source: trunk/doc/src/howtos/exceptionsafety.qdoc@ 651

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/*!
    \page exceptionsafety.html
    \title Exception Safety
    \ingroup best-practices
    \brief A guide to exception safety in Qt.

    \bold {Preliminary warning}: Exception safety is not feature complete!
    Common cases should work, but classes might still leak or even crash.

    Qt itself will not throw exceptions. Instead, error codes are used.
    In addition, some classes have user visible error messages, for example
    \l QIODevice::errorString() or \l QSqlQuery::lastError().
    This has historical and practical reasons - turning on exceptions
    can increase the library size by over 20%.

    The following sections describe Qt's behavior if exception support is
    enabled at compile time.

    \tableofcontents

    \section1 Exception safe modules

    \section2 Containers
    
    Qt's \l{container classes} are generally exception neutral. They pass any
    exception that happens within their contained type \c T to the user
    while keeping their internal state valid.

    Example:

    \code
    QList<QString> list;
    ...
    try {
        list.append("hello");
    } catch (...) {
    }
    // list is safe to use - the exception did not affect it.
    \endcode

    Exceptions to that rule are containers for types that can throw during assignment
    or copy constructions. For those types, functions that modify the container as well as
    returning a value, are unsafe to use:

    \code
    MyType s = list.takeAt(2);
    \endcode

    If an exception occurs during the assignment of \c s, the value at index 2 is already
    removed from the container, but hasn't been assigned to \c s yet. It is lost
    without chance of recovery.

    The correct way to write it:

    \code
    MyType s = list.at(2);
    list.removeAt(2);
    \endcode

    If the assignment throws, the container still contains the value, no data loss occured.

    Note that implicitly shared Qt classes will not throw in their assignment
    operators or copy constructors, so the limitation above does not apply.

    \section1 Out of Memory Handling

    Most desktop operating systems overcommit memory. This means that \c malloc()
    or \c{operator new} return a valid pointer, even though there is not enough
    memory available at allocation time. On such systems, no exception of type
    \c std::bad_alloc is thrown.

    On all other operating systems, Qt will throw an exception of type std::bad_alloc
    if any allocation fails. Allocations can fail if the system runs out of memory or
    doesn't have enough continuous memory to allocate the requested size.

    Exceptions to that rule are documented. As an example, \l QImage::create()
    returns false if not enough memory exists instead of throwing an exception.

    \section1 Recovering from exceptions

    Currently, the only supported use case for recovering from exceptions thrown
    within Qt (for example due to out of memory) is to exit the event loop and do 
    some cleanup before exiting the application.

    Typical use case:

    \code
    QApplication app(argc, argv);
    ...
    try {
        app.exec();
    } catch (const std::bad_alloc &) {
        // clean up here, e.g. save the session
        // and close all config files.

        return 0; // exit the application
    }
    \endcode

    After an exception is thrown, the connection to the windowing server
    might already be closed. It is not safe to call a GUI related function
    after catching an exception.

    \section1 Platform-Specific Exception Handling

    \section2 The Symbian platform

    The Symbian platform implements its own exception system that differs from the standard
    C++ mechanism. When using Qt for the Symbian platform, and especially when writing code to 
    access Symbian functionality directly, it may be necessary to know about the underlying 
    implementation and how it interacts with Qt.

    The \l{Exception Safety with Symbian} document shows how to use the facilities provided
    by Qt to use exceptions as safely as possible.
*/