iterator Class

Detailed Description

QMultiMap 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.

QMultiMap<Key, T>::iterator allows you to iterate over a QMultiMap and to modify the value (but not the key) stored under a particular key. If you want to iterate over a const QMultiMap, you should use QMultiMap::const_iterator. It is generally good practice to use QMultiMap::const_iterator on a non-const QMultiMap as well, unless you need to change the QMultiMap through the iterator. Const iterators are slightly faster, and can improve code readability.

The default QMultiMap::iterator constructor creates an uninitialized iterator. You must initialize it using a QMultiMap function like QMultiMap::begin(), QMultiMap::end(), or QMultiMap::find() before you can start iterating. Here's a typical loop that prints all the (key, value) pairs stored in a map:

QMultiMap<QString, int> multimap;
multimap.insert("January", 1);
multimap.insert("February", 2);
...
multimap.insert("December", 12);

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

Unlike QMultiHash, which stores its items in an arbitrary order, QMultiMap stores its items ordered by key. Items that share the same key will appear consecutively, from the most recently to the least recently inserted value.

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

QMultiMap<QString, int>::iterator i;
for (i = multimap.begin(); i != multimap.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:

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

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

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

It might be tempting to write code like this:

// WRONG
while (i != multimap.end()) {
    if (i.key().startsWith('_'))
        multimap.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 map. If you add items to the map, existing iterators will remain valid. If you remove items from the map, iterators that point to the removed items will become dangling iterators.