source: trunk/src/corelib/tools/qvarlengtharray.qdoc@ 651

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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