Add QScopedValueRollback tools class.

This template allows a value to be automatically rolled back to a previous
state when the class goes out of scope. This can be used to maintain a
valid state when an exception is thrown.

Reviewed-by: mread
Reviewed-by: joao

3 files changed, 169 insertions, 3 deletions

diff --git a/src/corelib/tools/qscopedvaluerollback.cpp b/src/corelib/tools/qscopedvaluerollback.cpp
new file mode 100644
index 0000000..8933efc
--- /dev/null
+++ b/src/corelib/tools/qscopedvaluerollback.cpp
@@ -0,0 +1,84 @@
+/****************************************************************************
+**
+** Copyright (C) 2011 Nokia Corporation and/or its subsidiary(-ies).
+** All rights reserved.
+** Contact: Nokia Corporation (qt-info@nokia.com)
+**
+** This file is part of the QtCore module of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:LGPL$
+** No Commercial Usage
+** This file contains pre-release code and may not be distributed.
+** You may use this file in accordance with the terms and conditions
+** contained in the Technology Preview License Agreement accompanying
+** this package.
+**
+** GNU Lesser General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU Lesser
+** General Public License version 2.1 as published by the Free Software
+** Foundation and appearing in the file LICENSE.LGPL included in the
+** packaging of this file.  Please review the following information to
+** ensure the GNU Lesser General Public License version 2.1 requirements
+** will be met: http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html.
+**
+** In addition, as a special exception, Nokia gives you certain additional
+** rights.  These rights are described in the Nokia Qt LGPL Exception
+** version 1.1, included in the file LGPL_EXCEPTION.txt in this package.
+**
+** If you have questions regarding the use of this file, please contact
+** Nokia at qt-info@nokia.com.
+**
+**
+**
+**
+**
+**
+**
+**
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+
+#include "qscopedvaluerollback.h"
+
+QT_BEGIN_NAMESPACE
+
+/*!
+    \class QScopedValueRollback
+    \brief The QScopedValueRollback resets a variable to its previous value on destruction
+    \since 4.8
+    \ingroup misc
+
+    The QScopedAssignment class can be used to revert state when an
+    exception is thrown without needing to write try-catch blocks.
+
+    It can also be used to manage variables that are temporarily set,
+    such as reentrancy guards. By using this class, the variable will
+    be reset whether the function is exited normally, exited early by
+    a return statement, or exited by an exception.
+
+    The template can only be instantiated with a type that supports assignment.
+
+    \sa QScopedPointer
+*/
+
+/*!
+    \fn QScopedValueRollback::QScopedValueRollback(T &var)
+
+    Stores the previous value of var internally, for revert on destruction.
+*/
+
+/*!
+    \fn QScopedValueRollback::~QScopedValueRollback()
+
+    Assigns the previous value to the managed variable.
+    This is the value at construction time, or at the last call to commit()
+*/
+
+/*!
+    \fn void QScopedValueRollback::commit()
+
+    Updates the previous value of the managed variable to its current value.
+*/
+
+QT_END_NAMESPACE
diff --git a/src/corelib/tools/qscopedvaluerollback.h b/src/corelib/tools/qscopedvaluerollback.h
new file mode 100644
index 0000000..d344ed8
--- /dev/null
+++ b/src/corelib/tools/qscopedvaluerollback.h
@@ -0,0 +1,81 @@
+/****************************************************************************
+**
+** Copyright (C) 2011 Nokia Corporation and/or its subsidiary(-ies).
+** All rights reserved.
+** Contact: Nokia Corporation (qt-info@nokia.com)
+**
+** This file is part of the QtCore module of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:LGPL$
+** No Commercial Usage
+** This file contains pre-release code and may not be distributed.
+** You may use this file in accordance with the terms and conditions
+** contained in the Technology Preview License Agreement accompanying
+** this package.
+**
+** GNU Lesser General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU Lesser
+** General Public License version 2.1 as published by the Free Software
+** Foundation and appearing in the file LICENSE.LGPL included in the
+** packaging of this file.  Please review the following information to
+** ensure the GNU Lesser General Public License version 2.1 requirements
+** will be met: http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html.
+**
+** In addition, as a special exception, Nokia gives you certain additional
+** rights.  These rights are described in the Nokia Qt LGPL Exception
+** version 1.1, included in the file LGPL_EXCEPTION.txt in this package.
+**
+** If you have questions regarding the use of this file, please contact
+** Nokia at qt-info@nokia.com.
+**
+**
+**
+**
+**
+**
+**
+**
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+
+#ifndef QSCOPEDVALUEROLLBACK_H
+#define QSCOPEDVALUEROLLBACK_H
+
+#include <QtCore/qglobal.h>
+
+QT_BEGIN_HEADER
+QT_BEGIN_NAMESPACE
+QT_MODULE(Core)
+
+template <typename T>
+class QScopedValueRollback
+{
+public:
+    QScopedValueRollback(T &var) :
+        varRef(var)
+    {
+        oldValue = varRef;
+    }
+
+    ~QScopedValueRollback()
+    {
+        varRef = oldValue;
+    }
+
+    void commit()
+    {
+        oldValue = varRef;
+    }
+
+private:
+    T& varRef;
+    T oldValue;
+
+    Q_DISABLE_COPY(QScopedValueRollback);
+};
+
+QT_END_NAMESPACE
+QT_END_HEADER
+
+#endif // QSCOPEDVALUEROLLBACK_H
diff --git a/src/corelib/tools/tools.pri b/src/corelib/tools/tools.pri
index 03bb32d..9d564a1 100644
--- a/src/corelib/tools/tools.pri
+++ b/src/corelib/tools/tools.pri
@@ -29,6 +29,9 @@ HEADERS +=  \
         tools/qrect.h \
         tools/qregexp.h \
         tools/qringbuffer_p.h \
+        tools/qscopedpointer.h \
+        tools/qscopedpointer_p.h \
+        tools/qscopedvaluerollback.h \
         tools/qshareddata.h \
         tools/qsharedpointer.h \
         tools/qsharedpointer_impl.h \
@@ -45,9 +48,7 @@ HEADERS +=  \
         tools/qelapsedtimer.h \
         tools/qunicodetables_p.h \
         tools/qvarlengtharray.h \
-        tools/qvector.h \
-        tools/qscopedpointer.h \
-        tools/qscopedpointer_p.h
+        tools/qvector.h
 
 
 SOURCES += \