- Qt 6.9
- Qt Core
- C++ Classes
- const_iterator
- Qt 6.9.2 Reference Documentation

# QMap::const_iterator Class

class QMap::const_iterator

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

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

## Public Types

| iterator_category

## Public Functions

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

## Related Non-Members

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

## Detailed Description

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

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

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

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

Here's an example that removes all the items whose value is greater than 10:

     QMap<QString, int>::const_iterator i = map.cbegin();
     while (i != map.cend()) {
         if (i.value() > 10)
             i = map.erase(i);
         else
             ++i;
     }

And here the same behavior with erase_if()

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

Unlike QHash, which stores its items in an arbitrary order, QMap stores its items ordered by key.

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 QMap::iterator, QMap::key_iterator, and QMap::const_key_value_iterator.

## 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()

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 QMap::constBegin() and QMap::constEnd().

### const_iterator::const_iterator(const QMap<Key, T>::iterator &other)

Constructs a copy of other.

### 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().

### QMap<Key, T>::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 QMap::end() leads to undefined results.

See also operator--().

### QMap<Key, T>::const_iterator const_iterator::operator++(int)

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

This is an overloaded function.

### QMap<Key, T>::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 QMap::begin() leads to undefined results.

See also operator++().

### QMap<Key, T>::const_iterator const_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.

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

Returns a pointer to the current item's value.

See also value().

## Related Non-Members

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

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

See also operator!=().

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

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

See also operator==().