- Qt 6.9
- Qt Core
- C++ Classes
- const_iterator
- Qt 6.9.3 Reference Documentation

# QHash::const_iterator Class

class QHash::const_iterator

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

- List of all members, including inherited members

## Public Functions

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

## Detailed Description

QHash<Key, T>::const_iterator allows you to iterate over a QHash. If you want to modify the QHash as you iterate over it, you must use QHash::iterator instead. 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::const_iterator constructor creates an uninitialized iterator. You must initialize it using a QHash function like QHash::cbegin(), QHash::cend(), or QHash::constFind() 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(i.key()) << ": " << i.value() << endl;

Unlike QMap, which orders its items by key, QHash stores its items in an arbitrary order. The only guarantee is that items that share the same key (because they were inserted using a QMultiHash) will appear consecutively, from the most recently to the least recently inserted value.

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.

You can however safely use iterators to remove entries from the hash using the QHash::erase() method. This function can safely be called while iterating, and won't affect the order of items in the hash.

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::iterator, QHash::key_iterator, and QHash::const_key_value_iterator.

## Member Function Documentation

### `[constexpr noexcept]` 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 QHash::constBegin() and QHash::constEnd().

### `[noexcept]` const_iterator::const_iterator(const QHash<Key, T>::iterator &other)

Constructs a copy of other.

### `[noexcept]` const Key &const_iterator::key() const

Returns the current item's key.

See also value().

### `[noexcept]` const T &const_iterator::value() const

Returns the current item's value.

See also key() and operator*().

### `[noexcept]` bool const_iterator::operator!=(const QHash<Key, T>::const_iterator &other) const

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

See also operator==().

### `[noexcept]` const T &const_iterator::operator*() const

Returns the current item's value.

Same as value().

See also key().

### `[noexcept]` QHash<Key, T>::const_iterator &const_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.

### `[noexcept]` QHash<Key, T>::const_iterator const_iterator::operator++(int)

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

This is an overloaded function.

### `[noexcept]` const T *const_iterator::operator->() const

Returns a pointer to the current item's value.

See also value().

### `[noexcept]` bool const_iterator::operator==(const QHash<Key, T>::const_iterator &other) const

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

See also operator!=().