path: root/src/corelib/tools/qvarlengtharray.qdoc

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/*!
    \class QVarLengthArray
    \brief The QVarLengthArray class provides a low-level variable-length array.

    \ingroup tools
    \reentrant

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

    \snippet doc/src/snippets/code/doc_src_qvarlengtharray.cpp 0

    The alternative is to allocate the array on the heap (with
    \c{new}):

    \snippet doc/src/snippets/code/doc_src_qvarlengtharray.cpp 1

    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:
    \snippet doc/src/snippets/code/doc_src_qvarlengtharray.cpp 2

    In the example above, QVarLengthArray will preallocate 1024
    elements on the stack and use them unless \c{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 \l{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:

    \list
    \o QVarLengthArray's API is much more low-level. It provides no
       iterators and lacks much of QVector's functionality.

    \o QVarLengthArray doesn't initialize the memory if the value is
       a basic type. (QVector always does.)

    \o QVector uses \l{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.
    \endlist

    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.

    \sa QVector, QList, QLinkedList
*/

/*! \fn QVarLengthArray::QVarLengthArray(int size)

    Constructs an array with an initial size of \a 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
    \l{default-constructed value}.
*/

/*! \fn QVarLengthArray::~QVarLengthArray()

    Destroys the array.
*/

/*! \fn int QVarLengthArray::size() const

    Returns the number of elements in the array.

    \sa isEmpty(), resize()
*/

/*! \fn int QVarLengthArray::count() const

    Same as size().

    \sa isEmpty(), resize()
*/

/*! \fn bool QVarLengthArray::isEmpty() const

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

    \sa size(), resize()
*/

/*! \fn void QVarLengthArray::clear()

    Removes all the elements from the array.

    Same as resize(0).
*/

/*! \fn void QVarLengthArray::resize(int size)

    Sets the size of the array to \a size. If \a size is greater than
    the current size, elements are added to the end. If \a 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
    \l{default-constructed value}.

    \sa size()
*/

/*! \fn 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().

    \sa reserve()
*/

/*! \fn void QVarLengthArray::reserve(int size)

    Attempts to allocate memory for at least \a 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 \a 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().

    \sa capacity()
*/

/*! \fn T &QVarLengthArray::operator[](int i)

    Returns a reference to the item at index position \a i.

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

    \sa data(), at()
*/

/*! \fn const T &QVarLengthArray::operator[](int i) const

    \overload
*/


/*!
    \fn void QVarLengthArray::append(const T &t)

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

    \sa removeLast()
*/


/*!
    \fn inline void QVarLengthArray::removeLast()
    \since 4.5

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

    \sa append()
*/

/*!
    \fn void QVarLengthArray::append(const T *buf, int size)

    Appends \a size amount of items referenced by \a buf to this array.
*/


/*! \fn 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:
    \snippet doc/src/snippets/code/doc_src_qvarlengtharray.cpp 3

    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.

    \sa constData(), operator[]()
*/

/*! \fn const T *QVarLengthArray::data() const

    \overload
*/

/*! \fn 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.

    \sa data(), operator[]()
*/

/*! \fn QVarLengthArray<T, Prealloc> &QVarLengthArray::operator=(const QVarLengthArray<T, Prealloc> &other)
    Assigns \a other to this array and returns a reference to this array.
 */

/*! \fn QVarLengthArray::QVarLengthArray(const QVarLengthArray<T, Prealloc> &other)
    Constructs a copy of \a other.
 */

/*! \fn const T &QVarLengthArray::at(int i) const

    Returns a reference to the item at index position \a i.

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

    \sa value(), operator[]()
*/

/*! \fn T QVarLengthArray::value(int i) const

    Returns the value at index position \a i.

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

    \sa at(), operator[]()
*/

/*! \fn T QVarLengthArray::value(int i, const T &defaultValue) const

    \overload

    If the index \a i is out of bounds, the function returns
    \a defaultValue.
*/

/*!
    \typedef QVarLengthArray::size_type
    \since 4.7

    Typedef for int. Provided for STL compatibility.
*/

/*!
    \typedef QVarLengthArray::value_type
    \since 4.7

    Typedef for T. Provided for STL compatibility.
*/

/*!
    \typedef QVarLengthArray::difference_type
    \since 4.7

    Typedef for ptrdiff_t. Provided for STL compatibility.
*/

/*!
    \typedef QVarLengthArray::pointer
    \since 4.7

    Typedef for T *. Provided for STL compatibility.
*/

/*!
    \typedef QVarLengthArray::const_pointer
    \since 4.7

    Typedef for const T *. Provided for STL compatibility.
*/

/*!
    \typedef QVarLengthArray::reference
    \since 4.7

    Typedef for T &. Provided for STL compatibility.
*/

/*!
    \typedef QVarLengthArray::const_reference
    \since 4.7

    Typedef for const T &. Provided for STL compatibility.
*/

/*! \fn void QVarLengthArray::prepend(const T &value)

    \since 4.8
    Inserts \a value at the beginning of the array.


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

    For large arrays, this operation can be slow (\l{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.

    \sa append(), insert()
*/

/*! \fn void QVarLengthArray::replace(int i, const T &value)

    \since 4.8
    Replaces the item at index position \a i with \a value.

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

    \sa operator[](), remove()
*/

/*! \fn void QVarLengthArray::remove(int i)

    \overload
    \since 4.8

    Removes the element at index position \a i.

    \sa insert(), replace()
*/

/*! \fn void QVarLengthArray::remove(int i, int count)

    \overload
    \since 4.8

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

    \sa insert(), replace()
*/

/*! \fn QVarLengthArray::iterator QVarLengthArray::begin()
    \since 4.8

    Returns an \l{STL-style iterator} pointing to the first item in
    the array.

    \sa constBegin(), end()
*/

/*! \fn QVarLengthArray::const_iterator QVarLengthArray::begin() const
    \since 4.8
    \overload
*/

/*! \fn QVarLengthArray::const_iterator QVarLengthArray::constBegin() const
    \since 4.8

    Returns a const \l{STL-style iterator} pointing to the first item
    in the array.

    \sa begin(), constEnd()
*/

/*! \fn QVarLengthArray::iterator QVarLengthArray::end()
    \since 4.8

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

    \sa begin(), constEnd()
*/

/*! \fn QVarLengthArray::const_iterator QVarLengthArray::end() const
    \since 4.8

    \overload
*/

/*! \fn QVarLengthArray::const_iterator QVarLengthArray::constEnd() const
    \since 4.8

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

    \sa constBegin(), end()
*/

/*! \fn QVarLengthArray::iterator QVarLengthArray::erase(iterator pos)
    \since 4.8

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

    \sa insert(), remove()
*/

/*! \fn QVarLengthArray::iterator QVarLengthArray::erase(iterator begin, iterator end)

    \overload
    \since 4.8

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

/*! \fn void QVarLengthArray::insert(int i, const T &value)
    \since 4.8

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

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

    \sa remove()
*/

/*! \fn void QVarLengthArray::insert(int i, int count, const T &value)

    \overload
    \since 4.8

    Inserts \a count copies of \a value at index position \a i in the
    vector.
*/

/*! \fn QVarLengthArray::iterator QVarLengthArray::insert(iterator before, const T &value)

    \overload
    \since 4.8

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

/*! \fn QVarLengthArray::iterator QVarLengthArray::insert(iterator before, int count, const T &value)

    \since 4.8
    Inserts \a count copies of \a value in front of the item pointed to
    by the iterator \a before. Returns an iterator pointing at the
    first of the inserted items.
*/



/*! \fn bool operator==(const QVarLengthArray<T, Prealloc1> &left, const QVarLengthArray<T, Prealloc2> &right)

    \relates QVarLengthArray
    \since 4.8
    Returns true if the two array 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 \c operator==().

    \sa operator!=()
*/

/*! \fn bool operator!=(const QVarLengthArray<T, Prealloc1> &left, const QVarLengthArray<T, Prealloc2> &right)

    \relates QVarLengthArray
    \since 4.8
    Returns true if the two array are different;

    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 \c operator==().

    \sa operator==()
*/

/*! \fn QVarLengthArray &QVarLengthArray::operator<<(const T &value)

    \since 4.8
    Appends \a value to the array and returns a reference to this
    vector.

    \sa append(), operator+=()
*/

/*! \fn QVarLengthArray &QVarLengthArray::operator+=(const T &value)

    \since 4.8
    Appends \a value to the array and returns a reference to this
    vector.

    \sa append(), operator<<()
*/