iterator Class

class QMap::iterator

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

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

Detailed Description

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);

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

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

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

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

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

erase_if(map, [](const QMap<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 QMap::const_iterator, QMap::key_iterator, and QMap::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 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 the same item as the rhs iterator; otherwise returns false.

See also operator!=().

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==().

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