- Qt 6.9
- Qt 5 Core Compatibility APIs
- C++ Classes
- QLinkedList
- Qt 6.9.0 Reference Documentation

# QLinkedList Class

template <typename T> class QLinkedList

The QLinkedList class is a template class that provides linked lists. More...

Header: | `#include <QLinkedList>`
CMake: | `find_package(Qt6 REQUIRED COMPONENTS Core5Compat)` `target_link_libraries(mytarget PRIVATE Qt6::Core5Compat)`
qmake: | `QT += core5compat`

- List of all members, including inherited members

Note: All functions in this class are reentrant.

## Public Types

class | const_iterator
class | iterator
| ConstIterator
| Iterator
| const_pointer
| const_reference
| const_reverse_iterator
| difference_type
| pointer
| reference
| reverse_iterator
| size_type
| value_type

## Public Functions

| QLinkedList()
| QLinkedList(std::initializer_list<T> list)
| QLinkedList(InputIterator first, InputIterator last)
| QLinkedList(const QLinkedList<T> &other)
| QLinkedList(QLinkedList<T> &&other)
| ~QLinkedList()
void | append(const T &value)
T & | back()
const T & | back() const
QLinkedList<T>::iterator | begin()
QLinkedList<T>::const_iterator | begin() const
QLinkedList<T>::const_iterator | cbegin() const
QLinkedList<T>::const_iterator | cend() const
void | clear()
QLinkedList<T>::const_iterator | constBegin() const
QLinkedList<T>::const_iterator | constEnd() const
bool | contains(const T &value) const
int | count() const
int | count(const T &value) const
QLinkedList<T>::const_reverse_iterator | crbegin() const
QLinkedList<T>::const_reverse_iterator | crend() const
bool | empty() const
QLinkedList<T>::iterator | end()
QLinkedList<T>::const_iterator | end() const
bool | endsWith(const T &value) const
QLinkedList<T>::iterator | erase(QLinkedList<T>::iterator pos)
QLinkedList<T>::iterator | erase(QLinkedList<T>::iterator begin, QLinkedList<T>::iterator end)
T & | first()
const T & | first() const
T & | front()
const T & | front() const
QLinkedList<T>::iterator | insert(QLinkedList<T>::iterator before, const T &value)
bool | isEmpty() const
T & | last()
const T & | last() const
void | pop_back()
void | pop_front()
void | prepend(const T &value)
void | push_back(const T &value)
void | push_front(const T &value)
QLinkedList<T>::reverse_iterator | rbegin()
QLinkedList<T>::const_reverse_iterator | rbegin() const
int | removeAll(const T &value)
void | removeFirst()
void | removeLast()
bool | removeOne(const T &value)
QLinkedList<T>::reverse_iterator | rend()
QLinkedList<T>::const_reverse_iterator | rend() const
int | size() const
bool | startsWith(const T &value) const
void | swap(QLinkedList<T> &other)
T | takeFirst()
T | takeLast()
std::list<T> | toStdList() const
bool | operator!=(const QLinkedList<T> &other) const
QLinkedList<T> | operator+(const QLinkedList<T> &other) const
QLinkedList<T> & | operator+=(const QLinkedList<T> &other)
QLinkedList<T> & | operator+=(const T &value)
QLinkedList<T> & | operator<<(const QLinkedList<T> &other)
QLinkedList<T> & | operator<<(const T &value)
QLinkedList<T> & | operator=(const QLinkedList<T> &other)
bool | operator==(const QLinkedList<T> &other) const

## Static Public Members

QLinkedList<T> | fromStdList(const std::list<T> &list)

## Related Non-Members

int | operator<<(int &out, const QLinkedList<T> &list)

## Detailed Description

QLinkedList<T> is one of Qt's generic container classes. It stores a list of values and provides iterator-based access as well as constant time insertions and removals.

QList<T> and QLinkedList<T> provide similar functionality. Here's an overview:

- For most purposes, QList is the right class to use. Its index-based API is more convenient than QLinkedList's iterator-based API. Its items occupy adjacent memory positions. It also expands to less code in your executable.
- If you need a real linked list, with guarantees of constant time insertions in the middle of the list and iterators to items rather than indexes, use QLinkedList.

Here's an example of a QLinkedList that stores integers and a QLinkedList that stores QTime values:

     QLinkedList<int> integerList;
     QLinkedList<QTime> timeList;

QLinkedList stores a list of items. The default constructor creates an empty list. To insert items into the list, you can use operator<<():

     QLinkedList<QString> list;
     list << "one" << "two" << "three";
     // list: ["one", "two", "three"]

If you want to get the first or last item in a linked list, use first() or last(). If you want to remove an item from either end of the list, use removeFirst() or removeLast(). If you want to remove all occurrences of a given value in the list, use removeAll().

A common requirement is to remove the first or last item in the list and do something with it. For this, QLinkedList provides takeFirst() and takeLast(). Here's a loop that removes the items from a list one at a time and calls `delete` on them:

     QLinkedList<QWidget *> list;
     ...
     while (!list.isEmpty())
         delete list.takeFirst();

QLinkedList's value type must be an assignable data type. This covers most data types that are commonly used, but the compiler won't let you, for example, store a QWidget as a value; instead, store a QWidget *. A few functions have additional requirements; for example, contains() and removeAll() expect the value type to support `operator==()`. These requirements are documented on a per-function basis.

If you want to insert, modify, or remove items in the middle of the list, you must use an iterator. QLinkedList provides both Java-style iterators (QLinkedListIterator and QMutableLinkedListIterator) and STL-style iterators (QLinkedList::const_iterator and QLinkedList::iterator). See the documentation for these classes for details.

See also QLinkedListIterator, QMutableLinkedListIterator, and QList.

## Member Type Documentation

### QLinkedList::ConstIterator

Qt-style synonym for QLinkedList::const_iterator.

### QLinkedList::Iterator

Qt-style synonym for QLinkedList::iterator.

### QLinkedList::const_pointer

Typedef for const T *. Provided for STL compatibility.

### QLinkedList::const_reference

Typedef for const T &. Provided for STL compatibility.

### QLinkedList::const_reverse_iterator

The QLinkedList::const_reverse_iterator typedef provides an STL-style const reverse iterator for QLinkedList.

It is simply a typedef for `std::reverse_iterator<QLinkedList::const_iterator>`.

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::rbegin(), QLinkedList::rend(), QLinkedList::reverse_iterator, and QLinkedList::const_iterator.

### QLinkedList::difference_type

Typedef for ptrdiff_t. Provided for STL compatibility.

### QLinkedList::pointer

Typedef for T *. Provided for STL compatibility.

### QLinkedList::reference

Typedef for T &. Provided for STL compatibility.

### QLinkedList::reverse_iterator

The QLinkedList::reverse_iterator typedef provides an STL-style non-const reverse iterator for QLinkedList.

It is simply a typedef for `std::reverse_iterator<QLinkedList::iterator>`.

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::rbegin(), QLinkedList::rend(), QLinkedList::const_reverse_iterator, and QLinkedList::iterator.

### QLinkedList::size_type

Typedef for int. Provided for STL compatibility.

### QLinkedList::value_type

Typedef for T. Provided for STL compatibility.

## Member Function Documentation

### `[noexcept]` QLinkedList::QLinkedList()

Constructs an empty list.

### QLinkedList::QLinkedList(std::initializer_list<T> list)

Constructs a list from the std::initializer_list specified by list.

This constructor is only enabled if the compiler supports C++11 initializer lists.

### template <typename InputIterator, int = true> QLinkedList::QLinkedList(InputIterator first, InputIterator last)

Constructs a list with the contents in the iterator range [first, last).

The value type of `InputIterator` must be convertible to `T`.

### QLinkedList::QLinkedList(const QLinkedList<T> &other)

Constructs a copy of other.

This operation occurs in constant time, because QLinkedList is implicitly shared. This makes returning a QLinkedList from a function very fast. If a shared instance is modified, it will be copied (copy-on-write), and this takes linear time.

See also operator=().

### `[noexcept]` QLinkedList::QLinkedList(QLinkedList<T> &&other)

Move-constructs a QLinkedList instance, making it point at the same object that other was pointing to.

### QLinkedList::~QLinkedList()

Destroys the list. References to the values in the list, and all iterators over this list, become invalid.

### void QLinkedList::append(const T &value)

Inserts value at the end of the list.

Example:

     QLinkedList<QString> list;
     list.append("one");
     list.append("two");
     list.append("three");
     // list: ["one", "two", "three"]

This is the same as list.insert(end(), value).

See also operator<<(), prepend(), and insert().

### T &QLinkedList::back()

This function is provided for STL compatibility. It is equivalent to last().

### const T &QLinkedList::back() const

This is an overloaded function.

### QLinkedList<T>::iterator QLinkedList::begin()

Returns an STL-style iterator pointing to the first item in the list.

See also constBegin() and end().

### `[noexcept]` QLinkedList<T>::const_iterator QLinkedList::begin() const

This is an overloaded function.

### `[noexcept]` QLinkedList<T>::const_iterator QLinkedList::cbegin() const

Returns a const STL-style iterator pointing to the first item in the list.

See also begin() and cend().

### `[noexcept]` QLinkedList<T>::const_iterator QLinkedList::cend() const

Returns a const STL-style iterator pointing to the imaginary item after the last item in the list.

See also cbegin() and end().

### void QLinkedList::clear()

Removes all the items in the list.

See also removeAll().

### `[noexcept]` QLinkedList<T>::const_iterator QLinkedList::constBegin() const

Returns a const STL-style iterator pointing to the first item in the list.

See also begin() and constEnd().

### `[noexcept]` QLinkedList<T>::const_iterator QLinkedList::constEnd() const

Returns a const STL-style iterator pointing to the imaginary item after the last item in the list.

See also constBegin() and end().

### bool QLinkedList::contains(const T &value) const

Returns `true` if the list contains an occurrence of value; otherwise returns `false`.

This function requires the value type to have an implementation of `operator==()`.

See also QLinkedListIterator::findNext() and QLinkedListIterator::findPrevious().

### int QLinkedList::count() const

Same as size().

### int QLinkedList::count(const T &value) const

Returns the number of occurrences of value in the list.

This function requires the value type to have an implementation of `operator==()`.

See also contains().

### `[noexcept]` QLinkedList<T>::const_reverse_iterator QLinkedList::crbegin() const

Returns a const STL-style reverse iterator pointing to the first item in the list, in reverse order.

See also begin(), rbegin(), and rend().

### `[noexcept]` QLinkedList<T>::const_reverse_iterator QLinkedList::crend() const

Returns a const STL-style reverse iterator pointing to one past the last item in the list, in reverse order.

See also end(), rend(), and rbegin().

### bool QLinkedList::empty() const

This function is provided for STL compatibility. It is equivalent to isEmpty() and returns `true` if the list is empty.

### QLinkedList<T>::iterator QLinkedList::end()

Returns an STL-style iterator pointing to the imaginary item after the last item in the list.

See also begin() and constEnd().

### `[noexcept]` QLinkedList<T>::const_iterator QLinkedList::end() const

This is an overloaded function.

### bool QLinkedList::endsWith(const T &value) const

Returns `true` if the list is not empty and its last item is equal to value; otherwise returns `false`.

See also isEmpty() and last().

### QLinkedList<T>::iterator QLinkedList::erase(QLinkedList<T>::iterator pos)

Removes the item pointed to by the iterator pos from the list, and returns an iterator to the next item in the list (which may be end()).

See also insert().

### QLinkedList<T>::iterator QLinkedList::erase(QLinkedList<T>::iterator begin, QLinkedList<T>::iterator end)

This is an overloaded function.

Removes all the items from begin up to (but not including) end.

### T &QLinkedList::first()

Returns a reference to the first item in the list. This function assumes that the list isn't empty.

See also last() and isEmpty().

### const T &QLinkedList::first() const

This is an overloaded function.

### `[static]` QLinkedList<T> QLinkedList::fromStdList(const std::list<T> &list)

Returns a QLinkedList object with the data contained in list. The order of the elements in the QLinkedList is the same as in list.

Example:

     std::list<double> stdlist;
     list.push_back(1.2);
     list.push_back(0.5);
     list.push_back(3.14);

     QLinkedList<double> list = QLinkedList<double>::fromStdList(stdlist);

See also toStdList().

### T &QLinkedList::front()

This function is provided for STL compatibility. It is equivalent to first().

### const T &QLinkedList::front() const

This is an overloaded function.

### QLinkedList<T>::iterator QLinkedList::insert(QLinkedList<T>::iterator before, const T &value)

Inserts value in front of the item pointed to by the iterator before. Returns an iterator pointing at the inserted item.

See also erase().

### bool QLinkedList::isEmpty() const

Returns `true` if the list contains no items; otherwise returns false.

See also size().

### T &QLinkedList::last()

Returns a reference to the last item in the list. This function assumes that the list isn't empty.

See also first() and isEmpty().

### const T &QLinkedList::last() const

This is an overloaded function.

### void QLinkedList::pop_back()

This function is provided for STL compatibility. It is equivalent to removeLast().

### void QLinkedList::pop_front()

This function is provided for STL compatibility. It is equivalent to removeFirst().

### void QLinkedList::prepend(const T &value)

Inserts value at the beginning of the list.

Example:

     QLinkedList<QString> list;
     list.prepend("one");
     list.prepend("two");
     list.prepend("three");
     // list: ["three", "two", "one"]

This is the same as list.insert(begin(), value).

See also append() and insert().

### void QLinkedList::push_back(const T &value)

This function is provided for STL compatibility. It is equivalent to append(value).

### void QLinkedList::push_front(const T &value)

This function is provided for STL compatibility. It is equivalent to prepend(value).

### QLinkedList<T>::reverse_iterator QLinkedList::rbegin()

Returns a STL-style reverse iterator pointing to the first item in the list, in reverse order.

See also begin(), crbegin(), and rend().

### `[noexcept]` QLinkedList<T>::const_reverse_iterator QLinkedList::rbegin() const

This is an overloaded function.

### int QLinkedList::removeAll(const T &value)

Removes all occurrences of value in the list.

Example:

     QList<QString> list;
     list << "sun" << "cloud" << "sun" << "rain";
     list.removeAll("sun");
     // list: ["cloud", "rain"]

This function requires the value type to have an implementation of `operator==()`.

See also insert().

### void QLinkedList::removeFirst()

Removes the first item in the list.

This is the same as erase(begin()).

See also removeLast() and erase().

### void QLinkedList::removeLast()

Removes the last item in the list.

See also removeFirst() and erase().

### bool QLinkedList::removeOne(const T &value)

Removes the first occurrences of value in the list. Returns `true` on success; otherwise returns `false`.

Example:

     QList<QString> list;
     list << "sun" << "cloud" << "sun" << "rain";
     list.removeOne("sun");
     // list: ["cloud", "sun", "rain"]

This function requires the value type to have an implementation of `operator==()`.

See also insert().

### QLinkedList<T>::reverse_iterator QLinkedList::rend()

Returns a STL-style reverse iterator pointing to one past the last item in the list, in reverse order.

See also end(), crend(), and rbegin().

### `[noexcept]` QLinkedList<T>::const_reverse_iterator QLinkedList::rend() const

This is an overloaded function.

### int QLinkedList::size() const

Returns the number of items in the list.

See also isEmpty() and count().

### bool QLinkedList::startsWith(const T &value) const

Returns `true` if the list is not empty and its first item is equal to value; otherwise returns `false`.

See also isEmpty() and first().

### `[noexcept]` void QLinkedList::swap(QLinkedList<T> &other)

Swaps list other with this list. This operation is very fast and never fails.

### T QLinkedList::takeFirst()

Removes the first item in the list and returns it.

If you don't use the return value, removeFirst() is more efficient.

See also takeLast() and removeFirst().

### T QLinkedList::takeLast()

Removes the last item in the list and returns it.

If you don't use the return value, removeLast() is more efficient.

See also takeFirst() and removeLast().

### std::list<T> QLinkedList::toStdList() const

Returns a std::list object with the data contained in this QLinkedList. Example:

     QLinkedList<double> list;
     list << 1.2 << 0.5 << 3.14;

     std::list<double> stdlist = list.toStdList();

See also fromStdList().

### bool QLinkedList::operator!=(const QLinkedList<T> &other) const

Returns `true` if other is not equal to this list; otherwise returns `false`.

Two lists are considered equal if they contain the same values in the same order.

This function requires the value type to implement `operator==()`.

See also operator==().

### QLinkedList<T> QLinkedList::operator+(const QLinkedList<T> &other) const

Returns a list that contains all the items in this list followed by all the items in the other list.

See also operator+=().

### QLinkedList<T> &QLinkedList::operator+=(const QLinkedList<T> &other)

Appends the items of the other list to this list and returns a reference to this list.

See also operator+() and append().

### QLinkedList<T> &QLinkedList::operator+=(const T &value)

This is an overloaded function.

Appends value to the list.

### QLinkedList<T> &QLinkedList::operator<<(const QLinkedList<T> &other)

Appends the items of the other list to this list and returns a reference to this list.

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

### QLinkedList<T> &QLinkedList::operator<<(const T &value)

This is an overloaded function.

Appends value to the list.

### QLinkedList<T> &QLinkedList::operator=(const QLinkedList<T> &other)

Assigns other to this list and returns a reference to this list.

### bool QLinkedList::operator==(const QLinkedList<T> &other) const

Returns `true` if other is equal to this list; otherwise returns false.

Two lists are considered equal if they contain the same values in the same order.

This function requires the value type to implement `operator==()`.

See also operator!=().

## Related Non-Members

### template <typename T> int operator<<(int &out, const QLinkedList<T> &list)

Writes the linked list list to stream out.

This function requires the value type to implement `operator<<()`.

See also Format of the QDataStream operators.