const_iterator Class

Detailed Description

QHash 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.

QHash<Key, T>::const_iterator allows you to iterate over a QHash (or a QMultiHash). If you want to modify the QHash as you iterate over it, you must use QHash::iterator instead. It is generally good practice to use QHash::const_iterator on a non-const QHash as well, unless you need to change the QHash through the iterator. Const iterators are slightly faster, and can improve code readability.

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

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

 QHash<QString, int>::const_iterator i;
 for (i = hash.constBegin(); i != hash.constEnd(); ++i)
     cout << i.key() << ": " << i.value() << Qt::endl;

Unlike QMap, which orders its items by key, QHash stores its items in an arbitrary order. The only guarantee is that items that share the same key (because they were inserted using a QMultiHash) will appear consecutively, from the most recently to the least recently inserted value.

Multiple iterators can be used on the same hash. However, be aware that any modification performed directly on the QHash has the potential of dramatically changing the order in which the items are stored in the hash, as they might cause QHash to rehash its internal data structure. If you need to keep iterators over a long period of time, we recommend that you use QMap rather than QHash.