Qt Documentation

Qt 5.14
Qt Core
C++ Classes
QListIterator

QListIterator Class

The QListIterator class provides a Java-style const iterator for QList and QQueue. More...

Header:	#include <QListIterator>
qmake:	QT += core

List of all members, including inherited members

Public Functions

	QListIterator(const QList<T> &list)
QListIterator<T> &	operator=(const QList<T> &container)
bool	findNext(const T &value)
bool	findPrevious(const T &value)
bool	hasNext() const
bool	hasPrevious() const
const T &	next()
const T &	peekNext() const
const T &	peekPrevious() const
const T &	previous()
void	toBack()
void	toFront()

Detailed Description

QList has both Java-style iterators and STL-style iterators. The Java-style iterators are more high-level and easier to use than the STL-style iterators; on the other hand, they are slightly less efficient.

An alternative to using iterators is to use index positions. Most QList member functions take an index as their first parameter, making it possible to access, modify, and remove items without using iterators.

QListIterator<T> allows you to iterate over a QList<T> (or a QQueue<T>). If you want to modify the list as you iterate over it, use QMutableListIterator<T> instead.

The QListIterator constructor takes a QList as argument. After construction, the iterator is located at the very beginning of the list (before the first item). Here's how to iterate over all the elements sequentially:

QList<float> list;
...
QListIterator<float> i(list);
while (i.hasNext())
    qDebug() << i.next();

The next() function returns the next item in the list and advances the iterator. Unlike STL-style iterators, Java-style iterators point between items rather than directly at items. The first call to next() advances the iterator to the position between the first and second item, and returns the first item; the second call to next() advances the iterator to the position between the second and third item, and returns the second item; and so on.

Here's how to iterate over the elements in reverse order:

QListIterator<float> i(list);
i.toBack();
while (i.hasPrevious())
    qDebug() << i.previous();

If you want to find all occurrences of a particular value, use findNext() or findPrevious() in a loop.

Multiple iterators can be used on the same list. If the list is modified while a QListIterator is active, the QListIterator will continue iterating over the original list, ignoring the modified copy.

See also QMutableListIterator and QList::const_iterator.

Member Function Documentation

bool QListIterator::findPrevious(const T &value)

Searches for value starting from the current iterator position backward. Returns true if value is found; otherwise returns false.

After the call, if value was found, the iterator is positioned just before the matching item; otherwise, the iterator is positioned at the front of the container.

See also findNext().

bool QListIterator::findNext(const T &value)

Searches for value starting from the current iterator position forward. Returns true if value is found; otherwise returns false.

After the call, if value was found, the iterator is positioned just after the matching item; otherwise, the iterator is positioned at the back of the container.

See also findPrevious().

const T &QListIterator::peekPrevious() const

Returns the previous item without moving the iterator.

Calling this function on an iterator located at the front of the container leads to undefined results.

See also hasPrevious(), previous(), and peekNext().

const T &QListIterator::previous()

Returns the previous item and moves the iterator back by one position.

Calling this function on an iterator located at the front of the container leads to undefined results.

See also hasPrevious(), peekPrevious(), and next().

bool QListIterator::hasPrevious() const

Returns true if there is at least one item behind the iterator, i.e. the iterator is not at the front of the container; otherwise returns false.

See also hasNext() and previous().

const T &QListIterator::peekNext() const

Returns the next item without moving the iterator.

Calling this function on an iterator located at the back of the container leads to undefined results.

See also hasNext(), next(), and peekPrevious().

const T &QListIterator::next()

Returns the next item and advances the iterator by one position.

Calling this function on an iterator located at the back of the container leads to undefined results.

See also hasNext(), peekNext(), and previous().

bool QListIterator::hasNext() const

Returns true if there is at least one item ahead of the iterator, i.e. the iterator is not at the back of the container; otherwise returns false.

See also hasPrevious() and next().

void QListIterator::toBack()

Moves the iterator to the back of the container (after the last item).

See also toFront() and previous().

void QListIterator::toFront()

Moves the iterator to the front of the container (before the first item).

See also toBack() and next().

QListIterator<T> &QListIterator::operator=(const QList<T> &container)

Makes the iterator operate on list. The iterator is set to be at the front of the list (before the first item).

See also toFront() and toBack().

QListIterator::QListIterator(const QList<T> &list)

Constructs an iterator for traversing list. The iterator is set to be at the front of the list (before the first item).

See also operator=().

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