Make STL iterators the preferred style

Emphasize STL iterators over Java style iterators.

Details about Java style iterators moved to it's own page and only
briefly mentioned on the containers page.

Task-number: QTBUG-86584
Change-Id: Id54863f79f90214aaae87e60cee1a66f53a044ab
Reviewed-by: Paul Wicking <paul.wicking@qt.io>

2 files changed, 212 insertions, 178 deletions

diff --git a/src/corelib/doc/src/containers.qdoc b/src/corelib/doc/src/containers.qdoc
index 5b9b700669..70c22b1407 100644
--- a/src/corelib/doc/src/containers.qdoc
+++ b/src/corelib/doc/src/containers.qdoc
@@ -56,12 +56,10 @@
     in situations where they are used as read-only containers
     by all threads used to access them.
 
-    For traversing the items stored in a container, you can use one
-    of two types of iterators: \l{Java-style iterators} and
-    \l{STL-style iterators}. The Java-style iterators are easier to
-    use and provide high-level functionality, whereas the STL-style
-    iterators are slightly more efficient and can be used together
-    with Qt's and STL's \l{generic algorithms}.
+    The containers provide iterators for traversal. \l{STL-style iterators}
+    are the most efficient ones and can be used together with Qt's and
+    STL's \l{generic algorithms}.
+    \l{Java-style Iterators} are provided for backwards compatibility.
 
     Qt also offers a \l{foreach} keyword that make it very
     easy to iterate over all the items stored in a container.
@@ -223,178 +221,12 @@
     \section1 The Iterator Classes
 
     Iterators provide a uniform means to access items in a container.
-    Qt's container classes provide two types of iterators: Java-style
-    iterators and STL-style iterators. Iterators of both types are
+    Qt's container classes provide two types of iterators: STL-style
+    iterators and Java-style iterators. Iterators of both types are
     invalidated when the data in the container is modified or detached
     from \l{Implicit Sharing}{implicitly shared copies} due to a call
     to a non-const member function.
 
-    \section2 Java-Style Iterators
-
-    The Java-style iterators are new in Qt 4 and are the standard
-    ones used in Qt applications. They are more convenient to use than
-    the STL-style iterators, at the price of being slightly less
-    efficient. Their API is modelled on Java's iterator classes.
-
-    For each container class, there are two Java-style iterator data
-    types: one that provides read-only access and one that provides
-    read-write access.
-
-    \table
-    \header \li Containers                        \li Read-only iterator
-                                                  \li Read-write iterator
-                                                  \li QMutableListIterator<T>
-    \row    \li QList<T>, QQueue<T>, QStack<T>,   \li QListIterator<T>
-                                                  \li QMutableListIterator<T>
-    \row    \li QSet<T>                           \li QSetIterator<T>
-                                                  \li QMutableSetIterator<T>
-    \row    \li QMap<Key, T>, QMultiMap<Key, T>   \li QMapIterator<Key, T>
-                                                  \li QMutableMapIterator<Key, T>
-    \row    \li QHash<Key, T>, QMultiHash<Key, T> \li QHashIterator<Key, T>
-                                                  \li QMutableHashIterator<Key, T>
-    \endtable
-
-    In this discussion, we will concentrate on QList and QMap. The
-    iterator types for QSet have exactly
-    the same interface as QList's iterators; similarly, the iterator
-    types for QHash have the same interface as QMap's iterators.
-
-    Unlike STL-style iterators (covered \l{STL-style
-    iterators}{below}), Java-style iterators point \e between items
-    rather than directly \e at items. For this reason, they are
-    either pointing to the very beginning of the container (before
-    the first item), at the very end of the container (after the last
-    item), or between two items. The diagram below shows the valid
-    iterator positions as red arrows for a list containing four
-    items:
-
-    \image javaiterators1.png
-
-    Here's a typical loop for iterating through all the elements of a
-    QList<QString> in order and printing them to the console:
-
-    \snippet code/doc_src_containers.cpp 1
-
-    It works as follows: The QList to iterate over is passed to the
-    QListIterator constructor. At that point, the iterator is located
-    just in front of the first item in the list (before item "A").
-    Then we call \l{QListIterator::hasNext()}{hasNext()} to
-    check whether there is an item after the iterator. If there is, we
-    call \l{QListIterator::next()}{next()} to jump over that
-    item. The next() function returns the item that it jumps over. For
-    a QList<QString>, that item is of type QString.
-
-    Here's how to iterate backward in a QList:
-
-    \snippet code/doc_src_containers.cpp 2
-
-    The code is symmetric with iterating forward, except that we
-    start by calling \l{QListIterator::toBack()}{toBack()}
-    to move the iterator after the last item in the list.
-
-    The diagram below illustrates the effect of calling
-    \l{QListIterator::next()}{next()} and
-    \l{QListIterator::previous()}{previous()} on an iterator:
-
-    \image javaiterators2.png
-
-    The following table summarizes the QListIterator API:
-
-    \table
-    \header \li Function \li Behavior
-    \row    \li \l{QListIterator::toFront()}{toFront()}
-            \li Moves the iterator to the front of the list (before the first item)
-    \row    \li \l{QListIterator::toBack()}{toBack()}
-            \li Moves the iterator to the back of the list (after the last item)
-    \row    \li \l{QListIterator::hasNext()}{hasNext()}
-            \li Returns \c true if the iterator isn't at the back of the list
-    \row    \li \l{QListIterator::next()}{next()}
-            \li Returns the next item and advances the iterator by one position
-    \row    \li \l{QListIterator::peekNext()}{peekNext()}
-            \li Returns the next item without moving the iterator
-    \row    \li \l{QListIterator::hasPrevious()}{hasPrevious()}
-            \li Returns \c true if the iterator isn't at the front of the list
-    \row    \li \l{QListIterator::previous()}{previous()}
-            \li Returns the previous item and moves the iterator back by one position
-    \row    \li \l{QListIterator::peekPrevious()}{peekPrevious()}
-            \li Returns the previous item without moving the iterator
-    \endtable
-
-    QListIterator provides no functions to insert or remove items
-    from the list as we iterate. To accomplish this, you must use
-    QMutableListIterator. Here's an example where we remove all
-    odd numbers from a QList<int> using QMutableListIterator:
-
-    \snippet code/doc_src_containers.cpp 3
-
-    The next() call in the loop is made every time. It jumps over the
-    next item in the list. The
-    \l{QMutableListIterator::remove()}{remove()} function removes the
-    last item that we jumped over from the list. The call to
-    \l{QMutableListIterator::remove()}{remove()} does not invalidate
-    the iterator, so it is safe to continue using it. This works just
-    as well when iterating backward:
-
-    \snippet code/doc_src_containers.cpp 4
-
-    If we just want to modify the value of an existing item, we can
-    use \l{QMutableListIterator::setValue()}{setValue()}. In the code
-    below, we replace any value larger than 128 with 128:
-
-    \snippet code/doc_src_containers.cpp 5
-
-    Just like \l{QMutableListIterator::remove()}{remove()},
-    \l{QMutableListIterator::setValue()}{setValue()} operates on the
-    last item that we jumped over. If we iterate forward, this is the
-    item just before the iterator; if we iterate backward, this is
-    the item just after the iterator.
-
-    The \l{QMutableListIterator::next()}{next()} function returns a
-    non-const reference to the item in the list. For simple
-    operations, we don't even need
-    \l{QMutableListIterator::setValue()}{setValue()}:
-
-    \snippet code/doc_src_containers.cpp 6
-
-    As mentioned above QSet's iterator
-    classes have exactly the same API as QList's. We will now turn to
-    QMapIterator, which is somewhat different because it iterates on
-    (key, value) pairs.
-
-    Like QListIterator, QMapIterator provides
-    \l{QMapIterator::toFront()}{toFront()},
-    \l{QMapIterator::toBack()}{toBack()},
-    \l{QMapIterator::hasNext()}{hasNext()},
-    \l{QMapIterator::next()}{next()},
-    \l{QMapIterator::peekNext()}{peekNext()},
-    \l{QMapIterator::hasPrevious()}{hasPrevious()},
-    \l{QMapIterator::previous()}{previous()}, and
-    \l{QMapIterator::peekPrevious()}{peekPrevious()}. The key and
-    value components are extracted by calling \l{QMapIterator::key()}{key()} and \l{QMapIterator::value()}{value()} on
-    the object returned by next(), peekNext(), previous(), or
-    peekPrevious().
-
-    The following example removes all (capital, country) pairs where
-    the capital's name ends with "City":
-
-    \snippet code/doc_src_containers.cpp 7
-
-    QMapIterator also provides a \l{QMapIterator::key()}{key()} and a \l{QMapIterator::value()}{value()} function that
-    operate directly on the iterator and that return the key and
-    value of the last item that the iterator jumped above. For
-    example, the following code copies the contents of a QMap into a
-    QHash:
-
-    \snippet code/doc_src_containers.cpp 8
-
-    If we want to iterate through all the items with the same
-    value, we can use \l{QMapIterator::findNext()}{findNext()}
-    or \l{QMapIterator::findPrevious()}{findPrevious()}.
-    Here's an example where we remove all the items with a particular
-    value:
-
-    \snippet code/doc_src_containers.cpp 9
-
     \section2 STL-Style Iterators
 
     STL-style iterators have been available since the release of Qt
@@ -438,10 +270,9 @@
 
     \snippet code/doc_src_containers.cpp 10
 
-    Unlike \l{Java-style iterators}, STL-style iterators point
-    directly at items. The \l{QList::begin()}{begin()} function of a container returns an
-    iterator that points to the first item in the container. The
-    \l{QList::end()}{end()} function of a container returns an iterator to the
+    STL-style iterators point directly at items. The \l{QList::begin()}{begin()}
+    function of a container returns an iterator that points to the first item in the
+    container. The \l{QList::end()}{end()} function of a container returns an iterator to the
     imaginary item one position past the last item in the container.
     \l {QList::end()}{end()} marks an invalid position; it must never be dereferenced.
     It is typically used in a loop's break condition. If the list is
@@ -524,6 +355,11 @@
      The above example only shows a problem with QList, but
      the problem exists for all the implicitly shared Qt containers.
 
+    \section2 Java-Style Iterators
+    \l{java-style-iterators}{Java-Style iterators} were introduced in Qt 4. Their API is modelled
+    on Java's iterator classes.
+    New code should should prefer \l{STL-Style Iterators}.
+
     \target foreach
     \section1 The foreach Keyword
 
diff --git a/src/corelib/doc/src/java-style-iterators.qdoc b/src/corelib/doc/src/java-style-iterators.qdoc
new file mode 100644
index 0000000000..69b9592996
--- /dev/null
+++ b/src/corelib/doc/src/java-style-iterators.qdoc
@@ -0,0 +1,198 @@
+/****************************************************************************
+**
+** Copyright (C) 2020 The Qt Company Ltd.
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of the documentation of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:FDL$
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see https://www.qt.io/terms-conditions. For further
+** information use the contact form at https://www.qt.io/contact-us.
+**
+** GNU Free Documentation License Usage
+** 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. Please review the following information to ensure
+** the GNU Free Documentation License version 1.3 requirements
+** will be met: https://www.gnu.org/licenses/fdl-1.3.html.
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+
+/*!
+    \page java-style-iterators.html
+    \title Java-style Iterators in Qt
+    \ingroup groups
+    \ingroup qt-basic-concepts
+    \brief Java style iterators for Qt's containers.
+
+    \tableofcontents
+
+    \section1 Java-Style Iterators
+
+    For each container class, there are two Java-style iterator data
+    types: one that provides read-only access and one that provides
+    read-write access.
+
+    \note New code should use \l{STL-Style iterators} since these are more efficient
+    and can be used together with Qt's and STL's \l{generic algorithms}.
+
+    \table
+    \header \li Containers                        \li Read-only iterator
+                                                  \li Read-write iterator
+    \row    \li QList<T>, QQueue<T>, QStack<T>,   \li QListIterator<T>
+                                                  \li QMutableListIterator<T>
+    \row    \li QSet<T>                           \li QSetIterator<T>
+                                                  \li QMutableSetIterator<T>
+    \row    \li QMap<Key, T>, QMultiMap<Key, T>   \li QMapIterator<Key, T>
+                                                  \li QMutableMapIterator<Key, T>
+    \row    \li QHash<Key, T>, QMultiHash<Key, T> \li QHashIterator<Key, T>
+                                                  \li QMutableHashIterator<Key, T>
+    \endtable
+
+    In this discussion, we will concentrate on QList and QMap. The
+    iterator types for QSet have exactly
+    the same interface as QList's iterators; similarly, the iterator
+    types for QHash have the same interface as QMap's iterators.
+
+    Unlike \l{STL-Style iterators}, Java-style iterators point \e between items
+    rather than directly \e at items. For this reason, they are either pointing
+    to the very beginning of the container (before the first item), at the very
+    end of the container (after the last item), or between two items. The
+    diagram below shows the valid iterator positions as red arrows for a list
+    containing four items:
+
+    \image javaiterators1.png
+
+    Here's a typical loop for iterating through all the elements of a
+    QList<QString> in order and printing them to the console:
+
+    \snippet code/doc_src_containers.cpp 1
+
+    It works as follows: The QList to iterate over is passed to the
+    QListIterator constructor. At that point, the iterator is located
+    just in front of the first item in the list (before item "A").
+    Then we call \l{QListIterator::hasNext()}{hasNext()} to
+    check whether there is an item after the iterator. If there is, we
+    call \l{QListIterator::next()}{next()} to jump over that
+    item. The next() function returns the item that it jumps over. For
+    a QList<QString>, that item is of type QString.
+
+    Here's how to iterate backward in a QList:
+
+    \snippet code/doc_src_containers.cpp 2
+
+    The code is symmetric with iterating forward, except that we
+    start by calling \l{QListIterator::toBack()}{toBack()}
+    to move the iterator after the last item in the list.
+
+    The diagram below illustrates the effect of calling
+    \l{QListIterator::next()}{next()} and
+    \l{QListIterator::previous()}{previous()} on an iterator:
+
+    \image javaiterators2.png
+
+    The following table summarizes the QListIterator API:
+
+    \table
+    \header \li Function \li Behavior
+    \row    \li \l{QListIterator::toFront()}{toFront()}
+            \li Moves the iterator to the front of the list (before the first item)
+    \row    \li \l{QListIterator::toBack()}{toBack()}
+            \li Moves the iterator to the back of the list (after the last item)
+    \row    \li \l{QListIterator::hasNext()}{hasNext()}
+            \li Returns \c true if the iterator isn't at the back of the list
+    \row    \li \l{QListIterator::next()}{next()}
+            \li Returns the next item and advances the iterator by one position
+    \row    \li \l{QListIterator::peekNext()}{peekNext()}
+            \li Returns the next item without moving the iterator
+    \row    \li \l{QListIterator::hasPrevious()}{hasPrevious()}
+            \li Returns \c true if the iterator isn't at the front of the list
+    \row    \li \l{QListIterator::previous()}{previous()}
+            \li Returns the previous item and moves the iterator back by one position
+    \row    \li \l{QListIterator::peekPrevious()}{peekPrevious()}
+            \li Returns the previous item without moving the iterator
+    \endtable
+
+    QListIterator provides no functions to insert or remove items
+    from the list as we iterate. To accomplish this, you must use
+    QMutableListIterator. Here's an example where we remove all
+    odd numbers from a QList<int> using QMutableListIterator:
+
+    \snippet code/doc_src_containers.cpp 3
+
+    The next() call in the loop is made every time. It jumps over the
+    next item in the list. The
+    \l{QMutableListIterator::remove()}{remove()} function removes the
+    last item that we jumped over from the list. The call to
+    \l{QMutableListIterator::remove()}{remove()} does not invalidate
+    the iterator, so it is safe to continue using it. This works just
+    as well when iterating backward:
+
+    \snippet code/doc_src_containers.cpp 4
+
+    If we just want to modify the value of an existing item, we can
+    use \l{QMutableListIterator::setValue()}{setValue()}. In the code
+    below, we replace any value larger than 128 with 128:
+
+    \snippet code/doc_src_containers.cpp 5
+
+    Just like \l{QMutableListIterator::remove()}{remove()},
+    \l{QMutableListIterator::setValue()}{setValue()} operates on the
+    last item that we jumped over. If we iterate forward, this is the
+    item just before the iterator; if we iterate backward, this is
+    the item just after the iterator.
+
+    The \l{QMutableListIterator::next()}{next()} function returns a
+    non-const reference to the item in the list. For simple
+    operations, we don't even need
+    \l{QMutableListIterator::setValue()}{setValue()}:
+
+    \snippet code/doc_src_containers.cpp 6
+
+    As mentioned above QSet's iterator
+    classes have exactly the same API as QList's. We will now turn to
+    QMapIterator, which is somewhat different because it iterates on
+    (key, value) pairs.
+
+    Like QListIterator, QMapIterator provides
+    \l{QMapIterator::toFront()}{toFront()},
+    \l{QMapIterator::toBack()}{toBack()},
+    \l{QMapIterator::hasNext()}{hasNext()},
+    \l{QMapIterator::next()}{next()},
+    \l{QMapIterator::peekNext()}{peekNext()},
+    \l{QMapIterator::hasPrevious()}{hasPrevious()},
+    \l{QMapIterator::previous()}{previous()}, and
+    \l{QMapIterator::peekPrevious()}{peekPrevious()}. The key and
+    value components are extracted by calling \l{QMapIterator::key()}{key()} and \l{QMapIterator::value()}{value()} on
+    the object returned by next(), peekNext(), previous(), or
+    peekPrevious().
+
+    The following example removes all (capital, country) pairs where
+    the capital's name ends with "City":
+
+    \snippet code/doc_src_containers.cpp 7
+
+    QMapIterator also provides a \l{QMapIterator::key()}{key()} and a \l{QMapIterator::value()}{value()} function that
+    operate directly on the iterator and that return the key and
+    value of the last item that the iterator jumped above. For
+    example, the following code copies the contents of a QMap into a
+    QHash:
+
+    \snippet code/doc_src_containers.cpp 8
+
+    If we want to iterate through all the items with the same
+    value, we can use \l{QMapIterator::findNext()}{findNext()}
+    or \l{QMapIterator::findPrevious()}{findPrevious()}.
+    Here's an example where we remove all the items with a particular
+    value:
+
+    \snippet code/doc_src_containers.cpp 9
+
+*/