Discourage the foreach keyword

Stop endorsing the foreach keyword, suggest C++11 range-based loops
instead.

Task-number: QTBUG-86584
Pick-to: 6.0
Change-Id: Icaf537aded6557b7864d731217e967ac56a84928
Reviewed-by: Paul Wicking <paul.wicking@qt.io>

src/corelib/doc/src/containers.qdoc

@@ -61,9 +61,6 @@
     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.
-
     \note Since Qt 5.14, range constructors are available for most of the
     container classes. QMultiMap is a notable exception. Their use is
     encouraged in place of the various from/to methods. For example:
@@ -360,68 +357,15 @@
     on Java's iterator classes.
     New code should should prefer \l{STL-Style Iterators}.
 
+    \section1 Container keywords
+
     \target foreach
-    \section1 The foreach Keyword
-
-    If you just want to iterate over all the items in a container
-    in order, you can use Qt's \c foreach keyword. The keyword is a
-    Qt-specific addition to the C++ language, and is implemented
-    using the preprocessor.
-
-    Its syntax is: \c foreach (\e variable, \e container) \e
-    statement. For example, here's how to use \c foreach to iterate
-    over a QList<QString>:
-
-    \snippet code/doc_src_containers.cpp 15
-
-    The \c foreach code is significantly shorter than the equivalent
-    code that uses iterators:
-
-    \snippet code/doc_src_containers.cpp 16
-
-    Unless the data type contains a comma (e.g., \c{QPair<int,
-    int>}), the variable used for iteration can be defined within the
-    \c foreach statement:
-
-    \snippet code/doc_src_containers.cpp 17
-
-    And like any other C++ loop construct, you can use braces around
-    the body of a \c foreach loop, and you can use \c break to leave
-    the loop:
-
-    \snippet code/doc_src_containers.cpp 18
-
-    With QMap and QHash, \c foreach accesses the value component of
-    the (key, value) pairs automatically, so you should not call
-    values() on the container (it would generate an unnecessary copy,
-    see below). If you want to iterate over both the keys and the
-    values, you can use iterators (which are faster), or you can
-    obtain the keys, and use them to get the values too:
-
-    \snippet code/doc_src_containers.cpp 19
-
-    For a multi-valued map:
-
-    \snippet code/doc_src_containers.cpp 20
-
-    Qt automatically takes a copy of the container when it enters a
-    \c foreach loop. If you modify the container as you are
-    iterating, that won't affect the loop. (If you do not modify the
-    container, the copy still takes place, but thanks to \l{implicit
-    sharing} copying a container is very fast.)
-
-    Since foreach creates a copy of the container, using a non-const
-    reference for the variable does not allow you to modify the original
-    container. It only affects the copy, which is probably not what you
-    want.
-
-    An alternative to Qt's \c foreach loop is the range-based \c for that is
-    part of C++ 11 and newer. However, keep in mind that the range-based
-    \c for might force a Qt container to \l{Implicit Sharing}{detach}, whereas
-    \c foreach would not. But using \c foreach always copies the container,
-    which is usually not cheap for STL containers. If in doubt, prefer
-    \c foreach for Qt containers, and range based \c for for STL ones.
+    \section2 The foreach Keyword
+    \l{foreach-keyword}{The foreach keyword} is discouraged, new code should
+    prefer C++11 range-based loops.
 
+    \target forever
+    \section2 The forever keyword.
     In addition to \c foreach, Qt also provides a \c forever
     pseudo-keyword for infinite loops:
 
@@ -432,6 +376,9 @@
 
     \snippet code/doc_src_containers.cpp 22
 
+    \note The alternative macros Q_FOREACH and Q_FOREVER remain defined
+    regardless.
+
     \section1 Other Container-Like Classes
 
     Qt includes other template classes that resemble containers in

src/corelib/doc/src/foreach-keyword.qdoc

@@ -0,0 +1,100 @@
+/****************************************************************************
+**
+** 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 foreach-keyword.html
+    \title Qt's foreach Keyword
+    \ingroup groups
+    \ingroup qt-basic-concepts
+
+    \brief Qt's foreach keyword.
+
+    \tableofcontents
+
+    \target foreach-keyword
+    \section1 The foreach Keyword
+
+    \note The foreach keyword was introduced before the C++11 range-based loops
+    existed. New code should prefer C++11 range-based loops.
+
+    The \c foreach keyword is a Qt-specific addition to the C++ language,
+    and is implemented using the preprocessor.
+
+    Its syntax is: \c foreach (\e variable, \e container) \e
+    statement. For example, here's how to use \c foreach to iterate
+    over a QList<QString>:
+
+    \snippet code/doc_src_containers.cpp 15
+
+    The \c foreach code is significantly shorter than the equivalent
+    code that uses iterators:
+
+    \snippet code/doc_src_containers.cpp 16
+
+    Unless the data type contains a comma (e.g., \c{QPair<int,
+    int>}), the variable used for iteration can be defined within the
+    \c foreach statement:
+
+    \snippet code/doc_src_containers.cpp 17
+
+    And like any other C++ loop construct, you can use braces around
+    the body of a \c foreach loop, and you can use \c break to leave
+    the loop:
+
+    \snippet code/doc_src_containers.cpp 18
+
+    With QMap and QHash, \c foreach accesses the value component of
+    the (key, value) pairs automatically, so you should not call
+    values() on the container (it would generate an unnecessary copy,
+    see below). If you want to iterate over both the keys and the
+    values, you can use iterators (which are faster), or you can
+    obtain the keys, and use them to get the values too:
+
+    \snippet code/doc_src_containers.cpp 19
+
+    For a multi-valued map:
+
+    \snippet code/doc_src_containers.cpp 20
+
+    Qt automatically takes a copy of the container when it enters a
+    \c foreach loop. If you modify the container as you are
+    iterating, that won't affect the loop. (If you do not modify the
+    container, the copy still takes place, but thanks to \l{implicit
+    sharing} copying a container is very fast.)
+
+    Since foreach creates a copy of the container, using a non-const
+    reference for the variable does not allow you to modify the original
+    container. It only affects the copy, which is probably not what you
+    want.
+
+    An alternative to Qt's \c foreach loop is the range-based \c for that is
+    part of C++11 and newer. However, keep in mind that the range-based
+    \c for might force a Qt container to \l{Implicit Sharing}{detach}, whereas
+    \c foreach would not. But using \c foreach always copies the container,
+    which is usually not cheap for STL containers. If in doubt, prefer
+    \c foreach for Qt containers, and range based \c for for STL ones.
+*/