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diff --git a/doc/src/platforms/symbian-exceptionsafety.qdoc b/doc/src/platforms/symbian-exceptionsafety.qdoc new file mode 100644 index 0000000..88f4d03 --- /dev/null +++ b/doc/src/platforms/symbian-exceptionsafety.qdoc @@ -0,0 +1,241 @@ +/**************************************************************************** +** +** Copyright (C) 2009 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. +** +** $QT_BEGIN_LICENSE:LGPL$ +** 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. +** +** +** +** +** +** +** +** +** $QT_END_LICENSE$ +** +****************************************************************************/ + +/*! + \page symbianexceptionsafety.html + \title Exception Safety with Symbian + \ingroup qts60 + \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 +*/ |