- Qt 5.15
- Qt Core
- C++ Classes
- const_iterator
- Qt 5.15.15 Reference Documentation

# const_iterator Class

class QList::const_iterator

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

- List of all members, including inherited members

## Public Types

typedef | iterator_category

## Public Functions

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

## Detailed Description

QList provides 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>::const_iterator allows you to iterate over a QList<T> (or a QQueue<T>). If you want to modify the QList as you iterate over it, use QList::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::const_iterator constructor creates an uninitialized iterator. You must initialize it using a QList function like QList::constBegin(), QList::constEnd(), 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>::const_iterator i;
     for (i = list.constBegin(); i != list.constEnd(); ++i)
         cout << *i << Qt::endl;

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.constBegin(), list.constEnd());

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::iterator and QListIterator.

## Member Type Documentation

### typedef const_iterator::iterator_category

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

## Member Function Documentation

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

Constructs a copy of other.

### const_iterator::const_iterator(const const_iterator &other)

Constructs a copy of other.

### const_iterator::const_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::constBegin() and QList::constEnd().

### 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==().

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

Returns the current item.

See also operator->().

### const_iterator const_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+=().

### const_iterator &const_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--().

### const_iterator const_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.

### const_iterator &const_iterator::operator+=(difference_type j)

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

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

### const_iterator const_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 const_iterator::operator-(const_iterator other) const

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

### const_iterator &const_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++().

### const_iterator const_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.

### const_iterator &const_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-().

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

Returns a pointer to the current item.

See also operator*().

### bool const_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 const_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 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!=().

### bool const_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 const_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.

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

Returns the item at position *this + j.

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

See also operator+().