const_iterator Class

Detailed Description

QHash<Key, T>::const_iterator allows you to iterate over a QHash. 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::cbegin(), QHash::cend(), or QHash::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, 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 (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.

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.