iterator Class | Qt Core 6.4.3

- Qt 6.4
- Qt Core
- C++ Classes
- iterator
- Qt 6.4.3 Reference Documentation

# iterator Class

class QMap::iterator

The QMap::iterator class provides an STL-style non-const iterator for QMap. 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
iterator & | operator++()
iterator | operator++(int)
iterator & | operator--()
iterator | operator--(int)
T * | operator->() const

## Related Non-Members

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

## 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>::iterator allows you to iterate over a QMap and to modify the value (but not the key) stored under a particular key. If you want to iterate over a const QMap, you should use QMap::const_iterator. 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::iterator constructor creates an uninitialized iterator. You must initialize it using a QMap function like QMap::begin(), QMap::end(), 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>::iterator i;
     for (i = map.begin(); i != map.end(); ++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.

Let's see a few examples of things we can do with a QMap::iterator that we cannot do with a QMap::const_iterator. Here's an example that increments every value stored in the QMap by 2:

     QMap<QString, int>::iterator i;
     for (i = map.begin(); i != map.end(); ++i)
         i.value() += 2;

Here's an example that removes all the items whose key is a string that starts with an underscore character:

     QMap<QString, int>::iterator i = map.begin();
     while (i != map.end()) {
         if (i.key().startsWith('_'))
             i = map.erase(i);
         else
             ++i;
     }

The call to QMap::erase() removes the item pointed to by the iterator from the map, and returns an iterator to the next item. Here's another way of removing an item while iterating:

     QMap<QString, int>::iterator i = map.begin();
     while (i != map.end()) {
         QMap<QString, int>::iterator prev = i;
         ++i;
         if (prev.key().startsWith('_'))
             map.erase(prev);
     }

It might be tempting to write code like this:

     // WRONG
     while (i != map.end()) {
         if (i.key().startsWith('_'))
             map.erase(i);
         ++i;
     }

However, this will potentially crash in `++i`, because `i` is a dangling iterator after the call to erase().

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::const_iterator, QMap::key_iterator, and QMutableMapIterator.

## 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 QMap::begin() and QMap::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 QMap::erase() followed by QMap::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().

### iterator &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--().

### iterator 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.

### 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 QMap::begin() leads to undefined results.

See also operator++().

### iterator 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.

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

Returns a pointer to the current item's value.

See also value().

## Related Non-Members

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

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

See also operator!=().