diff options
Diffstat (limited to 'src/corelib/tools/qsharedpointer.cpp')
| -rw-r--r-- | src/corelib/tools/qsharedpointer.cpp | 123 |
1 files changed, 108 insertions, 15 deletions
diff --git a/src/corelib/tools/qsharedpointer.cpp b/src/corelib/tools/qsharedpointer.cpp index 4fecc9ac6b..f0e85d2066 100644 --- a/src/corelib/tools/qsharedpointer.cpp +++ b/src/corelib/tools/qsharedpointer.cpp @@ -34,7 +34,7 @@ ** met: http://www.gnu.org/copyleft/gpl.html. ** ** If you are unsure which license is appropriate for your use, please -** contact the sales department at http://www.qtsoftware.com/contact. +** contact the sales department at http://qt.nokia.com/contact. ** $QT_END_LICENSE$ ** ****************************************************************************/ @@ -98,20 +98,19 @@ access made to the data being guarded: if it's a non-const access, it creates a copy atomically for the operation to complete. - QExplicitlySharedDataPointer behaves like QSharedDataPointer, - except that it only detaches if - QExplicitlySharedDataPointer::detach() is explicitly called. + QExplicitlySharedDataPointer is a variant of QSharedDataPointer, except + that it only detaches if QExplicitlySharedDataPointer::detach() is + explicitly called (hence the name). QScopedPointer simply holds a pointer to a heap allocated object and deletes it in its destructor. This class is useful when an object needs to be heap allocated and deleted, but no more. QScopedPointer is lightweight, it makes no use of additional structure or reference counting. - Finally, QPointer holds a pointer to a QObject-derived object, but - it does so weakly. QPointer is similar, in that behaviour, to - QWeakPointer: it does not allow you to prevent the object from - being destroyed. All you can do is query whether it has been - destroyed or not. + Finally, QPointer holds a pointer to a QObject-derived object, but it + does so weakly. QPointer can be replaced by QWeakPointer in almost all + cases, since they have the same functionality. See + \l{QWeakPointer#tracking-qobject} for more information. \sa QSharedDataPointer, QWeakPointer, QScopedPointer */ @@ -128,12 +127,65 @@ 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. - - To access the pointer that QWeakPointer is tracking, you - must first create a QSharedPointer object and verify if the pointer - is null or not. See QWeakPointer::toStrongRef() for more information. + QWeakPointer objects can only be created by assignment from a + QSharedPointer. The exception is pointers derived from QObject: in that + case, QWeakPointer serves as a replacement to QPointer. + + 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 more 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. + + That function can also be used to obtain the tracked pointer for + QWeakPointers that cannot be promoted to QSharedPointer, such as those + created directly from a QObject pointer (not via QSharedPointer). + + \section1 Tracking QObject + + QWeakPointer can be used to track deletion classes derives from QObject, + even if they are not managed by QSharedPointer. When used in that role, + QWeakPointer replaces the older QPointer in all use-cases. QWeakPointer + is also more efficient than QPointer, so it should be preferred in all + new code. + + To do that, QWeakPointer provides a special constructor that is only + available if the template parameter \tt T is either QObject or a class + deriving from it. Trying to use that constructor if \tt T does not derive + from QObject will result in compilation errors. + + To obtain the QObject being tracked by QWeakPointer, you must use the + QWeakPointer::data() function, but only if you can guarantee that the + object cannot get deleted by another context. It should be noted that + QPointer had the same constraint, so use of QWeakPointer forces you to + consider whether the pointer is still valid. + + QObject-derived classes can only be deleted in the thread they have + affinity to (which is the thread they were created in or moved to, using + QObject::moveToThread()). In special, QWidget-derived classes cannot be + created in non-GUI threads nor moved there. Therefore, guaranteeing that + the tracked QObject has affinity to the current thread is enough to also + guarantee that it won't be deleted asynchronously. + + Note that QWeakPointer's size and data layout do not match QPointer, so + it cannot replace that class in a binary-compatible manner. + + Care must also be taken with QWeakPointers created directly from QObject + pointers when dealing with code that was compiled with Qt versions prior + to 4.6. Those versions may not track the reference counters correctly, so + QWeakPointers created from QObject should never be passed to code that + hasn't been recompiled. \sa QSharedPointer, QScopedPointer */ @@ -417,6 +469,35 @@ */ /*! + \fn QWeakPointer::QWeakPointer(const QObject *obj) + \since 4.6 + + Creates a QWeakPointer that holds a weak reference directly to the + QObject \a obj. This constructor is only available if the template type + \tt T is QObject or derives from it (otherwise a compilation error will + result). + + You can use this constructor with any QObject, even if they were not + created with \l QSharedPointer. + + Note that QWeakPointers created this way on arbitrary QObjects usually + cannot be promoted to QSharedPointer. + + \sa QSharedPointer, QWeakPointer#tracking-qobject +*/ + +/*! + \fn QWeakPointer &QWeakPointer::operator=(const QObject *obj) + \since 4.6 + + Makes this QWeakPointer hold a weak reference to directly to the QObject + \a obj. This function is only available if the template type \tt T is + QObject or derives from it. + + \sa QWeakPointer#tracking-qobject +*/ + +/*! \fn QWeakPointer &QWeakPointer::operator=(const QWeakPointer<T> &other) Makes this object share \a other's pointer. The current pointer @@ -872,6 +953,8 @@ #if !defined(QT_NO_QOBJECT) #include "../kernel/qobject_p.h" +QT_BEGIN_NAMESPACE + /*! \internal This function is called for a just-created QObject \a obj, to enable @@ -899,6 +982,8 @@ QtSharedPointer::ExternalRefCountData *QtSharedPointer::ExternalRefCountData::ge { Q_ASSERT(obj); QObjectPrivate *d = QObjectPrivate::get(const_cast<QObject *>(obj)); + Q_ASSERT_X(!d->wasDeleted, "QWeakPointer", "Detected QWeakPointer creation in a QObject being deleted"); + ExternalRefCountData *that = d->sharedRefcount; if (that) { that->weakref.ref(); @@ -915,6 +1000,9 @@ QtSharedPointer::ExternalRefCountData *QtSharedPointer::ExternalRefCountData::ge } return d->sharedRefcount; } + +QT_END_NAMESPACE + #endif @@ -937,6 +1025,8 @@ QtSharedPointer::ExternalRefCountData *QtSharedPointer::ExternalRefCountData::ge # include <unistd.h> # include <sys/wait.h> +QT_BEGIN_NAMESPACE + static inline QByteArray saveBacktrace() __attribute__((always_inline)); static inline QByteArray saveBacktrace() { @@ -997,6 +1087,9 @@ static void printBacktrace(QByteArray stacktrace) waitpid(child, 0, 0); } } + +QT_END_NAMESPACE + # endif // BACKTRACE_SUPPORTED namespace { |