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  5. QPointer

QPointer 類

模闆 <typename T> 類 QPointer

QPointer 類是提供守衛指針的模闆類，指嚮 QObject . 更多...

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

此類 相等可比較 .

此類 相等可比較 with QPointer<X>, X*, and std::nullptr_t.

Where X and T are compatible types, which means that they are either the same (except for their cv-qualifiers), or one is a base type of the other.

公共函數

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

詳細描述

守衛指針 QPointer<T>，行為像正常 C++ 指針 T * ，除瞭它會被自動清零，當銷毀引用對象時 (不像正常 C++ 指針，在這種情況下，會變成 "懸空指針")。 T 必須是子類化的 QObject .

守衛指針很有用，每當需要存儲的指針指嚮 QObject 由他人擁有，因此仍然可能在保持其引用時被銷毀。可以安全地測試，指針的有效性。

注意，Qt 5 引入瞭輕微的行為變化，當使用 QPointer 時。

  • 當使用 QPointer 在 QWidget (或子類化的 QWidget )，先前，QPointer 的清零是通過 QWidget 析構函數。現在，QPointer 的清零是通過 QObject 析構函數 (由於這是在 QWeakPointer 對象被清零時)。追蹤 Widget 的任何 QPointer 都會 NOT 被清零先於 QWidget 析構函數銷毀用於追蹤 Widget 的子級。

Qt 還提供 QSharedPointer ，計數引用共享指針對象的實現，可以用於維護單個指針的引用集閤。

範例： 

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

若 QLabel 被同時刪除， label 變量將保持 nullptr 而不是無效地址，且從不會執行最後一行。

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.

使用 QPointer 像正常指針，且不需要閱讀此類文檔編製。

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 錶示法。

守衛指針將自動鑄造成 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.

注意，類 T 必須繼承 QObject ，否則會導緻編譯 (或鏈接) 錯誤。

另請參閱 QSharedPointer , QObject ，和 QObjectCleanupHandler .

成員函數文檔編製

[noexcept] QPointer:: QPointer ()

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

構造守衛指針采用值 nullptr .

另請參閱 isNull ().

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

[noexcept, since 6.6] template <typename X, QPointer<T>::if_convertible<X> = 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* .

該函數在 Qt 6.6 引入。

QPointer:: QPointer ( T * p )

構造守衛指針指嚮同一對象 p 所指嚮。

QPointer:: ~QPointer ()

銷毀守衛指針。就像正常指針，銷毀守衛指針 not 銷毀所指嚮的對象。

[noexcept] void QPointer:: clear ()

清零此 QPointer 對象。

另請參閱 isNull ().

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

返迴指嚮被守衛對象的指針。

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

如同 data ()。此函數為兼容 STL (標準模闆庫) 提供。

該函數在 Qt 6.0 引入。

[noexcept] bool QPointer:: isNull () const

返迴 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 this pointer with other 。此操作很快且從不失敗。

[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 6.6.1] template <typename X, QPointer<T>::if_convertible<X> = 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 )

賦值運算符。此守衛指針現在將指嚮同一對象 p 所指嚮。

[noexcept, since 6.6] template <typename X, QPointer<T>::if_convertible<X> = 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* .

該函數在 Qt 6.6 引入。

相關非成員

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

不相等操作符。返迴 true if lhs 和守衛指針 rhs 未指嚮同一對象，否則返迴 false .

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

不相等操作符。返迴 true if rhs 和守衛指針 lhs 未指嚮同一對象，否則返迴 false .

[noexcept(...)] template <typename X> bool operator!= (const QPointer < T > & lhs , const QPointer < X > & rhs )

不相等操作符。返迴 true 若守衛指針 lhs and rhs 未指嚮同一對象，否則返迴 false .

注意： This function is noexcept when true is true .

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

不相等操作符。返迴 true if the pointer guarded by lhs is a valid (ie not nullptr ) pointer, otherwise returns false .

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

不相等操作符。返迴 true if the pointer guarded by rhs is a valid (ie not nullptr ) pointer, otherwise returns false .

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

相等運算符。返迴 true if lhs 和守衛指針 rhs 指嚮同一對象，否則返迴 false .

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

相等運算符。返迴 true if rhs 和守衛指針 lhs 指嚮同一對象，否則返迴 false .

[noexcept(...)] template <typename X> bool operator== (const QPointer < T > & lhs , const QPointer < X > & rhs )

相等運算符。返迴 true 若守衛指針 lhs and rhs 指嚮同一對象，否則返迴 false .

注意： This function is noexcept when true is true .

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

相等運算符。返迴 true if the pointer guarded by lhs is nullptr ，否則返迴 false .

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

相等運算符。返迴 true if the pointer guarded by rhs is nullptr ，否則返迴 false .

內容

  1. 公共函數

  2. 相關非成員

  3. 詳細描述