QVarLengthArray Class

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

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

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

Member Function Documentation

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.

This function was introduced in Qt 6.8.

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().

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.

[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).

This constructor only participates in overload resolution if InputIterator meets the requirements of an LegacyInputIterator.

The value type of InputIterator must be convertible to T.

[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.

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.

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)

This function overloads append.

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

[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.

This function overload only participates in overload resolution if InputIterator meets the requirements of an LegacyInputIterator.

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

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)

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 &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().

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.

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

This is an overloaded function.

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

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().

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)

This function overloads push_back.

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

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)

This is an overloaded function.

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.

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

This is an overloaded function.

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

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.

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.