Back to Qt.io
Contact Us Blog Download Qt

iterator Class

class QHash::iterator

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

Public Functions

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

Detailed Description

QHash<Key, T>::iterator allows you to iterate over a QHash and to modify the value (but not the key) associated with a particular key. If you want to iterate over a const QHash, you should use QHash::const_iterator. It is generally good practice to use QHash::const_iterator on a non-const QHash as well, unless you need to change the QHash through the iterator. Const iterators are slightly faster, and can improve code readability.

The default QHash::iterator constructor creates an uninitialized iterator. You must initialize it using a QHash function like QHash::begin(), QHash::end(), or QHash::find() before you can start iterating. Here's a typical loop that prints all the (key, value) pairs stored in a hash:

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

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

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

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

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

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

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

Multiple iterators can be used on the same hash. However, be aware that any modification performed directly on the QHash (inserting and removing items) can cause the iterators to become invalid.

Inserting items into the hash or calling methods such as QHash::reserve() or QHash::squeeze() can invalidate all iterators pointing into the hash. Iterators are guaranteed to stay valid only as long as the QHash doesn't have to grow/shrink its internal hash table. Using any iterator after a rehashing operation has occurred will lead to undefined behavior.

If you need to keep iterators over a long period of time, we recommend that you use QMap rather than QHash.

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 QHash::const_iterator, QHash::key_iterator, and QHash::key_value_iterator.

Member Function Documentation

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

bool iterator::operator==(const iterator &other) const

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

See also operator!=().

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

bool iterator::operator!=(const iterator &other) const

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

See also operator==().

[constexpr] 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 QHash::begin() and QHash::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 QHash::erase() followed by QHash::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 hash and returns an iterator to the new current item.

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

iterator iterator::operator++(int)

This is an overloaded function.

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

T *iterator::operator->() const

Returns a pointer to the current item's value.

See also value().

© 2024 The Qt Company Ltd. Documentation contributions included herein are the copyrights of their respective owners. The documentation provided herein is licensed under the terms of the GNU Free Documentation License version 1.3 as published by the Free Software Foundation. Qt and respective logos are trademarks of The Qt Company Ltd. in Finland and/or other countries worldwide. All other trademarks are property of their respective owners.