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/*!

    \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

*/