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/****************************************************************************
**
** 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 documentation of the Qt Toolkit.
**
** $QT_BEGIN_LICENSE:FDL$
** 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 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 qt-info@nokia.com.
** $QT_END_LICENSE$
**
****************************************************************************/
/*!
\page symbianexceptionsafety.html
\title Exception Safety with Symbian
\ingroup qtsymbian
\brief A guide to integrating exception safety in Qt with Symbian.
The following sections describe how Qt code can interoperate with Symbian's
exception safety system.
\tableofcontents
\section1 What the problem is
Qt and Symbian have different exception systems. Qt works with standard C++
exceptions, whereas Symbian has its TRAP/Leave/CleanupStack system. So, what would
happen if you mix the two systems? It could go wrong in a number of ways.
Clean-up ordering would be different between the two. When Symbian code
leaves, the clean-up stack is cleaned up before anything else happens. After
that, the objects on the call stack would be cleaned up as with a normal
exception. So if there are any dependencies between stack-based and
objects owned by the clean-up stack, there could be problems due to this
ordering.
Symbian's \c XLeaveException, which is used when Symbian implements leaves as
exceptions, is not derived from \c std::exception, so would not be caught in
Qt catch statements designed to catch \c std::exception.
Qt's and standard C++'s \c std::exception derived exceptions result in program
termination if they fall back to a Symbian TRAP.
These problems can be solved with barrier macros and helper functions that
will translate between the two exception systems. Use them, in Qt code,
whenever calling into or being called from Symbian code.
\section1 Qt calls to Symbian
When calling Symbian leaving functions from Qt code, we want to translate
Symbian leaves to standard C++ exceptions. The following help is provided:
\list
\o \l qt_symbian_throwIfError() takes a Symbian
error code and throws an appropriate exception to represent it.
This will do nothing if the error code is not in fact an error. The
function is equivalent to Symbian's \c User::LeaveIfError.
\o \l q_check_ptr() takes a pointer and throws a std::bad_alloc
exception if it is 0, otherwise the pointer is returned. This can be
used to check the success of a non-throwing allocation, eg from
\c malloc(). The function is equivalent to Symbian's \c
User::LeaveIfNull.
\o \l QT_TRAP_THROWING() takes a Symbian leaving
code fragment f and runs it under a trap harness converting any resulting
error into an exception.
\o \c TRAP and \c TRAPD from the Symbian libraries can be used to convert
leaves to error codes.
\endlist
\code
HBufC* buf=0;
// this will throw a std::bad_alloc because we've asked for too much memory
QT_TRAP_THROWING(buf = HBufC::NewL(100000000));
_LIT(KStr,"abc");
TInt pos = KStr().Locate('c');
// pos is a good value, >= 0, so no exception is thrown
qt_symbian_throwIfError(pos);
pos = KStr().Locate('d');
// pos == KErrNotFound, so this throws an exception
qt_symbian_throwIfError(pos);
// we are asking for a lot of memory, HBufC::New may return NULL, so check it
HBufC *buffer = q_check_ptr(HBufC::New(1000000));
\endcode
\section2 Be careful with new and CBase
When writing Qt code, \c new will normally throw a \c std::bad_alloc if the
allocation fails. However this may not happen if the object being created
has its own \c {operator new}. For example, CBase and derived classes have
their own \c {operator new} which returns 0 and the \c {new(ELeave)}
overload for a leaving \c {operator new}, neither of which does what we want.
When using 2-phase construction of CBase derived objects, use \c new and
\l q_check_ptr().
\oldcode
CFbsBitmap* fbsBitmap = new(ELeave) CFbsBitmap;
\newcode
CFbsBitmap* fbsBitmap = q_check_ptr(new CFbsBitmap);
\endcode
\section1 Qt called from Symbian
When Qt code is called from Symbian, we want to translate standard C++
exceptions to Symbian leaves or error codes. The following help is
provided:
\list
\o \l qt_symbian_exception2Error() -
this takes a standard exception and gives an appropriate Symbian
error code. If no mapping is known for the exception type,
\c KErrGeneral is returned.
\o \l qt_symbian_exception2LeaveL() -
this takes a standard exception and generates an appropriate Symbian
leave.
\o \l QT_TRYCATCH_ERROR() - this macro
takes the standard C++ code fragment \c f, catches any std::exceptions
thrown from it, and sets err to the corresponding Symbian error code.
err is set to \c KErrNone otherwise.
\o \l QT_TRYCATCH_LEAVING() - this macro takes the
standard C++ code fragment \c f, catches any std::exceptions thrown from
it, and throws a corresponding Symbian leave.
\endlist
\code
TInt DoTickL() // called from an active object RunL, ie Symbian leaves expected
{
// without the translation to Symbian Leave, we get a USER:0 panic
QT_TRYCATCH_LEAVING({
int* x = new int[100000000]; // compiled as Qt code, will throw std::bad_alloc
delete [] x;
});
return 0;
}
\endcode
\section1 Common sense things
Try to minimise the interleaving of Symbian and Qt code, every switch
requires a barrier. Grouping the code styles in different blocks will
minimise the problems. For instance, examine the following code.
\code
1. TRAPD(err, m_playUtility = CMdaAudioPlayerUtility::NewL(*this);
2. QString filepath = QFileInfo( m_sound->fileName() ).absoluteFilePath();
3. filepath = QDir::toNativeSeparators(filepath);
4. m_playUtility->OpenFileL(qt_QString2TPtrC(filepath)));
\endcode
Line 1 starts a Symbian leave handling block, which is good because it
also uses a Symbian leave generating function.
Line 2 creates a \l QString, uses \l QFileInfo and various member functions.
These could all throw exceptions, which is not good inside a \c TRAP block.
Line 3 is unclear as to whether it might throw an exception, but since
it's dealing with strings it probably does, again bad.
Line 4 is tricky, it calls a leaving function which is ok within a \c TRAP,
but it also uses a helper function to convert string types. In this case
the helper function may cause an unwelcome exception.
We could rewrite this with nested exception translations, but it's much
easier to refactor it.
\code
QString filepath = QFileInfo( m_sound->fileName() ).absoluteFilePath();
filepath = QDir::toNativeSeparators(filepath);
TPtrC filepathPtr(qt_QString2TPtrC(filepath));
TRAPD(err, m_playUtility = CMdaAudioPlayerUtility::NewL(*this);
m_playUtility->OpenFileL(filepathPtr));
\endcode
Now the exception generating functions are separated from the leaving
functions.
\section1 Advanced technique
When using Symbian APIs in Qt code, you may find that Symbian leaving
code and Qt exception throwing code are just too mixed up to have
them interoperate through barriers. In some circumstances you can allow
code to both leave and throw exceptions. But you must be aware of the
following issues:
\list
\o Depending on whether a leave or exception is thrown, or a normal
exit happens, the cleanup order will vary. If the code leaves,
cleanup stack cleanup will happen first. On an exception however,
cleanup stack cleanup will happen last.
\o There must not be any destructor dependencies between different
code styles. That is, you must not have symbian objects using Qt
objects in their destructors, and vice versa. This is because the
cleanup order varies, and may result in objects being used after
they are deleted.
\o The cleanup stack must not refer to any stack based object. For
instance, in Symbian you may use \c CleanupClosePushL() to push
stack based R-classes onto the cleanup stack. However if the
stack has unwound due to an exception before the cleanup stack
cleanup happens, stack based objects will now be invalid.
Instead of using the cleanup stack, consider Symbian's new
\c LManagedHandle<> (or a custom cleanup object) to tie R-class
cleanup to the stack.
\o Mixed throwing code must be called within both a TRAP and a
try/catch harness. Standard exceptions must not propagate to
the TRAP and cleanup stack cleanup will only happen if a leave
is thrown, so the correct pattern is either \c {TRAPD(err,
QT_TRYCATCH_LEAVING( f ));} or \c {QT_TRAP_THROWING(
QT_TRYCATCH_LEAVING( f ));}, depending if you want an error
code or exception as a result.
\endlist
*/
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