Qt 5.11 | Qt Core | C++ Classes | QHashIterator

Qt 5.11.3 Reference Documentation

# QHashIterator Class

The QHashIterator class provides a Java-style const iterator for QHash and QMultiHash. More...

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

- List of all members, including inherited members

## Public Functions

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

## Detailed Description

The QHashIterator class provides a Java-style const iterator for QHash and QMultiHash.

QHash 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.

QHashIterator<Key, T> allows you to iterate over a QHash (or a QMultiHash). If you want to modify the hash as you iterate over it, use QMutableHashIterator instead.

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

      QHash<int, QWidget *> hash;
      ...
      QHashIterator<int, QWidget *> i(hash);
      while (i.hasNext()) {
          i.next();
          qDebug() << i.key() << ": " << i.value();
      }

The next() function returns the next item in the hash 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:

      QHashIterator<int, QWidget *> i(hash);
      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:

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

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

See also QMutableHashIterator and QHash::const_iterator.

## Member Function Documentation

### bool QHashIterator::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 QHashIterator::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 &QHashIterator::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 &QHashIterator::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().

### QHashIterator::Item QHashIterator::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().

### QHashIterator::Item QHashIterator::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 QHashIterator::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().

### QHashIterator::Item QHashIterator::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().

### QHashIterator::Item QHashIterator::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 QHashIterator::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 QHashIterator::toBack()

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

See also toFront() and previous().

### void QHashIterator::toFront()

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

See also toBack() and next().

### QHashIterator<Key, T> &QHashIterator::operator=(const QHash<Key, T> &container)

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

See also toFront() and toBack().

### QHashIterator::QHashIterator(const QHash<Key, T> &hash)

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

See also operator=().