QVarLengthArray Class

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

All functions in this class are reentrant.

Article lu fois.

L'auteur

The Qt Company Ltd.

L'article

Publié le 27 décembre 2024 - Mis à jour le 27 décembre 2024

Liens sociaux

QVarLengthArray Class▲

Header: QVarLengthArray
qmake: QT += core
Group: QVarLengthArray is part of tools

Detailed Description▲

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

Sélectionnez

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

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

Sélectionnez

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:

Sélectionnez

int myfunc(int n)
{
    QVarLengthArray&lt;int, 1024&gt; 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.

Member Type Documentation▲

[since 4.7] QVarLengthArray::const_iterator▲

Typedef for const T *. Provided for STL compatibility.

This typedef was introduced in Qt 4.7.

[since 4.7] QVarLengthArray::const_pointer▲

Typedef for const T *. Provided for STL compatibility.

This typedef was introduced in Qt 4.7.

[since 4.7] QVarLengthArray::const_reference▲

Typedef for const T &. Provided for STL compatibility.

This typedef was introduced in Qt 4.7.

[since 5.6] QVarLengthArray::const_reverse_iterator▲

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

This typedef was introduced in Qt 5.6.

[since 4.7] QVarLengthArray::difference_type▲

Typedef for ptrdiff_t. Provided for STL compatibility.

This typedef was introduced in Qt 4.7.

[since 4.7] QVarLengthArray::iterator▲

Typedef for T *. Provided for STL compatibility.

This typedef was introduced in Qt 4.7.

[since 4.7] QVarLengthArray::pointer▲

Typedef for T *. Provided for STL compatibility.

This typedef was introduced in Qt 4.7.

[since 4.7] QVarLengthArray::reference▲

Typedef for T &. Provided for STL compatibility.

This typedef was introduced in Qt 4.7.

[since 5.6] QVarLengthArray::reverse_iterator▲

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

This typedef was introduced in Qt 5.6.

[since 4.7] QVarLengthArray::size_type▲

Typedef for int. Provided for STL compatibility.

This typedef was introduced in Qt 4.7.

[since 4.7] QVarLengthArray::value_type▲

Typedef for T. Provided for STL compatibility.

This typedef was introduced in Qt 4.7.

Member Function Documentation▲

[since 4.8] void QVarLengthArray::prepend(T &&value)▲

[since 4.8] 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.

[since 4.8] void QVarLengthArray::insert(int i, T &&value)▲

[since 4.8] 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.

[since 4.8] QVarLengthArray::iterator QVarLengthArray::insert(QVarLengthArray::const_iterator before, T &&value)▲

[since 4.8] QVarLengthArray::iterator QVarLengthArray::insert(QVarLengthArray::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.

[explicit] QVarLengthArray::QVarLengthArray(int size = 0)▲

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

Constructs a copy of other.

[since 5.5] QVarLengthArray::QVarLengthArray(std::initializer_list<T> args)▲

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

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

This function was introduced in Qt 5.5.

QVarLengthArray::~QVarLengthArray()▲

Destroys the array.

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

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

[since 5.9] void QVarLengthArray::append(T &&t)▲

This function overloads append.

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

Sélectionnez

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()).

[since 5.0] T &QVarLengthArray::back()▲

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

This function was introduced in Qt 5.0.

[since 5.0] const T &QVarLengthArray::back() const▲

This is an overloaded function.

This function was introduced in Qt 5.0.

[since 4.8] QVarLengthArray::iterator QVarLengthArray::begin()▲

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

This function was introduced in Qt 4.8.

[since 4.8] QVarLengthArray::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().

[since 5.0] QVarLengthArray::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.

[since 5.0] QVarLengthArray::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.

void QVarLengthArray::clear()▲

Removes all the elements from the array.

Same as resize(0).

[since 4.8] QVarLengthArray::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.

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.

[since 4.8] QVarLengthArray::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.

[since 5.3] 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.

int QVarLengthArray::count() const▲

Same as size().

[since 5.6] QVarLengthArray::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.

[since 5.6] QVarLengthArray::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.

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:

Sélectionnez

QVarLengthArray&lt;int&gt; array(10);
int *data = array.data();
for (int i = 0; i &lt; 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.

const T *QVarLengthArray::data() const▲

This is an overloaded function.

[since 5.0] 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.

[since 4.8] QVarLengthArray::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.

[since 4.8] QVarLengthArray::const_iterator QVarLengthArray::end() const▲

This is an overloaded function.

This function was introduced in Qt 4.8.

[since 4.8] QVarLengthArray::iterator QVarLengthArray::erase(QVarLengthArray::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.

[since 4.8] QVarLengthArray::iterator QVarLengthArray::erase(QVarLengthArray::const_iterator begin, QVarLengthArray::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.

const T &QVarLengthArray::first() const▲

This is an overloaded function.

[since 5.0] T &QVarLengthArray::front()▲

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

This function was introduced in Qt 5.0.

[since 5.0] const T &QVarLengthArray::front() const▲

This is an overloaded function.

This function was introduced in Qt 5.0.

[since 5.3] 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.

[since 4.8] 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.

[since 4.8] QVarLengthArray::iterator QVarLengthArray::insert(QVarLengthArray::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.

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.

const T &QVarLengthArray::last() const▲

This is an overloaded function.

[since 5.3] 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.

[since 5.0] int QVarLengthArray::length() const▲

Same as size().

This function was introduced in Qt 5.0.

[since 5.0] void QVarLengthArray::pop_back()▲

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

This function was introduced in Qt 5.0.

[since 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.

[since 5.9] void QVarLengthArray::push_back(T &&t)▲

This function overloads push_back.

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

Sélectionnez

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.

[since 5.6] QVarLengthArray::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.

[since 5.6] QVarLengthArray::const_reverse_iterator QVarLengthArray::rbegin() const▲

This is an overloaded function.

This function was introduced in Qt 5.6.

[since 4.8] 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.

[since 4.8] 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.

[since 4.5] 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.

[since 5.6] QVarLengthArray::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.

[since 5.6] QVarLengthArray::const_reverse_iterator QVarLengthArray::rend() const▲

This is an overloaded function.

This function was introduced in Qt 5.6.

[since 4.8] 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.

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().

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.

[since 5.10] 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.

[since 5.1] 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.

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.

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.

[since 4.8] 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.

[since 5.11] QVarLengthArray<T, Prealloc> &QVarLengthArray::operator+=(T &&value)▲

This is an overloaded function.

This function was introduced in Qt 5.11.

[since 4.8] 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.

[since 5.11] QVarLengthArray<T, Prealloc> &QVarLengthArray::operator<<(T &&value)▲

This is an overloaded function.

This function was introduced in Qt 5.11.

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

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

[since 5.5] 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 constructor is only enabled if the compiler supports C++11 initializer lists.

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()).

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

This is an overloaded function.

Related Non-Members▲

[since 4.8] 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.

[since 5.6] 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.

[since 4.8] 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.

QVarLengthArray Class

L'auteur

L'article

Liens sociaux

QVarLengthArray Class▲

Detailed Description▲

See Also▲

Member Type Documentation▲

[since 4.7] QVarLengthArray::const_iterator▲

[since 4.7] QVarLengthArray::const_pointer▲

[since 4.7] QVarLengthArray::const_reference▲

[since 5.6] QVarLengthArray::const_reverse_iterator▲

[since 4.7] QVarLengthArray::difference_type▲

[since 4.7] QVarLengthArray::iterator▲

[since 4.7] QVarLengthArray::pointer▲

[since 4.7] QVarLengthArray::reference▲

[since 5.6] QVarLengthArray::reverse_iterator▲

[since 4.7] QVarLengthArray::size_type▲

[since 4.7] QVarLengthArray::value_type▲

Member Function Documentation▲

[since 4.8] void QVarLengthArray::prepend(T &&value)▲

See Also▲

[since 4.8] void QVarLengthArray::insert(int i, T &&value)▲

See Also▲

[since 4.8] QVarLengthArray::iterator QVarLengthArray::insert(QVarLengthArray::const_iterator before, T &&value)▲

[explicit] QVarLengthArray::QVarLengthArray(int size = 0)▲

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

[since 5.5] QVarLengthArray::QVarLengthArray(std::initializer_list<T> args)▲

QVarLengthArray::~QVarLengthArray()▲

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

See Also▲

[since 5.9] void QVarLengthArray::append(T &&t)▲

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

const T &QVarLengthArray::at(int i) const▲

See Also▲

[since 5.0] T &QVarLengthArray::back()▲

[since 5.0] const T &QVarLengthArray::back() const▲

[since 4.8] QVarLengthArray::iterator QVarLengthArray::begin()▲

See Also▲

[since 4.8] QVarLengthArray::const_iterator QVarLengthArray::begin() const▲

int QVarLengthArray::capacity() const▲

See Also▲

[since 5.0] QVarLengthArray::const_iterator QVarLengthArray::cbegin() const▲

See Also▲

[since 5.0] QVarLengthArray::const_iterator QVarLengthArray::cend() const▲

See Also▲

void QVarLengthArray::clear()▲

[since 4.8] QVarLengthArray::const_iterator QVarLengthArray::constBegin() const▲

See Also▲

const T *QVarLengthArray::constData() const▲

See Also▲

[since 4.8] QVarLengthArray::const_iterator QVarLengthArray::constEnd() const▲

See Also▲

[since 5.3] bool QVarLengthArray::contains(const T &value) const▲

See Also▲

int QVarLengthArray::count() const▲

See Also▲

[since 5.6] QVarLengthArray::const_reverse_iterator QVarLengthArray::crbegin() const▲

See Also▲

[since 5.6] QVarLengthArray::const_reverse_iterator QVarLengthArray::crend() const▲

See Also▲

T *QVarLengthArray::data()▲

See Also▲

const T *QVarLengthArray::data() const▲

[since 5.0] bool QVarLengthArray::empty() const▲

[since 4.8] QVarLengthArray::iterator QVarLengthArray::end()▲

See Also▲

[since 4.8] QVarLengthArray::const_iterator QVarLengthArray::end() const▲

[since 4.8] QVarLengthArray::iterator QVarLengthArray::erase(QVarLengthArray::const_iterator pos)▲

See Also▲

[since 4.8] QVarLengthArray::iterator QVarLengthArray::erase(QVarLengthArray::const_iterator begin, QVarLengthArray::const_iterator end)▲

T &QVarLengthArray::first()▲

See Also▲

const T &QVarLengthArray::first() const▲

[since 5.0] T &QVarLengthArray::front()▲

[since 5.0] const T &QVarLengthArray::front() const▲

[since 5.3] int QVarLengthArray::indexOf(const T &value, int from = 0) const▲

See Also▲

[since 4.8] void QVarLengthArray::insert(int i, int count, const T &value)▲

[since 4.8] QVarLengthArray::iterator QVarLengthArray::insert(QVarLengthArray::const_iterator before, int count, const T &value)▲