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>::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);

QHash<QString, int>::iterator i;
for (i = hash.begin(); i != hash.end(); ++i)
    cout << i.key() << ": " << i.value() << Qt::endl;

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

Let's see a few examples of things we can do with a QMultiHash::iterator that we cannot do with a QMultiHash::const_iterator. Here's an example that increments every value stored in the QMultiHash by 2:

QHash<QString, int>::iterator i;
for (i = hash.begin(); i != hash.end(); ++i)
    i.value() += 2;

Here's an example that removes all the items whose key is a string that starts with an underscore character:

QHash<QString, int>::iterator i = hash.begin();
while (i != hash.end()) {
    if (i.key().startsWith('_'))
        i = hash.erase(i);
    else
        ++i;
}

The call to QMultiHash::erase() removes the item pointed to by the iterator from the hash, and returns an iterator to the next item. Here's another way of removing an item while iterating:

QHash<QString, int>::iterator i = hash.begin();
while (i != hash.end()) {
    QHash<QString, int>::iterator prev = i;
    ++i;
    if (prev.key().startsWith('_'))
        hash.erase(prev);
}

It might be tempting to write code like this:

// WRONG
while (i != hash.end()) {
    if (i.key().startsWith('_'))
        hash.erase(i);
    ++i;
}

However, this will potentially crash in ++i, because i is a dangling iterator after the call to erase().

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.

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