iterator Class

Detailed Description

QLinkedList 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.

QLinkedList<T>::iterator allows you to iterate over a QLinkedList<T> and to modify the list item associated with the iterator. If you want to iterate over a const QLinkedList, use QLinkedList::const_iterator instead. It is generally good practice to use QLinkedList::const_iterator on a non-const QLinkedList as well, unless you need to change the QLinkedList through the iterator. Const iterators are slightly faster, and can improve code readability.

The default QLinkedList::iterator constructor creates an uninitialized iterator. You must initialize it using a function like QLinkedList::begin(), QLinkedList::end(), or QLinkedList::insert() before you can start iterating. Here's a typical loop that prints all the items stored in a list:

 QLinkedList<QString> list;
 list.append("January");
 list.append("February");
 ...
 list.append("December");

 QLinkedList<QString>::iterator i;
 for (i = list.begin(); i != list.end(); ++i)
     cout << *i << Qt::endl;

STL-style iterators can be used as arguments to generic algorithms. For example, here's how to find an item in the list:

 QLinkedList<QString> list;
 ...
 QLinkedList<QString>::iterator it = std::find(list.begin(),
                                               list.end(), "Joel");
 if (it != list.end())
     cout << "Found Joel" << Qt::endl;

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

 QLinkedList<int>::iterator i;
 for (i = list.begin(); i != list.end(); ++i)
     *i += 2;

Here's an example that removes all the items that start with an underscore character in a QLinkedList<QString>:

 QLinkedList<QString> list;
 ...
 QLinkedList<QString>::iterator i = list.begin();
 while (i != list.end()) {
     if ((*i).startsWith('_'))
         i = list.erase(i);
     else
         ++i;
 }

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

 QLinkedList<QString>::iterator i = list.begin();
 while (i != list.end()) {
     QLinkedList<QString>::iterator previous = i;
     ++i;
     if ((*previous).startsWith('_'))
         list.erase(previous);
 }

It might be tempting to write code like this:

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