const_iterator Class▲
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Header: const_iterator
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qmake: QT += core
Detailed Description▲
QMap features both STL-style iterators and Java-style iterators. The STL-style iterators are more low-level and more cumbersome to use; on the other hand, they are slightly faster and, for developers who already know STL, have the advantage of familiarity.
QMap<Key, T>::const_iterator allows you to iterate over a QMap (or a QMultiMap). If you want to modify the QMap as you iterate over it, you must use QMap::iterator instead. It is generally good practice to use QMap::const_iterator on a non-const QMap as well, unless you need to change the QMap through the iterator. Const iterators are slightly faster, and can improve code readability.
The default QMap::const_iterator constructor creates an uninitialized iterator. You must initialize it using a QMap function like QMap::constBegin(), QMap::constEnd(), or QMap::find() before you can start iterating. Here's a typical loop that prints all the (key, value) pairs stored in a map:
QMap<QString, int> map;
map.insert("January", 1);
map.insert("February", 2);
...
map.insert("December", 12);
QMap<QString, int>::const_iterator i;
for (i = map.constBegin(); i != map.constEnd(); ++i)
cout << i.key() << ": " << i.value() << endl;Unlike QHash, which stores its items in an arbitrary order, QMap stores its items ordered by key. Items that share the same key (because they were inserted using QMap::insertMulti()) will appear consecutively, from the most recently to the least recently inserted value.
Multiple iterators can be used on the same map. If you add items to the map, existing iterators will remain valid. If you remove items from the map, iterators that point to the removed items will become dangling iterators.
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▲
See also QMap::iterator, QMap::key_iterator, QMapIterator
Member Type Documentation▲
const_iterator::iterator_category▲
A synonym for std::bidirectional_iterator_tag indicating this iterator is a bidirectional iterator.
Member Function Documentation▲
const_iterator::const_iterator(const iterator &other)▲
Constructs a copy of other.
[default] 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▲
See also QMap::constBegin(), QMap::constEnd()
const Key &const_iterator::key() const▲
const T &const_iterator::value() const▲
int const_iterator::operator!=(const const_iterator &other) const▲
Returns true if other points to a different item than this iterator; otherwise returns false.
See Also▲
See also operator==()
const T &const_iterator::operator*() const▲
const_iterator const_iterator::operator+(int j) const▲
Returns an iterator to the item at j positions forward from this iterator. (If j is negative, the iterator goes backward.)
This operation can be slow for large j values.
See Also▲
See also operator-()
const_iterator &const_iterator::operator++()▲
The prefix ++ operator (++i) advances the iterator to the next item in the map and returns an iterator to the new current item.
Calling this function on QMap::end() leads to undefined results.
See Also▲
See also operator--()
const_iterator const_iterator::operator++(int)▲
This is an overloaded function.
The postfix ++ operator (i++) advances the iterator to the next item in the map and returns an iterator to the previously current item.
const_iterator &const_iterator::operator+=(int j)▲
Advances the iterator by j items. (If j is negative, the iterator goes backward.)
This operation can be slow for large j values.
See Also▲
See also operator-=(), operator+()
const_iterator const_iterator::operator-(int j) const▲
Returns an iterator to the item at j positions backward from this iterator. (If j is negative, the iterator goes forward.)
This operation can be slow for large j values.
See Also▲
See also operator+()
const_iterator &const_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::begin() leads to undefined results.
See Also▲
See also operator++()
const_iterator const_iterator::operator--(int)▲
This is an overloaded function.
The postfix – operator (i--) makes the preceding item current and returns an iterator pointing to the previously current item.
const_iterator &const_iterator::operator-=(int j)▲
Makes the iterator go back by j items. (If j is negative, the iterator goes forward.)
This operation can be slow for large j values.
See Also▲
See also operator+=(), operator-()
const T *const_iterator::operator->() const▲
int const_iterator::operator==(const const_iterator &other) const▲
Returns true if other points to the same item as this iterator; otherwise returns false.
See Also▲
See also operator!=()