iterator Class

Detailed Description

QMultiHash<Key, T>::iterator allows you to iterate over a QMultiHash and to modify the value (but not the key) associated with a particular key. If you want to iterate over a const QMultiHash, you should use QMultiHash::const_iterator. 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::iterator constructor creates an uninitialized iterator. You must initialize it using a QMultiHash function like QMultiHash::begin(), QMultiHash::end(), 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);

 for (auto i = hash.cbegin(), end = hash.cend(); i != end; ++i)
     cout << qPrintable(key()) << ": " << i.value() << endl;

Unlike QMap, which orders its items by key, QMultiHash stores its items in an arbitrary order.

Here's an example that increments every value stored in the QMultiHash by 2:

 for (auto i = hash.begin(), end = hash.end(); i != end; ++i)
     i.value() += 2;

To remove elements from a QMultiHash you can use erase_if(QMultiHash<Key, T> &map, Predicate pred):

 erase_if(hash, [](const QHash<QString, int>::iterator it) { return it.value() > 10; });

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 its internal hash table. Using any iterator after a rehashing operation has occurred will lead to undefined behavior.

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