iterator Class

Detailed Description

The QSet::iterator class provides an STL-style non-const iterator for QSet.

QSet 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.

QSet<T>::iterator allows you to iterate over a QSet and to remove items (using QSet::erase()) while you iterate. (QSet doesn't let you modify a value through an iterator, because that would potentially require moving the value in the internal hash table used by QSet.) If you want to iterate over a const QSet, you should use QSet::const_iterator. It is generally good practice to use QSet::const_iterator on a non-const QSet as well, unless you need to change the QSet through the iterator. Const iterators are slightly faster, and can improve code readability.

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

 QSet<QString> set;
 set << "January" << "February" << ... << "December";

 QSet<QString>::iterator i;
 for (i = set.begin(); i != set.end(); ++i)
     qDebug() << *i;

Here's a loop that removes certain items (all those that start with 'J') from a set while iterating:

 QSet<QString> set;
 set << "January" << "February" << ... << "December";

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

STL-style iterators can be used as arguments to generic algorithms. For example, here's how to find an item in the set using the qFind() algorithm:

 QSet<QString> set;
 ...
 const auto predicate = [](const QString &s) { return s.compare("Jeanette", Qt::CaseInsensitive) == 0; };
 QSet<QString>::iterator it = std::find_if(set.begin(), set.end(), predicate);
 if (it != set.end())
     cout << "Found Jeanette" << endl;

Multiple iterators can be used on the same set.

Warning: Iterators on implicitly shared containers do not work exactly like STL-iterators. You should avoid copying a container while iterators are active on that container. For more information, read Implicit sharing iterator problem.