const_iterator Class

Detailed Description

QMultiHash 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.

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

The default QMultiHash::const_iterator constructor creates an uninitialized iterator. You must initialize it using a QMultiHash function like QMultiHash::constBegin(), QMultiHash::constEnd(), or QMultiHash::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, QMultiHash 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 (inserting and removing items) can cause the iterators to become invalid.

Inserting items into the hash or calling methods such as QHash::reserve() or QHash::squeeze() can invalidate all iterators pointing into the hash. Iterators are guaranteed to stay valid only as long as the QHash doesn't have to grow/shrink it's internal hash table. Using any iterator after a rehashing operation ahs occurred will lead to undefined behavior.

You can however safely use iterators to remove entries from the hash using the QHash::erase() method. This function can safely be called while iterating, and won't affect the order of items in the hash.

If you need to keep iterators over a long period of time, we recommend that you use QMap rather than QHash.

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.