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# QMultiMap Class Reference

The QMultiMap class is a convenience QMap subclass that provides multi-valued maps. More...

    #include <QMultiMap>

Part of the QtCore module.

Inherits QMap<Key, T>.

Note: All the functions in this class are reentrant.

- List of all members, including inherited members

### Public Functions

- QMultiMap ()
- QMultiMap ( const QMap<Key, T> & other )
- QMap<Key, T>::iterator insert ( const Key & key, const T & value )
- QMap<Key, T>::iterator replace ( const Key & key, const T & value )
- QMultiMap operator+ ( const QMultiMap & other ) const
- QMultiMap & operator+= ( const QMultiMap & other )

- 38 public functions inherited from QMap

## Detailed Description

The QMultiMap class is a convenience QMap subclass that provides multi-valued maps.

QMultiMap<Key, T> is one of Qt's generic container classes. It inherits QMap and extends it with a few convenience functions that make it more suitable than QMap for storing multi-valued maps. A multi-valued map is a map that allows multiple values with the same key; QMap normally doesn't allow that, unless you call QMap::insertMulti().

Because QMultiMap inherits QMap, all of QMap's functionality also applies to QMultiMap. For example, you can use isEmpty() to test whether the map is empty, and you can traverse a QMultiMap using QMap's iterator classes (for example, QMapIterator). But in addition, it provides an insert() function that corresponds to QMap::insertMulti(), and a replace() function that corresponds to QMap::insert(). It also provides convenient operator+() and 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

Unlike QMap, QMultiMap provides no operator[]. Use value() or replace() if you want to access the most recently inserted item with a certain key.

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) << endl;

The items that share the same key are available from most recently to least recently inserted.

A more efficient approach is to use QMapIterator::findNextKey() or QMutableMapIterator::findNextKey():

        QMapIterator<QString, int> i(map);
        while (i.findNextKey("plenty"))
            cout << i.value() << endl;

If you prefer the STL-style iterators, you can call find() to get the iterator for the first item with a key and iterate from there:

        QMultiMap<QString, int>::iterator i = map.find("plenty");
        while (i != map.end() && i.key() == "plenty") {
            cout << i.value() << endl;
            ++i;
        }

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<(). See the QMap documentation for details.

See also QMap, QMapIterator, QMutableMapIterator, and QMultiMap.

## Member Function Documentation

### QMultiMap::QMultiMap ()

Constructs an empty map.

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

Constructs a copy of other (which can be a QMap or a QMultiMap).

See also operator=().

### QMap<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.)

See also replace().

### QMap<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.

See also insert().

### QMultiMap QMultiMap::operator+ ( const QMultiMap & other ) const

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

See also operator+=().

### QMultiMap & QMultiMap::operator+= ( const QMultiMap & other )

Inserts all the items in the other map into this map and returns a reference to this map.

See also insert() and operator+().