QSet Class
The QSet class is a template class that provides a hash-table-based set. More...
Header: | #include <QSet> |
qmake: | QT += core |
Note: All functions in this class are reentrant.
Public Types
class | const_iterator |
class | iterator |
typedef | ConstIterator |
typedef | Iterator |
typedef | const_pointer |
typedef | const_reference |
typedef | const_reverse_iterator |
typedef | difference_type |
typedef | key_type |
typedef | pointer |
typedef | reference |
typedef | reverse_iterator |
typedef | size_type |
typedef | value_type |
Detailed Description
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()) qDebug() << i.next();
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.
See also QSetIterator, QMutableSetIterator, QHash, and QMap.
Member Type Documentation
typedef QSet::ConstIterator
Qt-style synonym for QSet::const_iterator.
typedef QSet::Iterator
Qt-style synonym for QSet::iterator.
This typedef was introduced in Qt 4.2.
typedef QSet::const_pointer
Typedef for const T *. Provided for STL compatibility.
typedef QSet::const_reference
Typedef for const T &. Provided for STL compatibility.
typedef QSet::const_reverse_iterator
The QSet::const_reverse_iterator typedef provides an STL-style const reverse iterator for QSet.
It is simply a typedef for std::reverse_iterator<QSet::const_iterator>
.
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.
This typedef was introduced in Qt 5.6.
See also QSet::rbegin(), QSet::rend(), QSet::reverse_iterator, and QSet::const_iterator.
typedef QSet::difference_type
Typedef for const ptrdiff_t. Provided for STL compatibility.
typedef QSet::key_type
Typedef for T. Provided for STL compatibility.
typedef QSet::pointer
Typedef for T *. Provided for STL compatibility.
typedef QSet::reference
Typedef for T &. Provided for STL compatibility.
typedef QSet::reverse_iterator
The QSet::reverse_iterator typedef provides an STL-style non-const reverse iterator for QSet.
It is simply a typedef for std::reverse_iterator<QSet::iterator>
.
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.
This typedef was introduced in Qt 5.6.
See also QSet::rbegin(), QSet::rend(), QSet::const_reverse_iterator, and QSet::iterator.
typedef QSet::size_type
Typedef for int. Provided for STL compatibility.
typedef QSet::value_type
Typedef for T. Provided for STL compatibility.