (const QWeakPointer<T> & lhs , std::nullptr_t)
bool operator!= (std::nullptr_t, const QWeakPointer<T> & rhs )
bool operator!= (const QWeakPointer<T> & ptr1 , const QSharedPointer<X> & ptr2 )
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 )

详细描述

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 () 范例。

另请参阅 QSharedPointer and QScopedPointer .

成员函数文档编制

QWeakPointer:: QWeakPointer ()

Creates a QWeakPointer that points to nothing.

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

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

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 QSharedPointer < T > & other )

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

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 ()

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

void QWeakPointer:: clear ()

清零此 QWeakPointer object, dropping the reference that it may have had to the pointer.

bool QWeakPointer:: isNull () const

返回 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

如同 toStrongRef ().

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

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

Swaps this weak pointer instance with other 。此函数非常快且从不失败。

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";
					

另请参阅 QSharedPointer::QSharedPointer ().

bool QWeakPointer:: operator bool () const

返回 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.

另请参阅 isNull ().

bool QWeakPointer:: operator! () const

返回 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.

另请参阅 isNull ().

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.

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 QSharedPointer < T > & other )

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

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.

相关非成员

template <typename X, typename T> QWeakPointer < X > qWeakPointerCast (const QWeakPointer < T > & src )

Returns a weak pointer to the pointer held by src , cast to type X . The types T and X must belong to one hierarchy for the static_cast to succeed.

注意, X must have the same cv-qualifiers ( const and volatile ) that T has, or the code will fail to compile. Use qSharedPointerConstCast to cast away the constness.

template <typename T, typename X> bool operator!= (const QSharedPointer < T > & ptr1 , const QWeakPointer < X > & ptr2 )

返回 true if ptr1 and ptr2 refer to distinct pointers.

ptr2 's template parameter is different from ptr1 's, QSharedPointer will attempt to perform an automatic static_cast to ensure that the pointers being compared are equal. If ptr2 's template parameter is not a base or a derived type from ptr1 's, you will get a compiler error.

template <typename T> bool operator!= (const QWeakPointer < T > & lhs , std::nullptr_t )

返回 true if lhs refers to a valid (i.e. non-null) pointer.

另请参阅 QWeakPointer::isNull ().

template <typename T> bool operator!= ( std::nullptr_t , const QWeakPointer < T > & rhs )

返回 true if rhs refers to a valid (i.e. non-null) pointer.

另请参阅 QWeakPointer::isNull ().

template <typename T, typename X> bool operator!= (const QWeakPointer < T > & ptr1 , const QSharedPointer < X > & ptr2 )

返回 true if ptr1 and ptr2 refer to distinct pointers.

ptr2 's template parameter is different from ptr1 's, QSharedPointer will attempt to perform an automatic static_cast to ensure that the pointers being compared are equal. If ptr2 's template parameter is not a base or a derived type from ptr1 's, you will get a compiler error.

template <typename T, typename X> bool operator== (const QSharedPointer < T > & ptr1 , const QWeakPointer < X > & ptr2 )

返回 true if ptr1 and ptr2 refer to the same pointer.

ptr2 's template parameter is different from ptr1 's, QSharedPointer will attempt to perform an automatic static_cast to ensure that the pointers being compared are equal. If ptr2 's template parameter is not a base or a derived type from ptr1 's, you will get a compiler error.

template <typename T, typename X> bool operator== (const QWeakPointer < T > & ptr1 , const QSharedPointer < X > & ptr2 )

返回 true if ptr1 and ptr2 refer to the same pointer.

ptr2 's template parameter is different from ptr1 's, QSharedPointer will attempt to perform an automatic static_cast to ensure that the pointers being compared are equal. If ptr2 's template parameter is not a base or a derived type from ptr1 's, you will get a compiler error.

template <typename T> bool operator== (const QWeakPointer < T > & lhs , std::nullptr_t )

返回 true if lhs refers to nullptr .

另请参阅 QWeakPointer::isNull ().

template <typename T> bool operator== ( std::nullptr_t , const QWeakPointer < T > & rhs )

返回 true if rhs refers to nullptr .

另请参阅 QWeakPointer::isNull ().