- Qt 5.15
- Qt Core
- C++ Classes
- const_iterator
- Qt 5.15.18 Reference Documentation

# const_iterator Class

class QMap::const_iterator

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

- List of all members, including inherited members

## Public Types

typedef | iterator_category

## Public Functions

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

## Detailed Description

QMap 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.

QMap<Key, T>::const_iterator allows you to iterate over a QMap (or a QMultiMap). 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::constBegin(), QMap::constEnd(), or QMap::find() 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);

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

Unlike QHash, which stores its items in an arbitrary order, QMap stores its items ordered by key. Items that share the same key (because the map is a QMultiMap) will appear consecutively, from the most recently to the least recently inserted value.

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 QMapIterator.

## Member Type Documentation

### typedef 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 QMap::constBegin() and QMap::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*().

### bool const_iterator::operator!=(const const_iterator &other) const

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

See also operator==().

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

Returns the current item's value.

Same as value().

See also key().

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

Returns an iterator to the item at j positions forward from this iterator. (If j is negative, the iterator goes backward.)

This operation can be slow for large j values.

See also operator-().

### 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--().

### 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+=(int j)

Advances the iterator by j items. (If j is negative, the iterator goes backward.)

This operation can be slow for large j values.

See also operator-=() and operator+().

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

Returns an iterator to the item at j positions backward from this iterator. (If j is negative, the iterator goes forward.)

This operation can be slow for large j values.

See also operator+().

### 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++().

### 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_iterator &const_iterator::operator-=(int j)

Makes the iterator go back by j items. (If j is negative, the iterator goes forward.)

This operation can be slow for large j values.

See also operator+=() and operator-().

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

Returns a pointer to the current item's value.

See also value().

### bool const_iterator::operator==(const const_iterator &other) const

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

See also operator!=().