path: root/doc/src/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.qdoc 0

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

    \snippet doc/src/snippets/code/doc_src_qvarlengtharray.qdoc 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.qdoc 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()
*/

/*! \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.qdoc 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.
 */