docs/doxygen/overviews/exceptions.h

   1 /////////////////////////////////////////////////////////////////////////////
   2 // Name:        exceptions.h
   3 // Purpose:     topic overview
   4 // Author:      wxWidgets team
   5 // Licence:     wxWindows licence
   6 /////////////////////////////////////////////////////////////////////////////
   7
   8 /**
   9
  10 @page overview_exceptions C++ Exceptions
  11
  12 @tableofcontents
  13
  14 wxWidgets had been started long before the exceptions were introduced in C++ so
  15 it is not very surprising that it is not built around using them as some more
  16 modern C++ libraries are. For instance, the library doesn't throw exceptions to
  17 signal about the errors. Moreover, up to (and including) the version 2.4 of
  18 wxWidgets, even using the exceptions in the user code was dangerous because the
  19 library code wasn't exception-safe and so an exception propagating through it
  20 could result in memory and/or resource leaks, and also not very convenient.
  21
  22 However the recent wxWidgets versions are exception-friendly. This means that
  23 while the library still doesn't use the exceptions by itself, it should be now
  24 safe to use the exceptions in the user code and the library tries to help you
  25 with this.
  26
  27
  28
  29 @section overview_exceptions_strategies Strategies for Exception Handling
  30
  31 There are several choice for using the exceptions in wxWidgets programs. First
  32 of all, you may not use them at all. As stated above, the library doesn't throw
  33 any exceptions by itself and so you don't have to worry about exceptions at all
  34 unless your own code throws them. This is, of course, the simplest solution but
  35 may be not the best one to deal with all possible errors.
  36
  37 The next simplest strategy is to only use exceptions inside non-GUI code, i.e.
  38 never let unhandled exceptions escape the event handler in which it happened.
  39 In this case using exceptions in wxWidgets programs is not different from using
  40 them in any other C++ program.
  41
  42 Things get more interesting if you decide to let (at least some) exceptions
  43 escape from the event handler in which they occurred. Such exceptions will be
  44 caught by wxWidgets and the special wxApp::OnExceptionInMainLoop() method will
  45 be called from the @c catch clause. This allows you to decide in a single place
  46 what to do about such exceptions: you may want to handle the exception somehow
  47 or terminate the program. In this sense, OnExceptionInMainLoop() is equivalent
  48 to putting a @c try/catch block around the entire @c main() function body in
  49 the traditional console programs. However notice that, as its name indicates,
  50 this method won't help you with the exceptions thrown before the main loop is
  51 started or after it is over, so you may still want to have @c try/catch in your
  52 overridden wxApp::OnInit() and wxApp::OnExit() methods too, otherwise
  53 wxApp::OnUnhandledException() will be called.
  54
  55 Finally, notice that even if you decide to not let any exceptions escape in
  56 this way, this still may happen unexpectedly in a program using exceptions as a
  57 result of a bug. So consider always overriding OnExceptionInMainLoop() in your
  58 wxApp-derived class if you use exceptions in your program, whether you expect
  59 it to be called or not. In the latter case you may simple re-throw the
  60 exception and let it bubble up to OnUnhandledException() as well.
  61
  62 To summarize, when you use exceptions in your code, you may handle them in the
  63 following places, in order of priority:
  64     -# In a @c try/catch block inside an event handler.
  65     -# In wxApp::OnExceptionInMainLoop().
  66     -# In wxApp::OnUnhandledException().
  67
  68 In the first two cases you may decide whether you want to handle the exception
  69 and continue execution or to exit the program. In the last one the program is
  70 about to exit already so you can just try to save any unsaved data and notify
  71 the user about the problem (while being careful not to throw any more
  72 exceptions as otherwise @c std::terminate() will be called).
  73
  74
  75 @section overview_exceptions_tech Technicalities
  76
  77 To use any kind of exception support in the library you need to build it
  78 with @c wxUSE_EXCEPTIONS set to 1. It is turned on by default but you may
  79 wish to check @c include/wx/msw/setup.h file under Windows or run @c configure
  80 with explicit @c --enable-exceptions argument under Unix.
  81
  82 On the other hand, if you do not plan to use exceptions, setting this
  83 flag to 0 or using @c --disable-exceptions could result in a leaner and
  84 slightly faster library.
  85
  86 As for any other library feature, there is a sample (@c except)
  87 showing how to use it. Please look at its sources for further information.
  88
  89 */