key_iterator Class | Qt Core 5.6

Qt 5.6 | Qt Core | C++ Classes | key_iterator

Qt 5.6.2 Reference Documentation

# key_iterator Class

(QHash::key_iterator)

The QHash::key_iterator class provides an STL-style const iterator for QHash and QMultiHash keys. More...

Header: | #include <key_iterator>
qmake: | QT += core
Since: | Qt 5.6

- List of all members, including inherited members

## Public Functions

| key_iterator(const_iterator o)
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

The QHash::key_iterator class provides an STL-style const iterator for QHash and QMultiHash keys.

QHash::key_iterator is essentially the same as QHash::const_iterator with the difference that operator*() and operator->() return a key instead of a value.

For most uses QHash::iterator and QHash::const_iterator should be used, you can easily access the key by calling QHash::iterator::key():

      for (QHash<int, QString>::const_iterator it = hash.cbegin(), end = hash.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 QHash's keys and STL-style algorithms we need an iterator that dereferences to a key instead of a value. With QHash::key_iterator we can apply an algorithm to a range of keys without having to call QHash::keys(), which is inefficient as it costs one QHash iteration and memory allocation to create a temporary QList.

      // Inefficient, keys() is expensive
      QList<int> keys = hash.keys();
      int numPrimes = std::count_if(keys.cbegin(), keys.cend(), isPrimeNumber);
      qDeleteAll(hash2.keys());

      // Efficient, no memory allocation needed
      int numPrimes = std::count_if(hash.keyBegin(), hash.keyEnd(), isPrimeNumber);
      qDeleteAll(hash2.keyBegin(), hash2.keyEnd());

QHash::key_iterator is const, it's not possible to modify the key.

The default QHash::key_iterator constructor creates an uninitialized iterator. You must initialize it using a QHash function like QHash::keyBegin() or QHash::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 QHash::const_iterator and QHash::iterator.

## Member Function Documentation

### key_iterator::key_iterator(const_iterator o)

Default constructs an instance of key_iterator.

### 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 QHash::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 QHash::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!=().