key_iterator Class
Class key_iterator is declared in class QMap.The QMap::key_iterator class provides an STL-style const iterator for QMap and QMultiMap keys. More...
This class was introduced in Qt 5.6.
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() << endl cout << "The value: " << it.value() << endl; cout << "Also the value: " << (*it) << 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.