- Qt 5.15
- Qt Core
- C++ Classes
- QMultiHash
- Qt 5.15.19 Reference Documentation

# QMultiHash Class

template <typename Key, typename T> class QMultiHash

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

Header: | #include <QMultiHash>
qmake: | QT += core
Inherits: | QHash

- List of all members, including inherited members

Note: All functions in this class are reentrant.

## Public Functions

| QMultiHash(const QHash<Key, T> &other)
| QMultiHash(InputIterator begin, InputIterator end)
| QMultiHash(std::initializer_list<std::pair<Key, T>> list)
| QMultiHash()
typename QHash<Key, T>::const_iterator | constFind(const Key &key, const T &value) const
bool | contains(const Key &key, const T &value) const
int | count(const Key &key, const T &value) const
typename QHash<Key, T>::iterator | find(const Key &key, const T &value)
typename QHash<Key, T>::const_iterator | find(const Key &key, const T &value) const
typename QHash<Key, T>::iterator | insert(const Key &key, const T &value)
int | remove(const Key &key, const T &value)
typename QHash<Key, T>::iterator | replace(const Key &key, const T &value)
void | swap(QMultiHash<K, V> &other)
QList<Key> | uniqueKeys() const
QMultiHash<K, V> & | unite(const QMultiHash<K, V> &other)
QList<T> | values(const Key &key) const
QMultiHash<K, V> | operator+(const QMultiHash<K, V> &other) const
QMultiHash<K, V> & | operator+=(const QMultiHash<K, V> &other)

## Related Non-Members

uint | qHash(const QMultiHash<Key, T> &key, uint seed = 0)

## Detailed Description

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.

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 opposed to QHash, it provides an insert() function will allow the insertion of multiple items with the same key. The replace() function corresponds to QHash::insert(). It also provides convenient operator+() and operator+=().

Unlike QMultiMap, QMultiHash does not provide and ordering of the inserted items. The only guarantee is that items that share the same key will appear consecutively, from the most recently to the least recently inserted value.

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) << Qt::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() << Qt::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 qHash() function in the type's namespace 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(const QHash<Key, T> &other)

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

See also operator=().

### template <typename InputIterator> QMultiHash::QMultiHash(InputIterator begin, InputIterator end)

Constructs a multi-hash with a copy of each of the elements in the iterator range [begin, end). Either the elements iterated by the range must be objects with `first` and `second` data members (like `QPair`, `std::pair`, etc.) convertible to `Key` and to `T` respectively; or the iterators must have `key()` and `value()` member functions, returning a key convertible to `Key` and a value convertible to `T` respectively.

This function was introduced in Qt 5.14.

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

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

This function was introduced in Qt 5.1.

### QMultiHash::QMultiHash()

Constructs an empty hash.

### typename QHash<Key, T>::const_iterator QMultiHash::constFind(const Key &key, const T &value) const

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

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

This function was introduced in Qt 4.3.

See also QHash::constFind().

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

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

This function was introduced in Qt 4.3.

See also QHash::contains().

### int QMultiHash::count(const Key &key, const T &value) const

Returns the number of items with the key and value.

This function was introduced in Qt 4.3.

See also QHash::count().

### typename QHash<Key, T>::iterator QMultiHash::find(const Key &key, const T &value)

Returns an iterator pointing to the item with the key and value. If the hash contains no such item, the function returns end().

If the hash contains multiple items with the key and value, the iterator returned points to the most recently inserted item.

This function was introduced in Qt 4.3.

See also QHash::find().

### typename QHash<Key, T>::const_iterator QMultiHash::find(const Key &key, const T &value) const

This is an overloaded function.

This function was introduced in Qt 4.3.

### typename QHash<Key, T>::iterator QMultiHash::insert(const Key &key, const T &value)

Inserts a new item with the 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().

### int QMultiHash::remove(const Key &key, const T &value)

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

This function was introduced in Qt 4.3.

See also QHash::remove().

### typename QHash<Key, T>::iterator QMultiHash::replace(const Key &key, const T &value)

Inserts a new item with the key and a value of value.

If there is already an item with the key, that item's value is replaced with value.

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

See also insert().

### void QMultiHash::swap(QMultiHash<K, V> &other)

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

This function was introduced in Qt 4.8.

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

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

This function was introduced in Qt 5.13.

See also keys() and values().

### QMultiHash<K, V> &QMultiHash::unite(const QMultiHash<K, V> &other)

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

This function was introduced in Qt 5.13.

See also insert().

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

This is an overloaded function.

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

See also count() and insert().

### QMultiHash<K, V> QMultiHash::operator+(const QMultiHash<K, V> &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<K, V> &QMultiHash::operator+=(const QMultiHash<K, V> &other)

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

See also unite() and insert().

## Related Non-Members

### template <typename Key, typename T> uint qHash(const QMultiHash<Key, T> &key, uint seed = 0)

Returns the hash value for the key, using seed to seed the calculation.

Type `T` must be supported by qHash().

This function was introduced in Qt 5.8.