- Qt 5.15
- Qt Core
- C++ Classes
- QMutableListIterator
- Qt 5.15.18 Reference Documentation

# QMutableListIterator Class

template <typename T> class QMutableListIterator

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

Header: | #include <QMutableListIterator>
qmake: | QT += core

- List of all members, including inherited members

## Public Functions

| QMutableListIterator(QList<T> &list)
QMutableListIterator<T> & | operator=(QList<T> &list)
bool | findNext(const T &value)
bool | findPrevious(const T &value)
bool | hasNext() const
bool | hasPrevious() const
void | insert(const T &value)
T & | next()
T & | peekNext() const
T & | peekPrevious() const
T & | previous()
void | remove()
void | setValue(const T &value) const
void | toBack()
void | toFront()
const T & | value() const
T & | value()

## 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, insert, and remove items without using iterators.

QMutableListIterator<T> allows you to iterate over a QList<T> (or a QQueue<T>) and modify the list. If you don't want to modify the list (or have a const QList), use the slightly faster QListIterator<T> instead.

The QMutableListIterator 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;
     ...
     QMutableListIterator<float> i(list);
     while (i.hasNext())
         float f = 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, returning the second item; and so on.

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

     QMutableListIterator<float> i(list);
     i.toBack();
     while (i.hasPrevious())
         float f = i.previous();

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

If you want to remove items as you iterate over the list, use remove(). If you want to modify the value of an item, use setValue(). If you want to insert a new item in the list, use insert().

Example:

     QMutableListIterator<int> i(list);
     while (i.hasNext()) {
         int val = i.next();
         if (val < 0) {
             i.setValue(-val);
         } else if (val == 0) {
             i.remove();
         }
     }

The example traverses a list, replacing negative numbers with their absolute values, and eliminating zeroes.

Only one mutable iterator can be active on a given list at any time. Furthermore, no changes should be done directly to the list while the iterator is active (as opposed to through the iterator), since this could invalidate the iterator and lead to undefined behavior.

See also QListIterator and QList::iterator.

## Member Function Documentation

### void QMutableListIterator::insert(const T &value)

Inserts value at the current iterator position. After the call, the iterator is located just after the inserted item.

See also remove() and setValue().

### T &QMutableListIterator::value()

This is an overloaded function.

Returns a non-const reference to the value of the last item that was jumped over using one of the traversal functions.

### const T &QMutableListIterator::value() const

Returns the value of the last item that was jumped over using one of the traversal functions (next(), previous(), findNext(), findPrevious()).

After a call to next() or findNext(), value() is equivalent to peekPrevious(). After a call to previous() or findPrevious(), value() is equivalent to peekNext().

### bool QMutableListIterator::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 QMutableListIterator::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().

### T &QMutableListIterator::peekPrevious() const

Returns a reference to 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().

### T &QMutableListIterator::previous()

Returns a reference to 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 QMutableListIterator::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().

### T &QMutableListIterator::peekNext() const

Returns a reference to 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().

### T &QMutableListIterator::next()

Returns a reference to 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 QMutableListIterator::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 QMutableListIterator::toBack()

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

See also toFront() and previous().

### void QMutableListIterator::toFront()

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

See also toBack() and next().

### QMutableListIterator<T> &QMutableListIterator::operator=(QList<T> &list)

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().

### QMutableListIterator::QMutableListIterator(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=().

### void QMutableListIterator::remove()

Removes the last item that was jumped over using one of the traversal functions (next(), previous(), findNext(), findPrevious()).

Example:

     QMutableListIterator<int> i(list);
     while (i.hasNext()) {
         int val = i.next();
         if (val < -32768 || val > 32767)
             i.remove();
     }

See also insert() and setValue().

### void QMutableListIterator::setValue(const T &value) const

Replaces the value of the last item that was jumped over using one of the traversal functions with value.

The traversal functions are next(), previous(), findNext(), and findPrevious().

Example:

     QMutableListIterator<double> i(list);
     while (i.hasNext()) {
         double val = i.next();
         i.setValue(std::sqrt(val));
     }

See also value(), remove(), and insert().