- Qt 6.9
- Qt Core
- C++ Classes
- QMutableMultiMapIterator
- Qt 6.9.3 Reference Documentation

# QMutableMultiMapIterator Class

template <typename Key, typename T> class QMutableMultiMapIterator

The QMutableMultiMapIterator class provides a Java-style non-const iterator for QMultiMap. More...

Header: | `#include <QMutableMultiMapIterator>`
CMake: | `find_package(Qt6 REQUIRED COMPONENTS Core)` `target_link_libraries(mytarget PRIVATE Qt6::Core)`
qmake: | `QT += core`

- List of all members, including inherited members

## Public Functions

| QMutableMultiMapIterator(QMultiMap<Key, T> &map)
bool | findNext(const T &value)
bool | findPrevious(const T &value)
bool | hasNext() const
bool | hasPrevious() const
const Key & | key() const
QMutableMultiMapIterator<Key, T>::Item | next()
QMutableMultiMapIterator<Key, T>::Item | peekNext() const
QMutableMultiMapIterator<Key, T>::Item | peekPrevious() const
QMutableMultiMapIterator<Key, T>::Item | previous()
void | remove()
void | setValue(const T &value)
void | toBack()
void | toFront()
const T & | value() const
T & | value()
QMutableMultiMapIterator<Key, T> & | operator=(QMultiMap<Key, T> &map)

## Detailed Description

QMultiMap has both Java-style iterators and STL-style iterators. STL-style iterators are more efficient and should be preferred.

QMutableMultiMapIterator<Key, T> allows you to iterate over a QMultiMap and modify the map. If you don't want to modify the map (or have a const QMultiMap), use the slightly faster QMultiMapIterator instead.

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

     QMultiMap<int, QWidget *> multimap;
     ...
     QMutableMultiMapIterator<int, QWidget *> i(multimap);
     while (i.hasNext()) {
         i.next();
         qDebug() << i.key() << ": " << i.value();
     }

The next() function returns the next item in the map and advances the iterator. The key() and value() functions return the key and value of the last item that was jumped over.

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 so on.

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

     QMutableMultiMapIterator<int, QWidget *> i(multimap);
     i.toBack();
     while (i.hasPrevious()) {
         i.previous();
         qDebug() << i.key() << ": " << i.value();
     }

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

     QMutableMultiMapIterator<int, QWidget *> i(multimap);
     while (i.findNext(widget)) {
         qDebug() << "Found widget " << widget << " under key "
                  << i.key();
     }

If you want to remove items as you iterate over the map, use remove(). If you want to modify the value of an item, use setValue().

Example:

     QMutableMultiMapIterator<QString, QString> i(multimap);
     while (i.hasNext()) {
         i.next();
         if (i.key() == i.value())
             i.remove();
     }

The example removes all (key, value) pairs where the key and the value are the same.

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

See also QMultiMapIterator and QMultiMap::iterator.

## Member Function Documentation

### QMutableMultiMapIterator::QMutableMultiMapIterator(QMultiMap<Key, T> &map)

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

See also operator=().

### QMutableMultiMapIterator<Key, T> &QMutableMultiMapIterator::operator=(QMultiMap<Key, T> &map)

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

See also toFront() and toBack().

### void QMutableMultiMapIterator::toFront()

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

See also toBack() and next().

### void QMutableMultiMapIterator::toBack()

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

See also toFront() and previous().

### bool QMutableMultiMapIterator::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().

### QMutableMultiMapIterator<Key, T>::Item QMutableMultiMapIterator::next()

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

Call key() on the return value to obtain the item's key, and value() to obtain the value.

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

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

### QMutableMultiMapIterator<Key, T>::Item QMutableMultiMapIterator::peekNext() const

Returns a reference to the next item without moving the iterator.

Call key() on the return value to obtain the item's key, and value() to obtain the value.

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

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

### bool QMutableMultiMapIterator::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().

### QMutableMultiMapIterator<Key, T>::Item QMutableMultiMapIterator::previous()

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

Call key() on the return value to obtain the item's key, and value() to obtain the value.

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

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

### QMutableMultiMapIterator<Key, T>::Item QMutableMultiMapIterator::peekPrevious() const

Returns the previous item without moving the iterator.

Call key() on the return value to obtain the item's key, and value() to obtain the value.

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 &QMutableMultiMapIterator::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().value(). After a call to previous() or findPrevious(), value() is equivalent to peekNext().value().

See also key() and setValue().

### T &QMutableMultiMapIterator::value()

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

### const Key &QMutableMultiMapIterator::key() const

Returns the key 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(), key() is equivalent to peekPrevious().key(). After a call to previous() or findPrevious(), key() is equivalent to peekNext().key().

See also value().

### bool QMutableMultiMapIterator::findNext(const T &value)

Searches for value starting from the current iterator position forward. Returns `true` if a (key, value) pair with value 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().

### bool QMutableMultiMapIterator::findPrevious(const T &value)

Searches for value starting from the current iterator position backward. Returns `true` if a (key, value) pair with value 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().

### void QMutableMultiMapIterator::remove()

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

See also setValue().

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

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().

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