/****************************************************************************
**
** Copyright (C) 2012 Digia Plc and/or its subsidiary(-ies).
** Contact: http://www.qt-project.org/legal
**
** This file is part of the documentation of the Qt Toolkit.
**
** $QT_BEGIN_LICENSE:FDL$
** 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 Digia.  For licensing terms and
** conditions see http://qt.digia.com/licensing.  For further information
** use the contact form at http://qt.digia.com/contact-us.
**
** GNU Free Documentation License Usage
** 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.  Please review the following information to ensure
** the GNU Free Documentation License version 1.3 requirements
** will be met: http://www.gnu.org/copyleft/fdl.html.
** $QT_END_LICENSE$
**
****************************************************************************/

/*!
    \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
*/
