/**************************************************************************** ** ** 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 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 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. */