iterator Class

(QMap::iterator)

The QMap::iterator class provides an STL-style non-const iterator for QMap and QMultiMap. More...

Header: #include <iterator>
qmake: QT += core

Public Types

Public Functions

iterator()
iterator(Node *node)
const Key &key() const
T &value() const
bool operator!=(const iterator &o) const
bool operator!=(const const_iterator &o) const
T &operator*() const
iterator operator+(int j) const
iterator &operator++()
iterator operator++(int)
iterator &operator+=(int j)
iterator operator-(int j) const
iterator &operator--()
iterator operator--(int)
iterator &operator-=(int j)
T *operator->() const
bool operator==(const iterator &o) const
bool operator==(const const_iterator &o) const

Detailed Description

The QMap::iterator class provides an STL-style non-const iterator for QMap and QMultiMap.

QMap features both STL-style iterators and Java-style iterators. The STL-style iterators are more low-level and more cumbersome to use; on the other hand, they are slightly faster and, for developers who already know STL, have the advantage of familiarity.

QMap<Key, T>::iterator allows you to iterate over a QMap (or QMultiMap) and to modify the value (but not the key) stored under a particular key. If you want to iterate over a const QMap, you should use QMap::const_iterator. It is generally good practice to use QMap::const_iterator on a non-const QMap as well, unless you need to change the QMap through the iterator. Const iterators are slightly faster, and can improve code readability.

The default QMap::iterator constructor creates an uninitialized iterator. You must initialize it using a QMap function like QMap::begin(), QMap::end(), or QMap::find() before you can start iterating. Here's a typical loop that prints all the (key, value) pairs stored in a map:


  QMap<QString, int> map;
  map.insert("January", 1);
  map.insert("February", 2);
  ...
  map.insert("December", 12);

  QMap<QString, int>::iterator i;
  for (i = map.begin(); i != map.end(); ++i)
      cout << i.key() << ": " << i.value() << endl;

Unlike QHash, which stores its items in an arbitrary order, QMap stores its items ordered by key. Items that share the same key (because they were inserted using QMap::insertMulti(), or due to a unite()) will appear consecutively, from the most recently to the least recently inserted value.

Let's see a few examples of things we can do with a QMap::iterator that we cannot do with a QMap::const_iterator. Here's an example that increments every value stored in the QMap by 2:


  QMap<QString, int>::iterator i;
  for (i = map.begin(); i != map.end(); ++i)
      i.value() += 2;

Here's an example that removes all the items whose key is a string that starts with an underscore character:


  QMap<QString, int>::iterator i = map.begin();
  while (i != map.end()) {
      if (i.key().startsWith('_'))
          i = map.erase(i);
      else
          ++i;
  }

The call to QMap::erase() removes the item pointed to by the iterator from the map, and returns an iterator to the next item. Here's another way of removing an item while iterating:


  QMap<QString, int>::iterator i = map.begin();
  while (i != map.end()) {
      QMap<QString, int>::iterator prev = i;
      ++i;
      if (prev.key().startsWith('_'))
          map.erase(prev);
  }

It might be tempting to write code like this:


  // WRONG
  while (i != map.end()) {
      if (i.key().startsWith('_'))
          map.erase(i);
      ++i;
  }

However, this will potentially crash in ++i, because i is a dangling iterator after the call to erase().

Multiple iterators can be used on the same map. If you add items to the map, existing iterators will remain valid. If you remove items from the map, iterators that point to the removed items will become dangling iterators.

Warning: Iterators on implicitly shared containers do not work exactly like STL-iterators. You should avoid copying a container while iterators are active on that container. For more information, read Implicit sharing iterator problem.

See also QMap::const_iterator, QMap::key_iterator, and QMutableMapIterator.

Member Type Documentation

typedef iterator::iterator_category

A synonym for std::bidirectional_iterator_tag indicating this iterator is a bidirectional iterator.

Member Function Documentation

iterator::iterator()

Default constructs an instance of iterator.

iterator::iterator(Node *node)

Default constructs an instance of iterator.

const Key &iterator::key() const

T &iterator::value() const

bool iterator::operator!=(const iterator &o) const

bool iterator::operator!=(const const_iterator &o) const

T &iterator::operator*() const

iterator iterator::operator+(int j) const

iterator &iterator::operator++()

iterator iterator::operator++(int)

iterator &iterator::operator+=(int j)

iterator iterator::operator-(int j) const

iterator &iterator::operator--()

iterator iterator::operator--(int)

iterator &iterator::operator-=(int j)

T *iterator::operator->() const

bool iterator::operator==(const iterator &o) const

bool iterator::operator==(const const_iterator &o) const