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Please review the following ** information to ensure the GNU General Public License requirements will ** be met: https://www.gnu.org/licenses/gpl-2.0.html and ** https://www.gnu.org/licenses/gpl-3.0.html. ** ** $QT_END_LICENSE$ ** ****************************************************************************/ /*! \class QMap \inmodule QtCore \brief The QMap class is a template class that provides an associative array. \ingroup tools \ingroup shared \reentrant QMap\ is one of Qt's generic \l{container classes}. It stores (key, value) pairs and provides fast lookup by key. QMap and QHash provide very similar functionality. The differences are: \list \li QHash provides average faster lookups than QMap. (See \l{Algorithmic Complexity} for details.) \li When iterating over a QHash, the items are arbitrarily ordered. With QMap, the items are always sorted by key. \li The key type of a QHash must provide operator==() and a global qHash(Key) function. The key type of a QMap must provide operator<() specifying a total order. Since Qt 5.8.1 it is also safe to use a pointer type as key, even if the underlying operator<() does not provide a total order. \endlist Here's an example QMap with QString keys and \c int values: \snippet code/src_corelib_tools_qmap.cpp 0 To insert a (key, value) pair into the map, you can use operator[](): \snippet code/src_corelib_tools_qmap.cpp 1 This inserts the following three (key, value) pairs into the QMap: ("one", 1), ("three", 3), and ("seven", 7). Another way to insert items into the map is to use insert(): \snippet code/src_corelib_tools_qmap.cpp 2 To look up a value, use operator[]() or value(): \snippet code/src_corelib_tools_qmap.cpp 3 If there is no item with the specified key in the map, these functions return a \l{default-constructed value}. If you want to check whether the map contains a certain key, use contains(): \snippet code/src_corelib_tools_qmap.cpp 4 There is also a value() overload that uses its second argument as a default value if there is no item with the specified key: \snippet code/src_corelib_tools_qmap.cpp 5 In general, we recommend that you use contains() and value() rather than operator[]() for looking up a key in a map. The reason is that operator[]() silently inserts an item into the map if no item exists with the same key (unless the map is const). For example, the following code snippet will create 1000 items in memory: \snippet code/src_corelib_tools_qmap.cpp 6 To avoid this problem, replace \c map[i] with \c map.value(i) in the code above. If you want to navigate through all the (key, value) pairs stored in a QMap, you can use an iterator. QMap provides both \l{Java-style iterators} (QMapIterator and QMutableMapIterator) and \l{STL-style iterators} (QMap::const_iterator and QMap::iterator). Here's how to iterate over a QMap using a Java-style iterator: \snippet code/src_corelib_tools_qmap.cpp 7 Here's the same code, but using an STL-style iterator this time: \snippet code/src_corelib_tools_qmap.cpp 8 The items are traversed in ascending key order. Normally, a QMap allows only one value per key. If you call insert() with a key that already exists in the QMap, the previous value will be erased. For example: \snippet code/src_corelib_tools_qmap.cpp 9 However, you can store multiple values per key by using QMultiMap. If you want to retrieve all the values for a single key, you can use values(const Key &key), which returns a QList: \snippet code/src_corelib_tools_qmap.cpp 10 Another approach is to call find() to get the STL-style iterator for the first item with a key and iterate from there: \snippet code/src_corelib_tools_qmap.cpp 11 If you only need to extract the values from a map (not the keys), you can also use \l{foreach}: \snippet code/src_corelib_tools_qmap.cpp 12 Items can be removed from the map in several ways. One way is to call remove(); this will remove any item with the given key. Another way is to use QMutableMapIterator::remove(). In addition, you can clear the entire map using clear(). QMap's key and value data types must be \l{assignable data types}. This covers most data types you are likely to encounter, but the compiler won't let you, for example, store a QWidget as a value; instead, store a QWidget *. In addition, QMap's key type must provide operator<(). QMap uses it to keep its items sorted, and assumes that two keys \c x and \c y are equivalent if neither \c{x < y} nor \c{y < x} is true. Example: \snippet code/src_corelib_tools_qmap.cpp 13 In the example, we start by comparing the employees' names. If they're equal, we compare their dates of birth to break the tie. \sa QMapIterator, QMutableMapIterator, QHash, QSet */ /*! \fn template QMap::QMap() Constructs an empty map. \sa clear() */ /*! \fn template QMap::QMap(const QMap &other) Constructs a copy of \a other. This operation occurs in \l{constant time}, because QMap is \l{implicitly shared}. This makes returning a QMap from a function very fast. If a shared instance is modified, it will be copied (copy-on-write), and this takes \l{linear time}. \sa operator= */ /*! \fn template QMap::QMap(QMap &&other) Move-constructs a QMap instance. \since 5.2 */ /*! \fn template QMap &QMap::operator=(const QMap &other) Assigns \a other to this map and returns a reference to this map. */ /*! \fn template QMap &QMap::operator=(QMap &&other) Move-assigns \a other to this QMap instance. \since 5.2 */ /*! \fn template QMap::~QMap() Destroys the map. References to the values in the map, and all iterators over this map, become invalid. */ /*! \fn template void QMap::swap(QMap &other) noexcept \since 4.8 Swaps map \a other with this map. This operation is very fast and never fails. */ /*! \fn template QMap::QMap(std::initializer_list > list) \since 5.1 Constructs a map with a copy of each of the elements in the initializer list \a list. */ /*! \fn template QMap::QMap(const std::map & other) Constructs a copy of \a other. \sa toStdMap() */ /*! \fn template QMap::QMap(std::map && other) Constructs a map by moving from \a other. \sa toStdMap() */ /*! \fn template std::map QMap::toStdMap() const Returns an STL map equivalent to this QMap. */ /*! \fn template bool QMap::operator==(const QMap &lhs, const QMap &rhs) Returns \c true if \a lhs is equal to \a rhs; otherwise returns false. Two maps are considered equal if they contain the same (key, value) pairs. This function requires the key and the value types to implement \c operator==(). \sa operator!=() */ /*! \fn template bool QMap::operator!=(const QMap &lhs, const QMap &rhs) Returns \c true if \a lhs is not equal to \a rhs; otherwise returns false. Two maps are considered equal if they contain the same (key, value) pairs. This function requires the key and the value types to implement \c operator==(). \sa operator==() */ /*! \fn template size_type QMap::size() const Returns the number of (key, value) pairs in the map. \sa isEmpty(), count() */ /*! \fn template bool QMap::isEmpty() const Returns \c true if the map contains no items; otherwise returns false. \sa size() */ /*! \fn template void QMap::detach() \internal Detaches this map from any other maps with which it may share data. \sa isDetached() */ /*! \fn template bool QMap::isDetached() const \internal Returns \c true if the map's internal data isn't shared with any other map object; otherwise returns \c false. \sa detach() */ /*! \fn template bool QMap::isSharedWith(const QMap &other) const \internal */ /*! \fn template void QMap::clear() Removes all items from the map. \sa remove() */ /*! \fn template size_type QMap::remove(const Key &key) Removes all the items that have the key \a key from the map. Returns the number of items removed which will be 1 if the key exists in the map, and 0 otherwise. \sa clear(), take() */ /*! \fn template template size_type QMap::removeIf(Predicate pred) \since 6.1 Removes all elements for which the predicate \a pred returns true from the map. The function supports predicates which take either an argument of type \c{QMap::iterator}, or an argument of type \c{std::pair}. Returns the number of elements removed, if any. \sa clear(), take() */ /*! \fn template T QMap::take(const Key &key) Removes the item with the key \a key from the map and returns the value associated with it. If the item does not exist in the map, the function simply returns a \l{default-constructed value}. If there are multiple items for \a key in the map, only the most recently inserted one is removed and returned. If you don't use the return value, remove() is more efficient. \sa remove() */ /*! \fn template bool QMap::contains(const Key &key) const Returns \c true if the map contains an item with key \a key; otherwise returns \c false. \sa count() */ /*! \fn template Key QMap::key(const T &value, const Key &defaultKey) const \since 4.3 \overload Returns the first key with value \a value, or \a defaultKey if the map contains no item with value \a value. If no \a defaultKey is provided the function returns a \l{default-constructed value}{default-constructed key}. This function can be slow (\l{linear time}), because QMap's internal data structure is optimized for fast lookup by key, not by value. \sa value(), keys() */ /*! \fn template T QMap::value(const Key &key, const T &defaultValue) const Returns the value associated with the key \a key. If the map contains no item with key \a key, the function returns \a defaultValue. If no \a defaultValue is specified, the function returns a \l{default-constructed value}. \sa key(), values(), contains(), operator[]() */ /*! \fn template T &QMap::operator[](const Key &key) Returns the value associated with the key \a key as a modifiable reference. If the map contains no item with key \a key, the function inserts a \l{default-constructed value} into the map with key \a key, and returns a reference to it. If the map contains multiple items with key \a key, this function returns a reference to the most recently inserted value. \sa insert(), value() */ /*! \fn template T QMap::operator[](const Key &key) const \overload Same as value(). */ /*! \fn template QList QMap::keys() const Returns a list containing all the keys in the map in ascending order. The order is guaranteed to be the same as that used by values(). This function creates a new list, in \l {linear time}. The time and memory use that entails can be avoided by iterating from \l keyBegin() to \l keyEnd(). \sa QMultiMap::uniqueKeys(), values(), key() */ /*! \fn template QList QMap::keys(const T &value) const \overload Returns a list containing all the keys associated with value \a value in ascending order. This function can be slow (\l{linear time}), because QMap's internal data structure is optimized for fast lookup by key, not by value. */ /*! \fn template QList QMap::values() const Returns a list containing all the values in the map, in ascending order of their keys. If a key is associated with multiple values, all of its values will be in the list, and not just the most recently inserted one. This function creates a new list, in \l {linear time}. The time and memory use that entails can be avoided by iterating from \l keyValueBegin() to \l keyValueEnd(). \sa keys(), value() */ /*! \fn template size_type QMap::count(const Key &key) const Returns the number of items associated with key \a key. \sa contains() */ /*! \fn template size_type QMap::count() const \overload Same as size(). */ /*! \fn template const Key &QMap::firstKey() const \since 5.2 Returns a reference to the smallest key in the map. This function assumes that the map is not empty. This executes in \l{constant time}. \sa lastKey(), first(), keyBegin(), isEmpty() */ /*! \fn template const Key &QMap::lastKey() const \since 5.2 Returns a reference to the largest key in the map. This function assumes that the map is not empty. This executes in \l{logarithmic time}. \sa firstKey(), last(), keyEnd(), isEmpty() */ /*! \fn template T &QMap::first() \since 5.2 Returns a reference to the first value in the map, that is the value mapped to the smallest key. This function assumes that the map is not empty. When unshared (or const version is called), this executes in \l{constant time}. \sa last(), firstKey(), isEmpty() */ /*! \fn template const T &QMap::first() const \since 5.2 \overload */ /*! \fn template T &QMap::last() \since 5.2 Returns a reference to the last value in the map, that is the value mapped to the largest key. This function assumes that the map is not empty. When unshared (or const version is called), this executes in \l{logarithmic time}. \sa first(), lastKey(), isEmpty() */ /*! \fn template const T &QMap::last() const \since 5.2 \overload */ /*! \fn template QMap::iterator QMap::begin() Returns an \l{STL-style iterators}{STL-style iterator} pointing to the first item in the map. \sa constBegin(), end() */ /*! \fn template QMap::const_iterator QMap::begin() const \overload */ /*! \fn template QMap::const_iterator QMap::cbegin() const \since 5.0 Returns a const \l{STL-style iterators}{STL-style iterator} pointing to the first item in the map. \sa begin(), cend() */ /*! \fn template QMap::const_iterator QMap::constBegin() const Returns a const \l{STL-style iterators}{STL-style iterator} pointing to the first item in the map. \sa begin(), constEnd() */ /*! \fn template QMap::key_iterator QMap::keyBegin() const \since 5.6 Returns a const \l{STL-style iterators}{STL-style iterator} pointing to the first key in the map. \sa keyEnd(), firstKey() */ /*! \fn template QMap::iterator QMap::end() Returns an \l{STL-style iterators}{STL-style iterator} pointing to the imaginary item after the last item in the map. \sa begin(), constEnd() */ /*! \fn template QMap::const_iterator QMap::end() const \overload */ /*! \fn template QMap::const_iterator QMap::cend() const \since 5.0 Returns a const \l{STL-style iterators}{STL-style iterator} pointing to the imaginary item after the last item in the map. \sa cbegin(), end() */ /*! \fn template QMap::const_iterator QMap::constEnd() const Returns a const \l{STL-style iterators}{STL-style iterator} pointing to the imaginary item after the last item in the map. \sa constBegin(), end() */ /*! \fn template QMap::key_iterator QMap::keyEnd() const \since 5.6 Returns a const \l{STL-style iterators}{STL-style iterator} pointing to the imaginary item after the last key in the map. \sa keyBegin(), lastKey() */ /*! \fn template QMap::key_value_iterator QMap::keyValueBegin() \since 5.10 Returns an \l{STL-style iterators}{STL-style iterator} pointing to the first entry in the map. \sa keyValueEnd() */ /*! \fn template QMap::key_value_iterator QMap::keyValueEnd() \since 5.10 Returns an \l{STL-style iterators}{STL-style iterator} pointing to the imaginary entry after the last entry in the map. \sa keyValueBegin() */ /*! \fn template QMap::const_key_value_iterator QMap::keyValueBegin() const \since 5.10 Returns a const \l{STL-style iterators}{STL-style iterator} pointing to the first entry in the map. \sa keyValueEnd() */ /*! \fn template QMap::const_key_value_iterator QMap::constKeyValueBegin() const \since 5.10 Returns a const \l{STL-style iterators}{STL-style iterator} pointing to the first entry in the map. \sa keyValueBegin() */ /*! \fn template QMap::const_key_value_iterator QMap::keyValueEnd() const \since 5.10 Returns a const \l{STL-style iterators}{STL-style iterator} pointing to the imaginary entry after the last entry in the map. \sa keyValueBegin() */ /*! \fn template QMap::const_key_value_iterator QMap::constKeyValueEnd() const \since 5.10 Returns a const \l{STL-style iterators}{STL-style iterator} pointing to the imaginary entry after the last entry in the map. \sa constKeyValueBegin() */ /*! \fn template QMap::iterator QMap::erase(const_iterator pos) Removes the (key, value) pair pointed to by the iterator \a pos from the map, and returns an iterator to the next item in the map. \note The iterator \a pos \e must be valid and dereferenceable. \sa remove() */ /*! \fn template QMap::iterator QMap::erase(const_iterator first, const_iterator last) \since 6.0 Removes the (key, value) pairs pointed to by the iterator range [\a first, \a last) from the map. Returns an iterator to the item in the map following the last removed element. \note The range \c {[first, last)} \e must be a valid range in \c {*this}. \sa remove() */ /*! \fn template QMap::iterator QMap::find(const Key &key) Returns an iterator pointing to the item with key \a key in the map. If the map contains no item with key \a key, the function returns end(). \sa constFind(), value(), values(), lowerBound(), upperBound() */ /*! \fn template QMap::const_iterator QMap::find(const Key &key) const \overload */ /*! \fn template QMap::const_iterator QMap::constFind(const Key &key) const \since 4.1 Returns an const iterator pointing to the item with key \a key in the map. If the map contains no item with key \a key, the function returns constEnd(). \sa find() */ /*! \fn template QMap::iterator QMap::lowerBound(const Key &key) Returns an iterator pointing to the first item with key \a key in the map. If the map contains no item with key \a key, the function returns an iterator to the nearest item with a greater key. \sa upperBound(), find() */ /*! \fn template QMap::const_iterator QMap::lowerBound(const Key &key) const \overload */ /*! \fn template QMap::iterator QMap::upperBound(const Key &key) Returns an iterator pointing to the item that immediately follows the last item with key \a key in the map. If the map contains no item with key \a key, the function returns an iterator to the nearest item with a greater key. Example: \snippet code/src_corelib_tools_qmap.cpp 17 \sa lowerBound(), find() */ /*! \fn template QMap::const_iterator QMap::upperBound(const Key &key) const \overload */ /*! \fn template QMap::iterator QMap::insert(const Key &key, const T &value) Inserts a new item with the key \a key and a value of \a value. If there is already an item with the key \a key, that item's value is replaced with \a value. \sa QMultiMap::insert() */ /*! \fn template QMap::iterator QMap::insert(const_iterator pos, const Key &key, const T &value) \overload \since 5.1 Inserts a new item with the key \a key and value \a value and with hint \a pos suggesting where to do the insert. If constBegin() is used as hint it indicates that the \a key is less than any key in the map while constEnd() suggests that the \a key is (strictly) larger than any key in the map. Otherwise the hint should meet the condition (\a pos - 1).key() < \a key <= pos.key(). If the hint \a pos is wrong it is ignored and a regular insert is done. If there is already an item with the key \a key, that item's value is replaced with \a value. If the hint is correct and the map is unshared, the insert executes in amortized \l{constant time}. When creating a map from sorted data inserting the largest key first with constBegin() is faster than inserting in sorted order with constEnd(), since constEnd() - 1 (which is needed to check if the hint is valid) needs \l{logarithmic time}. \b {Note:} Be careful with the hint. Providing an iterator from an older shared instance might crash but there is also a risk that it will silently corrupt both the map and the \a pos map. \sa QMultiMap::insert() */ /*! \fn template void QMap::insert(const QMap &map) \since 5.15 Inserts all the items in \a map into this map. If a key is common to both maps, its value will be replaced with the value stored in \a map. \note If \a map contains multiple entries with the same key then the final value of the key is undefined. \sa QMultiMap::insert() */ /*! \fn template void QMap::insert(QMap &&map) \since 5.15 Moves all the items from \a map into this map. If a key is common to both maps, its value will be replaced with the value stored in \a map. If \a map is shared, then the items will be copied instead. */ /*! \typedef QMap::Iterator Qt-style synonym for QMap::iterator. */ /*! \typedef QMap::ConstIterator Qt-style synonym for QMap::const_iterator. */ /*! \typedef QMap::difference_type Typedef for ptrdiff_t. Provided for STL compatibility. */ /*! \typedef QMap::key_type Typedef for Key. Provided for STL compatibility. */ /*! \typedef QMap::mapped_type Typedef for T. Provided for STL compatibility. */ /*! \typedef QMap::size_type Typedef for int. Provided for STL compatibility. */ /*! \fn template bool QMap::empty() const This function is provided for STL compatibility. It is equivalent to isEmpty(), returning true if the map is empty; otherwise returning false. */ /*! \fn template QPair::iterator, typename QMap::iterator> QMap::equal_range(const Key &key) Returns a pair of iterators delimiting the range of values \c{[first, second)}, that are stored under \a key. */ /*! \fn template QPair::const_iterator, typename QMap::const_iterator> QMap::equal_range(const Key &key) const \overload \since 5.6 */ /*! \class QMap::iterator \inmodule QtCore \brief The QMap::iterator class provides an STL-style non-const iterator for QMap. QMap features both \l{STL-style iterators} and \l{Java-style iterators}. The STL-style iterators are more low-level and more cumbersome to use; on the other hand, they are slightly faster and, for developers who already know STL, have the advantage of familiarity. QMap\::iterator allows you to iterate over a QMap and to modify the value (but not the key) stored under a particular key. If you want to iterate over a const QMap, you should use QMap::const_iterator. It is generally good practice to use QMap::const_iterator on a non-const QMap as well, unless you need to change the QMap through the iterator. Const iterators are slightly faster, and can improve code readability. The default QMap::iterator constructor creates an uninitialized iterator. You must initialize it using a QMap function like QMap::begin(), QMap::end(), or QMap::find() before you can start iterating. Here's a typical loop that prints all the (key, value) pairs stored in a map: \snippet code/src_corelib_tools_qmap.cpp 18 Unlike QHash, which stores its items in an arbitrary order, QMap stores its items ordered by key. Let's see a few examples of things we can do with a QMap::iterator that we cannot do with a QMap::const_iterator. Here's an example that increments every value stored in the QMap by 2: \snippet code/src_corelib_tools_qmap.cpp 19 Here's an example that removes all the items whose key is a string that starts with an underscore character: \snippet code/src_corelib_tools_qmap.cpp 20 The call to QMap::erase() removes the item pointed to by the iterator from the map, and returns an iterator to the next item. Here's another way of removing an item while iterating: \snippet code/src_corelib_tools_qmap.cpp 21 It might be tempting to write code like this: \snippet code/src_corelib_tools_qmap.cpp 22 However, this will potentially crash in \c{++i}, because \c i is a dangling iterator after the call to erase(). Multiple iterators can be used on the same map. If you add items to the map, existing iterators will remain valid. If you remove items from the map, iterators that point to the removed items will become dangling iterators. \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 \l{Implicit sharing iterator problem}. \sa QMap::const_iterator, QMap::key_iterator, QMutableMapIterator */ /*! \typedef QMap::iterator::difference_type \internal */ /*! \typedef QMap::iterator::iterator_category A synonym for \e {std::bidirectional_iterator_tag} indicating this iterator is a bidirectional iterator. */ /*! \typedef QMap::iterator::pointer \internal */ /*! \typedef QMap::iterator::reference \internal */ /*! \typedef QMap::iterator::value_type \internal */ /*! \fn template QMap::iterator::iterator() Constructs an uninitialized iterator. Functions like key(), value(), and operator++() must not be called on an uninitialized iterator. Use operator=() to assign a value to it before using it. \sa QMap::begin(), QMap::end() */ /*! \fn template const Key &QMap::iterator::key() const Returns the current item's key as a const reference. There is no direct way of changing an item's key through an iterator, although it can be done by calling QMap::erase() followed by QMap::insert(). \sa value() */ /*! \fn template T &QMap::iterator::value() const Returns a modifiable reference to the current item's value. You can change the value of an item by using value() on the left side of an assignment, for example: \snippet code/src_corelib_tools_qmap.cpp 23 \sa key(), operator*() */ /*! \fn template T &QMap::iterator::operator*() const Returns a modifiable reference to the current item's value. Same as value(). \sa key() */ /*! \fn template T *QMap::iterator::operator->() const Returns a pointer to the current item's value. \sa value() */ /*! \fn template bool QMap::iterator::operator==(const iterator &lhs, const iterator &rhs) \fn template bool QMap::const_iterator::operator==(const const_iterator &lhs, const const_iterator &rhs) Returns \c true if \a lhs points to the same item as the \a rhs iterator; otherwise returns \c false. \sa operator!=() */ /*! \fn template bool QMap::iterator::operator!=(const iterator &lhs, const iterator &rhs) \fn template bool QMap::const_iterator::operator!=(const const_iterator &lhs, const const_iterator &rhs) Returns \c true if \a lhs points to a different item than the \a rhs iterator; otherwise returns \c false. \sa operator==() */ /*! \fn template QMap::iterator &QMap::iterator::operator++() The prefix ++ operator (\c{++i}) advances the iterator to the next item in the map and returns an iterator to the new current item. Calling this function on QMap::end() leads to undefined results. \sa operator--() */ /*! \fn template QMap::iterator QMap::iterator::operator++(int) \overload The postfix ++ operator (\c{i++}) advances the iterator to the next item in the map and returns an iterator to the previously current item. */ /*! \fn template QMap::iterator &QMap::iterator::operator--() The prefix -- operator (\c{--i}) makes the preceding item current and returns an iterator pointing to the new current item. Calling this function on QMap::begin() leads to undefined results. \sa operator++() */ /*! \fn template QMap::iterator QMap::iterator::operator--(int) \overload The postfix -- operator (\c{i--}) makes the preceding item current and returns an iterator pointing to the previously current item. */ /*! //! friends \fn [qmap-op-it-plus-step] template typename QMap::iterator QMap::iterator::operator+(QMap::iterator, difference_type n) \fn [qmap-op-step-plus-it] template QMap::iterator QMap::iterator::operator+(difference_type n, QMap::iterator) \fn [qmap-op-it-minus-step] template QMap::iterator QMap::iterator::operator-(QMap::iterator, difference_type n) \fn [qmap-op-step-minus-it] template QMap::iterator QMap::iterator::operator-(difference_type n, QMap::iterator) //! members \fn template typename QMap::iterator QMap::iterator::operator+=(QMap::iterator::difference_type n) \fn template typename QMap::iterator QMap::iterator::operator-=(QMap::iterator::difference_type n) \deprecated [6.2] Use \c{std::next}, \c{std::prev} or \c{std::advance} instead. Moves an iterator by \e{n} positions. These operations can be expensive for large values of \e{n}; QMap iterators are not random access. */ /*! \class QMap::const_iterator \inmodule QtCore \brief The QMap::const_iterator class provides an STL-style const iterator for QMap. QMap features both \l{STL-style iterators} and \l{Java-style iterators}. The STL-style iterators are more low-level and more cumbersome to use; on the other hand, they are slightly faster and, for developers who already know STL, have the advantage of familiarity. QMap\::const_iterator allows you to iterate over a QMap. If you want to modify the QMap as you iterate over it, you must use QMap::iterator instead. It is generally good practice to use QMap::const_iterator on a non-const QMap as well, unless you need to change the QMap through the iterator. Const iterators are slightly faster, and can improve code readability. The default QMap::const_iterator constructor creates an uninitialized iterator. You must initialize it using a QMap function like QMap::constBegin(), QMap::constEnd(), or QMap::find() before you can start iterating. Here's a typical loop that prints all the (key, value) pairs stored in a map: \snippet code/src_corelib_tools_qmap.cpp 24 Unlike QHash, which stores its items in an arbitrary order, QMap stores its items ordered by key. Multiple iterators can be used on the same map. If you add items to the map, existing iterators will remain valid. If you remove items from the map, iterators that point to the removed items will become dangling iterators. \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 \l{Implicit sharing iterator problem}. \sa QMap::iterator, QMap::key_iterator, QMapIterator */ /*! \typedef QMap::const_iterator::difference_type \internal */ /*! \typedef QMap::const_iterator::iterator_category A synonym for \e {std::bidirectional_iterator_tag} indicating this iterator is a bidirectional iterator. */ /*! \typedef QMap::const_iterator::pointer \internal */ /*! \typedef QMap::const_iterator::reference \internal */ /*! \typedef QMap::const_iterator::value_type \internal */ /*! \fn template QMap::const_iterator::const_iterator() Constructs an uninitialized iterator. Functions like key(), value(), and operator++() must not be called on an uninitialized iterator. Use operator=() to assign a value to it before using it. \sa QMap::constBegin(), QMap::constEnd() */ /*! \fn template QMap::const_iterator::const_iterator(const iterator &other) Constructs a copy of \a other. */ /*! \fn template const Key &QMap::const_iterator::key() const Returns the current item's key. \sa value() */ /*! \fn template const T &QMap::const_iterator::value() const Returns the current item's value. \sa key(), operator*() */ /*! \fn template const T &QMap::const_iterator::operator*() const Returns the current item's value. Same as value(). \sa key() */ /*! \fn template const T *QMap::const_iterator::operator->() const Returns a pointer to the current item's value. \sa value() */ /*! \fn template QMap::const_iterator &QMap::const_iterator::operator++() The prefix ++ operator (\c{++i}) advances the iterator to the next item in the map and returns an iterator to the new current item. Calling this function on QMap::end() leads to undefined results. \sa operator--() */ /*! \fn template QMap::const_iterator QMap::const_iterator::operator++(int) \overload The postfix ++ operator (\c{i++}) advances the iterator to the next item in the map and returns an iterator to the previously current item. */ /*! \fn template QMap::const_iterator &QMap::const_iterator::operator--() The prefix -- operator (\c{--i}) makes the preceding item current and returns an iterator pointing to the new current item. Calling this function on QMap::begin() leads to undefined results. \sa operator++() */ /*! \fn template QMap::const_iterator QMap::const_iterator::operator--(int) \overload The postfix -- operator (\c{i--}) makes the preceding item current and returns an iterator pointing to the previously current item. */ /*! //! friends \fn [qmap-op-it-plus-step-const] template QMap::const_iterator::operator+(QMap::const_iterator, difference_type n) \fn [qmap-op-step-plus-it-const] template QMap::operator+(difference_type n, QMap::const_iterator) \fn [qmap-op-it-minus-step-const] template QMap::operator-(QMap::const_iterator, difference_type n) \fn [qmap-op-step-minus-it-const] template QMap::operator-(difference_type n, QMap::const_iterator) //! members \fn template typename QMap::const_iterator QMap::const_iterator::operator+=(QMap::const_iterator::difference_type n) \fn template typename QMap::const_iterator QMap::const_iterator::operator-=(QMap::const_iterator::difference_type n) \deprecated [6.2] Use \c{std::next}, \c{std::prev} or \c{std::advance} instead. Moves an iterator by \e{n} positions. These operations can be expensive for large values of \e{n}. QMap iterators are not random access. */ /*! \class QMap::key_iterator \inmodule QtCore \since 5.6 \brief The QMap::key_iterator class provides an STL-style const iterator for QMap keys. QMap::key_iterator is essentially the same as QMap::const_iterator with the difference that operator*() and operator->() return a key instead of a value. For most uses QMap::iterator and QMap::const_iterator should be used, you can easily access the key by calling QMap::iterator::key(): \snippet code/src_corelib_tools_qmap.cpp keyiterator1 However, to have interoperability between QMap's keys and STL-style algorithms we need an iterator that dereferences to a key instead of a value. With QMap::key_iterator we can apply an algorithm to a range of keys without having to call QMap::keys(), which is inefficient as it costs one QMap iteration and memory allocation to create a temporary QList. \snippet code/src_corelib_tools_qmap.cpp keyiterator2 QMap::key_iterator is const, it's not possible to modify the key. The default QMap::key_iterator constructor creates an uninitialized iterator. You must initialize it using a QMap function like QMap::keyBegin() or QMap::keyEnd(). \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 \l{Implicit sharing iterator problem}. \sa QMap::const_iterator, QMap::iterator */ /*! \typedef QMap::key_iterator::difference_type \internal */ /*! \typedef QMap::key_iterator::iterator_category \internal */ /*! \typedef QMap::key_iterator::pointer \internal */ /*! \typedef QMap::key_iterator::reference \internal */ /*! \typedef QMap::key_iterator::value_type \internal */ /*! \fn template const T &QMap::key_iterator::operator*() const Returns the current item's key. */ /*! \fn template const T *QMap::key_iterator::operator->() const Returns a pointer to the current item's key. */ /*! \fn template bool QMap::key_iterator::operator==(key_iterator other) const Returns \c true if \a other points to the same item as this iterator; otherwise returns \c false. \sa operator!=() */ /*! \fn template bool QMap::key_iterator::operator!=(key_iterator other) const Returns \c true if \a other points to a different item than this iterator; otherwise returns \c false. \sa operator==() */ /*! \fn template QMap::key_iterator &QMap::key_iterator::operator++() The prefix ++ operator (\c{++i}) advances the iterator to the next item in the hash and returns an iterator to the new current item. Calling this function on QMap::keyEnd() leads to undefined results. \sa operator--() */ /*! \fn template QMap::key_iterator QMap::key_iterator::operator++(int) \overload The postfix ++ operator (\c{i++}) advances the iterator to the next item in the hash and returns an iterator to the previous item. */ /*! \fn template QMap::key_iterator &QMap::key_iterator::operator--() The prefix -- operator (\c{--i}) makes the preceding item current and returns an iterator pointing to the new current item. Calling this function on QMap::keyBegin() leads to undefined results. \sa operator++() */ /*! \fn template QMap::key_iterator QMap::key_iterator::operator--(int) \overload The postfix -- operator (\c{i--}) makes the preceding item current and returns an iterator pointing to the previous item. */ /*! \fn template const_iterator QMap::key_iterator::base() const Returns the underlying const_iterator this key_iterator is based on. */ /*! \typedef QMap::const_key_value_iterator \inmodule QtCore \since 5.10 \brief The QMap::const_key_value_iterator typedef provides an STL-style iterator for QMap. QMap::const_key_value_iterator is essentially the same as QMap::const_iterator with the difference that operator*() returns a key/value pair instead of a value. \sa QKeyValueIterator */ /*! \typedef QMap::key_value_iterator \inmodule QtCore \since 5.10 \brief The QMap::key_value_iterator typedef provides an STL-style iterator for QMap. QMap::key_value_iterator is essentially the same as QMap::iterator with the difference that operator*() returns a key/value pair instead of a value. \sa QKeyValueIterator */ /*! \fn template QDataStream &operator<<(QDataStream &out, const QMap &map) \relates QMap Writes the map \a map to stream \a out. This function requires the key and value types to implement \c operator<<(). \sa{Serializing Qt Data Types}{Format of the QDataStream operators} */ /*! \fn template QDataStream &operator>>(QDataStream &in, QMap &map) \relates QMap Reads a map from stream \a in into \a map. This function requires the key and value types to implement \c operator>>(). \sa{Serializing Qt Data Types}{Format of the QDataStream operators} */ /*! \fn template qsizetype erase_if(QMap &map, Predicate pred) \relates QMap \since 6.1 Removes all elements for which the predicate \a pred returns true from the map \a map. The function supports predicates which take either an argument of type \c{QMap::iterator}, or an argument of type \c{std::pair}. Returns the number of elements removed, if any. */