key_iterator Class
class QMap::key_iteratorThe QMap::key_iterator class provides an STL-style const iterator for QMap 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) |
| key_iterator & | operator--() |
| key_iterator | operator--(int) |
| const Key * | operator->() const |
| bool | operator==(key_iterator other) const |
Detailed Description
QMap::key_iterator is essentially the same as QMap::const_iterator with the difference that operator*() and operator->() return a key instead of a value.
For most uses QMap::iterator and QMap::const_iterator should be used, you can easily access the key by calling QMap::iterator::key():
for (QMap<int, QString>::const_iterator it = map.cbegin(), end = map.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 QMap's keys and STL-style algorithms we need an iterator that dereferences to a key instead of a value. With QMap::key_iterator we can apply an algorithm to a range of keys without having to call QMap::keys(), which is inefficient as it costs one QMap iteration and memory allocation to create a temporary QList.
// Inefficient, keys() is expensive QList<int> keys = map.keys(); int numPrimes = std::count_if(map.cbegin(), map.cend(), isPrimeNumber); qDeleteAll(map2.keys()); // Efficient, no memory allocation needed int numPrimes = std::count_if(map.keyBegin(), map.keyEnd(), isPrimeNumber); qDeleteAll(map2.keyBegin(), map2.keyEnd());
QMap::key_iterator is const, it's not possible to modify the key.
The default QMap::key_iterator constructor creates an uninitialized iterator. You must initialize it using a QMap function like QMap::keyBegin() or QMap::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 QMap::const_iterator and QMap::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 QMap::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.
key_iterator &key_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::keyBegin() leads to undefined results.
See also operator++().
key_iterator key_iterator::operator--(int)
This is an overloaded function.
The postfix -- operator (i--) makes the preceding item current and returns an iterator pointing 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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