const_iterator Class

class QMultiHash::const_iterator

The QMultiHash::const_iterator class provides an STL-style const iterator for QMultiHash. More...

Public Types

Public Functions

const_iterator(const iterator &other)
const_iterator()
const Key &key() const
T &value() const
bool operator!=(const const_iterator &other) const
T &operator*() const
const_iterator &operator++()
const_iterator operator++(int)
T *operator->() const
bool operator==(const const_iterator &other) const

Detailed Description

QMultiHash 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.

QMultiHash<Key, T>::const_iterator allows you to iterate over a QMultiHash. If you want to modify the QMultiHash as you iterate over it, you must use QMultiHash::iterator instead. It is generally good practice to use QMultiHash::const_iterator on a non-const QMultiHash as well, unless you need to change the QMultiHash through the iterator. Const iterators are slightly faster, and can improve code readability.

The default QMultiHash::const_iterator constructor creates an uninitialized iterator. You must initialize it using a QMultiHash function like QMultiHash::constBegin(), QMultiHash::constEnd(), or QMultiHash::find() before you can start iterating. Here's a typical loop that prints all the (key, value) pairs stored in a hash:

QHash<QString, int> hash;
hash.insert("January", 1);
hash.insert("February", 2);
...
hash.insert("December", 12);

QHash<QString, int>::const_iterator i;
for (i = hash.constBegin(); i != hash.constEnd(); ++i)
    cout << i.key() << ": " << i.value() << Qt::endl;

Unlike QMap, which orders its items by key, QMultiHash stores its items in an arbitrary order. The only guarantee is that items that share the same key (because they were inserted using a QMultiHash) will appear consecutively, from the most recently to the least recently inserted value.

Multiple iterators can be used on the same hash. However, be aware that any modification performed directly on the QHash (inserting and removing items) can cause the iterators to become invalid.

Inserting items into the hash or calling methods such as QHash::reserve() or QHash::squeeze() can invalidate all iterators pointing into the hash. Iterators are guaranteed to stay valid only as long as the QHash doesn't have to grow/shrink it's internal hash table. Using any iterator after a rehashing operation ahs occurred will lead to undefined behavior.

You can however safely use iterators to remove entries from the hash using the QHash::erase() method. This function can safely be called while iterating, and won't affect the order of items in the hash.

If you need to keep iterators over a long period of time, we recommend that you use QMap rather than QHash.

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 QMultiHash::iterator.

Member Type Documentation

const_iterator::difference_type

const_iterator::iterator_category

const_iterator::pointer

const_iterator::reference

const_iterator::value_type

Member Function Documentation

const_iterator::const_iterator(const iterator &other)

Constructs a copy of other.

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 QMultiHash::constBegin() and QMultiHash::constEnd().

const Key &const_iterator::key() const

Returns the current item's key.

See also value().

T &const_iterator::value() const

Returns the current item's value.

See also key() and operator*().

bool 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 operator==().

T &const_iterator::operator*() const

Returns the current item's value.

Same as value().

See also key().

const_iterator &const_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 QMultiHash::end() leads to undefined results.

const_iterator const_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 previously current item.

T *const_iterator::operator->() const

Returns a pointer to the current item's value.

See also value().

bool 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 operator!=().

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