QGuardedPtr Class Reference

Detailed Description

A guarded pointer, QGuardedPtr<X>, behaves like a normal C++ pointer X*, 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). X 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:

        QGuardedPtr<QLabel> label = new QLabel( 0, "label" );
        label->setText( "I like guarded pointers" );

        delete (QLabel*) label; // simulate somebody destroying the label

        if ( label)
            label->show();
        else
            qDebug("The label has been destroyed");
    

The program will output The label has been destroyed rather than dereferencing an invalid address in label->show().

The functions and operators available with a QGuardedPtr 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 them 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 an X* 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(). And you can dereference them using either the *x or the x->member notation.

A guarded pointer will automatically cast to an X*, so you can freely mix guarded and unguarded pointers. This means that if you have a QGuardedPtr, 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 QGuardedPtr as a parameter; just use normal pointers. Use a QGuardedPtr when you are storing a pointer over time.

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

See also Object Model.

Member Function Documentation

QGuardedPtr::QGuardedPtr ()

Constructs a 0 guarded pointer.

See also isNull().

QGuardedPtr::QGuardedPtr ( T * p )

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

QGuardedPtr::QGuardedPtr ( const QGuardedPtr<T> & p )

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

QGuardedPtr::~QGuardedPtr ()

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

bool QGuardedPtr::isNull () const

Returns TRUE if the referenced object has been destroyed or if there is no referenced object; otherwise returns FALSE.

QGuardedPtr::operator T * () const

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

bool QGuardedPtr::operator!= ( const QGuardedPtr<T> & p ) const

Inequality operator; implements pointer semantics, the negation of operator==(). Returns TRUE if p and this guarded pointer are not pointing to the same object; otherwise returns FALSE.

T & QGuardedPtr::operator* () const

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

T * QGuardedPtr::operator-> () const

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

QGuardedPtr<T> & QGuardedPtr::operator= ( const QGuardedPtr<T> & p )

Assignment operator. This guarded pointer then points to the same object as p points to.

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

Assignment operator. This guarded pointer then points to the same object as p points to.

bool QGuardedPtr::operator== ( const QGuardedPtr<T> & p ) const

Equality operator; implements traditional pointer semantics. Returns TRUE if both p and this guarded pointer are 0, or if both p and this pointer point to the same object; otherwise returns FALSE.

See also operator!=().

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