Qt 4.3: QPointer Class Reference

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# QPointer Class Reference
[QtCore module]

The QPointer class is a template class that provides guarded pointers to QObjects. More...

     #include <QPointer>

- List of all members, including inherited members

### Public Functions

- QPointer ()
- QPointer ( T * p )
- QPointer ( const QPointer<T> & p )
- ~QPointer ()
- bool isNull () const
- operator T * () const
- T & operator* () const
- T * operator-> () const
- QPointer<T> & operator= ( const QPointer<T> & p )
- QPointer<T> & operator= ( T * p )

## Detailed Description

The QPointer class is a template class that provides guarded pointers to QObjects.

A guarded pointer, QPointer<T>, behaves like a normal C++ pointer `T *`, except that it is automatically set to 0 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.

Example:

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

If the QLabel is deleted in the meantime, the `label` variable will hold 0 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 (`+`, `-`, `++`, and `--`), 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 0 with isNull(). You can dereference them using either the `*x` or the `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.

See also QObject and QObjectCleanupHandler.

## Member Function Documentation

### QPointer::QPointer ()

Constructs a 0 guarded pointer.

See also isNull().

### QPointer::QPointer ( T * p )

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

### QPointer::QPointer ( const QPointer<T> & p )

Copies one guarded pointer from another. The constructed guarded pointer points to the same object that p points to (which may be 0).

### QPointer::~QPointer ()

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

### bool QPointer::isNull () const

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

### 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.

### T & QPointer::operator* () const

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

### T * QPointer::operator-> () const

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

### QPointer<T> & QPointer::operator= ( const QPointer<T> & p )

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

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

This is an overloaded member function, provided for convenience.

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