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iterator Class

Class iterator is declared in class QList.

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

Public Types

typedef iterator_category

Public Functions

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

Detailed Description

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

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

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.

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

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

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

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

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==().

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

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!=().

iterator::iterator(const iterator &other)

Constructs a copy of other.

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().

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+(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+=().

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+=(iterator::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-(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 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-=(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-().

T *iterator::operator->() const

Returns a pointer to the current item.

See also operator*().

T &iterator::operator[](iterator::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+().

© The Qt Company Ltd
Licensed under the GNU Free Documentation License, Version 1.3.
https://doc.qt.io/qt-5.13/qlist-iterator.html