QSet Class▲
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Header: QSet
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CMake:
find_package(Qt6 REQUIRED COMPONENTS Core)
target_link_libraries(mytarget PRIVATE Qt6::Core)
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qmake: QT += core
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Group: QSet is part of tools, Implicitly Shared Classes
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▲
See also QSetIterator, QMutableSetIterator, QHash, QMap
Member Type Documentation▲
QSet::ConstIterator▲
Qt-style synonym for QSet::const_iterator.
QSet::Iterator▲
Qt-style synonym for QSet::iterator.
QSet::const_pointer▲
Typedef for const T *. Provided for STL compatibility.
QSet::const_reference▲
Typedef for const T &. Provided for STL compatibility.
QSet::difference_type▲
Typedef for const ptrdiff_t. Provided for STL compatibility.
QSet::key_type▲
Typedef for T. Provided for STL compatibility.
QSet::pointer▲
Typedef for T *. Provided for STL compatibility.
QSet::reference▲
Typedef for T &. Provided for STL compatibility.
QSet::size_type▲
Typedef for int. Provided for STL compatibility.
QSet::value_type▲
Typedef for T. Provided for STL compatibility.
Member Function Documentation▲
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▲
See also insert()
QSet<T> &QSet::operator+=(const QSet<T> &other)▲
QSet<T> &QSet::operator|=(const QSet<T> &other)
Same as unite(other).
See Also▲
See also operator|(), operator&=(), operator-=()
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▲
See also unite(), operator|=(), operator&(), operator-()
QSet::QSet()▲
[since 5.1] 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.
[since 5.14] 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.
If the range [first, last) contains duplicate elements, the first one is retained.
This function was introduced in Qt 5.14.
QSet::const_iterator QSet::begin() const▲
Returns a const STL-style iterator positioned at the first item in the set.
See Also▲
See also constBegin(), 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.
qsizetype 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▲
[since 5.0] 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▲
[since 5.0] 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▲
void QSet::clear()▲
QSet::const_iterator QSet::constBegin() const▲
Returns a const STL-style iterator positioned at the first item in the set.
See Also▲
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▲
See also constBegin(), 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().
See Also▲
bool QSet::contains(const T &value) const▲
Returns true if the set contains item value; otherwise returns false.
See Also▲
bool QSet::contains(const QSet<T> &other) const▲
Returns true if the set contains all items from the other set; otherwise returns false.
See Also▲
qsizetype 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().
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▲
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.
[since 5.7] 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.
The iterator pos must be valid and dereferenceable. Calling this method on any other iterator, including its own end(), results in undefined behavior. In particular, even the begin() iterator of an empty set cannot be dereferenced.
This function was introduced in Qt 5.7.
See Also▲
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().
See Also▲
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().
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▲
See also operator<<(), remove(), contains()
[since 6.1] QSet::iterator QSet::insert(QSet::const_iterator it, const T &value)▲
This is an overloaded function.
Inserts item value into the set, if value isn't already in the set, and returns an iterator pointing at the inserted item.
The iterator it is ignored.
This function is provided for compatibility with the STL.
This function was introduced in Qt 6.1.
See Also▲
See also operator<<(), remove(), 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▲
See also intersects(), operator&=(), unite(), subtract()
[since 5.6] 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▲
bool QSet::isEmpty() const▲
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▲
void QSet::reserve(qsizetype 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▲
qsizetype QSet::size() const▲
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▲
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▲
See also operator-=(), unite(), 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▲
See also operator|=(), intersect(), 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.
Since Qt 5.14, range constructors are available for Qt's generic container classes and should be used in place of this method.
This function creates a new list, in linear time. The time and memory use that entails can be avoided by iterating from constBegin() to constEnd().
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▲
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▲
See also intersect(), operator&=(), operator|(), operator-()
QSet<T> &QSet::operator&=(const QSet<T> &other)▲
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▲
See also subtract(), operator-=(), operator|(), operator&()
QSet<T> &QSet::operator-=(const QSet<T> &other)▲
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▲
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▲
See also operator!=()
Related Non-Members▲
[since 6.1] qsizetype erase_if(QSet<T> &set, Predicate pred)▲
Removes all elements for which the predicate pred returns true from the set set. Returns the number of elements removed, if any.
This function was introduced in Qt 6.1.
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▲
See also Format of the QDataStream operators
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▲
See also Format of the QDataStream operators