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# QMultiHash Class Reference
[QtCore module]

The QMultiHash class is a convenience QHash subclass that provides multi-valued hashes. More...

     #include <QMultiHash>

Inherits QHash<Key, T>.

Note: All the functions in this class are reentrant.

- List of all members, including inherited members

### Public Functions

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

- 38 public functions inherited from QHash

## Detailed Description

The QMultiHash class is a convenience QHash subclass that provides multi-valued hashes.

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

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

Example:

     QMultiHash<QString, int> hash1, hash2, hash3;

     hash1.insert("plenty", 100);
     hash1.insert("plenty", 2000);
     // hash1.size() == 2

     hash2.insert("plenty", 5000);
     // hash2.size() == 1

     hash3 = hash1 + hash2;
     // hash3.size() == 3

Unlike QHash, QMultiHash 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 = hash.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 call find() to get the STL-style iterator for the first item with a key and iterate from there:

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

QMultiHash's key and value data types must be assignable data types. You cannot, for example, store a QWidget as a value; instead, store a QWidget *. In addition, QMultiHash's key type must provide operator==(), and there must also be a global qHash() function that returns a hash value for an argument of the key's type. See the QHash documentation for details.

See also QHash, QHashIterator, QMutableHashIterator, and QMultiMap.

## Member Function Documentation

### QMultiHash::QMultiHash ()

Constructs an empty hash.

### QMultiHash::QMultiHash ( const QHash<Key, T> & other )

Constructs a copy of other (which can be a QHash or a QMultiHash).

See also operator=().

### QHash<Key, T>::iterator QMultiHash::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 hash, 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().

### QHash<Key, T>::iterator QMultiHash::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().

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

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

See also operator+=().

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

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

See also insert().