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1%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
2%% Name: texcept.tex
3%% Purpose: C++ exceptions and wxWidgets overview
4%% Author: Vadim Zeitlin
5%% Modified by:
6%% Created: 17.09.03
7%% RCS-ID: $Id$
8%% Copyright: (c) 2003 Vadim Zeitlin
9%% License: wxWidgets license
10%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
11
12\section{C++ exceptions overview}\label{exceptionsoverview}
13
14\subsection{Introduction}
15
16wxWidgets had been started long before the exceptions were introduced in C++ so
17it is not very surprizing that it is not built around using them as some more
18modern C++ libraries are. For instance, the library doesn't throw exceptions to
19signal about the errors. Moreover, up to (and including) the version 2.4 of
20wxWidgets, even using the exceptions in the user code was dangerous because the
21library code wasn't exception-safe and so an exception propagating through it
22could result in memory and/or resource leaks, and also not very convenient.
23
24Starting from the version 2.5.1 wxWidgets becomes more exception-friendly. It
25still doesn't use the exceptions by itself but it should be now safe to use the
26exceptions in the user code and the library tries to help you with this. Please
27note that making the library exception-safe is still work in progress.
28
29
30\subsection{Strategies for exceptions handling}
31
32There are several choice for using the exceptions in wxWidgets programs. First
33of all, you may not use them at all. As stated above, the library doesn't throw
34any exceptions by itself and so you don't have to worry about exceptions at all
35unless your own code throws them. This is, of course, the simplest solution but
36may be not the best one to deal with all possible errors.
37
38Another strategy is to use exceptions only to signal truly fatal errors. In
39this case you probably don't expect to recover from them and the default
40behaviour -- to simply terminate the program -- may be appropriate. If it is
41not, you may override \helpref{OnUnhandledException()}{wxapponunhandledexception}
42in your wxApp-derived class to perform any clean up tasks. Note, however, that
43any information about the exact exception type is lost when this function is
44called, so if you need you should override \helpref{OnRun()}{wxapponrun} and
45add a try/catch clause around the call of the base class version. This would
46allow you to catch any exceptions generated during the execution of the main
47event loop. To deal with the exceptions which may arise during the program
48startup and/or shutdown you should insert try/catch clauses in
49\helpref{OnInit()}{wxapponinit} and/or \helpref{OnExit()}{wxapponexit} as well.
50
51Finally, you may also want to continue running even when certain exceptions
52occur. If all of your exceptions may happen only in the event handlers of a
53single class (or only in the classes derived from it), you may centralize your
54exception handling code in \helpref{ProcessEvent}{wxevthandlerprocessevent}
55method of this class. If this is impractical, you may also consider overriding
56the \helpref{wxApp::HandleEvent()}{wxapphandleevent} which allows you to handle
57all the exceptions thrown by any event handler.
58
59
60\subsection{Technicalities}
61
62To use any kind of exception support in the library you need to build it with
63\texttt{wxUSE\_EXCEPTIONS} set to $1$. This should be the case by default but
64if it isn't, you should edit the \texttt{include/wx/msw/setup.h} file under
65Windows or run \texttt{configure} with \texttt{--enable-exceptions} argument
66under Unix.
67
68On the other hand, if you do \emph{not} plan to use exceptions, setting this
69flag to $0$ or using \texttt{--disable-exceptions} could result in a leaner and
70slightly faster library.
71
72As for any other library feature, there is a \helpref{sample}{sampleexcept}
73showing how to use it. Please look at its sources for further information.
74
75