QMapIterator Class | Qt Core 6.4.1

- Qt 6.4
- Qt Core
- C++ Classes
- QMapIterator
- Qt 6.4.1 Reference Documentation

# QMapIterator Class

template <typename Key, typename T> class QMapIterator

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

Header: | #include <QMapIterator>
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

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

## Detailed Description

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

QMapIterator<Key, T> allows you to iterate over a QMap. If you want to modify the map as you iterate over it, use QMutableMapIterator instead.

The QMapIterator 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;
     ...
     QMapIterator<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:

     QMapIterator<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:

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

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

See also QMutableMapIterator and QMap::const_iterator.

## Member Function Documentation

### bool QMapIterator::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().

### bool QMapIterator::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().

### const Key &QMapIterator::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().

### const T &QMapIterator::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().

### QMapIterator::Item QMapIterator::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().

### QMapIterator::Item QMapIterator::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().

### bool QMapIterator::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().

### QMapIterator::Item QMapIterator::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().

### QMapIterator::Item QMapIterator::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().

### bool QMapIterator::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 QMapIterator::toBack()

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

See also toFront() and previous().

### void QMapIterator::toFront()

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

See also toBack() and next().

### QMapIterator<Key, T> &QMapIterator::operator=(const 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().

### QMapIterator::QMapIterator(const 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=().