QMultiMap Class | Qt Core | Qt 6.10.3
Description: The QMultiMap class is a template class that provides an associative array with multiple equivalent keys.

- Qt 6.10
- Qt Core
- C++ Classes
- QMultiMap
- Qt 6.10.3 Reference Documentation

# QMultiMap Class

template <typename Key, typename T> class QMultiMap

The QMultiMap class is a template class that provides an associative array with multiple equivalent keys. More...

Header: | `#include <QMultiMap>`
CMake: | `find_package(Qt6 REQUIRED COMPONENTS Core)` `target_link_libraries(mytarget PRIVATE Qt6::Core)`
qmake: | `QT += core`

- List of all members, including inherited members
- Deprecated members
- QMultiMap is part of Implicitly Shared Classes.

This class is equality-comparable.

Note: All functions in this class are reentrant.

## Public Types

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

## Public Functions

| QMultiMap()
`(since 6.0)` | QMultiMap(QMap<Key, T> &&other)
`(since 6.0)` | QMultiMap(const QMap<Key, T> &other)
| QMultiMap(const std::multimap<Key, T> &other)
| QMultiMap(std::initializer_list<std::pair<Key, T>> list)
| QMultiMap(std::multimap<Key, T> &&other)
| QMultiMap(const QMultiMap<Key, T> &other)
| QMultiMap(QMultiMap<Key, T> &&other)
| ~QMultiMap()
`(since 6.4)` auto | asKeyValueRange() &&
`(since 6.4)` auto | asKeyValueRange() &
`(since 6.4)` auto | asKeyValueRange() const &&
`(since 6.4)` auto | asKeyValueRange() const &
QMultiMap<Key, T>::iterator | begin()
QMultiMap<Key, T>::const_iterator | begin() const
QMultiMap<Key, T>::const_iterator | cbegin() const
QMultiMap<Key, T>::const_iterator | cend() const
void | clear()
QMultiMap<Key, T>::const_iterator | constBegin() const
QMultiMap<Key, T>::const_iterator | constEnd() const
QMultiMap<Key, T>::const_iterator | constFind(const Key &key) const
QMultiMap<Key, T>::const_iterator | constFind(const Key &key, const T &value) const
QMultiMap<Key, T>::const_key_value_iterator | constKeyValueBegin() const
QMultiMap<Key, T>::const_key_value_iterator | constKeyValueEnd() const
bool | contains(const Key &key) const
bool | contains(const Key &key, const T &value) const
QMultiMap<Key, T>::size_type | count(const Key &key) const
QMultiMap<Key, T>::size_type | count(const Key &key, const T &value) const
QMultiMap<Key, T>::size_type | count() const
bool | empty() const
QMultiMap<Key, T>::iterator | end()
QMultiMap<Key, T>::const_iterator | end() const
std::pair<QMultiMap<Key, T>::iterator, QMultiMap<Key, T>::iterator> | equal_range(const Key &key)
std::pair<QMultiMap<Key, T>::const_iterator, QMultiMap<Key, T>::const_iterator> | equal_range(const Key &key) const
QMultiMap<Key, T>::iterator | erase(QMultiMap<Key, T>::const_iterator pos)
`(since 6.0)` QMultiMap<Key, T>::iterator | erase(QMultiMap<Key, T>::const_iterator first, QMultiMap<Key, T>::const_iterator last)
QMultiMap<Key, T>::iterator | find(const Key &key)
QMultiMap<Key, T>::const_iterator | find(const Key &key) const
QMultiMap<Key, T>::const_iterator | find(const Key &key, const T &value) const
T & | first()
const T & | first() const
const Key & | firstKey() const
QMultiMap<Key, T>::iterator | insert(const Key &key, const T &value)
QMultiMap<Key, T>::iterator | insert(QMultiMap<Key, T>::const_iterator pos, const Key &key, const T &value)
bool | isEmpty() const
Key | key(const T &value, const Key &defaultKey = Key()) const
QMultiMap<Key, T>::key_iterator | keyBegin() const
QMultiMap<Key, T>::key_iterator | keyEnd() const
QMultiMap<Key, T>::key_value_iterator | keyValueBegin()
QMultiMap<Key, T>::const_key_value_iterator | keyValueBegin() const
QMultiMap<Key, T>::key_value_iterator | keyValueEnd()
QMultiMap<Key, T>::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
QMultiMap<Key, T>::iterator | lowerBound(const Key &key)
QMultiMap<Key, T>::const_iterator | lowerBound(const Key &key) const
QMultiMap<Key, T>::size_type | remove(const Key &key)
QMultiMap<Key, T>::size_type | remove(const Key &key, const T &value)
`(since 6.1)` QMultiMap<Key, T>::size_type | removeIf(Predicate pred)
QMultiMap<Key, T>::iterator | replace(const Key &key, const T &value)
QMultiMap<Key, T>::size_type | size() const
void | swap(QMultiMap<Key, T> &other)
T | take(const Key &key)
std::multimap<Key, T> | toStdMultiMap() const &
QList<Key> | uniqueKeys() const
QMultiMap<Key, T> & | unite(QMultiMap<Key, T> &&other)
QMultiMap<Key, T> & | unite(const QMultiMap<Key, T> &other)
QMultiMap<Key, T>::iterator | upperBound(const Key &key)
QMultiMap<Key, T>::const_iterator | upperBound(const Key &key) const
T | value(const Key &key, const T &defaultValue = T()) const
QList<T> | values() const
QList<T> | values(const Key &key) const
QMultiMap<Key, T> & | operator=(QMultiMap<Key, T> &&other)
QMultiMap<Key, T> & | operator=(const QMultiMap<Key, T> &other)

## Related Non-Members

`(since 6.1)` qsizetype | erase_if(QMultiMap<Key, T> &map, Predicate pred)
bool | operator!=(const QMultiMap<Key, T> &lhs, const QMultiMap<Key, T> &rhs)
QMultiMap<Key, T> | operator+(const QMultiMap<Key, T> &lhs, const QMultiMap<Key, T> &rhs)
QMultiMap<Key, T> | operator+=(QMultiMap<Key, T> &lhs, const QMultiMap<Key, T> &rhs)
QDataStream & | operator<<(QDataStream &out, const QMultiMap<Key, T> &map)
bool | operator==(const QMultiMap<Key, T> &lhs, const QMultiMap<Key, T> &rhs)
QDataStream & | operator>>(QDataStream &in, QMultiMap<Key, T> &map)

## Detailed Description

QMultiMap<Key, T> is one of Qt's generic container classes. It stores (key, value) pairs and provides fast lookup by key.

QMultiMap and QMultiHash provide very similar functionality. The differences are:

- QMultiHash provides average faster lookups than QMultiMap. (See Algorithmic Complexity for details.)
- When iterating over a QMultiHash, the items are arbitrarily ordered. With QMultiMap, the items are always sorted by key.
- The key type of a QMultiHash must provide operator==() and a global qHash(Key) function. The key type of a QMultiMap 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 QMultiMap with QString keys and `int` values:

     QMultiMap<QString, int> multimap;

To insert a (key, value) pair into the multi map, you can use insert():

     multimap.insert("a", 1);
     multimap.insert("b", 3);
     multimap.insert("c", 7);
     multimap.insert("c", -5);

This inserts the following four (key, value) pairs into the QMultiMap: ("a", 1), ("b", 3), ("c", 7), and ("c", -5); note that duplicate keys are allowed.

To look up a value, use find() or value():

     int num2 = multimap.value("a"); // 1
     int num3 = multimap.value("thirteen"); // not found; 0
     int num3 = 0;
     auto it = multimap.constFind("b");
     if (it != multimap.cend()) {
         num3 = it.value();
     }

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 (multimap.contains("TIMEOUT"))
         timeout = multimap.value("TIMEOUT");

     // better:
     auto it = multimap.find("TIMEOUT");
     if (it != multimap.end())
         timeout = it.value();

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 = multimap.value("TIMEOUT", 30);

If you want to navigate through all the (key, value) pairs stored in a QMultiMap, you can use an iterator. QMultiMap provides both Java-style iterators (QMultiMapIterator and QMutableMultiMapIterator) and STL-style iterators (QMultiMap::const_iterator and QMultiMap::iterator). Here's how to iterate over a QMultiMap<QString, int> using a Java-style iterator:

     QMultiMapIterator<QString, int> i(multimap);
     while (i.hasNext()) {
         i.next();
         cout << qPrintable(i.key()) << ": " << i.value() << endl;
     }

Here's the same code, but using an STL-style iterator this time:

     for (auto i = multimap.cbegin(), end = multimap.cend(); i != end; ++i)
         cout << qPrintable(i.key()) << ": " << i.value() << endl;

The items are traversed in ascending key order.

A QMultiMap allows multiple values per key. If you call insert() with a key that already exists in the map, a new (key, value) pair will be inserted. For example:

     multimap.insert("plenty", 100);
     multimap.insert("plenty", 2000);
     // multimap.size() == 2

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 = multimap.values("plenty");
     for (auto i : std::as_const(values))
         cout << i << endl;

The items that share the same key are available from most recently to least recently inserted. Another approach is to call find() to get the STL-style iterator for the first item with a key and iterate from there:

     auto i = multimap.find("plenty");
     while (i != map.end() && i.key() == "plenty") {
         cout << i.value() << endl;
         ++i;
     }

     // better:
     auto [i, end] = multimap.equal_range("plenty");
     while (i != end) {
         cout << i.value() << endl;
         ++i;
     }

If you only need to extract the values from a map (not the keys), you can also use range-based for:

     QMap<QString, int> multimap;
     ...
     for (int value : std::as_const(multimap))
         cout << value << endl;

Items can be removed from the multi map in several ways. One way is to call remove(); this will remove any item with the given key. Another way is to use QMutableMultiMapIterator::remove(). In addition, you can clear the entire map using clear().

It is possible to merge two multi maps by calling unite(), by using operator+(), and by using operator+=(). Example:

     QMultiMap<QString, int> map1, map2, map3;

     map1.insert("plenty", 100);
     map1.insert("plenty", 2000);
     // map1.size() == 2

     map2.insert("plenty", 5000);
     // map2.size() == 1

     map3 = map1 + map2;
     // map3.size() == 3

QMultiMap'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, QMultiMap's key type must provide operator<(). QMap uses it to keep its items sorted, and assumes that two keys `x` and `y` are equal 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 QMultiMapIterator, QMutableMultiMapIterator, and QMultiHash.

## Member Type Documentation

### QMultiMap::ConstIterator

Qt-style synonym for QMultiMap::const_iterator.

### QMultiMap::Iterator

Qt-style synonym for QMultiMap::iterator.

### QMultiMap::const_key_value_iterator

The QMultiMap::const_key_value_iterator typedef provides an STL-style iterator for QMultiMap.

QMultiMap::const_key_value_iterator is essentially the same as QMultiMap::const_iterator with the difference that operator*() returns a key/value pair instead of a value.

See also QKeyValueIterator.

### `[alias]` QMultiMap::difference_type

Typedef for ptrdiff_t. Provided for STL compatibility.

### `[alias]` QMultiMap::key_type

Typedef for Key. Provided for STL compatibility.

### QMultiMap::key_value_iterator

The QMultiMap::key_value_iterator typedef provides an STL-style iterator for QMultiMap.

QMultiMap::key_value_iterator is essentially the same as QMultiMap::iterator with the difference that operator*() returns a key/value pair instead of a value.

See also QKeyValueIterator.

### `[alias]` QMultiMap::mapped_type

Typedef for T. Provided for STL compatibility.

### `[alias]` QMultiMap::size_type

Typedef for int. Provided for STL compatibility.

## Member Function Documentation

### `[since 6.4]` auto QMultiMap::asKeyValueRange() &

### `[since 6.4]` auto QMultiMap::asKeyValueRange() &&

### `[since 6.4]` auto QMultiMap::asKeyValueRange() const &

### `[since 6.4]` auto QMultiMap::asKeyValueRange() const &&

Returns a range object that allows iteration over this multi map as key/value pairs. For instance, this range object can be used in a range-based for loop, in combination with a structured binding declaration:

     QMultiMap<QString, int> map;
     map.insert("January", 1);
     map.insert("February", 2);
     // ...
     map.insert("December", 12);

     for (auto [key, value] : map.asKeyValueRange()) {
         cout << qPrintable(key) << ": " << value << endl;
         --value; // convert to JS month indexing
     }

Note that both the key and the value obtained this way are references to the ones in the multi map. Specifically, mutating the value will modify the map itself.

These functions were introduced in Qt 6.4.

See also QKeyValueIterator.

### QMultiMap::QMultiMap()

Constructs an empty multi map.

See also clear().

### `[explicit, since 6.0]` QMultiMap::QMultiMap(QMap<Key, T> &&other)

If other is shared, constructs a multi map as a copy of other. Otherwise, constructs a multi map by moving the elements from other.

This function was introduced in Qt 6.0.

### `[explicit, since 6.0]` QMultiMap::QMultiMap(const QMap<Key, T> &other)

Constructs a multi map as a copy of other.

This function was introduced in Qt 6.0.

### `[explicit]` QMultiMap::QMultiMap(const std::multimap<Key, T> &other)

Constructs a copy of other.

See also toStdMultiMap().

### QMultiMap::QMultiMap(std::initializer_list<std::pair<Key, T>> list)

Constructs a multi map with a copy of each of the elements in the initializer list list.

### `[explicit]` QMultiMap::QMultiMap(std::multimap<Key, T> &&other)

Constructs a multi map by moving from other.

See also toStdMultiMap().

### `[default]` QMultiMap::QMultiMap(const QMultiMap<Key, T> &other)

Constructs a copy of other.

This operation occurs in constant time, because QMultiMap is implicitly shared. This makes returning a QMultiMap 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=().

### `[default]` QMultiMap::QMultiMap(QMultiMap<Key, T> &&other)

Move-constructs a QMultiMap instance, making it point at the same object that other was pointing to.

### `[default]` QMultiMap::~QMultiMap()

Destroys the multi map. References to the values in the multi map, and all iterators over this multi map, become invalid.

### QMultiMap<Key, T>::iterator QMultiMap::begin()

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

See also constBegin() and end().

### QMultiMap<Key, T>::const_iterator QMultiMap::begin() const

This is an overloaded function.

### QMultiMap<Key, T>::const_iterator QMultiMap::cbegin() const

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

See also begin() and cend().

### QMultiMap<Key, T>::const_iterator QMultiMap::cend() const

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

See also cbegin() and end().

### void QMultiMap::clear()

Removes all items from the multi map.

See also remove().

### QMultiMap<Key, T>::const_iterator QMultiMap::constBegin() const

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

See also begin() and constEnd().

### QMultiMap<Key, T>::const_iterator QMultiMap::constEnd() const

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

See also constBegin() and end().

### QMultiMap<Key, T>::const_iterator QMultiMap::constFind(const Key &key) const

Returns an const iterator pointing to the item with key key in the multi map.

If the multi map contains no item with key key, the function returns constEnd().

See also find() and QMultiMap::constFind(const Key &key, const T &value).

### QMultiMap<Key, T>::const_iterator QMultiMap::constFind(const Key &key, const T &value) const

Returns an iterator pointing to the item with key key and the value value in the map.

If the map contains no such item, the function returns constEnd().

See also QMap::constFind().

### QMultiMap<Key, T>::const_key_value_iterator QMultiMap::constKeyValueBegin() const

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

See also keyValueBegin().

### QMultiMap<Key, T>::const_key_value_iterator QMultiMap::constKeyValueEnd() const

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

See also constKeyValueBegin().

### bool QMultiMap::contains(const Key &key) const

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

See also count().

### bool QMultiMap::contains(const Key &key, const T &value) const

Returns `true` if the multi map contains an item with key key and value value; otherwise returns `false`.

See also count().

### QMultiMap<Key, T>::size_type QMultiMap::count(const Key &key) const

Returns the number of items associated with key key.

See also contains() and QMultiMap::count().

### QMultiMap<Key, T>::size_type QMultiMap::count(const Key &key, const T &value) const

Returns the number of items with key key and value value.

See also contains() and QMultiMap::count().

### QMultiMap<Key, T>::size_type QMultiMap::count() const

Same as size().

This is an overloaded function.

### bool QMultiMap::empty() const

This function is provided for STL compatibility. It is equivalent to isEmpty(), returning true if the map is empty; otherwise returning false.

### QMultiMap<Key, T>::iterator QMultiMap::end()

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

See also begin() and constEnd().

### QMultiMap<Key, T>::const_iterator QMultiMap::end() const

This is an overloaded function.

### std::pair<QMultiMap<Key, T>::iterator, QMultiMap<Key, T>::iterator> QMultiMap::equal_range(const Key &key)

Returns a pair of iterators delimiting the range of values `[first, second)`, that are stored under key.

### std::pair<QMultiMap<Key, T>::const_iterator, QMultiMap<Key, T>::const_iterator> QMultiMap::equal_range(const Key &key) const

This is an overloaded function.

### QMultiMap<Key, T>::iterator QMultiMap::erase(QMultiMap<Key, T>::const_iterator pos)

Removes the (key, value) pair pointed to by the iterator pos from the multi 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]` QMultiMap<Key, T>::iterator QMultiMap::erase(QMultiMap<Key, T>::const_iterator first, QMultiMap<Key, T>::const_iterator last)

Removes the (key, value) pairs pointed to by the iterator range [first, last) from the multi map. Returns an iterator to the item in the multi 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().

### QMultiMap<Key, T>::iterator QMultiMap::find(const Key &key)

Returns an iterator pointing to the item with key key in the multi map.

If the multi map contains no item with key key, the function returns end().

If the map contains multiple items with key key, this function returns an iterator that points to the most recently inserted value. The other values are accessible by incrementing the iterator. For example, here's some code that iterates over all the items with the same key:

     auto i = multimap.find("plenty");
     while (i != map.end() && i.key() == "plenty") {
         cout << i.value() << endl;
         ++i;
     }

     // better:
     auto [i, end] = multimap.equal_range("plenty");
     while (i != end) {
         cout << i.value() << endl;
         ++i;
     }

See also constFind(), value(), values(), lowerBound(), and upperBound().

### QMultiMap<Key, T>::const_iterator QMultiMap::find(const Key &key) const

This is an overloaded function.

### QMultiMap<Key, T>::const_iterator QMultiMap::find(const Key &key, const T &value) const

Returns a const iterator pointing to the item with the given key and value in the map.

If the map contains no such item, the function returns end().

If the map contains multiple items with the specified key, this function returns a const iterator that points to the most recently inserted value.

This is an overloaded function.

### T &QMultiMap::first()

Returns a reference to the first value in the multi map, that is the value mapped to the smallest key. This function assumes that the multi map is not empty.

When unshared (or const version is called), this executes in constant time.

See also last(), firstKey(), and isEmpty().

### const T &QMultiMap::first() const

This is an overloaded function.

### const Key &QMultiMap::firstKey() const

Returns a reference to the smallest key in the multi map. This function assumes that the multi map is not empty.

This executes in constant time.

See also lastKey(), first(), keyBegin(), and isEmpty().

### QMultiMap<Key, T>::iterator QMultiMap::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 same key in the map, this function will simply create a new one. (This behavior is different from replace(), which overwrites the value of an existing item.)

Returns an iterator pointing to the new element.

See also replace().

### QMultiMap<Key, T>::iterator QMultiMap::insert(QMultiMap<Key, T>::const_iterator pos, const Key &key, const T &value)

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 multi map while constEnd() suggests that the key is (strictly) larger than any key in the multi 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 the hint is correct and the multi map is unshared, the insert executes in amortized constant time.

If there is already an item with the same key in the map, this function will simply create a new one.

When creating a multi 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.

Returns an iterator pointing to the new element.

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 multi map and the pos multi map.

This is an overloaded function.

### bool QMultiMap::isEmpty() const

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

See also size().

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

Returns the first key with value value, or defaultKey if the multi 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 QMultiMap's internal data structure is optimized for fast lookup by key, not by value.

See also value() and keys().

### QMultiMap<Key, T>::key_iterator QMultiMap::keyBegin() const

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

See also keyEnd() and firstKey().

### QMultiMap<Key, T>::key_iterator QMultiMap::keyEnd() const

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

See also keyBegin() and lastKey().

### QMultiMap<Key, T>::key_value_iterator QMultiMap::keyValueBegin()

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

See also keyValueEnd().

### QMultiMap<Key, T>::const_key_value_iterator QMultiMap::keyValueBegin() const

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

See also keyValueEnd().

### QMultiMap<Key, T>::key_value_iterator QMultiMap::keyValueEnd()

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

See also keyValueBegin().

### QMultiMap<Key, T>::const_key_value_iterator QMultiMap::keyValueEnd() const

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

See also keyValueBegin().

### QList<Key> QMultiMap::keys() const

Returns a list containing all the keys in the multi map in ascending order. Keys that occur multiple times in the multi map also occur multiple times in the list.

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 values() and key().

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

Returns a list containing all the keys associated with value value in ascending order.

This function can be slow (linear time), because QMultiMap's internal data structure is optimized for fast lookup by key, not by value.

This is an overloaded function.

### T &QMultiMap::last()

Returns a reference to the last value in the multi 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 constant time.

See also first(), lastKey(), and isEmpty().

### const T &QMultiMap::last() const

This is an overloaded function.

### const Key &QMultiMap::lastKey() const

Returns a reference to the largest key in the multi map. This function assumes that the multi map is not empty.

This executes in constant time.

See also firstKey(), last(), keyEnd(), and isEmpty().

### QMultiMap<Key, T>::iterator QMultiMap::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.

Example:

     QMultiMap<int, QString> multimap;
     multimap.insert(1, "one");
     multimap.insert(5, "five");
     multimap.insert(5, "five (2)");
     multimap.insert(10, "ten");

     multimap.lowerBound(0);      // returns iterator to (1, "one")
     multimap.lowerBound(1);      // returns iterator to (1, "one")
     multimap.lowerBound(2);      // returns iterator to (5, "five")
     multimap.lowerBound(5);      // returns iterator to (5, "five")
     multimap.lowerBound(6);      // returns iterator to (10, "ten")
     multimap.lowerBound(10);     // returns iterator to (10, "ten")
     multimap.lowerBound(999);    // returns end()

If the map contains multiple items with key key, this function returns an iterator that points to the most recently inserted value. The other values are accessible by incrementing the iterator. For example, here's some code that iterates over all the items with the same key:

     QMap<QString, int> multimap;
     ...
     QMap<QString, int>::const_iterator i = multimap.lowerBound("HDR");
     QMap<QString, int>::const_iterator upperBound = multimap.upperBound("HDR");
     while (i != upperBound) {
         cout << i.value() << endl;
         ++i;
     }

See also upperBound() and find().

### QMultiMap<Key, T>::const_iterator QMultiMap::lowerBound(const Key &key) const

This is an overloaded function.

### QMultiMap<Key, T>::size_type QMultiMap::remove(const Key &key)

Removes all the items that have the key key from the multi map. Returns the number of items removed.

See also clear() and take().

### QMultiMap<Key, T>::size_type QMultiMap::remove(const Key &key, const T &value)

Removes all the items that have the key key and value value from the multi map. Returns the number of items removed.

See also clear() and take().

### `[since 6.1]` template <typename Predicate> QMultiMap<Key, T>::size_type QMultiMap::removeIf(Predicate pred)

Removes all elements for which the predicate pred returns true from the multi map.

The function supports predicates which take either an argument of type `QMultiMap<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().

### QMultiMap<Key, T>::iterator QMultiMap::replace(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.

If there are multiple items with the key key, the most recently inserted item's value is replaced with value.

Returns an iterator pointing to the new/updated element.

See also insert().

### QMultiMap<Key, T>::size_type QMultiMap::size() const

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

See also isEmpty() and count().

### `[noexcept]` void QMultiMap::swap(QMultiMap<Key, T> &other)

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

### T QMultiMap::take(const Key &key)

Removes the item with the key key from the multi map and returns the value associated with it.

If the item does not exist in the multi 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::multimap<Key, T> QMultiMap::toStdMultiMap() const &

Returns an STL multi map equivalent to this QMultiMap.

### QList<Key> QMultiMap::uniqueKeys() const

Returns a list containing all the keys in the map in ascending order. Keys that occur multiple times in the map occur only once in the returned list.

### QMultiMap<Key, T> &QMultiMap::unite(QMultiMap<Key, T> &&other)

Moves all the items from the other map into this map. If a key is common to both maps, the resulting map will contain the key multiple times.

If other is shared, then the items will be copied instead.

### QMultiMap<Key, T> &QMultiMap::unite(const QMultiMap<Key, T> &other)

Inserts all the items in the other map into this map. If a key is common to both maps, the resulting map will contain the key multiple times.

### QMultiMap<Key, T>::iterator QMultiMap::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:

     QMultiMap<int, QString> multimap;
     multimap.insert(1, "one");
     multimap.insert(5, "five");
     multimap.insert(5, "five (2)");
     multimap.insert(10, "ten");

     multimap.upperBound(0);      // returns iterator to (1, "one")
     multimap.upperBound(1);      // returns iterator to (5, "five")
     multimap.upperBound(2);      // returns iterator to (5, "five")
     multimap.lowerBound(5);      // returns iterator to (5, "five (2)")
     multimap.lowerBound(6);      // returns iterator to (10, "ten")
     multimap.upperBound(10);     // returns end()
     multimap.upperBound(999);    // returns end()

See also lowerBound() and find().

### QMultiMap<Key, T>::const_iterator QMultiMap::upperBound(const Key &key) const

This is an overloaded function.

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

Returns the value associated with the key key.

If the multi map contains no item with key key, the function returns defaultValue. If no defaultValue is specified, the function returns a default-constructed value. If there are multiple items for key in the multi map, the value of the most recently inserted one is returned.

See also key(), values(), and contains().

### QList<T> QMultiMap::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.

See also keys() and value().

### QList<T> QMultiMap::values(const Key &key) const

Returns a list containing all the values associated with key key, from the most recently inserted to the least recently inserted one.

See also keys() and value().

### `[default]` QMultiMap<Key, T> &QMultiMap::operator=(QMultiMap<Key, T> &&other)

Move-assigns other to this QMultiMap instance.

### `[default]` QMultiMap<Key, T> &QMultiMap::operator=(const QMultiMap<Key, T> &other)

Assigns other to this multi map and returns a reference to this multi map.

## Related Non-Members

### `[since 6.1]` template <typename Key, typename T, typename Predicate> qsizetype erase_if(QMultiMap<Key, T> &map, Predicate pred)

Removes all elements for which the predicate pred returns true from the multi map map.

The function supports predicates which take either an argument of type `QMultiMap<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 QMultiMap<Key, T> &lhs, const QMultiMap<Key, T> &rhs)

Returns `true` if lhs is not equal to rhs; otherwise returns `false`.

Two multi maps are considered equal if they contain the same (key, value) pairs, in the same order (which matters for duplicate keys).

This function requires the key and the value types to implement `operator==()`.

See also operator==().

### template <typename Key, typename T> QMultiMap<Key, T> operator+(const QMultiMap<Key, T> &lhs, const QMultiMap<Key, T> &rhs)

Returns a map that contains all the items in the lhs map in addition to all the items in rhs. If a key is common to both maps, the resulting map will contain the key multiple times.

See also operator+=().

### template <typename Key, typename T> QMultiMap<Key, T> operator+=(QMultiMap<Key, T> &lhs, const QMultiMap<Key, T> &rhs)

Inserts all the items in the rhs map into the lhs map and returns the resulting map.

See also insert() and operator+().

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

Writes the multi 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 QMultiMap<Key, T> &lhs, const QMultiMap<Key, T> &rhs)

Returns `true` if lhs is equal to rhs; otherwise returns false.

Two multi maps are considered equal if they contain the same (key, value) pairs, in the same order (which matters for duplicate keys).

This function requires the key and the value types to implement `operator==()`.

See also operator!=().

### template <typename Key, typename T> QDataStream &operator>>(QDataStream &in, QMultiMap<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.