iterator Class | Qt Core 5.6

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

Qt 5.6.2 Reference Documentation

# iterator Class

(QList::iterator)

The QList::iterator class provides an STL-style non-const iterator for QList and QQueue. More...

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

- 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
T & | operator*() const
iterator | operator+(difference_type j) const
iterator & | operator++()
iterator | operator++(int)
iterator & | operator+=(difference_type j)
iterator | operator-(difference_type j) const
int | operator-(iterator other) const
iterator & | operator--()
iterator | operator--(int)
iterator & | operator-=(difference_type j)
T * | operator->() const
bool | operator<(const iterator &other) const
bool | operator<(const const_iterator &other) const
bool | operator<=(const iterator &other) const
bool | operator<=(const const_iterator &other) const
bool | operator==(const iterator &other) const
bool | operator==(const const_iterator &other) const
bool | operator>(const iterator &other) const
bool | operator>(const const_iterator &other) const
bool | operator>=(const iterator &other) const
bool | operator>=(const const_iterator &other) const
T & | operator[](difference_type j) const

## Detailed Description

The QList::iterator class provides an STL-style non-const iterator for QList and QQueue.

QList 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.

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

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

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

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

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

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

Most QList functions accept an integer index rather than an iterator. For that reason, iterators are rarely useful in connection with QList. One place where STL-style iterators do make sense is as arguments to generic algorithms.

For example, here's how to delete all the widgets stored in a QList<QWidget *>:

      QList<QWidget *> list;
      ...
      qDeleteAll(list.begin(), list.end());

Multiple iterators can be used on the same list. However, be aware that any non-const function call performed on the QList will render all existing iterators undefined. If you need to keep iterators over a long period of time, we recommend that you use QLinkedList rather than QList.

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 QList::const_iterator and QMutableListIterator.

## Member Type Documentation

### typedef iterator::iterator_category

A synonym for std::random_access_iterator_tag indicating this iterator is a random access iterator.

## 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 QList::begin() and QList::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+(difference_type j) const

Returns an iterator to the item at j positions forward from this iterator. (If j is negative, the iterator goes backward.)

See also operator-() and 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 QList::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+=(difference_type j)

Advances the iterator by j items. (If j is negative, the iterator goes backward.)

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

### iterator iterator::operator-(difference_type j) const

Returns an iterator to the item at j positions backward from this iterator. (If j is negative, the iterator goes forward.)

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

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

Returns the number of items between the item pointed to by other and the item pointed to by this iterator.

### 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 QList::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-=(difference_type 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*().

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

Returns `true` if the item pointed to by this iterator is less than the item pointed to by the other iterator.

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

Returns `true` if the item pointed to by this iterator is less than the item pointed to by the other iterator.

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

Returns `true` if the item pointed to by this iterator is less than or equal to the item pointed to by the other iterator.

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

Returns `true` if the item pointed to by this iterator is less than or equal to the item pointed to by the other 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!=().

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

Returns `true` if the item pointed to by this iterator is greater than the item pointed to by the other iterator.

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

Returns `true` if the item pointed to by this iterator is greater than the item pointed to by the other iterator.

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

Returns `true` if the item pointed to by this iterator is greater than or equal to the item pointed to by the other iterator.

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

Returns `true` if the item pointed to by this iterator is greater than or equal to the item pointed to by the other iterator.

### T &iterator::operator[](difference_type j) const

Returns a modifiable reference to the item at position *this + j.

This function is provided to make QList iterators behave like C++ pointers.

See also operator+().