Qt Reference Documentation

# const_iterator Class Reference

(QList::const_iterator)

The QList::const_iterator class provides an STL-style const iterator for QList and QQueue. More...

     #include <QList>

- List of all members, including inherited members

## Public Types

typedef | iterator_category

## Public Functions

| const_iterator ()
| const_iterator ( const const_iterator & other )
| const_iterator ( const iterator & other )
bool | operator!= ( const const_iterator & other ) const
const T & | operator* () const
const_iterator | operator+ ( int j ) const
const_iterator & | operator++ ()
const_iterator | operator++ ( int )
const_iterator & | operator+= ( int j )
const_iterator | operator- ( int j ) const
int | operator- ( const_iterator other ) const
const_iterator & | operator-- ()
const_iterator | operator-- ( int )
const_iterator & | operator-= ( int j )
const T * | operator-> () const
bool | operator< ( const const_iterator & other ) const
bool | operator<= ( const const_iterator & other ) const
bool | operator== ( const const_iterator & other ) const
bool | operator> ( const const_iterator & other ) const
bool | operator>= ( const const_iterator & other ) const
const T & | operator[] ( int j ) const

## Detailed Description

The QList::const_iterator class provides an STL-style const iterator for QList and QQueue.

QList provides both STL-style iterators and Java-style iterators. The STL-style iterators are more low-level and more cumbersome to use; on the other hand, they are slightly faster and, for developers who already know STL, have the advantage of familiarity.

QList<T>::const_iterator allows you to iterate over a QList<T> (or a QQueue<T>). If you want to modify the QList as you iterate over it, use QList::iterator instead. It is generally good practice to use QList::const_iterator on a non-const QList as well, unless you need to change the QList through the iterator. Const iterators are slightly faster, and can improve code readability.

The default QList::const_iterator constructor creates an uninitialized iterator. You must initialize it using a QList function like QList::constBegin(), QList::constEnd(), or QList::insert() before you can start iterating. Here's a typical loop that prints all the items stored in a list:

     QList<QString> list;
     list.append("January");
     list.append("February");
     ...
     list.append("December");

     QList<QString>::const_iterator i;
     for (i = list.constBegin(); i != list.constEnd(); ++i)
         cout << *i << endl;

Most QList functions accept an integer index rather than an iterator. For that reason, iterators are rarely useful in connection with QList. One place where STL-style iterators do make sense is as arguments to generic algorithms.

For example, here's how to delete all the widgets stored in a QList<QWidget *>:

     QList<QWidget *> list;
     ...
     qDeleteAll(list.constBegin(), list.constEnd());

Multiple iterators can be used on the same list. However, be aware that any non-const function call performed on the QList will render all existing iterators undefined. If you need to keep iterators over a long period of time, we recommend that you use QLinkedList rather than QList.

See also QList::iterator and QListIterator.

## Member Type Documentation

### typedef const_iterator::iterator_category

A synonym for std::random_access_iterator_tag indicating this iterator is a random access iterator.

## Member Function Documentation

### const_iterator::const_iterator ()

Constructs an uninitialized iterator.

Functions like operator*() and operator++() should not be called on an uninitialized iterator. Use operator=() to assign a value to it before using it.

See also QList::constBegin() and QList::constEnd().

### const_iterator::const_iterator ( const const_iterator & other )

Constructs a copy of other.

### const_iterator::const_iterator ( const iterator & other )

Constructs a copy of other.

### bool const_iterator::operator!= ( const const_iterator & other ) const

Returns true if other points to a different item than this iterator; otherwise returns false.

See also operator==().

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

Returns the current item.

See also operator->().

### const_iterator const_iterator::operator+ ( int j ) const

Returns an iterator to the item at j positions forward from this iterator. (If j is negative, the iterator goes backward.)

See also operator-() and operator+=().

### const_iterator & const_iterator::operator++ ()

The prefix ++ operator (`++it`) advances the iterator to the next item in the list and returns an iterator to the new current item.

Calling this function on QList::end() leads to undefined results.

See also operator--().

### const_iterator const_iterator::operator++ ( int )

This is an overloaded function.

The postfix ++ operator (`it++`) advances the iterator to the next item in the list and returns an iterator to the previously current item.

### const_iterator & const_iterator::operator+= ( int j )

Advances the iterator by j items. (If j is negative, the iterator goes backward.)

See also operator-=() and operator+().

### const_iterator const_iterator::operator- ( int j ) const

Returns an iterator to the item at j positions backward from this iterator. (If j is negative, the iterator goes forward.)

See also operator+() and operator-=().

### int const_iterator::operator- ( const_iterator other ) const

Returns the number of items between the item pointed to by other and the item pointed to by this iterator.

### const_iterator & const_iterator::operator-- ()

The prefix -- operator (`--it`) makes the preceding item current and returns an iterator to the new current item.

Calling this function on QList::begin() leads to undefined results.

See also operator++().

### const_iterator const_iterator::operator-- ( int )

This is an overloaded function.

The postfix -- operator (`it--`) makes the preceding item current and returns an iterator to the previously current item.

### const_iterator & const_iterator::operator-= ( int j )

Makes the iterator go back by j items. (If j is negative, the iterator goes forward.)

See also operator+=() and operator-().

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

Returns a pointer to the current item.

See also operator*().

### bool const_iterator::operator< ( const const_iterator & other ) const

Returns true if the item pointed to by this iterator is less than the item pointed to by the other iterator.

### bool const_iterator::operator<= ( const const_iterator & other ) const

Returns true if the item pointed to by this iterator is less than or equal to the item pointed to by the other iterator.

### bool const_iterator::operator== ( const const_iterator & other ) const

Returns true if other points to the same item as this iterator; otherwise returns false.

See also operator!=().

### bool const_iterator::operator> ( const const_iterator & other ) const

Returns true if the item pointed to by this iterator is greater than the item pointed to by the other iterator.

### bool const_iterator::operator>= ( const const_iterator & other ) const

Returns true if the item pointed to by this iterator is greater than or equal to the item pointed to by the other iterator.

### const T & const_iterator::operator[] ( int j ) const

Returns the item at position *this + j.

This function is provided to make QList iterators behave like C++ pointers.

See also operator+().