QImageWriter: improve documentation regarding use of resources

QImageWriter's plugin can open devices while operating and only free
them on destruction which means that if one wants to act on the file
written if must first destroy the writer's instance. This patch adds
the suggestion to use a scope to avoid that problem.

Change-Id: I239157ea86c4a93faab237fe1860312a3c7ac7a2
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>

src/gui/doc/snippets/qimagewriter/main.cpp

@@ -0,0 +1,71 @@
+/****************************************************************************
+**
+** Copyright (C) 2018 Samuel Gaist <samuel.gaist@idiap.ch>
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of the documentation of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:BSD$
+** 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.
+**
+** BSD License Usage
+** Alternatively, you may use this file under the terms of the BSD license
+** as follows:
+**
+** "Redistribution and use in source and binary forms, with or without
+** modification, are permitted provided that the following conditions are
+** met:
+**   * Redistributions of source code must retain the above copyright
+**     notice, this list of conditions and the following disclaimer.
+**   * Redistributions in binary form must reproduce the above copyright
+**     notice, this list of conditions and the following disclaimer in
+**     the documentation and/or other materials provided with the
+**     distribution.
+**   * Neither the name of The Qt Company Ltd nor the names of its
+**     contributors may be used to endorse or promote products derived
+**     from this software without specific prior written permission.
+**
+**
+** THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+** "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+** LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+** A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+** OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+** SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+** LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+** DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+** THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+** (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+** OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE."
+**
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+
+#include <QtGui>
+
+int main(int argc, char **argv)
+{
+    QApplication app(argc, argv);
+
+//! [0]
+    QString imagePath(QStringLiteral("path/image.jpeg"));
+    QImage image(64, 64, QImage::Format_RGB32);
+    image.fill(Qt::red);
+    {
+        QImageWriter writer(imagePath);
+        writer.write(image);
+    }
+
+    QFile::rename(imagePath,
+                  QStringLiteral("path/other_image.jpeg"));
+//! [0]
+
+    return 0;
+}

src/gui/image/qimagewriter.cpp

@@ -71,6 +71,16 @@
     formats, in addition to any image format plugins that support
     writing.
 
+    \note QImageWriter assumes exclusive control over the file or
+    device that is assigned. Any attempts to modify the assigned file
+    or device during the lifetime of the QImageWriter object will
+    yield undefined results. If immediate access to a resource is
+    desired, the use of a scope is the recommended method.
+
+    For example:
+
+    \snippet qimagewriter/main.cpp 0
+
     \sa QImageReader, QImageIOHandler, QImageIOPlugin
 */