QList Class

template <typename T> class QList

The QList class is a template class that provides lists. More...

Header:	#include <QList>
qmake:	QT += core
Inherited By:	QByteArrayList, QItemSelection, QQueue, and QStringList

• List of all members, including inherited members
• Obsolete members

Note: All functions in this class are reentrant.

Public Types

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

Public Functions

	QList(InputIterator first, InputIterator last)
	QList(std::initializer_list<T> args)
	QList(QList<T> &&other)
	QList(const QList<T> &other)
	QList()
QList<T> &	operator=(QList<T> &&other)
QList<T> &	operator=(const QList<T> &other)
	~QList()
void	append(const T &value)
void	append(const QList<T> &value)
const T &	at(int i) const
T &	back()
const T &	back() const
QList::iterator	begin()
QList::const_iterator	begin() const
QList::const_iterator	cbegin() const
QList::const_iterator	cend() const
void	clear()
QList::const_iterator	constBegin() const
QList::const_iterator	constEnd() const
const T &	constFirst() const
const T &	constLast() const
bool	contains(const T &value) const
int	count(const T &value) const
int	count() const
QList::const_reverse_iterator	crbegin() const
QList::const_reverse_iterator	crend() const
bool	empty() const
QList::iterator	end()
QList::const_iterator	end() const
bool	endsWith(const T &value) const
QList::iterator	erase(QList::iterator pos)
QList::iterator	erase(QList::iterator begin, QList::iterator end)
T &	first()
const T &	first() const
T &	front()
const T &	front() const
int	indexOf(const T &value, int from = 0) const
void	insert(int i, const T &value)
QList::iterator	insert(QList::iterator before, const T &value)
bool	isEmpty() const
T &	last()
const T &	last() const
int	lastIndexOf(const T &value, int from = -1) const
int	length() const
QList<T>	mid(int pos, int length = -1) const
void	move(int from, int to)
void	pop_back()
void	pop_front()
void	prepend(const T &value)
void	push_back(const T &value)
void	push_front(const T &value)
QList::reverse_iterator	rbegin()
QList::const_reverse_iterator	rbegin() const
int	removeAll(const T &value)
void	removeAt(int i)
void	removeFirst()
void	removeLast()
bool	removeOne(const T &value)
QList::reverse_iterator	rend()
QList::const_reverse_iterator	rend() const
void	replace(int i, const T &value)
void	reserve(int alloc)
int	size() const
bool	startsWith(const T &value) const
void	swap(QList<T> &other)
void	swapItemsAt(int i, int j)
T	takeAt(int i)
T	takeFirst()
T	takeLast()
QSet<T>	toSet() const
std::list<T>	toStdList() const
QVector<T>	toVector() const
T	value(int i) const
T	value(int i, const T &defaultValue) const
bool	operator!=(const QList<T> &other) const
QList<T>	operator+(const QList<T> &other) const
QList<T> &	operator+=(const QList<T> &other)
QList<T> &	operator+=(const T &value)
QList<T> &	operator<<(const QList<T> &other)
QList<T> &	operator<<(const T &value)
bool	operator==(const QList<T> &other) const
T &	operator[](int i)
const T &	operator[](int i) const

Static Public Members

QList<T>	fromSet(const QSet<T> &set)
QList<T>	fromStdList(const std::list<T> &list)
QList<T>	fromVector(const QVector<T> &vector)

Related Non-Members

uint	qHash(const QList<T> &key, uint seed = 0)
bool	operator<(const QList<T> &lhs, const QList<T> &rhs)
QDataStream &	operator<<(QDataStream &out, const QList<T> &list)
bool	operator<=(const QList<T> &lhs, const QList<T> &rhs)
bool	operator>(const QList<T> &lhs, const QList<T> &rhs)
bool	operator>=(const QList<T> &lhs, const QList<T> &rhs)
QDataStream &	operator>>(QDataStream &in, QList<T> &list)

Detailed Description

QList<T> is one of Qt's generic container classes. It stores items in a list that provides fast index-based access and index-based insertions and removals.

QList<T>, QLinkedList<T>, and QVector<T> provide similar APIs and functionality. They are often interchangeable, but there are performance consequences. Here is an overview of use cases:

• QVector should be your default first choice. QVector<T> will usually give better performance than QList<T>, because QVector<T> always stores its items sequentially in memory, where QList<T> will allocate its items on the heap unless sizeof(T) <= sizeof(void*) and T has been declared to be either a Q_MOVABLE_TYPE or a Q_PRIMITIVE_TYPE using Q_DECLARE_TYPEINFO. See the Pros and Cons of Using QList for an explanation.
• However, QList is used throughout the Qt APIs for passing parameters and for returning values. Use QList to interface with those APIs.
• If you need a real linked list, which guarantees constant time insertions mid-list and uses iterators to items rather than indexes, use QLinkedList.

Note: QVector and QVarLengthArray both guarantee C-compatible array layout. QList does not. This might be important if your application must interface with a C API.

Note: Iterators into a QLinkedList and references into heap-allocating QLists remain valid as long as the referenced items remain in the container. This is not true for iterators and references into a QVector and non-heap-allocating QLists.

Internally, QList<T> is represented as an array of T if sizeof(T) <= sizeof(void*) and T has been declared to be either a Q_MOVABLE_TYPE or a Q_PRIMITIVE_TYPE using Q_DECLARE_TYPEINFO. Otherwise, QList<T> is represented as an array of T* and the items are allocated on the heap.

The array representation allows very fast insertions and index-based access. The prepend() and append() operations are also very fast because QList preallocates memory at both ends of its internal array. (See Algorithmic Complexity for details.

Note, however, that when the conditions specified above are not met, each append or insert of a new item requires allocating the new item on the heap, and this per item allocation will make QVector a better choice for use cases that do a lot of appending or inserting, because QVector can allocate memory for many items in a single heap allocation.

Note that the internal array only ever gets bigger over the life of the list. It never shrinks. The internal array is deallocated by the destructor and by the assignment operator, when one list is assigned to another.

Here's an example of a QList that stores integers and a QList that stores QDate values:

QList<int> integerList;
QList<QDate> dateList;

Qt includes a QStringList class that inherits QList<QString> and adds a few convenience functions, such as QStringList::join() and QStringList::filter(). QString::split() creates QStringLists from strings.

QList stores a list of items. The default constructor creates an empty list. You can use the initializer-list constructor to create a list with elements:

QList<QString> list = { "one", "two", "three" };

QList provides these basic functions to add, move, and remove items: insert(), replace(), removeAt(), move(), and swap(). In addition, it provides the following convenience functions: append(), operator<<(), operator+=(), prepend(), removeFirst(), and removeLast().

operator<<() allows to conveniently add multiple elements to a list:

list << "four" << "five";

QList uses 0-based indexes, just like C++ arrays. To access the item at a particular index position, you can use operator[](). On non-const lists, operator[]() returns a reference to the item and can be used on the left side of an assignment:

if (list[0] == "Bob")
    list[0] = "Robert";

Because QList is implemented as an array of pointers for types that are larger than a pointer or are not movable, this operation requires (constant time). For read-only access, an alternative syntax is to use at():

for (int i = 0; i < list.size(); ++i) {
    if (list.at(i) == "Jane")
        cout << "Found Jane at position " << i << Qt::endl;
}

at() can be faster than operator[](), because it never causes a deep copy to occur.

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

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

Inserting and removing items at either end of the list is very fast (constant time in most cases), because QList preallocates extra space on both sides of its internal buffer to allow for fast growth at both ends of the list.

If you want to find all occurrences of a particular value in a list, use indexOf() or lastIndexOf(). The former searches forward starting from a given index position, the latter searches backward. Both return the index of a matching item if they find it; otherwise, they return -1. For example:

int i = list.indexOf("Jane");
if (i != -1)
    cout << "First occurrence of Jane is at position " << i << Qt::endl;

If you simply want to check whether a list contains a particular value, use contains(). If you want to find out how many times a particular value occurs in the list, use count(). If you want to replace all occurrences of a particular value with another, use replace().

QList'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, indexOf() and lastIndexOf() expect the value type to support operator==(). These requirements are documented on a per-function basis.

Like the other container classes, QList provides Java-style iterators (QListIterator and QMutableListIterator) and STL-style iterators (QList::const_iterator and QList::iterator). In practice, these are rarely used, because you can use indexes into the QList. QList is implemented in such a way that direct index-based access is just as fast as using iterators.

QList does not support inserting, prepending, appending or replacing with references to its own values. Doing so will cause your application to abort with an error message.

To make QList as efficient as possible, its member functions don't validate their input before using it. Except for isEmpty(), member functions always assume the list is not empty. Member functions that take index values as parameters always assume their index value parameters are in the valid range. This means QList member functions can fail. If you define QT_NO_DEBUG when you compile, failures will not be detected. If you don't define QT_NO_DEBUG, failures will be detected using Q_ASSERT() or Q_ASSERT_X() with an appropriate message.

To avoid failures when your list can be empty, call isEmpty() before calling other member functions. If you must pass an index value that might not be in the valid range, check that it is less than the value returned by size() but not less than 0.

More Members

If T is a QByteArray, this class has a couple more members that can be used. See the documentation for QByteArrayList for more information.

If T is QString, this class has the following additional members: filter, join, removeDuplicates, sort.

More Information on Using Qt Containers

For a detailed discussion comparing Qt containers with each other and with STL containers, see Understand the Qt Containers.

See also QListIterator, QMutableListIterator, QLinkedList, and QVector.

Member Type Documentation

typedef QList::ConstIterator

Qt-style synonym for QList::const_iterator.

typedef QList::Iterator

Qt-style synonym for QList::iterator.

typedef QList::const_pointer

Typedef for const T *. Provided for STL compatibility.

typedef QList::const_reference

Typedef for const T &. Provided for STL compatibility.

typedef QList::const_reverse_iterator

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

It is simply a typedef for std::reverse_iterator<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.

This typedef was introduced in Qt 5.6.

See also QList::rbegin(), QList::rend(), QList::reverse_iterator, and QList::const_iterator.

typedef QList::difference_type

Typedef for ptrdiff_t. Provided for STL compatibility.

typedef QList::pointer

Typedef for T *. Provided for STL compatibility.

typedef QList::reference

Typedef for T &. Provided for STL compatibility.

typedef QList::reverse_iterator

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

It is simply a typedef for std::reverse_iterator<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.

This typedef was introduced in Qt 5.6.

See also QList::rbegin(), QList::rend(), QList::const_reverse_iterator, and QList::iterator.

typedef QList::size_type

Typedef for int. Provided for STL compatibility.

typedef QList::value_type

Typedef for T. Provided for STL compatibility.

Member Function Documentation

template <typename InputIterator> QList::QList(InputIterator first, InputIterator last)

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

The value type of InputIterator must be convertible to T.

This function was introduced in Qt 5.14.

QList::QList(std::initializer_list<T> args)

Construct a list from the std::initializer_list specified by args.

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

This function was introduced in Qt 4.8.

QList::QList(QList<T> &&other)

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

This function was introduced in Qt 5.2.

QList::QList(const QList<T> &other)

Constructs a copy of other.

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

See also operator=().

QList::QList()

Constructs an empty list.

QList<T> &QList::operator=(QList<T> &&other)

Move-assigns other to this QList instance.

This function was introduced in Qt 5.2.

QList<T> &QList::operator=(const QList<T> &other)

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

QList::~QList()

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

void QList::append(const T &value)

Inserts value at the end of the list.

Example:

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

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

If this list is not shared, this operation is typically very fast (amortized constant time), because QList preallocates extra space on both sides of its internal buffer to allow for fast growth at both ends of the list.

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

void QList::append(const QList<T> &value)

This is an overloaded function.

Appends the items of the value list to this list.

This function was introduced in Qt 4.5.

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

const T &QList::at(int i) const

Returns the item at index position i in the list. i must be a valid index position in the list (i.e., 0 <= i < size()).

This function is very fast (constant time).

See also value() and operator[]().

T &QList::back()

This function is provided for STL compatibility. It is equivalent to last(). The list must not be empty. If the list can be empty, call isEmpty() before calling this function.

const T &QList::back() const

This is an overloaded function.

QList::iterator QList::begin()

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

See also constBegin() and end().

QList::const_iterator QList::begin() const

This is an overloaded function.

QList::const_iterator QList::cbegin() const

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

This function was introduced in Qt 5.0.

See also begin() and cend().

QList::const_iterator QList::cend() const

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

This function was introduced in Qt 5.0.

See also cbegin() and end().

void QList::clear()

Removes all items from the list.

See also removeAll().

QList::const_iterator QList::constBegin() const

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

See also begin() and constEnd().

QList::const_iterator QList::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().

const T &QList::constFirst() const

Returns a const reference to the first item in the list. The list must not be empty. If the list can be empty, call isEmpty() before calling this function.

This function was introduced in Qt 5.6.

See also constLast(), isEmpty(), and first().

const T &QList::constLast() const

Returns a reference to the last item in the list. The list must not be empty. If the list can be empty, call isEmpty() before calling this function.

This function was introduced in Qt 5.6.

See also constFirst(), isEmpty(), and last().

bool QList::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 indexOf() and count().

int QList::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() and indexOf().

int QList::count() const

Returns the number of items in the list. This is effectively the same as size().

QList::const_reverse_iterator QList::crbegin() const

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

This function was introduced in Qt 5.6.

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

QList::const_reverse_iterator QList::crend() const

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

This function was introduced in Qt 5.6.

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

bool QList::empty() const

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

QList::iterator QList::end()

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

See also begin() and constEnd().

QList::const_iterator QList::end() const

This is an overloaded function.

bool QList::endsWith(const T &value) const

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

This function was introduced in Qt 4.5.

See also isEmpty() and contains().

QList::iterator QList::erase(QList::iterator pos)

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

See also insert() and removeAt().

QList::iterator QList::erase(QList::iterator begin, QList::iterator end)

This is an overloaded function.

Removes all the items from begin up to (but not including) end. Returns an iterator to the same item that end referred to before the call.

T &QList::first()

Returns a reference to the first item in the list. The list must not be empty. If the list can be empty, call isEmpty() before calling this function.

See also constFirst(), last(), and isEmpty().

const T &QList::first() const

This is an overloaded function.

[static] QList<T> QList::fromSet(const QSet<T> &set)

Returns a QList object with the data contained in set. The order of the elements in the QList is undefined.

Example:

QSet<int> set;
set << 20 << 30 << 40 << ... << 70;

QList<int> list = QList<int>::fromSet(set);
std::sort(list.begin(), list.end());

Note: Since Qt 5.14, range constructors are available for Qt's generic container classes and should be used in place of this method.

See also fromVector(), toSet(), and QSet::toList().

[static] QList<T> QList::fromStdList(const std::list<T> &list)

Returns a QList object with the data contained in list. The order of the elements in the QList 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);

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

Note: Since Qt 5.14, range constructors are available for Qt's generic container classes and should be used in place of this method.

See also toStdList() and QVector::fromStdVector().

[static] QList<T> QList::fromVector(const QVector<T> &vector)

Returns a QList object with the data contained in vector.

Example:

QVector<double> vect;
vect << 20.0 << 30.0 << 40.0 << 50.0;

QList<double> list = QVector<T>::fromVector(vect);
// list: [20.0, 30.0, 40.0, 50.0]

Note: Since Qt 5.14, range constructors are available for Qt's generic container classes and should be used in place of this method.

See also fromSet(), toVector(), and QVector::toList().

T &QList::front()

This function is provided for STL compatibility. It is equivalent to first(). The list must not be empty. If the list can be empty, call isEmpty() before calling this function.

const T &QList::front() const

This is an overloaded function.

int QList::indexOf(const T &value, int from = 0) const

Returns the index position of the first occurrence of value in the list, searching forward from index position from. Returns -1 if no item matched.

Example:

QList<QString> list;
list << "A" << "B" << "C" << "B" << "A";
list.indexOf("B");          // returns 1
list.indexOf("B", 1);       // returns 1
list.indexOf("B", 2);       // returns 3
list.indexOf("X");          // returns -1

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

Note that QList uses 0-based indexes, just like C++ arrays. Negative indexes are not supported with the exception of the value mentioned above.

See also lastIndexOf() and contains().

void QList::insert(int i, const T &value)

Inserts value at index position i in the list.

If i == 0, the value is prepended to the list. If i == size(), the value is appended to the list.

Example:

QList<QString> list;
list << "alpha" << "beta" << "delta";
list.insert(2, "gamma");
// list: ["alpha", "beta", "gamma", "delta"]

See also append(), prepend(), replace(), and removeAt().

QList::iterator QList::insert(QList::iterator before, const T &value)

This is an overloaded function.

Inserts value in front of the item pointed to by the iterator before. Returns an iterator pointing at the inserted item. Note that the iterator passed to the function will be invalid after the call; the returned iterator should be used instead.

bool QList::isEmpty() const

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

See also size().

T &QList::last()

Returns a reference to the last item in the list. The list must not be empty. If the list can be empty, call isEmpty() before calling this function.

See also constLast(), first(), and isEmpty().

const T &QList::last() const

This is an overloaded function.

int QList::lastIndexOf(const T &value, int from = -1) const

Returns the index position of the last occurrence of value in the list, searching backward from index position from. If from is -1 (the default), the search starts at the last item. Returns -1 if no item matched.

Example:

QList<QString> list;
list << "A" << "B" << "C" << "B" << "A";
list.lastIndexOf("B");      // returns 3
list.lastIndexOf("B", 3);   // returns 3
list.lastIndexOf("B", 2);   // returns 1
list.lastIndexOf("X");      // returns -1

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

Note that QList uses 0-based indexes, just like C++ arrays. Negative indexes are not supported with the exception of the value mentioned above.

See also indexOf().

int QList::length() const

This function is identical to count().

This function was introduced in Qt 4.5.

See also count().

QList<T> QList::mid(int pos, int length = -1) const

Returns a sub-list which includes elements from this list, starting at position pos. If length is -1 (the default), all elements from pos are included; otherwise length elements (or all remaining elements if there are less than length elements) are included.

void QList::move(int from, int to)

Moves the item at index position from to index position to.

Example:

QList<QString> list;
list << "A" << "B" << "C" << "D" << "E" << "F";
list.move(1, 4);
// list: ["A", "C", "D", "E", "B", "F"]

This is the same as insert(to, takeAt(from)).This function assumes that both from and to are at least 0 but less than size(). To avoid failure, test that both from and to are at least 0 and less than size().

See also swap(), insert(), and takeAt().

void QList::pop_back()

This function is provided for STL compatibility. It is equivalent to removeLast(). The list must not be empty. If the list can be empty, call isEmpty() before calling this function.

void QList::pop_front()

This function is provided for STL compatibility. It is equivalent to removeFirst(). The list must not be empty. If the list can be empty, call isEmpty() before calling this function.

void QList::prepend(const T &value)

Inserts value at the beginning of the list.

Example:

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

This is the same as list.insert(0, value).

If this list is not shared, this operation is typically very fast (amortized constant time), because QList preallocates extra space on both sides of its internal buffer to allow for fast growth at both ends of the list.

See also append() and insert().

void QList::push_back(const T &value)

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

void QList::push_front(const T &value)

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

QList::reverse_iterator QList::rbegin()

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

This function was introduced in Qt 5.6.

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

QList::const_reverse_iterator QList::rbegin() const

This is an overloaded function.

This function was introduced in Qt 5.6.

int QList::removeAll(const T &value)

Removes all occurrences of value in the list and returns the number of entries removed.

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 removeOne(), removeAt(), takeAt(), and replace().

void QList::removeAt(int i)

Removes the item at index position i. i must be a valid index position in the list (i.e., 0 <= i < size()).

See also takeAt(), removeFirst(), removeLast(), and removeOne().

void QList::removeFirst()

Removes the first item in the list. Calling this function is equivalent to calling removeAt(0). The list must not be empty. If the list can be empty, call isEmpty() before calling this function.

See also removeAt() and takeFirst().

void QList::removeLast()

Removes the last item in the list. Calling this function is equivalent to calling removeAt(size() - 1). The list must not be empty. If the list can be empty, call isEmpty() before calling this function.

See also removeAt() and takeLast().

bool QList::removeOne(const T &value)

Removes the first occurrence of value in the list and 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==().

This function was introduced in Qt 4.4.

See also removeAll(), removeAt(), takeAt(), and replace().

QList::reverse_iterator QList::rend()

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

This function was introduced in Qt 5.6.

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

QList::const_reverse_iterator QList::rend() const

This is an overloaded function.

This function was introduced in Qt 5.6.

void QList::replace(int i, const T &value)

Replaces the item at index position i with value. i must be a valid index position in the list (i.e., 0 <= i < size()).

See also operator[]() and removeAt().

void QList::reserve(int alloc)

Reserve space for alloc elements.

If alloc is smaller than the current size of the list, nothing will happen.

Use this function to avoid repetetive reallocation of QList's internal data if you can predict how many elements will be appended. Note that the reservation applies only to the internal pointer array.

This function was introduced in Qt 4.7.

int QList::size() const

Returns the number of items in the list.

See also isEmpty() and count().

bool QList::startsWith(const T &value) const

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

This function was introduced in Qt 4.5.

See also isEmpty() and contains().

void QList::swap(QList<T> &other)

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

This function was introduced in Qt 4.8.

void QList::swapItemsAt(int i, int j)

Exchange the item at index position i with the item at index position j. This function assumes that both i and j are at least 0 but less than size(). To avoid failure, test that both i and j are at least 0 and less than size().

Example:

QList<QString> list;
list << "A" << "B" << "C" << "D" << "E" << "F";
list.swapItemsAt(1, 4);
// list: ["A", "E", "C", "D", "B", "F"]

This function was introduced in Qt 5.13.

See also move().

T QList::takeAt(int i)

Removes the item at index position i and returns it. i must be a valid index position in the list (i.e., 0 <= i < size()).

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

See also removeAt(), takeFirst(), and takeLast().

T QList::takeFirst()

Removes the first item in the list and returns it. This is the same as takeAt(0). This function assumes the list is not empty. To avoid failure, call isEmpty() before calling this function.

If this list is not shared, this operation takes constant time.

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

See also takeLast(), takeAt(), and removeFirst().

T QList::takeLast()

Removes the last item in the list and returns it. This is the same as takeAt(size() - 1). This function assumes the list is not empty. To avoid failure, call isEmpty() before calling this function.

If this list is not shared, this operation takes constant time.

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

See also takeFirst(), takeAt(), and removeLast().

QSet<T> QList::toSet() const

Returns a QSet object with the data contained in this QList. Since QSet doesn't allow duplicates, the resulting QSet might be smaller than the original list was.

Example:

QStringList list;
list << "Julia" << "Mike" << "Mike" << "Julia" << "Julia";

QSet<QString> set = list.toSet();
set.contains("Julia");  // returns true
set.contains("Mike");   // returns true
set.size();             // returns 2

Note: Since Qt 5.14, range constructors are available for Qt's generic container classes and should be used in place of this method.

See also toVector(), fromSet(), and QSet::fromList().

std::list<T> QList::toStdList() const

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

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

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

Note: Since Qt 5.14, range constructors are available for Qt's generic container classes and should be used in place of this method.

See also fromStdList() and QVector::toStdVector().

QVector<T> QList::toVector() const

Returns a QVector object with the data contained in this QList.

Example:

QStringList list;
list << "Sven" << "Kim" << "Ola";

QVector<QString> vect = list.toVector();
// vect: ["Sven", "Kim", "Ola"]

Note: Since Qt 5.14, range constructors are available for Qt's generic container classes and should be used in place of this method.

See also toSet(), fromVector(), and QVector::fromList().

T QList::value(int i) const

Returns the value at index position i in the list.

If the index i is out of bounds, the function returns a default-constructed value. If you are certain that the index is going to be within bounds, you can use at() instead, which is slightly faster.

See also at() and operator[]().

T QList::value(int i, const T &defaultValue) const

This is an overloaded function.

If the index i is out of bounds, the function returns defaultValue.

bool QList::operator!=(const QList<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 have an implementation of operator==().

See also operator==().

QList<T> QList::operator+(const QList<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+=().

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

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

See also operator+() and append().

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

This is an overloaded function.

Appends value to the list.

See also append() and operator<<().

QList<T> &QList::operator<<(const QList<T> &other)

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

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

QList<T> &QList::operator<<(const T &value)

This is an overloaded function.

Appends value to the list.

bool QList::operator==(const QList<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 have an implementation of operator==().

See also operator!=().

T &QList::operator[](int i)

Returns the item at index position i as a modifiable reference. i must be a valid index position in the list (i.e., 0 <= i < size()).

If this function is called on a list that is currently being shared, it will trigger a copy of all elements. Otherwise, this function runs in constant time. If you do not want to modify the list you should use QList::at().

See also at() and value().

const T &QList::operator[](int i) const

This is an overloaded function.

Same as at(). This function runs in constant time.