- Qt 6.10
- Qt Core
- C++ Classes
- iterator
- Qt 6.10.2 Reference Documentation

# QMultiMap::iterator Class

class QMultiMap::iterator

The QMultiMap::iterator class provides an STL-style non-const iterator for QMultiMap. More...

- List of all members, including inherited members
- Deprecated members

## Public Types

| iterator_category

## Public Functions

| iterator()
const Key & | key() const
T & | value() const
T & | operator*() const
QMultiMap<Key, T>::iterator & | operator++()
QMultiMap<Key, T>::iterator | operator++(int)
QMultiMap<Key, T>::iterator & | operator--()
QMultiMap<Key, T>::iterator | operator--(int)
T * | operator->() const

## Related Non-Members

bool | operator!=(const QMultiMap<Key, T>::iterator &lhs, const QMultiMap<Key, T>::iterator &rhs)
bool | operator==(const QMultiMap<Key, T>::iterator &lhs, const QMultiMap<Key, T>::iterator &rhs)

## Detailed Description

QMultiMap<Key, T>::iterator allows you to iterate over a QMultiMap and to modify the value (but not the key) stored under a particular key. If you want to iterate over a const QMultiMap, you should use QMultiMap::const_iterator. It is generally good practice to use QMultiMap::const_iterator on a non-const QMultiMap as well, unless you need to change the QMultiMap through the iterator. Const iterators are slightly faster, and can improve code readability.

The default QMultiMap::iterator constructor creates an uninitialized iterator. You must initialize it using a QMultiMap function like QMultiMap::begin(), QMultiMap::end(), or QMultiMap::find() before you can start iterating. Here's a typical loop that prints all the (key, value) pairs stored in a map:

Unlike QMultiHash, which stores its items in an arbitrary order, QMultiMap stores its items ordered by key. Items that share the same key will appear consecutively, from the most recently to the least recently inserted value.

Here's an example that increments every value stored in the QMultiMap by 2:

     for (auto it = multimap.begin(), end = multimap.end(); i != end; ++i)
         i.value() += 2;

To remove elements from a QMultiMap you can use erase_if(QMultiMap<Key, T> &map, Predicate pred):

     erase_if(multimap, [](const QMultiMap<QString, int>::iterator it) { return it.value() > 10; });

Multiple iterators can be used on the same map. If you add items to the map, existing iterators will remain valid. If you remove items from the map, iterators that point to the removed items will become dangling iterators.

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 QMultiMap::const_iterator, QMultiMap::key_iterator, and QMultiMap::key_value_iterator.

## Member Type Documentation

### `[alias]` iterator::iterator_category

A synonym for std::bidirectional_iterator_tag indicating this iterator is a bidirectional iterator.

## Member Function Documentation

### iterator::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 QMultiMap::begin() and QMultiMap::end().

### const Key &iterator::key() const

Returns the current item's key as a const reference.

There is no direct way of changing an item's key through an iterator, although it can be done by calling QMultiMap::erase() followed by QMultiMap::insert().

See also value().

### T &iterator::value() const

Returns a modifiable reference to the current item's value.

You can change the value of an item by using value() on the left side of an assignment, for example:

     if (i.key() == "Hello")
         i.value() = "Bonjour";

See also key() and operator*().

### T &iterator::operator*() const

Returns a modifiable reference to the current item's value.

Same as value().

See also key().

### QMultiMap<Key, T>::iterator &iterator::operator++()

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

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

See also operator--().

### QMultiMap<Key, T>::iterator iterator::operator++(int)

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

This is an overloaded function.

### QMultiMap<Key, T>::iterator &iterator::operator--()

The prefix `--` operator (`--i`) makes the preceding item current and returns an iterator pointing to the new current item.

Calling this function on QMultiMap::begin() leads to undefined results.

See also operator++().

### QMultiMap<Key, T>::iterator iterator::operator--(int)

The postfix `--` operator (`i--`) makes the preceding item current and returns an iterator pointing to the previously current item.

This is an overloaded function.

### T *iterator::operator->() const

Returns a pointer to the current item's value.

See also value().

## Related Non-Members

### bool operator==(const QMultiMap<Key, T>::iterator &lhs, const QMultiMap<Key, T>::iterator &rhs)

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

See also operator!=().

### bool operator!=(const QMultiMap<Key, T>::iterator &lhs, const QMultiMap<Key, T>::iterator &rhs)

Returns `true` if lhs points to a different item than the rhs iterator; otherwise returns `false`.

See also operator==().