QSet Class

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

Header:	#include <QSet>
qmake:	QT += core

• List of all members, including inherited members

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

Public Functions

	QSet(InputIterator first, InputIterator last)
	QSet(std::initializer_list<T> list)
	QSet()
QSet::const_iterator	begin() const
QSet::iterator	begin()
int	capacity() const
QSet::const_iterator	cbegin() const
QSet::const_iterator	cend() const
void	clear()
QSet::const_iterator	constBegin() const
QSet::const_iterator	constEnd() const
QSet::const_iterator	constFind(const T &value) const
bool	contains(const T &value) const
bool	contains(const QSet<T> &other) const
int	count() const
QSet::const_reverse_iterator	crbegin() const
QSet::const_reverse_iterator	crend() const
bool	empty() const
QSet::const_iterator	end() const
QSet::iterator	end()
QSet::iterator	erase(QSet::const_iterator pos)
QSet::iterator	erase(QSet::iterator pos)
QSet::const_iterator	find(const T &value) const
QSet::iterator	find(const T &value)
QSet::iterator	insert(const T &value)
QSet<T> &	intersect(const QSet<T> &other)
bool	intersects(const QSet<T> &other) const
bool	isEmpty() const
QSet::reverse_iterator	rbegin()
QSet::const_reverse_iterator	rbegin() const
bool	remove(const T &value)
QSet::reverse_iterator	rend()
QSet::const_reverse_iterator	rend() const
void	reserve(int size)
int	size() const
void	squeeze()
QSet<T> &	subtract(const QSet<T> &other)
void	swap(QSet<T> &other)
QList<T>	toList() const
QSet<T> &	unite(const QSet<T> &other)
QList<T>	values() const
bool	operator!=(const QSet<T> &other) const
QSet<T>	operator&(const QSet<T> &other) const
QSet<T> &	operator&=(const QSet<T> &other)
QSet<T> &	operator&=(const T &value)
QSet<T>	operator+(const QSet<T> &other) const
QSet<T> &	operator+=(const QSet<T> &other)
QSet<T> &	operator+=(const T &value)
QSet<T>	operator-(const QSet<T> &other) const
QSet<T> &	operator-=(const QSet<T> &other)
QSet<T> &	operator-=(const T &value)
QSet<T> &	operator<<(const T &value)
bool	operator==(const QSet<T> &other) const
QSet<T>	operator|(const QSet<T> &other) const
QSet<T> &	operator|=(const QSet<T> &other)
QSet<T> &	operator|=(const T &value)

Static Public Members

QSet<T>

fromList(const QList<T> &list)

Related Non-Members

QDataStream &	operator<<(QDataStream &out, const QSet<T> &set)
QDataStream &	operator>>(QDataStream &in, QSet<T> &set)

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.

Member Function Documentation

Returns a new QSet that is the union of this set and the other set.

See also unite(), operator|=(), operator&(), and operator-().

Same as unite(other).

See also operator|(), operator&=(), and operator-=().

Inserts a new item value and returns a reference to the set. If value already exists in the set, the set is left unchanged.

See also insert().

QSet::QSet(InputIterator first, InputIterator last)

Constructs a set with the contents in the iterator range [first, last).

The value type of InputIterator must be convertible to T.

Note: If the range [first, last) contains duplicate elements, the first one is retained.

This function was introduced in Qt 5.14.

QSet::QSet(std::initializer_list<T> list)

Constructs a set with a copy of each of the elements in the initializer list list.

This function was introduced in Qt 5.1.

QSet::QSet()

Constructs an empty set.

See also clear().

QSet::const_iterator QSet::begin() const

Returns a const STL-style iterator positioned at the first item in the set.

See also constBegin() and end().

QSet::iterator QSet::begin()

This is an overloaded function.

Returns a non-const STL-style iterator positioned at the first item in the set.

This function was introduced in Qt 4.2.

int QSet::capacity() const

Returns the number of buckets in the set's internal hash table.

The sole purpose of this function is to provide a means of fine tuning QSet's memory usage. In general, you will rarely ever need to call this function. If you want to know how many items are in the set, call size().

See also reserve() and squeeze().

QSet::const_iterator QSet::cbegin() const

Returns a const STL-style iterator positioned at the first item in the set.

This function was introduced in Qt 5.0.

See also begin() and cend().

QSet::const_iterator QSet::cend() const

Returns a const STL-style iterator pointing to the imaginary item after the last item in the set.

This function was introduced in Qt 5.0.

See also cbegin() and end().

void QSet::clear()

Removes all elements from the set.

See also remove().

QSet::const_iterator QSet::constBegin() const

Returns a const STL-style iterator positioned at the first item in the set.

See also begin() and constEnd().

QSet::const_iterator QSet::constEnd() const

Returns a const STL-style iterator pointing to the imaginary item after the last item in the set.

See also constBegin() and end().

QSet::const_iterator QSet::constFind(const T &value) const

Returns a const iterator positioned at the item value in the set. If the set contains no item value, the function returns constEnd().

This function was introduced in Qt 4.2.

See also find() and contains().

bool QSet::contains(const T &value) const

Returns true if the set contains item value; otherwise returns false.

See also insert(), remove(), and find().

bool QSet::contains(const QSet<T> &other) const

Returns true if the set contains all items from the other set; otherwise returns false.

This function was introduced in Qt 4.6.

See also insert(), remove(), and find().

int QSet::count() const

Same as size().

QSet::const_reverse_iterator QSet::crbegin() const

Returns a const STL-style reverse iterator pointing to the first item in the set, in reverse order.

This function was introduced in Qt 5.6.

See also begin(), rbegin(), and rend().

QSet::const_reverse_iterator QSet::crend() const

Returns a const STL-style reverse iterator pointing to one past the last item in the set, in reverse order.

This function was introduced in Qt 5.6.

See also end(), rend(), and rbegin().

bool QSet::empty() const

Returns true if the set is empty. This function is provided for STL compatibility. It is equivalent to isEmpty().

QSet::const_iterator QSet::end() const

Returns a const STL-style iterator positioned at the imaginary item after the last item in the set.

See also constEnd() and begin().

QSet::iterator QSet::end()

This is an overloaded function.

Returns a non-const STL-style iterator pointing to the imaginary item after the last item in the set.

This function was introduced in Qt 4.2.

QSet::iterator QSet::erase(QSet::const_iterator pos)

Removes the item at the iterator position pos from the set, and returns an iterator positioned at the next item in the set.

Unlike remove(), this function never causes QSet to rehash its internal data structure. This means that it can safely be called while iterating, and won't affect the order of items in the set.

This function was introduced in Qt 5.7.

See also remove() and find().

QSet::iterator QSet::erase(QSet::iterator pos)

This is an overloaded function.

This function was introduced in Qt 4.2.

QSet::const_iterator QSet::find(const T &value) const

Returns a const iterator positioned at the item value in the set. If the set contains no item value, the function returns constEnd().

This function was introduced in Qt 4.2.

See also constFind() and contains().

QSet::iterator QSet::find(const T &value)

This is an overloaded function.

Returns a non-const iterator positioned at the item value in the set. If the set contains no item value, the function returns end().

This function was introduced in Qt 4.2.

[static] QSet<T> QSet::fromList(const QList<T> &list)

Returns a new QSet object containing the data contained in list. Since QSet doesn't allow duplicates, the resulting QSet might be smaller than the list, because QList can contain duplicates.

Example:

QStringList list;
list << "Julia" << "Mike" << "Mike" << "Julia" << "Julia";

QSet<QString> set = QSet<QString>::fromList(list);
set.contains("Julia");  // returns true
set.contains("Mike");   // returns true
set.size();             // returns 2

Note: Since Qt 5.14, range constructors are available for Qt's generic container classes and should be used in place of this method.

See also toList() and QList::toSet().

QSet::iterator QSet::insert(const T &value)

Inserts item value into the set, if value isn't already in the set, and returns an iterator pointing at the inserted item.

See also operator<<(), remove(), and contains().

QSet<T> &QSet::intersect(const QSet<T> &other)

Removes all items from this set that are not contained in the other set. A reference to this set is returned.

See also intersects(), operator&=(), unite(), and subtract().

bool QSet::intersects(const QSet<T> &other) const

Returns true if this set has at least one item in common with other.

This function was introduced in Qt 5.6.

See also contains() and intersect().

bool QSet::isEmpty() const

Returns true if the set contains no elements; otherwise returns false.

See also size().

QSet::reverse_iterator QSet::rbegin()

Returns a STL-style reverse iterator pointing to the first item in the set, in reverse order.

This function was introduced in Qt 5.6.

See also begin(), crbegin(), and rend().

QSet::const_reverse_iterator QSet::rbegin() const

This is an overloaded function.

This function was introduced in Qt 5.6.

bool QSet::remove(const T &value)

Removes any occurrence of item value from the set. Returns true if an item was actually removed; otherwise returns false.

See also contains() and insert().

QSet::reverse_iterator QSet::rend()

Returns a STL-style reverse iterator pointing to one past the last item in the set, in reverse order.

This function was introduced in Qt 5.6.

See also end(), crend(), and rbegin().

QSet::const_reverse_iterator QSet::rend() const

This is an overloaded function.

This function was introduced in Qt 5.6.

void QSet::reserve(int size)

Ensures that the set's internal hash table consists of at least size buckets.

This function is useful for code that needs to build a huge set and wants to avoid repeated reallocation. For example:

QSet<QString> set;
set.reserve(20000);
for (int i = 0; i < 20000; ++i)
    set.insert(values[i]);

Ideally, size should be slightly more than the maximum number of elements expected in the set. size doesn't have to be prime, because QSet will use a prime number internally anyway. If size is an underestimate, the worst that will happen is that the QSet will be a bit slower.

In general, you will rarely ever need to call this function. QSet's internal hash table automatically shrinks or grows to provide good performance without wasting too much memory.

See also squeeze() and capacity().

int QSet::size() const

Returns the number of items in the set.

See also isEmpty() and count().

void QSet::squeeze()

Reduces the size of the set's internal hash table to save memory.

The sole purpose of this function is to provide a means of fine tuning QSet's memory usage. In general, you will rarely ever need to call this function.

See also reserve() and capacity().

QSet<T> &QSet::subtract(const QSet<T> &other)

Removes all items from this set that are contained in the other set. Returns a reference to this set.

See also operator-=(), unite(), and intersect().

void QSet::swap(QSet<T> &other)

Swaps set other with this set. This operation is very fast and never fails.

QList<T> QSet::toList() const

Returns a new QList containing the elements in the set. The order of the elements in the QList is undefined.

Example:

QSet<QString> set;
set << "red" << "green" << "blue" << ... << "black";

QList<QString> list = set.toList();
std::sort(list.begin(), list.end());

Note: Since Qt 5.14, range constructors are available for Qt's generic container classes and should be used in place of this method.

See also fromList() and QList::fromSet().

QSet<T> &QSet::unite(const QSet<T> &other)

Each item in the other set that isn't already in this set is inserted into this set. A reference to this set is returned.

See also operator|=(), intersect(), and subtract().

QList<T> QSet::values() const

Returns a new QList containing the elements in the set. The order of the elements in the QList is undefined.

This is the same as toList().

Note: Since Qt 5.14, range constructors are available for Qt's generic container classes and should be used in place of this method.

See also fromList() and QList::fromSet().

bool QSet::operator!=(const QSet<T> &other) const

Returns true if the other set is not equal to this set; otherwise returns false.

Two sets are considered equal if they contain the same elements.

This function requires the value type to implement operator==().

See also operator==().

QSet<T> QSet::operator&(const QSet<T> &other) const

Returns a new QSet that is the intersection of this set and the other set.

See also intersect(), operator&=(), operator|(), and operator-().

QSet<T> &QSet::operator&=(const QSet<T> &other)

Same as intersect(other).

See also operator&(), operator|=(), and operator-=().

QSet<T> &QSet::operator&=(const T &value)

This is an overloaded function.

Same as intersect(other), if we consider other to be a set that contains the singleton value.

QSet<T> QSet::operator-(const QSet<T> &other) const

Returns a new QSet that is the set difference of this set and the other set, i.e., this set - other set.

See also subtract(), operator-=(), operator|(), and operator&().

QSet<T> &QSet::operator-=(const QSet<T> &other)

Same as subtract(other).

See also operator-(), operator|=(), and operator&=().

QSet<T> &QSet::operator-=(const T &value)

Removes the occurrence of item value from the set, if it is found, and returns a reference to the set. If the value is not contained the set, nothing is removed.

See also remove().

bool QSet::operator==(const QSet<T> &other) const

Returns true if the other set is equal to this set; otherwise returns false.

Two sets are considered equal if they contain the same elements.

This function requires the value type to implement operator==().

See also operator!=().