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1 /////////////////////////////////////////////////////////////////////////////
2 // Name: exceptions.h
3 // Purpose: topic overview
4 // Author: wxWidgets team
5 // RCS-ID: $Id$
6 // Licence: wxWindows licence
7 /////////////////////////////////////////////////////////////////////////////
8
9 /**
10
11 @page overview_exceptions C++ Exceptions
12
13 @li @ref overview_exceptions_introduction
14 @li @ref overview_exceptions_strategies
15 @li @ref overview_exceptions_tech
16
17
18 <hr>
19
20
21 @section overview_exceptions_introduction Introduction
22
23 wxWidgets had been started long before the exceptions were introduced in C++ so
24 it is not very surprising that it is not built around using them as some more
25 modern C++ libraries are. For instance, the library doesn't throw exceptions to
26 signal about the errors. Moreover, up to (and including) the version 2.4 of
27 wxWidgets, even using the exceptions in the user code was dangerous because the
28 library code wasn't exception-safe and so an exception propagating through it
29 could result in memory and/or resource leaks, and also not very convenient.
30
31 However the recent wxWidgets versions are exception-friendly. This means that
32 while the library still doesn't use the exceptions by itself, it should be now
33 safe to use the exceptions in the user code and the library tries to help you
34 with this.
35
36
37 @section overview_exceptions_strategies Strategies for exceptions handling
38
39 There are several choice for using the exceptions in wxWidgets programs. First
40 of all, you may not use them at all. As stated above, the library doesn't throw
41 any exceptions by itself and so you don't have to worry about exceptions at all
42 unless your own code throws them. This is, of course, the simplest solution but
43 may be not the best one to deal with all possible errors.
44
45 The next simplest strategy is to only use exceptions inside non-GUI code, i.e.
46 never let unhandled exceptions escape the event handler in which it happened.
47 In this case using exceptions in wxWidgets programs is not different from using
48 them in any other C++ program.
49
50 Things get more interesting if you decide to let (at least some) exceptions
51 escape from the event handler in which they occurred. Such exceptions will be
52 caught by wxWidgets and the special wxApp::OnExceptionInMainLoop() method will
53 be called from the @c catch clause. This allows you to decide in a single place
54 what to do about such exceptions: you may want to handle the exception somehow
55 or terminate the program. In this sense, OnExceptionInMainLoop() is equivalent
56 to putting a @c try/catch block around the entire @c main() function body in
57 the traditional console programs. However notice that, as its name indicates,
58 this method won't help you with the exceptions thrown before the main loop is
59 started or after it is over, so you may still want to have @c try/catch in your
60 overridden wxApp::OnInit() and wxApp::OnExit() methods too, otherwise
61 wxApp::OnUnhandledException() will be called.
62
63 Finally, notice that even if you decide to not let any exceptions escape in
64 this way, this still may happen unexpectedly in a program using exceptions as a
65 result of a bug. So consider always overriding OnExceptionInMainLoop() in your
66 wxApp-derived class if you use exceptions in your program, whether you expect
67 it to be called or not. In the latter case you may simple re-throw the
68 exception and let it bubble up to OnUnhandledException() as well.
69
70 To summarize, when you use exceptions in your code, you may handle them in the
71 following places, in order of priority:
72 -# In a @c try/catch block inside an event handler.
73 -# In wxApp::OnExceptionInMainLoop().
74 -# In wxApp::OnUnhandledException().
75
76 In the first two cases you may decide whether you want to handle the exception
77 and continue execution or to exit the program. In the last one the program is
78 about to exit already so you can just try to save any unsaved data and notify
79 the user about the problem (while being careful not to throw any more
80 exceptions as otherwise @c std::terminate() will be called).
81
82
83 @section overview_exceptions_tech Technicalities
84
85 To use any kind of exception support in the library you need to build it
86 with @c wxUSE_EXCEPTIONS set to 1. It is turned on by default but you may
87 wish to check @c include/wx/msw/setup.h file under Windows or run @c configure
88 with explicit @c --enable-exceptions argument under Unix.
89
90 On the other hand, if you do not plan to use exceptions, setting this
91 flag to 0 or using @c --disable-exceptions could result in a leaner and
92 slightly faster library.
93
94 As for any other library feature, there is a sample (@c except)
95 showing how to use it. Please look at its sources for further information.
96
97 */