- Qt 5.15
- Qt Core
- C++ Classes
- QSet
- Qt 5.15.18 Reference Documentation

# QSet Class

template <typename T> class QSet

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
- Obsolete 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 | difference_type
typedef | key_type
typedef | pointer
typedef | reference
typedef | size_type
typedef | value_type

## Public Functions

| QSet(InputIterator first, InputIterator last)
| QSet(std::initializer_list<T> list)
| QSet()
const_iterator | begin() const
iterator | begin()
int | capacity() const
const_iterator | cbegin() const
const_iterator | cend() const
void | clear()
const_iterator | constBegin() const
const_iterator | constEnd() const
const_iterator | constFind(const T &value) const
bool | contains(const T &value) const
bool | contains(const QSet<T> &other) const
int | count() const
bool | empty() const
const_iterator | end() const
iterator | end()
iterator | erase(const_iterator pos)
iterator | erase(iterator pos)
const_iterator | find(const T &value) const
iterator | find(const T &value)
QSet<T> & | intersect(const QSet<T> &other)
bool | intersects(const QSet<T> &other) const
bool | isEmpty() const
bool | remove(const T &value)
void | reserve(int size)
int | size() const
void | squeeze()
QSet<T> & | subtract(const QSet<T> &other)
void | swap(QSet<T> &other)
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)

## 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()) {
         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.

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::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::size_type

Typedef for int. Provided for STL compatibility.

### typedef QSet::value_type

Typedef for T. Provided for STL compatibility.

## Member Function Documentation

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

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

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

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

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

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

Same as unite(other).

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

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

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

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

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().

### template <typename InputIterator> 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().

### const_iterator QSet::begin() const

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

See also constBegin() and end().

### 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().

### 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().

### 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().

### const_iterator QSet::constBegin() const

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

See also begin() and constEnd().

### 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().

### 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().

### bool QSet::empty() const

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

### 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().

### 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.

### iterator QSet::erase(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().

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

This is an overloaded function.

This function was introduced in Qt 4.2.

### 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().

### 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.

### 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().

### 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().

### 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.

### 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.

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

For example, if you have code like

     QSet<QString> set;
     // ...
     QList<QString> list = set.values();

you can rewrite it as

     QSet<QString> set;
     // ...
     QList<QString> list(set.begin(), set.end());

See also QList::QList(InputIterator, InputIterator).

### 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!=().

## Related Non-Members

### template <typename T> QDataStream &operator<<(QDataStream &out, const QSet<T> &set)

Writes the set to stream out.

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

See also Format of the QDataStream operators.

### template <typename T> QDataStream &operator>>(QDataStream &in, QSet<T> &set)

Reads a set from stream in into set.

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

See also Format of the QDataStream operators.