const_iterator Class

(QMap::const_iterator)

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

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

Public Types

Detailed Description

The QMap::const_iterator class provides an STL-style 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>::const_iterator allows you to iterate over a QMap (or a QMultiMap). If you want to modify the QMap as you iterate over it, you must use QMap::iterator instead. 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::const_iterator constructor creates an uninitialized iterator. You must initialize it using a QMap function like QMap::constBegin(), QMap::constEnd(), 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>::const_iterator i;
  for (i = map.constBegin(); i != map.constEnd(); ++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()) will appear consecutively, from the most recently to the least recently inserted value.

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::iterator, QMap::key_iterator, and QMapIterator.

Member Type Documentation

typedef const_iterator::iterator_category

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