The QMap class is a template class that provides an associative array. More...
| Header: | #include <QMap> |
| CMake: | find_package(Qt6 COMPONENTS Core REQUIRED) target_link_libraries(mytarget PRIVATE Qt6::Core) |
| qmake: | QT += core |
Note: All functions in this class are reentrant.
| class | const_iterator |
| class | iterator |
| class | key_iterator |
| ConstIterator | |
| Iterator | |
| const_key_value_iterator | |
| difference_type | |
| key_type | |
| key_value_iterator | |
| mapped_type | |
| size_type |
| QMap(QMap<Key, T> &&other) | |
| QMap(const QMap<Key, T> &other) | |
| QMap(std::map<Key, T> &&other) | |
| QMap(const std::map<Key, T> &other) | |
| QMap(std::initializer_list<std::pair<Key, T> > list) | |
| QMap() | |
| QMap<Key, T> & | operator=(QMap<Key, T> &&other) |
| QMap<Key, T> & | operator=(const QMap<Key, T> &other) |
| ~QMap() | |
| QMap::iterator | begin() |
| QMap::const_iterator | begin() const |
| QMap::const_iterator | cbegin() const |
| QMap::const_iterator | cend() const |
| void | clear() |
| QMap::const_iterator | constBegin() const |
| QMap::const_iterator | constEnd() const |
| QMap::const_iterator | constFind(const Key &key) const |
| QMap::const_key_value_iterator | constKeyValueBegin() const |
| QMap::const_key_value_iterator | constKeyValueEnd() const |
| bool | contains(const Key &key) const |
| QMap::size_type | count(const Key &key) const |
| QMap::size_type | count() const |
| bool | empty() const |
| QMap::iterator | end() |
| QMap::const_iterator | end() const |
| QPair<QMap::iterator, QMap::iterator> | equal_range(const Key &key) |
| QPair<QMap::const_iterator, QMap::const_iterator> | equal_range(const Key &key) const |
| QMap::iterator | erase(QMap::const_iterator pos) |
| QMap::iterator | erase(QMap::const_iterator first, QMap::const_iterator last) |
| QMap::iterator | find(const Key &key) |
| QMap::const_iterator | find(const Key &key) const |
| T & | first() |
| const T & | first() const |
| const Key & | firstKey() const |
| QMap::iterator | insert(const Key &key, const T &value) |
| QMap::iterator | insert(QMap::const_iterator pos, const Key &key, const T &value) |
| void | insert(const QMap<Key, T> &map) |
| void | insert(QMap<Key, T> &&map) |
| bool | isEmpty() const |
| Key | key(const T &value, const Key &defaultKey = Key()) const |
| QMap::key_iterator | keyBegin() const |
| QMap::key_iterator | keyEnd() const |
| QMap::key_value_iterator | keyValueBegin() |
| QMap::const_key_value_iterator | keyValueBegin() const |
| QMap::key_value_iterator | keyValueEnd() |
| QMap::const_key_value_iterator | keyValueEnd() const |
| QList<Key> | keys() const |
| QList<Key> | keys(const T &value) const |
| T & | last() |
| const T & | last() const |
| const Key & | lastKey() const |
| QMap::iterator | lowerBound(const Key &key) |
| QMap::const_iterator | lowerBound(const Key &key) const |
| QMap::size_type | remove(const Key &key) |
| QMap::size_type | removeIf(Predicate pred) |
| QMap::size_type | size() const |
| void | swap(QMap<Key, T> &other) |
| T | take(const Key &key) |
| std::map<Key, T> | toStdMap() const & |
| QMap::iterator | upperBound(const Key &key) |
| QMap::const_iterator | upperBound(const Key &key) const |
| T | value(const Key &key, const T &defaultValue = T()) const |
| QList<T> | values() const |
| T & | operator[](const Key &key) |
| T | operator[](const Key &key) const |
| qsizetype | erase_if(QMap<Key, T> &map, Predicate pred) |
| bool | operator!=(const QMap<Key, T> &lhs, const QMap<Key, T> &rhs) |
| QDataStream & | operator<<(QDataStream &out, const QMap<Key, T> &map) |
| bool | operator==(const QMap<Key, T> &lhs, const QMap<Key, T> &rhs) |
| QDataStream & | operator>>(QDataStream &in, QMap<Key, T> &map) |
QMap<Key, T> is one of Qt's generic container classes. It stores (key, value) pairs and provides fast lookup by key.
QMap and QHash provide very similar functionality. The differences are:
Here's an example QMap with QString keys and int values:
QMap<QString, int> map;
To insert a (key, value) pair into the map, you can use operator[]():
map["one"] = 1; map["three"] = 3; map["seven"] = 7;
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():
map.insert("twelve", 12); To look up a value, use operator[]() or value():
int num1 = map["thirteen"];
int num2 = map.value("thirteen"); If there is no item with the specified key in the map, these functions return a default-constructed value.
If you want to check whether the map contains a certain key, use contains():
int timeout = 30;
if (map.contains("TIMEOUT"))
timeout = map.value("TIMEOUT"); There is also a value() overload that uses its second argument as a default value if there is no item with the specified key:
int timeout = map.value("TIMEOUT", 30); 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:
// WRONG
QMap<int, QWidget *> map;
...
for (int i = 0; i < 1000; ++i) {
if (map[i] == okButton)
cout << "Found button at index " << i << Qt::endl;
} To avoid this problem, replace map[i] with 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 Java-style iterators (QMapIterator and QMutableMapIterator) and STL-style iterators (QMap::const_iterator and QMap::iterator). Here's how to iterate over a QMap<QString, int> using a Java-style iterator:
QMapIterator<QString, int> i(map);
while (i.hasNext()) {
i.next();
cout << i.key() << ": " << i.value() << Qt::endl;
} Here's the same code, but using an STL-style iterator this time:
QMap<QString, int>::const_iterator i = map.constBegin();
while (i != map.constEnd()) {
cout << i.key() << ": " << i.value() << Qt::endl;
++i;
} 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:
map.insert("plenty", 100);
map.insert("plenty", 2000);
// map.value("plenty") == 2000 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<T>:
QList<int> values = map.values("plenty");
for (int i = 0; i < values.size(); ++i)
cout << values.at(i) << Qt::endl; Another approach is to call find() to get the STL-style iterator for the first item with a key and iterate from there:
QMap<QString, int>::iterator i = map.find("plenty");
while (i != map.end() && i.key() == "plenty") {
cout << i.value() << Qt::endl;
++i;
} If you only need to extract the values from a map (not the keys), you can also use foreach:
QMap<QString, int> map;
...
foreach (int value, map)
cout << value << Qt::endl; 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 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 x and y are equivalent if neither x < y nor y < x is true.
Example:
#ifndef EMPLOYEE_H
#define EMPLOYEE_H
class Employee
{
public:
Employee() {}
Employee(const QString &name, QDate dateOfBirth);
...
private:
QString myName;
QDate myDateOfBirth;
};
inline bool operator<(const Employee &e1, const Employee &e2)
{
if (e1.name() != e2.name())
return e1.name() < e2.name();
return e1.dateOfBirth() < e2.dateOfBirth();
}
#endif // EMPLOYEE_H In the example, we start by comparing the employees' names. If they're equal, we compare their dates of birth to break the tie.
See also QMapIterator, QMutableMapIterator, QHash, and QSet.
Qt-style synonym for QMap<Key, T>::const_iterator.
Qt-style synonym for QMap<Key, T>::iterator.
[since 5.10] QMap::const_key_value_iterator
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.
This typedef was introduced in Qt 5.10.
See also QKeyValueIterator.
[alias] QMap::difference_type
Typedef for ptrdiff_t. Provided for STL compatibility.
[alias] QMap::key_type
Typedef for Key. Provided for STL compatibility.
[since 5.10] QMap::key_value_iterator
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.
This typedef was introduced in Qt 5.10.
See also QKeyValueIterator.
[alias] QMap::mapped_type
Typedef for T. Provided for STL compatibility.
[alias] QMap::size_type
Typedef for int. Provided for STL compatibility.
[default, since 5.2] QMap::QMap(QMap<Key, T> &&other)
Move-constructs a QMap instance.
This function was introduced in Qt 5.2.
[default] QMap::QMap(const QMap<Key, T> &other)
Constructs a copy of other.
This operation occurs in constant time, because QMap is 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 linear time.
See also operator=.
Constructs a map by moving from other.
See also toStdMap().
Constructs a copy of other.
See also toStdMap().
[since 5.1] QMap::QMap(std::initializer_list<std::pair<Key, T> > list)
Constructs a map with a copy of each of the elements in the initializer list list.
This function was introduced in Qt 5.1.
Constructs an empty map.
See also clear().
[default, since 5.2] QMap<Key, T> &QMap::operator=(QMap<Key, T> &&other)
Move-assigns other to this QMap instance.
This function was introduced in Qt 5.2.
[default] QMap<Key, T> &QMap::operator=(const QMap<Key, T> &other)
Assigns other to this map and returns a reference to this map.
[default] QMap::~QMap()
Destroys the map. References to the values in the map, and all iterators over this map, become invalid.
Returns an STL-style iterator pointing to the first item in the map.
See also constBegin() and end().
This is an overloaded function.
[since 5.0] QMap::const_iterator QMap::cbegin() const
Returns a const STL-style iterator pointing to the first item in the map.
This function was introduced in Qt 5.0.
[since 5.0] QMap::const_iterator QMap::cend() const
Returns a const STL-style iterator pointing to the imaginary item after the last item in the map.
This function was introduced in Qt 5.0.
Removes all items from the map.
See also remove().
Returns a const STL-style iterator pointing to the first item in the map.
See also begin() and constEnd().
Returns a const STL-style iterator pointing to the imaginary item after the last item in the map.
See also constBegin() and end().
Returns an const iterator pointing to the item with key key in the map.
If the map contains no item with key key, the function returns constEnd().
See also find().
[since 5.10] QMap::const_key_value_iterator QMap::constKeyValueBegin() const
Returns a const STL-style iterator pointing to the first entry in the map.
This function was introduced in Qt 5.10.
See also keyValueBegin().
[since 5.10] QMap::const_key_value_iterator QMap::constKeyValueEnd() const
Returns a const STL-style iterator pointing to the imaginary entry after the last entry in the map.
This function was introduced in Qt 5.10.
See also constKeyValueBegin().
Returns true if the map contains an item with key key; otherwise returns false.
See also count().
Returns the number of items associated with key key.
See also contains().
This is an overloaded function.
Same as size().
This function is provided for STL compatibility. It is equivalent to isEmpty(), returning true if the map is empty; otherwise returning false.
Returns an STL-style iterator pointing to the imaginary item after the last item in the map.
See also begin() and constEnd().
This is an overloaded function.
Returns a pair of iterators delimiting the range of values [first, second), that are stored under key.
[since 5.6] QPair<QMap::const_iterator, QMap::const_iterator> QMap::equal_range(const Key &key) const
This is an overloaded function.
This function was introduced in Qt 5.6.
Removes the (key, value) pair pointed to by the iterator pos from the map, and returns an iterator to the next item in the map.
Note: The iterator pos must be valid and dereferenceable.
See also remove().
[since 6.0] QMap::iterator QMap::erase(QMap::const_iterator first, QMap::const_iterator last)
Removes the (key, value) pairs pointed to by the iterator range [first, last) from the map. Returns an iterator to the item in the map following the last removed element.
Note: The range [first, last) must be a valid range in *this.
This function was introduced in Qt 6.0.
See also remove().
Returns an iterator pointing to the item with key key in the map.
If the map contains no item with key key, the function returns end().
See also constFind(), value(), values(), lowerBound(), and upperBound().
This is an overloaded function.
[since 5.2] T &QMap::first()
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 constant time.
This function was introduced in Qt 5.2.
See also last(), firstKey(), and isEmpty().
[since 5.2] const T &QMap::first() const
This is an overloaded function.
This function was introduced in Qt 5.2.
[since 5.2] const Key &QMap::firstKey() const
Returns a reference to the smallest key in the map. This function assumes that the map is not empty.
This executes in constant time.
This function was introduced in Qt 5.2.
See also lastKey(), first(), keyBegin(), and isEmpty().
Inserts a new item with the key key and a value of value.
If there is already an item with the key key, that item's value is replaced with value.
See also QMultiMap::insert().
[since 5.1] QMap::iterator QMap::insert(QMap::const_iterator pos, const Key &key, const T &value)
This is an overloaded function.
Inserts a new item with the key key and value value and with hint pos suggesting where to do the insert.
If constBegin() is used as hint it indicates that the key is less than any key in the map while constEnd() suggests that the key is (strictly) larger than any key in the map. Otherwise the hint should meet the condition (pos - 1).key() < key <= pos.key(). If the hint pos is wrong it is ignored and a regular insert is done.
If there is already an item with the key key, that item's value is replaced with value.
If the hint is correct and the map is unshared, the insert executes in amortized 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 logarithmic time.
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 pos map.
This function was introduced in Qt 5.1.
See also QMultiMap::insert().
[since 5.15] void QMap::insert(const QMap<Key, T> &map)
Inserts all the items in map into this map.
If a key is common to both maps, its value will be replaced with the value stored in map.
Note: If map contains multiple entries with the same key then the final value of the key is undefined.
This function was introduced in Qt 5.15.
See also QMultiMap::insert().
[since 5.15] void QMap::insert(QMap<Key, T> &&map)
Moves all the items from map into this map.
If a key is common to both maps, its value will be replaced with the value stored in map.
If map is shared, then the items will be copied instead.
This function was introduced in Qt 5.15.
Returns true if the map contains no items; otherwise returns false.
See also size().
This is an overloaded function.
Returns the first key with value value, or defaultKey if the map contains no item with value value. If no defaultKey is provided the function returns a default-constructed key.
This function can be slow (linear time), because QMap's internal data structure is optimized for fast lookup by key, not by value.
[since 5.6] QMap::key_iterator QMap::keyBegin() const
Returns a const STL-style iterator pointing to the first key in the map.
This function was introduced in Qt 5.6.
See also keyEnd() and firstKey().
[since 5.6] QMap::key_iterator QMap::keyEnd() const
Returns a const STL-style iterator pointing to the imaginary item after the last key in the map.
This function was introduced in Qt 5.6.
See also keyBegin() and lastKey().
[since 5.10] QMap::key_value_iterator QMap::keyValueBegin()
Returns an STL-style iterator pointing to the first entry in the map.
This function was introduced in Qt 5.10.
See also keyValueEnd().
[since 5.10] QMap::const_key_value_iterator QMap::keyValueBegin() const
Returns a const STL-style iterator pointing to the first entry in the map.
This function was introduced in Qt 5.10.
See also keyValueEnd().
[since 5.10] QMap::key_value_iterator QMap::keyValueEnd()
Returns an STL-style iterator pointing to the imaginary entry after the last entry in the map.
This function was introduced in Qt 5.10.
See also keyValueBegin().
[since 5.10] QMap::const_key_value_iterator QMap::keyValueEnd() const
Returns a const STL-style iterator pointing to the imaginary entry after the last entry in the map.
This function was introduced in Qt 5.10.
See also keyValueBegin().
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 linear time. The time and memory use that entails can be avoided by iterating from keyBegin() to keyEnd().
See also QMultiMap::uniqueKeys(), values(), and key().
This is an overloaded function.
Returns a list containing all the keys associated with value value in ascending order.
This function can be slow (linear time), because QMap's internal data structure is optimized for fast lookup by key, not by value.
[since 5.2] T &QMap::last()
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 logarithmic time.
This function was introduced in Qt 5.2.
See also first(), lastKey(), and isEmpty().
[since 5.2] const T &QMap::last() const
This is an overloaded function.
This function was introduced in Qt 5.2.
[since 5.2] const Key &QMap::lastKey() const
Returns a reference to the largest key in the map. This function assumes that the map is not empty.
This executes in logarithmic time.
This function was introduced in Qt 5.2.
See also firstKey(), last(), keyEnd(), and isEmpty().
Returns an iterator pointing to the first item with key key in the map. If the map contains no item with key key, the function returns an iterator to the nearest item with a greater key.
See also upperBound() and find().
This is an overloaded function.
Removes all the items that have the key key from the map. Returns the number of items removed which will be 1 if the key exists in the map, and 0 otherwise.
[since 6.1] template <typename Predicate> QMap::size_type QMap::removeIf(Predicate pred)
Removes all elements for which the predicate pred returns true from the map.
The function supports predicates which take either an argument of type QMap<Key, T>::iterator, or an argument of type std::pair<const Key &, T &>.
Returns the number of elements removed, if any.
This function was introduced in Qt 6.1.
Returns the number of (key, value) pairs in the map.
See also isEmpty() and count().
Swaps map other with this map. This operation is very fast and never fails.
Removes the item with the key 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 default-constructed value. If there are multiple items for 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.
See also remove().
Returns an STL map equivalent to this QMap.
Returns an iterator pointing to the item that immediately follows the last item with key key in the map. If the map contains no item with key key, the function returns an iterator to the nearest item with a greater key.
Example:
QMap<int, QString> map; map.insert(1, "one"); map.insert(5, "five"); map.insert(10, "ten"); map.upperBound(0); // returns iterator to (1, "one") map.upperBound(1); // returns iterator to (5, "five") map.upperBound(2); // returns iterator to (5, "five") map.upperBound(10); // returns end() map.upperBound(999); // returns end()
See also lowerBound() and find().
This is an overloaded function.
Returns the value associated with the key key.
If the map contains no item with key key, the function returns defaultValue. If no defaultValue is specified, the function returns a default-constructed value.
See also key(), values(), contains(), and operator[]().
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 linear time. The time and memory use that entails can be avoided by iterating from keyValueBegin() to keyValueEnd().
Returns the value associated with the key key as a modifiable reference.
If the map contains no item with key key, the function inserts a default-constructed value into the map with key key, and returns a reference to it. If the map contains multiple items with key key, this function returns a reference to the most recently inserted value.
See also insert() and value().
This is an overloaded function.
Same as value().
[since 6.1] template <typename Key, typename T, typename Predicate> qsizetype erase_if(QMap<Key, T> &map, Predicate pred)
Removes all elements for which the predicate pred returns true from the map map.
The function supports predicates which take either an argument of type QMap<Key, T>::iterator, or an argument of type std::pair<const Key &, T &>.
Returns the number of elements removed, if any.
This function was introduced in Qt 6.1.
Returns true if lhs is not equal to 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 operator==().
See also operator==().
Writes the map map to stream out.
This function requires the key and value types to implement operator<<().
See also Format of the QDataStream operators.
Returns true if lhs is equal to 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 operator==().
See also operator!=().
Reads a map from stream in into map.
This function requires the key and value types to implement operator>>().
See also Format of the QDataStream operators.
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Licensed under the GNU Free Documentation License, Version 1.3.
https://doc.qt.io/qt-6.2/qmap.html