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  5. QPointer

QPointer 类

template <typename T> class QPointer

QPointer 类是提供守卫指针的模板类，指向 QObject . 更多...

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

公共函数

QPointer ()
QPointer (std::nullptr_t)
QPointer (T * p )
(从 6.6 起) QPointer (QPointer<X> && other )
(从 6.6 起) QPointer (const QPointer<X> & other )
~QPointer ()
void clear ()
T * data () const
(从 6.0 起) T * get () const
bool isNull () const
void swap (QPointer<T> & other )
T * operator T * () const
T & operator* () const
T * operator-> () const
(从 6.6 起) QPointer<T> & operator= (const QPointer<X> & other )
(since 6.6.1) QPointer<T> & operator= (QPointer<X> && other )
QPointer<T> & operator= (T * p )
bool operator!= (const QPointer<T> & p1 , const QPointer<X> & p2 )
bool operator!= (const QPointer<T> & p , X * o )
bool operator!= (X * o , const QPointer<T> & p )
bool operator!= (const QPointer<T> & lhs , std::nullptr_t)
bool operator!= (std::nullptr_t, const QPointer<T> & rhs )
bool operator== (const QPointer<T> & p1 , const QPointer<X> & p2 )
bool operator== (const QPointer<T> & p , X * o )
bool operator== (X * o , const QPointer<T> & p )
bool operator== (const QPointer<T> & lhs , std::nullptr_t)
bool operator== (std::nullptr_t, const QPointer<T> & rhs )

详细描述

A guarded pointer, QPointer<T>, behaves like a normal C++ pointer T * , except that it is automatically cleared when the referenced object is destroyed (unlike normal C++ pointers, which become "dangling pointers" in such cases). T must be a subclass of QObject .

Guarded pointers are useful whenever you need to store a pointer to a QObject that is owned by someone else, and therefore might be destroyed while you still hold a reference to it. You can safely test the pointer for validity.

Note that Qt 5 introduces a slight change in behavior when using QPointer.

  • When using QPointer on a QWidget (or a subclass of QWidget ), previously the QPointer would be cleared by the QWidget destructor. Now, the QPointer is cleared by the QObject destructor (since this is when QWeakPointer objects are cleared). Any QPointers tracking a widget will NOT be cleared before the QWidget destructor destroys the children for the widget being tracked.

Qt also provides QSharedPointer , an implementation of a reference-counted shared pointer object, which can be used to maintain a collection of references to an individual pointer.

范例： 

    QPointer<QLabel> label = new QLabel;
    label->setText("&Status:");
    ...
    if (label)
        label->show();

若 QLabel is deleted in the meantime, the label variable will hold nullptr instead of an invalid address, and the last line will never be executed.

The functions and operators available with a QPointer are the same as those available with a normal unguarded pointer, except the pointer arithmetic operators ( + , - , ++ ，和 -- ), which are normally used only with arrays of objects.

Use QPointers like normal pointers and you will not need to read this class documentation.

For creating guarded pointers, you can construct or assign to them from a T* or from another guarded pointer of the same type. You can compare them with each other using operator==() and operator!=(), or test for nullptr with isNull (). You can dereference them using either the *x 或 x->member notation.

A guarded pointer will automatically cast to a T *, so you can freely mix guarded and unguarded pointers. This means that if you have a QPointer< QWidget >, you can pass it to a function that requires a QWidget *. For this reason, it is of little value to declare functions to take a QPointer as a parameter; just use normal pointers. Use a QPointer when you are storing a pointer over time.

Note that class T must inherit QObject , or a compilation or link error will result.

另请参阅 QSharedPointer , QObject ，和 QObjectCleanupHandler .

成员函数文档编制

[noexcept] QPointer:: QPointer ()

[constexpr noexcept] QPointer:: QPointer ( std::nullptr_t )

构造守卫指针采用值 nullptr .

另请参阅 isNull ().

[noexcept, since 6.6] template <typename X, QPointer<T>::if_convertible = true> QPointer:: QPointer ( QPointer < X > && other )

[noexcept, since 6.6] template <typename X, QPointer<T>::if_convertible = true> QPointer:: QPointer (const QPointer < X > & other )

Conversion constructor. Constructs a new QPointer by moving or copying from other .

The moved-from QPointer is reset to nullptr.

注意： These constructors participate in overload resolution only if X* is convertible to T* .

该函数在 Qt 6.6 引入。

QPointer:: QPointer ( T * p )

Constructs a guarded pointer that points to the same object that p points to.

QPointer:: ~QPointer ()

Destroys the guarded pointer. Just like a normal pointer, destroying a guarded pointer does not destroy the object being pointed to.

[noexcept] void QPointer:: clear ()

清零此 QPointer 对象。

另请参阅 isNull ().

[noexcept] T *QPointer:: data () const

返回指向被守卫对象的指针。

[noexcept, since 6.0] T *QPointer:: get () const

如同 data ()。此函数为兼容 STL (标准模板库) 提供。

该函数在 Qt 6.0 引入。

[noexcept] bool QPointer:: isNull () const

返回 true if the referenced object has been destroyed or if there is no referenced object; otherwise returns false .

[noexcept] void QPointer:: swap ( QPointer < T > & other )

Swaps the contents of this QPointer with the contents of other 。此操作非常快且从不失败。

[noexcept] T *QPointer:: operator T * () const

Cast operator; implements pointer semantics. Because of this function you can pass a QPointer <T> to a function where a T* is required.

[noexcept] T &QPointer:: operator* () const

Dereference operator; implements pointer semantics. Just use this operator as you would with a normal C++ pointer.

[noexcept] T *QPointer:: operator-> () const

Overloaded arrow operator; implements pointer semantics. Just use this operator as you would with a normal C++ pointer.

[noexcept, since 6.6] template <typename X, QPointer<T>::if_convertible = true> QPointer < T > &QPointer:: operator= (const QPointer < X > & other )

Conversion assignment operator. Makes this guarded pointer guard the same object guarded by other .

注意： This operator participates in overload resolution only if X* is convertible to T* .

该函数在 Qt 6.6 引入。

[noexcept, since 6.6.1] template <typename X, QPointer<T>::if_convertible = true> QPointer < T > &QPointer:: operator= ( QPointer < X > && other )

Conversion move-assignment operator. Makes this guarded pointer guard the same object guarded by other and resets other to nullptr.

注意： This operator participates in overload resolution only if X* is convertible to T* .

This function was introduced in Qt 6.6.1.

QPointer < T > &QPointer:: operator= ( T * p )

Assignment operator. This guarded pointer will now point to the same object that p points to.

相关非成员

[noexcept] template <typename X> bool operator!= (const QPointer < T > & p1 , const QPointer < X > & p2 )

Inequality operator. Returns true if the guarded pointers p1 and p2 are not pointing to the same object, otherwise returns false .

[noexcept] template <typename X> bool operator!= (const QPointer < T > & p , X * o )

Inequality operator. Returns true if o and the guarded pointer p are not pointing to the same object, otherwise returns false .

[noexcept] template <typename X> bool operator!= ( X * o , const QPointer < T > & p )

Inequality operator. Returns true if o and the guarded pointer p are not pointing to the same object, otherwise returns false .

[noexcept] bool operator!= (const QPointer < T > & lhs , std::nullptr_t )

Inequality operator. Returns true if the pointer guarded by lhs is a valid (ie not nullptr ) pointer, otherwise returns false .

[noexcept] bool operator!= ( std::nullptr_t , const QPointer < T > & rhs )

Inequality operator. Returns true if the pointer guarded by rhs is a valid (ie not nullptr ) pointer, otherwise returns false .

[noexcept] template <typename X> bool operator== (const QPointer < T > & p1 , const QPointer < X > & p2 )

相等运算符。返回 true if the guarded pointers p1 and p2 are pointing to the same object, otherwise returns false .

[noexcept] template <typename X> bool operator== (const QPointer < T > & p , X * o )

相等运算符。返回 true if o and the guarded pointer p are pointing to the same object, otherwise returns false .

[noexcept] template <typename X> bool operator== ( X * o , const QPointer < T > & p )

相等运算符。返回 true if o and the guarded pointer p are pointing to the same object, otherwise returns false .

[noexcept] bool operator== (const QPointer < T > & lhs , std::nullptr_t )

相等运算符。返回 true if the pointer guarded by lhs is nullptr ，否则返回 false .

[noexcept] bool operator== ( std::nullptr_t , const QPointer < T > & rhs )

相等运算符。返回 true if the pointer guarded by rhs is nullptr ，否则返回 false .

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