QVarLengthArray Class | Qt Core | Qt 6.11.0
Description: The QVarLengthArray class provides a low-level variable-length array.

- Qt 6.11
- Qt Core
- C++ Classes
- QVarLengthArray
- Qt 6.11.0 Reference Documentation

# QVarLengthArray Class

template <typename T, qsizetype Prealloc = QVarLengthArrayDefaultPrealloc> class QVarLengthArray

The QVarLengthArray class provides a low-level variable-length array. More...

Header: | `#include <QVarLengthArray>`
CMake: | `find_package(Qt6 REQUIRED COMPONENTS Core)` `target_link_libraries(mytarget PRIVATE Qt6::Core)`
qmake: | `QT += core`
Inherits: | QVLABase and QVLAStorage

- List of all members, including inherited members
- Deprecated members

Note: All functions in this class are reentrant.

## Public Types

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

## Public Functions

| QVarLengthArray()
| QVarLengthArray(qsizetype size)
| QVarLengthArray(std::initializer_list<T> args)
| QVarLengthArray(InputIterator first, InputIterator last)
`(since 6.4)` | QVarLengthArray(qsizetype size, const T &v)
| QVarLengthArray(const QVarLengthArray<T, Prealloc> &other)
`(since 6.0)` | QVarLengthArray(QVarLengthArray<T, Prealloc> &&other)
| ~QVarLengthArray()
void | append(const T &t)
void | append(const T *buf, qsizetype size)
void | append(T &&t)
`(since 6.6)` QVarLengthArray<T, Prealloc> & | assign(std::initializer_list<T> list)
`(since 6.6)` QVarLengthArray<T, Prealloc> & | assign(InputIterator first, InputIterator last)
`(since 6.6)` QVarLengthArray<T, Prealloc> & | assign(qsizetype n, const T &t)
const T & | at(qsizetype i) const
T & | back()
const T & | back() const
QVarLengthArray<T, Prealloc>::iterator | begin()
QVarLengthArray<T, Prealloc>::const_iterator | begin() const
qsizetype | capacity() const
QVarLengthArray<T, Prealloc>::const_iterator | cbegin() const
QVarLengthArray<T, Prealloc>::const_iterator | cend() const
void | clear()
QVarLengthArray<T, Prealloc>::const_iterator | constBegin() const
const T * | constData() const
QVarLengthArray<T, Prealloc>::const_iterator | constEnd() const
bool | contains(const AT &value) const
qsizetype | count() const
QVarLengthArray<T, Prealloc>::const_reverse_iterator | crbegin() const
QVarLengthArray<T, Prealloc>::const_reverse_iterator | crend() const
T * | data()
const T * | data() const
`(since 6.3)` QVarLengthArray<T, Prealloc>::iterator | emplace(QVarLengthArray<T, Prealloc>::const_iterator pos, Args &&... args)
`(since 6.3)` T & | emplace_back(Args &&... args)
bool | empty() const
QVarLengthArray<T, Prealloc>::iterator | end()
QVarLengthArray<T, Prealloc>::const_iterator | end() const
QVarLengthArray<T, Prealloc>::iterator | erase(QVarLengthArray<T, Prealloc>::const_iterator pos)
QVarLengthArray<T, Prealloc>::iterator | erase(QVarLengthArray<T, Prealloc>::const_iterator begin, QVarLengthArray<T, Prealloc>::const_iterator end)
T & | first()
const T & | first() const
T & | front()
const T & | front() const
qsizetype | indexOf(const AT &value, qsizetype from = 0) const
void | insert(qsizetype i, T &&value)
void | insert(qsizetype i, const T &value)
QVarLengthArray<T, Prealloc>::iterator | insert(QVarLengthArray<T, Prealloc>::const_iterator before, qsizetype count, const T &value)
QVarLengthArray<T, Prealloc>::iterator | insert(QVarLengthArray<T, Prealloc>::const_iterator before, T &&value)
QVarLengthArray<T, Prealloc>::iterator | insert(QVarLengthArray<T, Prealloc>::const_iterator before, const T &value)
void | insert(qsizetype i, qsizetype count, const T &value)
bool | isEmpty() const
T & | last()
const T & | last() const
qsizetype | lastIndexOf(const AT &value, qsizetype from = -1) const
qsizetype | length() const
`(since 6.8)` qsizetype | max_size() const
void | pop_back()
void | push_back(const T &t)
void | push_back(T &&t)
QVarLengthArray<T, Prealloc>::reverse_iterator | rbegin()
QVarLengthArray<T, Prealloc>::const_reverse_iterator | rbegin() const
void | remove(qsizetype i, qsizetype count = 1)
`(since 6.1)` qsizetype | removeAll(const AT &t)
`(since 6.1)` qsizetype | removeIf(Predicate pred)
void | removeLast()
`(since 6.1)` bool | removeOne(const AT &t)
QVarLengthArray<T, Prealloc>::reverse_iterator | rend()
QVarLengthArray<T, Prealloc>::const_reverse_iterator | rend() const
void | replace(qsizetype i, const T &value)
void | reserve(qsizetype size)
void | resize(qsizetype size)
`(since 6.4)` void | resize(qsizetype size, const T &v)
void | shrink_to_fit()
qsizetype | size() const
void | squeeze()
T | value(qsizetype i) const
T | value(qsizetype i, const T &defaultValue) const
QVarLengthArray<T, Prealloc> & | operator+=(const T &value)
QVarLengthArray<T, Prealloc> & | operator+=(T &&value)
QVarLengthArray<T, Prealloc> & | operator<<(const T &value)
QVarLengthArray<T, Prealloc> & | operator<<(T &&value)
`(since 6.0)` QVarLengthArray<T, Prealloc> & | operator=(QVarLengthArray<T, Prealloc> &&other)
QVarLengthArray<T, Prealloc> & | operator=(const QVarLengthArray<T, Prealloc> &other)
QVarLengthArray<T, Prealloc> & | operator=(std::initializer_list<T> list)
T & | operator[](qsizetype i)
const T & | operator[](qsizetype i) const

## Static Public Members

`(since 6.8)` qsizetype | maxSize()

## Related Non-Members

`(since 6.1)` qsizetype | erase(QVarLengthArray<T, Prealloc> &array, const AT &t)
`(since 6.1)` qsizetype | erase_if(QVarLengthArray<T, Prealloc> &array, Predicate pred)
size_t | qHash(const QVarLengthArray<T, Prealloc> &key, size_t seed = 0)
bool | operator!=(const QVarLengthArray<T, Prealloc1> &left, const QVarLengthArray<T, Prealloc2> &right)
bool | operator<(const QVarLengthArray<T, Prealloc1> &lhs, const QVarLengthArray<T, Prealloc2> &rhs)
bool | operator<=(const QVarLengthArray<T, Prealloc1> &lhs, const QVarLengthArray<T, Prealloc2> &rhs)
bool | operator==(const QVarLengthArray<T, Prealloc1> &left, const QVarLengthArray<T, Prealloc2> &right)
bool | operator>(const QVarLengthArray<T, Prealloc1> &lhs, const QVarLengthArray<T, Prealloc2> &rhs)
bool | operator>=(const QVarLengthArray<T, Prealloc1> &lhs, const QVarLengthArray<T, Prealloc2> &rhs)

## Detailed Description

The C++ language doesn't support variable-length arrays on the stack. For example, the following code won't compile:

     int myfunc_wrong(int n)
     {
         int table[n + 1];  // WRONG
         //...
         return table[n];
     }

The alternative is to allocate the array on the heap (with `new`):

     int myfunc_correct(int n)
     {
         int *table = new int[n + 1];
         //...
         int ret = table[n];
         delete[] table;
         return ret;
     }

However, if myfunc() is called very frequently from the application's inner loop, heap allocation can be a major source of slowdown.

QVarLengthArray is an attempt to work around this gap in the C++ language. It allocates a certain number of elements on the stack, and if you resize the array to a larger size, it automatically uses the heap instead. Stack allocation has the advantage that it is much faster than heap allocation.

Example:

     int myfunc_q(int n)
     {
         QVarLengthArray<int, 1024> array(n + 1);
         //...
         return array[n];
     }

In the example above, QVarLengthArray will preallocate 1024 elements on the stack and use them unless `n + 1` is greater than 1024. If you omit the second template argument, QVarLengthArray's default of 256 is used.

QVarLengthArray'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 *.

QVarLengthArray, like QList, provides a resizable array data structure. The main differences between the two classes are:

- QVarLengthArray's API is much more low-level and it lacks some of QList's functionality.
- QVarLengthArray doesn't initialize the memory if the value is a basic type. (QList always does.)
- QList uses implicit sharing as a memory optimization. QVarLengthArray doesn't provide that feature; however, it usually produces slightly better performance due to reduced overhead, especially in tight loops.

In summary, QVarLengthArray is a low-level optimization class that only makes sense in very specific cases. It is used a few places inside Qt and was added to Qt's public API for the convenience of advanced users.

See also QList.

## Member Type Documentation

### `[alias]` QVarLengthArray::const_iterator

Typedef for const T *. Provided for STL compatibility.

### `[alias]` QVarLengthArray::const_pointer

Typedef for const T *. Provided for STL compatibility.

### `[alias]` QVarLengthArray::const_reference

Typedef for const T &. Provided for STL compatibility.

### `[alias]` QVarLengthArray::const_reverse_iterator

Typedef for `std::reverse_iterator<const T*>`. Provided for STL compatibility.

### `[alias]` QVarLengthArray::difference_type

Typedef for ptrdiff_t. Provided for STL compatibility.

### `[alias]` QVarLengthArray::iterator

Typedef for T *. Provided for STL compatibility.

### `[alias]` QVarLengthArray::pointer

Typedef for T *. Provided for STL compatibility.

### `[alias]` QVarLengthArray::reference

Typedef for T &. Provided for STL compatibility.

### `[alias]` QVarLengthArray::reverse_iterator

Typedef for `std::reverse_iterator<T*>`. Provided for STL compatibility.

### `[alias]` QVarLengthArray::size_type

Typedef for int. Provided for STL compatibility.

### `[alias]` QVarLengthArray::value_type

Typedef for T. Provided for STL compatibility.

## Member Function Documentation

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

Constructs an array with an initial size of zero.

### `[explicit]` QVarLengthArray::QVarLengthArray(qsizetype size)

Constructs an array with an initial size of size elements.

If the value type is a primitive type (e.g., char, int, float) or a pointer type (e.g., QWidget *), the elements are not initialized. For other types, the elements are initialized with a default-constructed value.

### QVarLengthArray::QVarLengthArray(std::initializer_list<T> args)

Constructs an array from the std::initializer_list given by args.

### template <typename InputIterator, QVarLengthArray<T, Prealloc>::if_input_iterator<InputIterator> = true> QVarLengthArray::QVarLengthArray(InputIterator first, InputIterator last)

Constructs an array with the contents in the iterator range [first, last).

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

#### Constraints

Participates in overload resolution only if `InputIterator` meets the requirements of an LegacyInputIterator.

### `[explicit, since 6.4]` QVarLengthArray::QVarLengthArray(qsizetype size, const T &v)

Constructs an array with an initial size of size elements filled with copies of v.

Note: This constructor is only available when `T` is copy-constructible.

This function was introduced in Qt 6.4.

See also size() and squeeze().

### QVarLengthArray::QVarLengthArray(const QVarLengthArray<T, Prealloc> &other)

Constructs a copy of other.

### `[noexcept(...), since 6.0]` QVarLengthArray::QVarLengthArray(QVarLengthArray<T, Prealloc> &&other)

Move-constructs this variable-length array from other. After the move, other is empty.

This function was introduced in Qt 6.0.

Note: This function is noexcept when `std::is_nothrow_move_constructible_v<T>` is `true`.

### QVarLengthArray::~QVarLengthArray()

Destroys the array.

### void QVarLengthArray::append(const T &t)

Appends item t to the array, extending the array if necessary.

See also removeLast().

### void QVarLengthArray::append(const T *buf, qsizetype size)

Appends size amount of items referenced by buf to this array.

### void QVarLengthArray::append(T &&t)

Note: Unlike the lvalue overload of append(), passing a reference to an object that is already an element of `*this` leads to undefined behavior:

     vla.append(std::move(vla[0])); // BUG: passing an object that is already in the container

This function overloads QVarLengthArray::append.

### `[since 6.6]` QVarLengthArray<T, Prealloc> &QVarLengthArray::assign(std::initializer_list<T> list)

Replaces the contents of this container with a copy of the elements of list.

The size of this container will be equal to the number of elements in list.

This function only allocates memory if the number of elements in list exceeds the capacity of the container.

This function was introduced in Qt 6.6.

### `[since 6.6]` template <typename InputIterator, QVarLengthArray<T, Prealloc>::if_input_iterator<InputIterator> = true> QVarLengthArray<T, Prealloc> &QVarLengthArray::assign(InputIterator first, InputIterator last)

Replaces the contents of this container with a copy of the elements in the iterator range [first, last).

The size of this container will be equal to the number of elements in the range [first, last). This function will only allocate memory if the number of elements in the range exceeds the capacity of the container.

The behavior is undefined if either argument is an iterator into *this.

#### Constraints

Participates in overload resolution only if `InputIterator` meets the requirements of an LegacyInputIterator.

This function was introduced in Qt 6.6.

### `[since 6.6]` QVarLengthArray<T, Prealloc> &QVarLengthArray::assign(qsizetype n, const T &t)

Replaces the contents of this container with n copies of t.

The size of this container will be equal to n. This function will only allocate memory if n exceeds the capacity of the container.

This function was introduced in Qt 6.6.

### const T &QVarLengthArray::at(qsizetype i) const

Returns a reference to the item at index position i.

i must be a valid index position in the array (i.e., 0 <= i < size()).

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

### T &QVarLengthArray::back()

Same as last(). Provided for STL-compatibility.

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

This is an overloaded function.

### QVarLengthArray<T, Prealloc>::iterator QVarLengthArray::begin()

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

See also constBegin() and end().

### QVarLengthArray<T, Prealloc>::const_iterator QVarLengthArray::begin() const

This is an overloaded function.

### qsizetype QVarLengthArray::capacity() const

Returns the maximum number of elements that can be stored in the array without forcing a reallocation.

The sole purpose of this function is to provide a means of fine tuning QVarLengthArray's memory usage. In general, you will rarely ever need to call this function. If you want to know how many items are in the array, call size().

See also reserve() and squeeze().

### QVarLengthArray<T, Prealloc>::const_iterator QVarLengthArray::cbegin() const

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

See also begin() and cend().

### QVarLengthArray<T, Prealloc>::const_iterator QVarLengthArray::cend() const

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

See also cbegin() and end().

### void QVarLengthArray::clear()

Removes all the elements from the array.

Same as resize(0).

### QVarLengthArray<T, Prealloc>::const_iterator QVarLengthArray::constBegin() const

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

See also begin() and constEnd().

### const T *QVarLengthArray::constData() const

Returns a const pointer to the data stored in the array. The pointer can be used to access the items in the array. The pointer remains valid as long as the array isn't reallocated.

This function is mostly useful to pass an array to a function that accepts a plain C++ array.

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

### QVarLengthArray<T, Prealloc>::const_iterator QVarLengthArray::constEnd() const

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

See also constBegin() and end().

### template <typename AT = T> bool QVarLengthArray::contains(const AT &value) const

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

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

See also indexOf() and lastIndexOf().

### qsizetype QVarLengthArray::count() const

Same as size().

See also isEmpty() and resize().

### QVarLengthArray<T, Prealloc>::const_reverse_iterator QVarLengthArray::crbegin() const

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

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

### QVarLengthArray<T, Prealloc>::const_reverse_iterator QVarLengthArray::crend() const

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

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

### T *QVarLengthArray::data()

Returns a pointer to the data stored in the array. The pointer can be used to access and modify the items in the array.

Example:

     QVarLengthArray<int> array(10);
     int *data = array.data();
     for (int i = 0; i < 10; ++i)
         data[i] = 2 * i;

The pointer remains valid as long as the array isn't reallocated.

This function is mostly useful to pass an array to a function that accepts a plain C++ array.

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

### const T *QVarLengthArray::data() const

This is an overloaded function.

### `[since 6.3]` template <typename... Args> QVarLengthArray<T, Prealloc>::iterator QVarLengthArray::emplace(QVarLengthArray<T, Prealloc>::const_iterator pos, Args &&... args)

Inserts an item in front of the item pointed to by the iterator pos, passing args to its constructor.

Returns an iterator pointing at the emplaced item.

This function was introduced in Qt 6.3.

### `[since 6.3]` template <typename... Args> T &QVarLengthArray::emplace_back(Args &&... args)

Inserts an item at the back of this QVarLengthArray, passing args to its constructor.

Returns a reference to the emplaced item.

This function was introduced in Qt 6.3.

### bool QVarLengthArray::empty() const

Returns `true` if the array has size 0; otherwise returns `false`.

Same as isEmpty(). Provided for STL-compatibility.

### QVarLengthArray<T, Prealloc>::iterator QVarLengthArray::end()

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

See also begin() and constEnd().

### QVarLengthArray<T, Prealloc>::const_iterator QVarLengthArray::end() const

This is an overloaded function.

### QVarLengthArray<T, Prealloc>::iterator QVarLengthArray::erase(QVarLengthArray<T, Prealloc>::const_iterator pos)

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

See also insert() and remove().

### QVarLengthArray<T, Prealloc>::iterator QVarLengthArray::erase(QVarLengthArray<T, Prealloc>::const_iterator begin, QVarLengthArray<T, Prealloc>::const_iterator end)

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.

This is an overloaded function.

### T &QVarLengthArray::first()

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

See also last() and isEmpty().

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

This is an overloaded function.

### T &QVarLengthArray::front()

Same as first(). Provided for STL-compatibility.

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

This is an overloaded function.

### template <typename AT = T> qsizetype QVarLengthArray::indexOf(const AT &value, qsizetype from = 0) const

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

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

See also lastIndexOf() and contains().

### void QVarLengthArray::insert(qsizetype i, const T &value)

### void QVarLengthArray::insert(qsizetype i, T &&value)

Inserts value at index position i in the array. If i is 0, the value is prepended to the vector. If i is size(), the value is appended to the vector.

For large arrays, this operation can be slow (linear time), because it requires moving all the items at indexes i and above by one position further in memory. If you want a container class that provides a fast insert() function, use std::list instead.

See also remove().

### QVarLengthArray<T, Prealloc>::iterator QVarLengthArray::insert(QVarLengthArray<T, Prealloc>::const_iterator before, qsizetype count, const T &value)

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

### QVarLengthArray<T, Prealloc>::iterator QVarLengthArray::insert(QVarLengthArray<T, Prealloc>::const_iterator before, const T &value)

### QVarLengthArray<T, Prealloc>::iterator QVarLengthArray::insert(QVarLengthArray<T, Prealloc>::const_iterator before, T &&value)

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

### void QVarLengthArray::insert(qsizetype i, qsizetype count, const T &value)

Inserts count copies of value at index position i in the vector.

This is an overloaded function.

### bool QVarLengthArray::isEmpty() const

Returns `true` if the array has size 0; otherwise returns `false`.

See also size() and resize().

### T &QVarLengthArray::last()

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

See also first() and isEmpty().

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

This is an overloaded function.

### template <typename AT = T> qsizetype QVarLengthArray::lastIndexOf(const AT &value, qsizetype from = -1) const

Returns the index position of the last occurrence of the value value in the array, 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.

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

See also indexOf() and contains().

### qsizetype QVarLengthArray::length() const

Same as size().

See also isEmpty() and resize().

### `[constexpr noexcept, since 6.8]` qsizetype QVarLengthArray::max_size() const

### `[static constexpr noexcept, since 6.8]` qsizetype QVarLengthArray::maxSize()

It returns the maximum number of elements that the array can theoretically hold. In practice, the number can be much smaller, limited by the amount of memory available to the system.

These functions were introduced in Qt 6.8.

### void QVarLengthArray::pop_back()

Same as removeLast(). Provided for STL-compatibility.

### void QVarLengthArray::push_back(const T &t)

Appends item t to the array, extending the array if necessary. Provided for STL-compatibility.

### void QVarLengthArray::push_back(T &&t)

Note: Unlike the lvalue overload of push_back(), passing a reference to an object that is already an element of `*this` leads to undefined behavior:

     vla.push_back(std::move(vla[0])); // BUG: passing an object that is already in the container

This function overloads QVarLengthArray::push_back.

### QVarLengthArray<T, Prealloc>::reverse_iterator QVarLengthArray::rbegin()

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

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

### QVarLengthArray<T, Prealloc>::const_reverse_iterator QVarLengthArray::rbegin() const

This is an overloaded function.

### void QVarLengthArray::remove(qsizetype i, qsizetype count = 1)

Removes count elements from the middle of the array, starting at index position i.

See also insert() and replace().

### `[since 6.1]` template <typename AT = T> qsizetype QVarLengthArray::removeAll(const AT &t)

Removes all elements that compare equal to t from the array. Returns the number of elements removed, if any.

This function was introduced in Qt 6.1.

See also removeOne().

### `[since 6.1]` template <typename Predicate> qsizetype QVarLengthArray::removeIf(Predicate pred)

Removes all elements for which the predicate pred returns true from the array. Returns the number of elements removed, if any.

This function was introduced in Qt 6.1.

See also removeAll().

### void QVarLengthArray::removeLast()

Decreases the size of the array by one. The allocated size is not changed.

See also append().

### `[since 6.1]` template <typename AT = T> bool QVarLengthArray::removeOne(const AT &t)

Removes the first element that compares equal to t from the array. Returns whether an element was, in fact, removed.

This function was introduced in Qt 6.1.

See also removeAll().

### QVarLengthArray<T, Prealloc>::reverse_iterator QVarLengthArray::rend()

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

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

### QVarLengthArray<T, Prealloc>::const_reverse_iterator QVarLengthArray::rend() const

This is an overloaded function.

### void QVarLengthArray::replace(qsizetype i, const T &value)

Replaces the item at index position i with value.

i must be a valid index position in the array (i.e., 0 <= i < size()).

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

### void QVarLengthArray::reserve(qsizetype size)

Attempts to allocate memory for at least size elements. If you know in advance how large the array can get, you can call this function and if you call resize() often, you are likely to get better performance. If size is an underestimate, the worst that will happen is that the QVarLengthArray will be a bit slower.

The sole purpose of this function is to provide a means of fine tuning QVarLengthArray's memory usage. In general, you will rarely ever need to call this function. If you want to change the size of the array, call resize().

See also capacity() and squeeze().

### void QVarLengthArray::resize(qsizetype size)

Sets the size of the array to size. If size is greater than the current size, elements are added to the end. If size is less than the current size, elements are removed from the end.

If the value type is a primitive type (e.g., char, int, float) or a pointer type (e.g., QWidget *), new elements are not initialized. For other types, the elements are initialized with a default-constructed value.

See also size() and squeeze().

### `[since 6.4]` void QVarLengthArray::resize(qsizetype size, const T &v)

Sets the size of the array to size. If size is greater than the current size, copies of v are added to the end. If size is less than the current size, elements are removed from the end.

Note: This function is only available when `T` is copy-constructible.

This function was introduced in Qt 6.4.

See also size() and squeeze().

### void QVarLengthArray::shrink_to_fit()

Same as squeeze(). Provided for STL-compatibility.

### qsizetype QVarLengthArray::size() const

Returns the number of elements in the array.

See also isEmpty() and resize().

### void QVarLengthArray::squeeze()

Releases any memory not required to store the items. If the container can fit its storage on the stack allocation, it will free the heap allocation and copy the elements back to the stack.

The sole purpose of this function is to provide a means of fine tuning QVarLengthArray's memory usage. In general, you will rarely ever need to call this function.

See also reserve(), capacity(), and resize().

### T QVarLengthArray::value(qsizetype i) const

Returns the value at index position i.

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

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

### T QVarLengthArray::value(qsizetype i, const T &defaultValue) const

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

This is an overloaded function.

### QVarLengthArray<T, Prealloc> &QVarLengthArray::operator+=(const T &value)

Appends value to the array and returns a reference to this vector.

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

### QVarLengthArray<T, Prealloc> &QVarLengthArray::operator+=(T &&value)

This is an overloaded function.

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

### QVarLengthArray<T, Prealloc> &QVarLengthArray::operator<<(const T &value)

Appends value to the array and returns a reference to this vector.

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

### QVarLengthArray<T, Prealloc> &QVarLengthArray::operator<<(T &&value)

This is an overloaded function.

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

### `[noexcept(...), since 6.0]` QVarLengthArray<T, Prealloc> &QVarLengthArray::operator=(QVarLengthArray<T, Prealloc> &&other)

Move-assigns other to this array and returns a reference to this array. After the move, other is empty.

This function was introduced in Qt 6.0.

Note: This function is noexcept when `std::is_nothrow_move_constructible_v<T>` is `true`.

### QVarLengthArray<T, Prealloc> &QVarLengthArray::operator=(const QVarLengthArray<T, Prealloc> &other)

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

### QVarLengthArray<T, Prealloc> &QVarLengthArray::operator=(std::initializer_list<T> list)

Assigns the values of list to this array, and returns a reference to this array.

### T &QVarLengthArray::operator[](qsizetype i)

Returns a reference to the item at index position i.

i must be a valid index position in the array (i.e., 0 <= i < size()).

See also data() and at().

### const T &QVarLengthArray::operator[](qsizetype i) const

This is an overloaded function.

## Related Non-Members

### `[since 6.1]` template < typename T, qsizetype Prealloc, typename AT > qsizetype erase(QVarLengthArray<T, Prealloc> &array, const AT &t)

Removes all elements that compare equal to t from the array array. Returns the number of elements removed, if any.

Note: t is not allowed to be a reference to an element inside array. If you cannot be sure that this is not the case, take a copy of t and call this function with the copy.

This function was introduced in Qt 6.1.

See also erase_if().

### `[since 6.1]` template < typename T, qsizetype Prealloc, typename Predicate > qsizetype erase_if(QVarLengthArray<T, Prealloc> &array, Predicate pred)

Removes all elements for which the predicate pred returns true from the list array. Returns the number of elements removed, if any.

This function was introduced in Qt 6.1.

See also erase().

### `[noexcept(...)]` template <typename T, qsizetype Prealloc> size_t qHash(const QVarLengthArray<T, Prealloc> &key, size_t seed = 0)

Returns the hash value for key, using seed to seed the calculation.

Type `T` must be supported by qHash().

Note: This function is noexcept when `QtPrivate::QNothrowHashable_v<T>` is `true`.

### template < typename T, qsizetype Prealloc1, qsizetype Prealloc2 > bool operator!=(const QVarLengthArray<T, Prealloc1> &left, const QVarLengthArray<T, Prealloc2> &right)

Returns `true` if the two arrays, specified by left and right, are not equal.

Two arrays 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==().

### template < typename T, qsizetype Prealloc1, qsizetype Prealloc2 > bool operator<(const QVarLengthArray<T, Prealloc1> &lhs, const QVarLengthArray<T, Prealloc2> &rhs)

Returns `true` if variable length array lhs is lexicographically less than rhs; otherwise returns `false`.

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

### template < typename T, qsizetype Prealloc1, qsizetype Prealloc2 > bool operator<=(const QVarLengthArray<T, Prealloc1> &lhs, const QVarLengthArray<T, Prealloc2> &rhs)

Returns `true` if variable length array lhs is lexicographically less than or equal to rhs; otherwise returns `false`.

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

### template < typename T, qsizetype Prealloc1, qsizetype Prealloc2 > bool operator==(const QVarLengthArray<T, Prealloc1> &left, const QVarLengthArray<T, Prealloc2> &right)

Returns `true` if the two arrays, specified by left and right, are equal.

Two arrays 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!=().

### template < typename T, qsizetype Prealloc1, qsizetype Prealloc2 > bool operator>(const QVarLengthArray<T, Prealloc1> &lhs, const QVarLengthArray<T, Prealloc2> &rhs)

Returns `true` if variable length array lhs is lexicographically greater than rhs; otherwise returns `false`.

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

### template < typename T, qsizetype Prealloc1, qsizetype Prealloc2 > bool operator>=(const QVarLengthArray<T, Prealloc1> &lhs, const QVarLengthArray<T, Prealloc2> &rhs)

Returns `true` if variable length array lhs is lexicographically greater than or equal to rhs; otherwise returns `false`.

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