key_iterator Class
class QMultiMap::key_iteratorThe QMultiMap::key_iterator class provides an STL-style const iterator for QMultiMap keys. More...
Public Functions
| QMultiMap<Key, T>::const_iterator | base() const |
| bool | operator!=(QMultiMap<Key, T>::key_iterator other) const |
| const Key & | operator*() const |
| QMultiMap<Key, T>::key_iterator & | operator++() |
| QMultiMap<Key, T>::key_iterator | operator++(int) |
| QMultiMap<Key, T>::key_iterator & | operator--() |
| QMultiMap<Key, T>::key_iterator | operator--(int) |
| const Key * | operator->() const |
| bool | operator==(QMultiMap<Key, T>::key_iterator other) const |
Detailed Description
QMultiMap::key_iterator is essentially the same as QMultiMap::const_iterator with the difference that operator*() and operator->() return a key instead of a value.
For most uses QMultiMap::iterator and QMultiMap::const_iterator should be used, you can easily access the key by calling QMultiMap::iterator::key():
for (auto it = multimap.cbegin(), end = multimap.cend(); it != end; ++it) { cout << "The key: " << it.key() << endl cout << "The value: " << qPrintable(it.value()) << endl; cout << "Also the value: " << qPrintable(*it) << endl; }
However, to have interoperability between QMultiMap's keys and STL-style algorithms we need an iterator that dereferences to a key instead of a value. With QMultiMap::key_iterator we can apply an algorithm to a range of keys without having to call QMultiMap::keys(), which is inefficient as it costs one QMultiMap iteration and memory allocation to create a temporary QList.
// Inefficient, keys() is expensive QList<int> keys = multimap.keys(); int numPrimes = std::count_if(multimap.cbegin(), multimap.cend(), isPrimeNumber); qDeleteAll(multimap2.keys()); // Efficient, no memory allocation needed int numPrimes = std::count_if(multimap.keyBegin(), multimap.keyEnd(), isPrimeNumber); qDeleteAll(multimap2.keyBegin(), multimap2.keyEnd());
QMultiMap::key_iterator is const, it's not possible to modify the key.
The default QMultiMap::key_iterator constructor creates an uninitialized iterator. You must initialize it using a QMultiMap function like QMultiMap::keyBegin() or QMultiMap::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 QMultiMap::const_iterator and QMultiMap::iterator.
Member Function Documentation
QMultiMap<Key, T>::const_iterator key_iterator::base() const
Returns the underlying const_iterator this key_iterator is based on.
bool key_iterator::operator!=(QMultiMap<Key, T>::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.
QMultiMap<Key, T>::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 QMultiMap::keyEnd() leads to undefined results.
See also operator--().
QMultiMap<Key, T>::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.
QMultiMap<Key, T>::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 QMultiMap::keyBegin() leads to undefined results.
See also operator++().
QMultiMap<Key, T>::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==(QMultiMap<Key, T>::key_iterator other) const
Returns true if other points to the same item as this iterator; otherwise returns false.
See also operator!=().