QMutableMapIterator Class

template <typename Key, typename T> class QMutableMapIterator

The QMutableMapIterator class provides a Java-style non-const iterator for QMap. More...

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

Public Functions

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

Detailed Description

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

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

The QMutableMapIterator constructor takes a QMap 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:

QMap<int, QWidget *> map;
...
QMutableMapIterator<int, QWidget *> i(map);
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:

QMutableMapIterator<int, QWidget *> i(map);
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:

QMutableMapIterator<int, QWidget *> i(map);
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:

QMutableMapIterator<QString, QString> i(map);
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 QMapIterator and QMap::iterator.

Member Function Documentation

QMutableMapIterator::QMutableMapIterator(QMap<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=().

QMutableMapIterator<Key, T> &QMutableMapIterator::operator=(QMap<Key, T> &container)

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 QMutableMapIterator::toFront()

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

See also toBack() and next().

void QMutableMapIterator::toBack()

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

See also toFront() and previous().

bool QMutableMapIterator::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().

QMutableMapIterator::Item QMutableMapIterator::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().

QMutableMapIterator::Item QMutableMapIterator::peekNext() const

Returns 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().

QMutableMapIterator::Item QMutableMapIterator::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 QMutableMapIterator::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().

QMutableMapIterator::Item QMutableMapIterator::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().

QMutableMapIterator::Item QMutableMapIterator::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 &QMutableMapIterator::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 &QMutableMapIterator::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 Key &QMutableMapIterator::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 QMutableMapIterator::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 QMutableMapIterator::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 QMutableMapIterator::remove()

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

See also setValue().

void QMutableMapIterator::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().

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