- Qt 5.15
- Qt Core
- C++ Classes
- QVarLengthArray
- Qt 5.15.18 Reference Documentation

# QVarLengthArray Class

template <typename T, int Prealloc> class QVarLengthArray

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

Header: | #include <QVarLengthArray>
qmake: | QT += core

- List of all members, including inherited members

Note: All functions in this class are reentrant.

## Public Types

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

## Public Functions

| QVarLengthArray(InputIterator first, InputIterator last)
| QVarLengthArray(std::initializer_list<T> args)
| QVarLengthArray(const QVarLengthArray<T, Prealloc> &other = Prealloc)
| QVarLengthArray(int size)
| QVarLengthArray()
QVarLengthArray<T, Prealloc> & | operator=(const QVarLengthArray<T, Prealloc> &other = Prealloc)
| ~QVarLengthArray()
void | append(const T &t)
void | append(T &&t)
void | append(const T *buf, int size)
const T & | at(int i) const
T & | back()
const T & | back() const
iterator | begin()
const_iterator | begin() const
int | capacity() const
const_iterator | cbegin() const
const_iterator | cend() const
void | clear()
const_iterator | constBegin() const
const T * | constData() const
const_iterator | constEnd() const
bool | contains(const T &value) const
int | count() const
const_reverse_iterator | crbegin() const
const_reverse_iterator | crend() const
T * | data()
const T * | data() const
bool | empty() const
iterator | end()
const_iterator | end() const
iterator | erase(const_iterator pos)
iterator | erase(const_iterator begin, const_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, T &&value)
void | insert(int i, const T &value)
void | insert(int i, int count, const T &value)
iterator | insert(const_iterator before, int count, const T &value)
iterator | insert(const_iterator before, T &&value)
iterator | insert(const_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
void | pop_back()
void | prepend(T &&value)
void | prepend(const T &value)
void | push_back(const T &t)
void | push_back(T &&t)
reverse_iterator | rbegin()
const_reverse_iterator | rbegin() const
void | remove(int i)
void | remove(int i, int count)
void | removeLast()
reverse_iterator | rend()
const_reverse_iterator | rend() const
void | replace(int i, const T &value)
void | reserve(int size)
void | resize(int size)
void | shrink_to_fit()
int | size() const
void | squeeze()
T | value(int i) const
T | value(int 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)
QVarLengthArray<T, Prealloc> & | operator=(std::initializer_list<T> list)
T & | operator[](int i)
const T & | operator[](int i) const

## Related Non-Members

uint | qHash(const QVarLengthArray<T, Prealloc> &key = Prealloc, uint seed = 0)
bool | operator!=(const QVarLengthArray<T, Prealloc1> &left = Prealloc1, const QVarLengthArray<T, Prealloc2> &right = Prealloc2)
bool | operator<(const QVarLengthArray<T, Prealloc1> &lhs = Prealloc1, const QVarLengthArray<T, Prealloc2> &rhs = Prealloc2)
bool | operator<=(const QVarLengthArray<T, Prealloc1> &lhs = Prealloc1, const QVarLengthArray<T, Prealloc2> &rhs = Prealloc2)
bool | operator==(const QVarLengthArray<T, Prealloc1> &left = Prealloc1, const QVarLengthArray<T, Prealloc2> &right = Prealloc2)
bool | operator>(const QVarLengthArray<T, Prealloc1> &lhs = Prealloc1, const QVarLengthArray<T, Prealloc2> &rhs = Prealloc2)
bool | operator>=(const QVarLengthArray<T, Prealloc1> &lhs = Prealloc1, const QVarLengthArray<T, Prealloc2> &rhs = Prealloc2)

## Detailed Description

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

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

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

     int myfunc(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(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 QVector, 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 QVector's functionality.
- QVarLengthArray doesn't initialize the memory if the value is a basic type. (QVector always does.)
- QVector 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 QVector, QList, and QLinkedList.

## Member Type Documentation

### typedef QVarLengthArray::const_iterator

Typedef for const T *. Provided for STL compatibility.

This typedef was introduced in Qt 4.7.

### typedef QVarLengthArray::const_pointer

Typedef for const T *. Provided for STL compatibility.

This typedef was introduced in Qt 4.7.

### typedef QVarLengthArray::const_reference

Typedef for const T &. Provided for STL compatibility.

This typedef was introduced in Qt 4.7.

### typedef QVarLengthArray::const_reverse_iterator

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

This typedef was introduced in Qt 5.6.

### typedef QVarLengthArray::difference_type

Typedef for ptrdiff_t. Provided for STL compatibility.

This typedef was introduced in Qt 4.7.

### typedef QVarLengthArray::iterator

Typedef for T *. Provided for STL compatibility.

This typedef was introduced in Qt 4.7.

### typedef QVarLengthArray::pointer

Typedef for T *. Provided for STL compatibility.

This typedef was introduced in Qt 4.7.

### typedef QVarLengthArray::reference

Typedef for T &. Provided for STL compatibility.

This typedef was introduced in Qt 4.7.

### typedef QVarLengthArray::reverse_iterator

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

This typedef was introduced in Qt 5.6.

### typedef QVarLengthArray::size_type

Typedef for int. Provided for STL compatibility.

This typedef was introduced in Qt 4.7.

### typedef QVarLengthArray::value_type

Typedef for T. Provided for STL compatibility.

This typedef was introduced in Qt 4.7.

## Member Function Documentation

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

### iterator QVarLengthArray::insert(const_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.

This function was introduced in Qt 4.8.

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

### void QVarLengthArray::insert(int i, const 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 QLinkedList instead.

This function was introduced in Qt 4.8.

See also remove().

### void QVarLengthArray::prepend(T &&value)

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

Inserts value at the beginning of the array.

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

For large arrays, this operation can be slow (linear time), because it requires moving all the items in the vector by one position further in memory. If you want a container class that provides a fast prepend() function, use QList or QLinkedList instead.

This function was introduced in Qt 4.8.

See also append() and insert().

### template <typename InputIterator> 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`.

This function was introduced in Qt 5.14.

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

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

This function was introduced in Qt 5.5.

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

Constructs a copy of other.

### QVarLengthArray::QVarLengthArray(int 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()

Constructs an array with an initial size of zero.

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

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

### 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(T &&t)

This function overloads append.

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 was introduced in Qt 5.9.

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

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

### const T &QVarLengthArray::at(int 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.

This function was introduced in Qt 5.0.

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

This is an overloaded function.

This function was introduced in Qt 5.0.

### iterator QVarLengthArray::begin()

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

This function was introduced in Qt 4.8.

See also constBegin() and end().

### const_iterator QVarLengthArray::begin() const

This is an overloaded function.

This function was introduced in Qt 4.8.

### int 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().

### const_iterator QVarLengthArray::cbegin() const

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

This function was introduced in Qt 5.0.

See also begin() and cend().

### const_iterator QVarLengthArray::cend() const

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

This function was introduced in Qt 5.0.

See also cbegin() and end().

### void QVarLengthArray::clear()

Removes all the elements from the array.

Same as resize(0).

### const_iterator QVarLengthArray::constBegin() const

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

This function was introduced in Qt 4.8.

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[]().

### const_iterator QVarLengthArray::constEnd() const

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

This function was introduced in Qt 4.8.

See also constBegin() and end().

### bool QVarLengthArray::contains(const T &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==()`.

This function was introduced in Qt 5.3.

See also indexOf() and lastIndexOf().

### int QVarLengthArray::count() const

Same as size().

See also isEmpty() and resize().

### 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.

This function was introduced in Qt 5.6.

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

### 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.

This function was introduced in Qt 5.6.

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.

### bool QVarLengthArray::empty() const

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

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

This function was introduced in Qt 5.0.

### iterator QVarLengthArray::end()

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

This function was introduced in Qt 4.8.

See also begin() and constEnd().

### const_iterator QVarLengthArray::end() const

This is an overloaded function.

This function was introduced in Qt 4.8.

### iterator QVarLengthArray::erase(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()).

This function was introduced in Qt 4.8.

See also insert() and remove().

### iterator QVarLengthArray::erase(const_iterator begin, 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.

This function was introduced in Qt 4.8.

### 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.

This function was introduced in Qt 5.0.

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

This is an overloaded function.

This function was introduced in Qt 5.0.

### int QVarLengthArray::indexOf(const T &value, int 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==()`.

This function was introduced in Qt 5.3.

See also lastIndexOf() and contains().

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

This is an overloaded function.

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

This function was introduced in Qt 4.8.

### iterator QVarLengthArray::insert(const_iterator before, int 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.

This function was introduced in Qt 4.8.

### 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.

### int QVarLengthArray::lastIndexOf(const T &value, int 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==()`.

This function was introduced in Qt 5.3.

See also indexOf() and contains().

### int QVarLengthArray::length() const

Same as size().

This function was introduced in Qt 5.0.

See also isEmpty() and resize().

### void QVarLengthArray::pop_back()

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

This function was introduced in Qt 5.0.

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

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

This function was introduced in Qt 5.0.

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

This function overloads push_back.

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 was introduced in Qt 5.9.

### reverse_iterator QVarLengthArray::rbegin()

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

This function was introduced in Qt 5.6.

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

### const_reverse_iterator QVarLengthArray::rbegin() const

This is an overloaded function.

This function was introduced in Qt 5.6.

### void QVarLengthArray::remove(int i)

This is an overloaded function.

Removes the element at index position i.

This function was introduced in Qt 4.8.

See also insert() and replace().

### void QVarLengthArray::remove(int i, int count)

This is an overloaded function.

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

This function was introduced in Qt 4.8.

See also insert() and replace().

### void QVarLengthArray::removeLast()

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

This function was introduced in Qt 4.5.

See also append().

### 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.

This function was introduced in Qt 5.6.

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

### const_reverse_iterator QVarLengthArray::rend() const

This is an overloaded function.

This function was introduced in Qt 5.6.

### void QVarLengthArray::replace(int 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()).

This function was introduced in Qt 4.8.

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

### void QVarLengthArray::reserve(int 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(int 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().

### void QVarLengthArray::shrink_to_fit()

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

This function was introduced in Qt 5.10.

### int 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.

This function was introduced in Qt 5.1.

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

### T QVarLengthArray::value(int 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(int 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.

This function was introduced in Qt 4.8.

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

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

This is an overloaded function.

This function was introduced in Qt 5.11.

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.

This function was introduced in Qt 4.8.

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

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

This is an overloaded function.

This function was introduced in Qt 5.11.

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

### 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.

This function was introduced in Qt 5.5.

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

This is an overloaded function.

## Related Non-Members

### template <typename T, int Prealloc> uint qHash(const QVarLengthArray<T, Prealloc> &key = Prealloc, uint seed = 0)

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

This function was introduced in Qt 5.14.

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

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==()`.

This function was introduced in Qt 4.8.

See also operator==().

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

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<()`.

This function was introduced in Qt 5.6.

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

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<()`.

This function was introduced in Qt 5.6.

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

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==()`.

This function was introduced in Qt 4.8.

See also operator!=().

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

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<()`.

This function was introduced in Qt 5.6.

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

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<()`.

This function was introduced in Qt 5.6.