// Purpose: topic overview
// Author: wxWidgets team
// RCS-ID: $Id$
-// Licence: wxWindows license
+// Licence: wxWindows licence
/////////////////////////////////////////////////////////////////////////////
-/*!
+/**
- @page overview_exceptions C++ exceptions overview
+@page overview_exceptions C++ Exceptions
- @li @ref overview_exceptions_introduction
- @li @ref overview_exceptions_strategies
- @li @ref overview_exceptions_tech
+@li @ref overview_exceptions_introduction
+@li @ref overview_exceptions_strategies
+@li @ref overview_exceptions_tech
- <hr>
+<hr>
- @section overview_exceptions_introduction Introduction
+@section overview_exceptions_introduction Introduction
- wxWidgets had been started long before the exceptions were introduced in C++ so
- it is not very surprising that it is not built around using them as some more
- modern C++ libraries are. For instance, the library doesn't throw exceptions to
- signal about the errors. Moreover, up to (and including) the version 2.4 of
- wxWidgets, even using the exceptions in the user code was dangerous because the
- library code wasn't exception-safe and so an exception propagating through it
- could result in memory and/or resource leaks, and also not very convenient.
+wxWidgets had been started long before the exceptions were introduced in C++ so
+it is not very surprising that it is not built around using them as some more
+modern C++ libraries are. For instance, the library doesn't throw exceptions to
+signal about the errors. Moreover, up to (and including) the version 2.4 of
+wxWidgets, even using the exceptions in the user code was dangerous because the
+library code wasn't exception-safe and so an exception propagating through it
+could result in memory and/or resource leaks, and also not very convenient.
- Starting from the version 2.5.1 wxWidgets becomes more exception-friendly. It
- still doesn't use the exceptions by itself but it should be now safe to use the
- exceptions in the user code and the library tries to help you with this. Please
- note that making the library exception-safe is still work in progress.
+However the recent wxWidgets versions are exception-friendly. This means that
+while the library still doesn't use the exceptions by itself, it should be now
+safe to use the exceptions in the user code and the library tries to help you
+with this.
- @section overview_exceptions_strategies Strategies for exceptions handling
+@section overview_exceptions_strategies Strategies for exceptions handling
- There are several choice for using the exceptions in wxWidgets programs. First
- of all, you may not use them at all. As stated above, the library doesn't throw
- any exceptions by itself and so you don't have to worry about exceptions at all
- unless your own code throws them. This is, of course, the simplest solution but
- may be not the best one to deal with all possible errors.
+There are several choice for using the exceptions in wxWidgets programs. First
+of all, you may not use them at all. As stated above, the library doesn't throw
+any exceptions by itself and so you don't have to worry about exceptions at all
+unless your own code throws them. This is, of course, the simplest solution but
+may be not the best one to deal with all possible errors.
- Another strategy is to use exceptions only to signal truly fatal errors. In
- this case you probably don't expect to recover from them and the default
- behaviour -- to simply terminate the program -- may be appropriate. If it is
- not, you may override #OnUnhandledException()
- in your wxApp-derived class to perform any clean up tasks. Note, however, that
- any information about the exact exception type is lost when this function is
- called, so if you need you should override #OnRun() and
- add a try/catch clause around the call of the base class version. This would
- allow you to catch any exceptions generated during the execution of the main
- event loop. To deal with the exceptions which may arise during the program
- startup and/or shutdown you should insert try/catch clauses in
- #OnInit() and/or #OnExit() as well.
+The next simplest strategy is to only use exceptions inside non-GUI code, i.e.
+never let unhandled exceptions escape the event handler in which it happened.
+In this case using exceptions in wxWidgets programs is not different from using
+them in any other C++ program.
- Finally, you may also want to continue running even when certain exceptions
- occur. If all of your exceptions may happen only in the event handlers of a
- single class (or only in the classes derived from it), you may centralize your
- exception handling code in #ProcessEvent
- method of this class. If this is impractical, you may also consider overriding
- the wxApp::HandleEvent() which allows you to handle
- all the exceptions thrown by any event handler.
+Things get more interesting if you decide to let (at least some) exceptions
+escape from the event handler in which they occurred. Such exceptions will be
+caught by wxWidgets and the special wxApp::OnExceptionInMainLoop() method will
+be called from the @c catch clause. This allows you to decide in a single place
+what to do about such exceptions: you may want to handle the exception somehow
+or terminate the program. In this sense, OnExceptionInMainLoop() is equivalent
+to putting a @c try/catch block around the entire @c main() function body in
+the traditional console programs. However notice that, as its name indicates,
+this method won't help you with the exceptions thrown before the main loop is
+started or after it is over, so you may still want to have @c try/catch in your
+overridden wxApp::OnInit() and wxApp::OnExit() methods too, otherwise
+wxApp::OnUnhandledException() will be called.
+Finally, notice that even if you decide to not let any exceptions escape in
+this way, this still may happen unexpectedly in a program using exceptions as a
+result of a bug. So consider always overriding OnExceptionInMainLoop() in your
+wxApp-derived class if you use exceptions in your program, whether you expect
+it to be called or not. In the latter case you may simple re-throw the
+exception and let it bubble up to OnUnhandledException() as well.
- @section overview_exceptions_tech Technicalities
+To summarize, when you use exceptions in your code, you may handle them in the
+following places, in order of priority:
+ -# In a @c try/catch block inside an event handler.
+ -# In wxApp::OnExceptionInMainLoop().
+ -# In wxApp::OnUnhandledException().
- To use any kind of exception support in the library you need to build it
- with @c wxUSE_EXCEPTIONS set to 1. This should be the case by default but
- if it isn't, you should edit the @c include/wx/msw/setup.h file under
- Windows or run @c configure with @c --enable-exceptions argument
- under Unix.
+In the first two cases you may decide whether you want to handle the exception
+and continue execution or to exit the program. In the last one the program is
+about to exit already so you can just try to save any unsaved data and notify
+the user about the problem (while being careful not to throw any more
+exceptions as otherwise @c std::terminate() will be called).
- On the other hand, if you do not plan to use exceptions, setting this
- flag to 0 or using @c --disable-exceptions could result in a leaner and
- slightly faster library.
- As for any other library feature, there is a sample (@c except)
- showing how to use it. Please look at its sources for further information.
+@section overview_exceptions_tech Technicalities
+
+To use any kind of exception support in the library you need to build it
+with @c wxUSE_EXCEPTIONS set to 1. It is turned on by default but you may
+wish to check @c include/wx/msw/setup.h file under Windows or run @c configure
+with explicit @c --enable-exceptions argument under Unix.
+
+On the other hand, if you do not plan to use exceptions, setting this
+flag to 0 or using @c --disable-exceptions could result in a leaner and
+slightly faster library.
+
+As for any other library feature, there is a sample (@c except)
+showing how to use it. Please look at its sources for further information.
*/
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