Qt Toolkit - QGuardedPtr Class

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# QGuardedPtr Class Reference

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

`#include <qguardedptr.h>`

List of all member functions.

## Public Members

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

## Detailed Description

The QGuardedPtr class is a template class that provides guarded pointers to QObjects

A guarded pointer, QGuardedPtr<X>, behaves like a normal C++ pointer X*, except that it is automatically set to null when the referenced object is destroyed, unlike normal C++ pointers which become "dangling pointers" in that case. 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 keep a reference to it. You can safely test the pointer for validity.

Example:

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

          delete (QLabel*) label; // emulate 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 only used 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 for equality (==) and inequality (!=), or test for null with isNull(). Finally, 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, then 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.

## Member Function Documentation

### QGuardedPtr::QGuardedPtr ()

Constructs a null guarded pointer.

See also isNull().

### QGuardedPtr::QGuardedPtr ( T * p )

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

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

Copy one guarded pointer from another. The constructed guarded pointer points to the same object that p pointed to (possibly null).

### QGuardedPtr::~QGuardedPtr ()

Destructs the guarded pointer. Note that the object pointed to by the pointer is not destructed, just as is the case with a normal unguarded pointer.

### QGuardedPtr::operator T* () const

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

### bool QGuardedPtr::isNull () const

Returns `TRUE` if the referenced object has been destroyed or if there is no referenced object.

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

Unequality operator, implements pointer semantics, the negation of operator==().

### 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= ( T * p )

Assignment operator. This guarded pointer then points to same object as is pointed to by p.

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

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

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

Equality operator, implements traditional pointer semantics: returns TRUE if both p and this are null, or if both p and this point to the same object.

See also operator!=().

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