QMap Class

template <typename Key, typename T> class QMap

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.

Public Types

Public Functions

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)

Detailed Description

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:

  • QHash provides average faster lookups than QMap. (See Algorithmic Complexity for details.)
  • When iterating over a QHash, the items are arbitrarily ordered. With QMap, the items are always sorted by key.
  • 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.

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.

Member Type Documentation

QMap::ConstIterator

Qt-style synonym for QMap<Key, T>::const_iterator.

QMap::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.

Member Function Documentation

[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=.

QMap::QMap(std::map<Key, T> &&other)

Constructs a map by moving from other.

See also toStdMap().

QMap::QMap(const std::map<Key, T> &other)

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.

QMap::QMap()

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.

QMap::iterator QMap::begin()

Returns an STL-style iterator pointing to the first item in the map.

See also constBegin() and end().

QMap::const_iterator QMap::begin() const

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.

See also begin() and cend().

[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.

See also cbegin() and end().

void QMap::clear()

Removes all items from the map.

See also remove().

QMap::const_iterator QMap::constBegin() const

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

See also begin() and constEnd().

QMap::const_iterator QMap::constEnd() const

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

See also constBegin() and end().

QMap::const_iterator QMap::constFind(const Key &key) const

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().

bool QMap::contains(const Key &key) const

Returns true if the map contains an item with key key; otherwise returns false.

See also count().

QMap::size_type QMap::count(const Key &key) const

Returns the number of items associated with key key.

See also contains().

QMap::size_type QMap::count() const

This is an overloaded function.

Same as size().

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.

QMap::iterator QMap::end()

Returns an STL-style iterator pointing to the imaginary item after the last item in the map.

See also begin() and constEnd().

QMap::const_iterator QMap::end() const

This is an overloaded function.

QPair<QMap::iterator, QMap::iterator> QMap::equal_range(const Key &key)

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.

QMap::iterator QMap::erase(QMap::const_iterator pos)

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().

QMap::iterator QMap::find(const Key &key)

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().

QMap::const_iterator QMap::find(const Key &key) const

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().

QMap::iterator QMap::insert(const Key &key, const T &value)

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.

bool QMap::isEmpty() const

Returns true if the map contains no items; otherwise returns false.

See also size().

Key QMap::key(const T &value, const Key &defaultKey = Key()) const

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.

See also value() and keys().

[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().

QList<Key> 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 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().

QList<Key> QMap::keys(const T &value) const

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().

QMap::iterator QMap::lowerBound(const Key &key)

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().

QMap::const_iterator QMap::lowerBound(const Key &key) const

This is an overloaded function.

QMap::size_type QMap::remove(const Key &key)

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.

See also clear() and take().

[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.

See also clear() and take().

QMap::size_type QMap::size() const

Returns the number of (key, value) pairs in the map.

See also isEmpty() and count().

void QMap::swap(QMap<Key, T> &other)

Swaps map other with this map. This operation is very fast and never fails.

T QMap::take(const Key &key)

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().

std::map<Key, T> QMap::toStdMap() const &

Returns an STL map equivalent to this QMap.

QMap::iterator QMap::upperBound(const Key &key)

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().

QMap::const_iterator QMap::upperBound(const Key &key) const

This is an overloaded function.

T QMap::value(const Key &key, const T &defaultValue = T()) const

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[]().

QList<T> 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 linear time. The time and memory use that entails can be avoided by iterating from keyValueBegin() to keyValueEnd().

See also keys() and value().

T &QMap::operator[](const Key &key)

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().

T QMap::operator[](const Key &key) const

This is an overloaded function.

Same as value().

Related Non-Members

[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.

bool operator!=(const QMap<Key, T> &lhs, const QMap<Key, T> &rhs)

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==().

template <typename Key, typename T> QDataStream &operator<<(QDataStream &out, const QMap<Key, T> &map)

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.

bool operator==(const QMap<Key, T> &lhs, const QMap<Key, T> &rhs)

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!=().

template <typename Key, typename T> QDataStream &operator>>(QDataStream &in, QMap<Key, T> &map)

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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