- Qt 6.4
- Qt Core
- C++ Classes
- const_iterator
- Qt 6.4.2 Reference Documentation

# const_iterator Class

class QMultiMap::const_iterator

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

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

## Public Types

| iterator_category

## Public Functions

| const_iterator(const iterator &other)
| const_iterator()
const Key & | key() const
const T & | value() const
const T & | operator*() const
const_iterator & | operator++()
const_iterator | operator++(int)
const_iterator & | operator--()
const_iterator | operator--(int)
const T * | operator->() const

## Related Non-Members

bool | operator!=(const const_iterator &lhs, const const_iterator &rhs)
bool | operator==(const const_iterator &lhs, const const_iterator &rhs)

## Detailed Description

QMultiMap features both STL-style iterators and Java-style iterators. The STL-style iterators are more low-level and more cumbersome to use; on the other hand, they are slightly faster and, for developers who already know STL, have the advantage of familiarity.

QMultiMap<Key, T>::const_iterator allows you to iterate over a QMultiMap. If you want to modify the QMultiMap as you iterate over it, you must use QMultiMap::iterator instead. 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::const_iterator constructor creates an uninitialized iterator. You must initialize it using a QMultiMap function like QMultiMap::constBegin(), QMultiMap::constEnd(), or QMultiMap::find() before you can start iterating. Here's a typical loop that prints all the (key, value) pairs stored in a map:

     QMultiMap<QString, int> multi;
     multimap.insert("January", 1);
     multimap.insert("February", 2);
     ...
     multimap.insert("December", 12);

     QMultiMap<QString, int>::const_iterator i;
     for (i = multimap.constBegin(); i != multimap.constEnd(); ++i)
         cout << i.key() << ": " << i.value() << Qt::endl;

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.

Multiple iterators can be used on the same multi 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::iterator, QMultiMap::key_iterator, and QMultiMapIterator.

## Member Type Documentation

### `[alias]` const_iterator::iterator_category

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

## Member Function Documentation

### const_iterator::const_iterator(const iterator &other)

Constructs a copy of other.

### 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 QMultiMap::constBegin() and QMultiMap::constEnd().

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

Returns the current item's key.

See also value().

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

Returns the current item's value.

See also key() and operator*().

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

Returns the current item's value.

Same as value().

See also key().

### const_iterator &const_iterator::operator++()

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

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

See also operator--().

### const_iterator const_iterator::operator++(int)

This is an overloaded function.

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

### const_iterator &const_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++().

### const_iterator const_iterator::operator--(int)

This is an overloaded function.

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

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

Returns a pointer to the current item's value.

See also value().

## Related Non-Members

### bool operator!=(const const_iterator &lhs, const const_iterator &rhs)

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

See also operator==().

### bool operator==(const const_iterator &lhs, const const_iterator &rhs)

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

See also operator!=().