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# QSet Class Reference
[QtCore module]

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

    #include <QSet>

Note: All the functions in this class are reentrant.

- List of all members, including inherited members

### 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 ()
- QSet ( const QSet<T> & other )
- const_iterator begin () const
- iterator begin ()
- int capacity () const
- void clear ()
- const_iterator constBegin () const
- const_iterator constEnd () const
- const_iterator constFind ( const T & value ) const
- bool contains ( const T & value ) const
- int count () const
- bool empty () const
- const_iterator end () const
- iterator end ()
- iterator erase ( iterator pos )
- const_iterator find ( const T & value ) const
- iterator find ( const T & value )
- const_iterator insert ( const T & value )
- QSet<T> & intersect ( const QSet<T> & other )
- bool isEmpty () const
- bool remove ( const T & value )
- void reserve ( int size )
- int size () const
- void squeeze ()
- QSet<T> & subtract ( const 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 )
- QSet<T> & operator&= ( const QSet<T> & other )
- QSet<T> & operator&= ( const T & value )
- QSet<T> operator+ ( const QSet<T> & other )
- QSet<T> & operator+= ( const QSet<T> & other )
- QSet<T> & operator+= ( const T & value )
- QSet<T> operator- ( const QSet<T> & other )
- QSet<T> & operator-= ( const QSet<T> & other )
- QSet<T> & operator-= ( const T & value )
- QSet<T> & operator<< ( const T & value )
- QSet<T> & operator= ( const QSet<T> & other )
- bool operator== ( const QSet<T> & other ) const
- QSet<T> operator| ( const QSet<T> & other )
- 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

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())
        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 (QString value, set)
        qDebug() << value;

Items can be removed from the set using remove(). There's 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 too much 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::QSet ()

Constructs an empty set.

See also clear().

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

Constructs a copy of other.

This operation occurs in constant time, because QSet is implicitly shared. This makes returning a QSet from a function very fast. If a shared instance is modified, it will be copied (copy-on-write), and this takes linear time.

See also operator=().

### const_iterator QSet::begin () const

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

See also constBegin() and end().

### iterator QSet::begin ()

This is an overloaded member function, provided for convenience.

Returns a non-const STL-style iterator pointing to 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 QSet'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().

### void QSet::clear ()

Removes all elements from the set.

See also remove().

### const_iterator QSet::constBegin () const

Returns a const STL-style iterator pointing to 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 pointing to 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().

### int QSet::count () const

Same as size().

### bool QSet::empty () const

This function is provided for STL compatibility. It is equivalent to isEmpty().

### const_iterator QSet::end () const

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

See also constEnd() and begin().

### iterator QSet::end ()

This is an overloaded member function, provided for convenience.

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 ( iterator pos )

Removes the item associated with the iterator pos from the set, and returns an iterator to 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 4.2.

See also remove() and find().

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

Returns a const iterator pointing to 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 member function, provided for convenience.

Returns a non-const iterator pointing to 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::fromList ( const QList<T> & list ) `[static]`

Returns a QSet object with the data contained in list. Since QSet doesn't allow duplicates, the resulting QSet might be smaller than the original list was.

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

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

### const_iterator QSet::insert ( const T & value )

Inserts a new item value. If value already exists in the set, nothing happens.

For historical reasons, the return type is a non-const iterator. This might change in a future version.

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

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

Removes any items in this set that don't exist in the other set.

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

### 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 QSet'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 QSet'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 the items in the other set from this set.

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

### QList<T> QSet::toList () const

Returns a QList object with the data contained in this QSet. The order of the elements in the QList is undefined.

Example:

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

    QList<QString> list = set.toList();
    qSort(list);

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

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

Inserts all the items in the other set into this set.

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

### QList<T> QSet::values () const

Returns a QList object with the data contained in this QSet. The order of the elements in the QList is undefined.

This is the same as toList().

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

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

Returns true if other 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 )

Returns the intersection of this set and other.

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

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

Syntactic sugar for intersect(other).

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

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

This is an overloaded member function, provided for convenience.

Syntactic sugar for intersect(other), with other being a set that only contains value.

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

Returns the union of this set and other.

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

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

Syntactic sugar for unite(other).

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

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

This is an overloaded member function, provided for convenience.

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

See also insert().

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

Returns the set difference of this set with other.

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

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

Syntactic sugar for subtract(other).

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

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

This is an overloaded member function, provided for convenience.

Removes any occurrence of item value from the set and returns a reference to this set. If value didn't exist in the set, the set is left unchanged.

See also remove().

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

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

See also insert().

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

Assigns other to this set and returns a reference to this set.

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

Returns true if other 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!=().

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

Returns the union of this set and other.

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

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

Syntactic sugar for unite(other).

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

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

This is an overloaded member function, provided for convenience.

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

See also insert().

## Related Non-Members

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

This is an overloaded member function, provided for convenience.

Writes the set set to stream out.

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

See also Format of the QDataStream operators.

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

This is an overloaded member function, provided for convenience.

Reads a set from stream in into set.

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

See also Format of the QDataStream operators.