source: trunk/src/qt3support/tools/q3valuevector.qdoc

/****************************************************************************
**
** Copyright (C) 2011 Nokia Corporation and/or its subsidiary(-ies).
** All rights reserved.
** Contact: Nokia Corporation ([email protected])
**
** This file is part of the documentation of the Qt Toolkit.
**
** $QT_BEGIN_LICENSE:FDL$
** Commercial Usage
** Licensees holding valid Qt Commercial licenses may use this file in
** accordance with the Qt Commercial License Agreement provided with the
** Software or, alternatively, in accordance with the terms contained in a
** written agreement between you and Nokia.
**
** GNU Free Documentation License
** Alternatively, this file may be used under the terms of the GNU Free
** Documentation License version 1.3 as published by the Free Software
** Foundation and appearing in the file included in the packaging of this
** file.
**
** If you have questions regarding the use of this file, please contact
** Nokia at [email protected].
** $QT_END_LICENSE$
**
****************************************************************************/

/*!
    \class Q3ValueVector
    \brief The Q3ValueVector class is a value-based template class that provides a dynamic array.
    \compat

    Q3ValueVector is a Qt implementation of an STL-like vector
    container. It can be used in your application if the standard \c
    vector is not available for your target platforms.

    Q3ValueVector\<T\> defines a template instance to create a vector
    of values that all have the class T. Q3ValueVector does not store
    pointers to the members of the vector; it holds a copy of every
    member. Q3ValueVector is said to be value based; in contrast,
    Q3PtrList and Q3Dict are pointer based.

    Q3ValueVector contains and manages a collection of objects of type
    T and provides random access iterators that allow the contained
    objects to be addressed. Q3ValueVector owns the contained
    elements. For more relaxed ownership semantics, see Q3PtrCollection
    and friends, which are pointer-based containers.

    Q3ValueVector provides good performance if you append or remove
    elements from the end of the vector. If you insert or remove
    elements from anywhere but the end, performance is very bad. The
    reason for this is that elements must to be copied into new
    positions.

    Some classes cannot be used within a Q3ValueVector: for example,
    all classes derived from QObject and thus all classes that
    implement widgets. Only values can be used in a Q3ValueVector. To
    qualify as a value the class must provide:
    \list
    \i a copy constructor;
    \i an assignment operator;
    \i a default constructor, i.e., a constructor that does not take any arguments.
    \endlist

    Note that C++ defaults to field-by-field assignment operators and
    copy constructors if no explicit version is supplied. In many
    cases this is sufficient.

    Q3ValueVector uses an STL-like syntax to manipulate and address the
    objects it contains.

    Example:
    \snippet doc/src/snippets/code/doc_src_q3valuevector.qdoc 0

    Program output:
    \snippet doc/src/snippets/code/doc_src_q3valuevector.qdoc 1

    As you can see, the most recent change to Joe's salary did not
    affect the value in the vector because the vector created a copy
    of Joe's entry.

    Many Qt functions return const value vectors; to iterate over
    these you should make a copy and iterate over the copy.

    There are several ways to find items in the vector. The begin()
    and end() functions return iterators to the beginning and end of
    the vector. The advantage of getting an iterator is that you can
    move forward or backward from this position by
    incrementing/decrementing the iterator. The iterator returned by
    end() points to the element which is one past the last element in
    the container. The past-the-end iterator is still associated with
    the vector it belongs to, however it is \e not dereferenceable;
    operator*() will not return a well-defined value. If the vector is
    empty(), the iterator returned by begin() will equal the iterator
    returned by end().

    The fastest way to access an element of a vector is by using
    operator[]. This function provides random access and will return
    a reference to the element located at the specified index. Thus,
    you can access every element directly, in constant time, providing
    you know the location of the element. It is undefined to access
    an element that does not exist (your application will probably
    crash). For example:

    \snippet doc/src/snippets/code/doc_src_q3valuevector.qdoc 2

    Whenever inserting, removing or referencing elements in a vector,
    always make sure you are referring to valid positions. For
    example:

    \snippet doc/src/snippets/code/doc_src_q3valuevector.qdoc 3

    The iterators provided by vector are random access iterators,
    therefore you can use them with many generic algorithms, for
    example, algorithms provided by the STL.

    It is safe to have multiple iterators on the vector at the same
    time. Since Q3ValueVector manages memory dynamically, all iterators
    can become invalid if a memory reallocation occurs. For example,
    if some member of the vector is removed, iterators that point to
    the removed element and to all following elements become
    invalidated. Inserting into the middle of the vector will
    invalidate all iterators. For convenience, the function back()
    returns a reference to the last element in the vector, and front()
    returns a reference to the first element. If the vector is
    empty(), both back() and front() have undefined behavior (your
    application will crash or do unpredictable things). Use back() and
    front() with caution, for example:

    \snippet doc/src/snippets/code/doc_src_q3valuevector.qdoc 4

    Because Q3ValueVector manages memory dynamically, it is recommended
    that you contruct a vector with an initial size. Inserting and
    removing elements happens fastest when:
    \list
    \i Inserting or removing elements happens at the end() of the
    vector;
    \i The vector does not need to allocate additional memory.
    \endlist

    By creating a Q3ValueVector with a sufficiently large initial size,
    there will be less memory allocations. Do not use an initial size
    that is too big, since it will still take time to construct all
    the empty entries, and the extra space will be wasted if it is
    never used.