const_iterator Class

Detailed Description

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::cbegin(), QMultiHash::cend(), or QMultiHash::constFind() 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(i.key()) << ": " << i.value() << 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 QMultiHash (inserting and removing items) can cause the iterators to become invalid.

Inserting items into the hash or calling methods such as QMultiHash::reserve() or QMultiHash::squeeze() can invalidate all iterators pointing into the hash. Iterators are guaranteed to stay valid only as long as the QMultiHash 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.

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