The QWeakPointer class holds a weak reference to a shared pointer 更多...
| 头： | #include <QWeakPointer> |
| Since: | Qt 4.5 |
注意： 此类的所有函数 可重入 .
| QWeakPointer () | |
| QWeakPointer (const QWeakPointer<T> & other ) | |
| QWeakPointer (const QSharedPointer<T> & other ) | |
| QWeakPointer (const QObject * obj ) | |
| ~QWeakPointer () | |
| void | clear () |
| T * | data () const |
| bool | isNull () const |
| QSharedPointer<T> | toStrongRef () const |
| operator bool () const | |
| bool | operator! () const |
| QWeakPointer<T> | operator= (const QWeakPointer<T> & other ) |
| QWeakPointer<T> | operator= (const QSharedPointer<T> & other ) |
| QWeakPointer<T> | operator= (const QObject * obj ) |
| QSharedPointer<X> | qSharedPointerCast (const QWeakPointer<T> & other ) |
| QSharedPointer<X> | qSharedPointerConstCast (const QWeakPointer<T> & other ) |
| QSharedPointer<X> | qSharedPointerDynamicCast (const QWeakPointer<T> & other ) |
| QSharedPointer<X> | qSharedPointerObjectCast (const QWeakPointer<T> & other ) |
| QWeakPointer<X> | qWeakPointerCast (const QWeakPointer<T> & other ) |
| 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 QSharedPointer<T> & ptr1 , const QWeakPointer<X> & ptr2 ) |
The QWeakPointer class holds a weak reference to a shared pointer
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 . 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 () 范例。
QWeakPointer 还提供 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 使用 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 ).
QWeakPointer can be used to track deletion of classes that derive 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
T
is either
QObject
or a class deriving from it. Trying to use that constructor if
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.
注意， 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.
另请参阅 QSharedPointer and QScopedPointer .
创建 QWeakPointer that points to nothing.
创建 QWeakPointer that holds a weak reference to the pointer referenced by other .
若
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.
创建 QWeakPointer that holds a weak reference to the pointer referenced by other .
若
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.
创建
QWeakPointer
that holds a weak reference directly to the
QObject
obj
. This constructor is only available if the template type
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 QSharedPointer .
Note that QWeakPointers created this way on arbitrary QObjects usually cannot be promoted to QSharedPointer .
该函数在 Qt 4.6 引入。
另请参阅 QSharedPointer and QWeakPointer#tracking-qobject .
销毁此 QWeakPointer object. The pointer referenced by this object will not be deleted.
清零此 QWeakPointer object, dropping the reference that it may have had to the pointer.
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 0.
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.
该函数在 Qt 4.6 引入。
另请参阅 isNull () 和 toStrongRef ().
Returns true if this object is holding a reference to a null pointer.
Note that, due to the nature of weak references, the pointer that QWeakPointer references can become null at any moment, so the value returned from this function can change from false to true from one call to the next.
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 ().
Returns true if this object is not null. 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 null 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 ().
Returns true if this object is null. 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 null 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 ().
Makes this object share other 's pointer. The current pointer reference is discarded but is not deleted.
若
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.
Makes this object share other 's pointer. The current pointer reference is discarded but is not deleted.
若
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.
Makes this
QWeakPointer
hold a weak reference directly to the
QObject
obj
. This function is only available if the template type
T
is
QObject
or derives from it.
该函数在 Qt 4.6 引入。
另请参阅 QWeakPointer#tracking-qobject .
Returns a shared pointer to the pointer held by
other
, cast to type
X
. The types
T
and
X
must belong to one hierarchy for the
static_cast
to succeed.
The other object is converted first to a strong reference. If that conversion fails (because the object it's pointing to has already been deleted), this function returns a null QSharedPointer .
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.
另请参阅 QWeakPointer::toStrongRef (), qSharedPointerDynamicCast ()，和 qSharedPointerConstCast ().
Returns a shared pointer to the pointer held by
other
, cast to type
X
. The types
T
and
X
must belong to one hierarchy for the
const_cast
to succeed. The
const
and
volatile
differences between
T
and
X
被忽略。
The other object is converted first to a strong reference. If that conversion fails (because the object it's pointing to has already been deleted), this function returns a null QSharedPointer .
另请参阅 QWeakPointer::toStrongRef (), qSharedPointerCast ()，和 qSharedPointerDynamicCast ().
Returns a shared pointer to the pointer held by
other
, using a dynamic cast to type
X
to obtain an internal pointer of the appropriate type. If the
dynamic_cast
fails, the object returned will be null.
The other object is converted first to a strong reference. If that conversion fails (because the object it's pointing to has already been deleted), this function also returns a null QSharedPointer .
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.
另请参阅 QWeakPointer::toStrongRef (), qSharedPointerCast ()，和 qSharedPointerConstCast ().
The qSharedPointerObjectCast function is for casting a shared pointer.
Returns a shared pointer to the pointer held by
other
, using a
qobject_cast
() to type
X
to obtain an internal pointer of the appropriate type. If the
qobject_cast
fails, the object returned will be null.
The other object is converted first to a strong reference. If that conversion fails (because the object it's pointing to has already been deleted), this function also returns a null QSharedPointer .
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.
该函数在 Qt 4.6 引入。
另请参阅 QWeakPointer::toStrongRef (), qSharedPointerCast ()，和 qSharedPointerConstCast ().
Returns a weak pointer to the pointer held by
other
, 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.
Returns true if the pointer referenced by ptr1 is not the same pointer as that referenced by ptr2 .
若
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.
Returns true if the pointer referenced by ptr1 is not the same pointer as that referenced by ptr2 .
若
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.
Returns true if the pointer referenced by ptr1 is the same pointer as that referenced by ptr2 .
若
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.
Returns true if the pointer referenced by ptr1 is the same pointer as that referenced by ptr2 .
若
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.
