- Qt 6.9
- Qt Core
- C++ Classes
- const_iterator
- Qt 6.9.3 Reference Documentation

# QMultiHash::const_iterator Class

class QMultiHash::const_iterator

The QMultiHash::const_iterator class provides an STL-style const iterator for QMultiHash. More...

- List of all members, including inherited members

## Public Functions

| const_iterator()
| const_iterator(const QMultiHash<Key, T>::iterator &other)
const Key & | key() const
T & | value() const
bool | operator!=(const QMultiHash<Key, T>::const_iterator &other) const
T & | operator*() const
QMultiHash<Key, T>::const_iterator & | operator++()
QMultiHash<Key, T>::const_iterator | operator++(int)
T * | operator->() const
bool | operator==(const QMultiHash<Key, T>::const_iterator &other) const

## Detailed Description

QMultiHash<Key, T>::const_iterator allows you to iterate over a QMultiHash. If you want to modify the QMultiHash as you iterate over it, you must use QMultiHash::iterator instead. It is generally good practice to use QMultiHash::const_iterator on a non-const QMultiHash as well, unless you need to change the QMultiHash through the iterator. Const iterators are slightly faster, and can improve code readability.

The default QMultiHash::const_iterator constructor creates an uninitialized iterator. You must initialize it using a QMultiHash function like QMultiHash::cbegin(), QMultiHash::cend(), or QMultiHash::constFind() before you can start iterating. Here's a typical loop that prints all the (key, value) pairs stored in a hash:

     QHash<QString, int> hash;
     hash.insert("January", 1);
     hash.insert("February", 2);
     ...
     hash.insert("December", 12);

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

Unlike QMap, which orders its items by key, QMultiHash stores its items in an arbitrary order. The only guarantee is that items that share the same key (because they were inserted using a QMultiHash) will appear consecutively, from the most recently to the least recently inserted value.

Multiple iterators can be used on the same hash. However, be aware that any modification performed directly on the QMultiHash (inserting and removing items) can cause the iterators to become invalid.

Inserting items into the hash or calling methods such as QMultiHash::reserve() or QMultiHash::squeeze() can invalidate all iterators pointing into the hash. Iterators are guaranteed to stay valid only as long as the QMultiHash doesn't have to grow/shrink it's internal hash table. Using any iterator after a rehashing operation ahs occurred will lead to undefined behavior.

If you need to keep iterators over a long period of time, we recommend that you use QMultiMap rather than QMultiHash.

Warning: Iterators on implicitly shared containers do not work exactly like STL-iterators. You should avoid copying a container while iterators are active on that container. For more information, read Implicit sharing iterator problem.

See also QMultiHash::iterator, QMultiHash::key_iterator, and QMultiHash::const_key_value_iterator.

## Member Function Documentation

### `[constexpr noexcept]` const_iterator::const_iterator()

Constructs an uninitialized iterator.

Functions like key(), value(), and operator++() must not be called on an uninitialized iterator. Use operator=() to assign a value to it before using it.

See also QMultiHash::constBegin() and QMultiHash::constEnd().

### `[noexcept]` const_iterator::const_iterator(const QMultiHash<Key, T>::iterator &other)

Constructs a copy of other.

### `[noexcept]` const Key &const_iterator::key() const

Returns the current item's key.

See also value().

### `[noexcept]` T &const_iterator::value() const

Returns the current item's value.

See also key() and operator*().

### `[noexcept]` bool const_iterator::operator!=(const QMultiHash<Key, T>::const_iterator &other) const

Returns `true` if other points to a different item than this iterator; otherwise returns `false`.

See also operator==().

### `[noexcept]` T &const_iterator::operator*() const

Returns the current item's value.

Same as value().

See also key().

### `[noexcept]` QMultiHash<Key, T>::const_iterator &const_iterator::operator++()

The prefix ++ operator (`++i`) advances the iterator to the next item in the hash and returns an iterator to the new current item.

Calling this function on QMultiHash::end() leads to undefined results.

### `[noexcept]` QMultiHash<Key, T>::const_iterator const_iterator::operator++(int)

The postfix ++ operator (`i++`) advances the iterator to the next item in the hash and returns an iterator to the previously current item.

This is an overloaded function.

### `[noexcept]` T *const_iterator::operator->() const

Returns a pointer to the current item's value.

See also value().

### `[noexcept]` bool const_iterator::operator==(const QMultiHash<Key, T>::const_iterator &other) const

Returns `true` if other points to the same item as this iterator; otherwise returns `false`.

See also operator!=().