QSet Class

Detailed Description

The QSet class is a template class that provides a hash-table-based set.

QSet<T> is one of Qt's generic container classes. It stores values in an unspecified order and provides very fast lookup of the values. Internally, QSet<T> is implemented as a QHash.

Here's an example QSet with QString values:

 QSet<QString> set;

To insert a value into the set, use insert():

 set.insert("one");
 set.insert("three");
 set.insert("seven");

Another way to insert items into the set is to use operator<<():

 set << "twelve" << "fifteen" << "nineteen";

To test whether an item belongs to the set or not, use contains():

 if (!set.contains("ninety-nine"))
     ...

If you want to navigate through all the values stored in a QSet, you can use an iterator. QSet supports both Java-style iterators (QSetIterator and QMutableSetIterator) and STL-style iterators (QSet::iterator and QSet::const_iterator). Here's how to iterate over a QSet<QWidget *> using a Java-style iterator:

 QSetIterator<QWidget *> i(set);
 while (i.hasNext()) {
     QWidget *w = i.next();
     qDebug() << w;
 }

Here's the same code, but using an STL-style iterator:

 QSet<QWidget *>::const_iterator i = set.constBegin();
 while (i != set.constEnd()) {
     qDebug() << *i;
     ++i;
 }

QSet is unordered, so an iterator's sequence cannot be assumed to be predictable. If ordering by key is required, use a QMap.

To navigate through a QSet, you can also use foreach:

 QSet<QString> set;
 ...
 foreach (const QString &value, set)
     qDebug() << value;

Items can be removed from the set using remove(). There is also a clear() function that removes all items.

QSet's value data type must be an assignable data type. You cannot, for example, store a QWidget as a value; instead, store a QWidget *. In addition, the type must provide operator==(), and there must also be a global qHash() function that returns a hash value for an argument of the key's type. See the QHash documentation for a list of types supported by qHash().

Internally, QSet uses a hash table to perform lookups. The hash table automatically grows and shrinks to provide fast lookups without wasting memory. You can still control the size of the hash table by calling reserve(), if you already know approximately how many elements the QSet will contain, but this isn't necessary to obtain good performance. You can also call capacity() to retrieve the hash table's size.