iterator Class | Qt Core 5.6

Qt 5.6 | Qt Core | C++ Classes | iterator

Qt 5.6.2 Reference Documentation

# iterator Class

(QSet::iterator)

The QSet::iterator class provides an STL-style non-const iterator for QSet. More...

Header: | #include <QSet>
qmake: | QT += core
Since: | Qt 4.2

- List of all members, including inherited members

## Public Types

typedef | iterator_category

## Public Functions

| iterator()
| iterator(const iterator &other)
bool | operator!=(const iterator &other) const
bool | operator!=(const const_iterator &other) const
const T & | operator*() const
iterator | operator+(int j) const
iterator & | operator++()
iterator | operator++(int)
iterator & | operator+=(int j)
iterator | operator-(int j) const
iterator & | operator--()
iterator | operator--(int)
iterator & | operator-=(int j)
const T * | operator->() const
iterator & | operator=(const iterator &other)
bool | operator==(const iterator &other) const
bool | operator==(const const_iterator &other) const

## Detailed Description

The QSet::iterator class provides an STL-style non-const iterator for QSet.

QSet 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.

QSet<T>::iterator allows you to iterate over a QSet and to remove items (using QSet::erase()) while you iterate. (QSet doesn't let you modify a value through an iterator, because that would potentially require moving the value in the internal hash table used by QSet.) If you want to iterate over a const QSet, you should use QSet::const_iterator. It is generally good practice to use QSet::const_iterator on a non-const QSet as well, unless you need to change the QSet through the iterator. Const iterators are slightly faster, and can improve code readability.

The default QSet::iterator constructor creates an uninitialized iterator. You must initialize it using a function like QSet::begin(), QSet::end(), or QSet::insert() before you can start iterating. Here's a typical loop that prints all the items stored in a set:

      QSet<QString> set;
      set << "January" << "February" << ... << "December";

      QSet<QString>::iterator i;
      for (i = set.begin(); i != set.end(); ++i)
          qDebug() << *i;

Here's a loop that removes certain items (all those that start with 'J') from a set while iterating:

      QSet<QString> set;
      set << "January" << "February" << ... << "December";

      QSet<QString>::iterator i = set.begin();
      while (i != set.end()) {
          if ((*i).startsWith('J')) {
              i = set.erase(i);
          } else {
              ++i;
          }
      }

STL-style iterators can be used as arguments to generic algorithms. For example, here's how to find an item in the set using the qFind() algorithm:

      QSet<QString> set;
      ...
      QSet<QString>::iterator it = qFind(set.begin(), set.end(), "Jeanette");
      if (it != set.end())
          cout << "Found Jeanette" << endl;

Multiple iterators can be used on the same set.

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 QSet::const_iterator and QMutableSetIterator.

## Member Type Documentation

### typedef iterator::iterator_category

Synonyms for std::bidirectional_iterator_tag indicating these iterators are bidirectional iterators.

## Member Function Documentation

### iterator::iterator()

Constructs an uninitialized iterator.

Functions like operator*() and operator++() should not be called on an uninitialized iterator. Use operator=() to assign a value to it before using it.

See also QSet::begin() and QSet::end().

### iterator::iterator(const iterator &other)

Constructs a copy of other.

### bool iterator::operator!=(const iterator &other) const

Returns `true` if other points to a different item than this iterator; otherwise returns `false`.

See also operator==().

### bool iterator::operator!=(const const_iterator &other) const

This is an overloaded function.

### const T &iterator::operator*() const

Returns a reference to the current item.

See also operator->().

### iterator 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 operator-().

### iterator &iterator::operator++()

The prefix ++ operator (`++it`) advances the iterator to the next item in the set and returns an iterator to the new current item.

Calling this function on QSet::constEnd() leads to undefined results.

See also operator--().

### iterator iterator::operator++(int)

This is an overloaded function.

The postfix ++ operator (`it++`) advances the iterator to the next item in the set and returns an iterator to the previously current item.

### iterator &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 operator-=() and operator+().

### iterator 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 operator+().

### iterator &iterator::operator--()

The prefix -- operator (`--it`) makes the preceding item current and returns an iterator to the new current item.

Calling this function on QSet::begin() leads to undefined results.

See also operator++().

### iterator iterator::operator--(int)

This is an overloaded function.

The postfix -- operator (`it--`) makes the preceding item current and returns an iterator to the previously current item.

### iterator &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 operator+=() and operator-().

### const T *iterator::operator->() const

Returns a pointer to the current item.

See also operator*().

### iterator &iterator::operator=(const iterator &other)

Assigns other to this iterator.

### bool iterator::operator==(const iterator &other) const

Returns `true` if other points to the same item as this iterator; otherwise returns `false`.

See also operator!=().

### bool iterator::operator==(const const_iterator &other) const

This is an overloaded function.