Qt Documentation

Qt 5.12
Qt Core
C++ Classes
const_iterator

const_iterator Class

(QList::const_iterator)

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

Header:	#include <const_iterator>
qmake:	QT += core

List of all members, including inherited members

Public Types

typedef iterator_category

Public Functions

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

Public Variables

QList::Node * i

Detailed Description

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

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 << 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()

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().

const_iterator::const_iterator(const const_iterator &other)

Constructs a copy of other.

const_iterator::const_iterator(const iterator &other)

Constructs a copy of other.

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+(const_iterator::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+=(const_iterator::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-(const_iterator::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-=(const_iterator::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[](const_iterator::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+().

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