iterator Class | Qt Core 5.6

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

Qt 5.6.2 Reference Documentation

# iterator Class

(QLinkedList::iterator)

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

Header: | #include <iterator>
qmake: | QT += core

- List of all members, including inherited members

## Public Functions

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

## Detailed Description

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

QLinkedList 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.

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

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

      QLinkedList<QString> list;
      list.append("January");
      list.append("February");
      ...
      list.append("December");

      QLinkedList<QString>::iterator i;
      for (i = list.begin(); i != list.end(); ++i)
          cout << *i << endl;

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

      QLinkedList<QString> list;
      ...
      QLinkedList<QString>::iterator it = qFind(list.begin(),
                                                list.end(), "Joel");
      if (it != list.end())
          cout << "Found Joel" << endl;

Let's see a few examples of things we can do with a QLinkedList::iterator that we cannot do with a QLinkedList::const_iterator. Here's an example that increments every value stored in a QLinkedList<int> by 2:

      QLinkedList<int>::iterator i;
      for (i = list.begin(); i != list.end(); ++i)
          *i += 2;

Here's an example that removes all the items that start with an underscore character in a QLinkedList<QString>:

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

The call to QLinkedList::erase() removes the item pointed to by the iterator from the list, and returns an iterator to the next item. Here's another way of removing an item while iterating:

      QLinkedList<QString>::iterator i = list.begin();
      while (i != list.end()) {
          QLinkedList<QString>::iterator previous = i;
          ++i;
          if ((*previous).startsWith('_'))
              list.erase(previous);
      }

It might be tempting to write code like this:

      // WRONG
      while (i != list.end()) {
          if ((*i).startsWith('_'))
              list.erase(i);
          ++i;
      }

However, this will potentially crash in `++i`, because `i` is a dangling iterator after the call to erase().

Multiple iterators can be used on the same list. If you add items to the list, existing iterators will remain valid. If you remove items from the list, iterators that point to the removed items will become dangling iterators.

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 QLinkedList::const_iterator and QMutableLinkedListIterator.

## 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 QLinkedList::begin() and QLinkedList::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

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

See also operator==().

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

Returns a modifiable reference to the current item.

You can change the value of an item by using operator*() on the left side of an assignment, for example:

      if (*it == "Hello")
          *it = "Bonjour";

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 list and returns an iterator to the new current item.

Calling this function on QLinkedList::end() 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 list 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.)

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 QLinkedList::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.)

See also operator+=() and operator-().

### 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

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

See also operator!=().