key_iterator Class

class QHash::key_iterator

The QHash::key_iterator class provides an STL-style const iterator for QHash and QMultiHash keys. More...

This class was introduced in Qt 5.6.

Public Functions

const_iterator base() const
bool operator!=(key_iterator other) const
const Key &operator*() const
key_iterator &operator++()
key_iterator operator++(int)
const Key *operator->() const
bool operator==(key_iterator other) const

Detailed Description

QHash::key_iterator is essentially the same as QHash::const_iterator with the difference that operator*() and operator->() return a key instead of a value.

For most uses QHash::iterator and QHash::const_iterator should be used, you can easily access the key by calling QHash::iterator::key():

for (QHash<int, QString>::const_iterator it = hash.cbegin(), end = hash.cend(); it != end; ++it) {
    cout << "The key: " << it.key() << Qt::endl
    cout << "The value: " << it.value() << Qt::endl;
    cout << "Also the value: " << (*it) << Qt::endl;

However, to have interoperability between QHash's keys and STL-style algorithms we need an iterator that dereferences to a key instead of a value. With QHash::key_iterator we can apply an algorithm to a range of keys without having to call QHash::keys(), which is inefficient as it costs one QHash iteration and memory allocation to create a temporary QList.

// Inefficient, keys() is expensive
QList<int> keys = hash.keys();
int numPrimes = std::count_if(keys.cbegin(), keys.cend(), isPrimeNumber);

// Efficient, no memory allocation needed
int numPrimes = std::count_if(hash.keyBegin(), hash.keyEnd(), isPrimeNumber);
qDeleteAll(hash2.keyBegin(), hash2.keyEnd());

QHash::key_iterator is const, it's not possible to modify the key.

The default QHash::key_iterator constructor creates an uninitialized iterator. You must initialize it using a QHash function like QHash::keyBegin() or QHash::keyEnd().

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 and QHash::iterator.

Member Function Documentation

const_iterator key_iterator::base() const

Returns the underlying const_iterator this key_iterator is based on.

bool key_iterator::operator!=(key_iterator other) const

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

See also operator==().

const Key &key_iterator::operator*() const

Returns the current item's key.

key_iterator &key_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::keyEnd() leads to undefined results.

See also operator--().

key_iterator key_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 previous item.

const Key *key_iterator::operator->() const

Returns a pointer to the current item's key.

bool key_iterator::operator==(key_iterator other) const

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

See also operator!=().

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