Improve documentation about handling C++ exceptions in wx programs.

Try to explain the different exception handling strategies more clearly in the
overview and also update OnUnhandledException() documentation.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66205 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

docs/doxygen/overviews/exceptions.h

@@ -28,10 +28,10 @@ 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 is exception-friendly.
+However the recent wxWidgets versions are exception-friendly. This means that
-It still doesn't use the exceptions by itself but it should be now safe to use the
+while the library still doesn't use the exceptions by itself, it should be now
-exceptions in the user code and the library tries to help you with this. Please
+safe to use the exceptions in the user code and the library tries to help you
-note that making the library exception-safe is still work in progress.
+with this.
 
 
 @section overview_exceptions_strategies Strategies for exceptions handling
@@ -42,35 +42,50 @@ 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
+The next simplest strategy is to only use exceptions inside non-GUI code, i.e.
-this case you probably don't expect to recover from them and the default
+never let unhandled exceptions escape the event handler in which it happened.
-behaviour -- to simply terminate the program -- may be appropriate. If it is
+In this case using exceptions in wxWidgets programs is not different from using
-not, you may override wxApp::OnUnhandledException()
+them in any other C++ program.
-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 wxApp::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
-wxApp::OnInit() and/or wxApp::OnExit() as well.
 
-Finally, you may also want to continue running even when certain exceptions
+Things get more interesting if you decide to let (at least some) exceptions
-occur. If all of your exceptions may happen only in the event handlers of a
+escape from the event handler in which they occurred. Such exceptions will be
-single class (or only in the classes derived from it), you may centralize your
+caught by wxWidgets and the special wxApp::OnExceptionInMainLoop() method will
-exception handling code in wxApp::ProcessEvent
+be called from the @c catch clause. This allows you to decide in a single place
-method of this class. If this is impractical, you may also consider overriding
+what to do about such exceptions: you may want to handle the exception somehow
-the wxApp::HandleEvent() which allows you to handle
+or terminate the program. In this sense, OnExceptionInMainLoop() is equivalent
-all the exceptions thrown by any event handler.
+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.
+
+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().
+
+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).
 
 
 @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. This should be the case by default but
+with @c wxUSE_EXCEPTIONS set to 1. It is turned on by default but you may
-if it isn't, you should edit the @c include/wx/msw/setup.h file under
+wish to check @c include/wx/msw/setup.h file under Windows or run @c configure
-Windows or run @c configure with @c --enable-exceptions argument
+with explicit @c --enable-exceptions argument under Unix.
-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

interface/wx/app.h

@@ -468,15 +468,21 @@ public:
     virtual int OnRun();
 
     /**
-        This function is called when an unhandled C++ exception occurs inside
+        This function is called when an unhandled C++ exception occurs in user
-        OnRun() (the exceptions which occur during the program startup and shutdown
+        code called by wxWidgets.
-        might not be caught at all). Notice that by now the main event loop has been
-        terminated and the program will exit, if you want to prevent this from happening
-        (i.e. continue running after catching an exception) you need to override
-        OnExceptionInMainLoop().
 
-        The default implementation shows information about the exception in debug build
+        Any unhandled exceptions thrown from (overridden versions of) OnInit()
-        but does nothing in the release build.
+        and OnExit() methods as well as any exceptions thrown from inside the
+        main loop and re-thrown by OnUnhandledException() will result in a call
+        to this function.
+
+        By the time this function is called, the program is already about to
+        exit and the exception can't be handled nor ignored any more, override
+        OnUnhandledException() or use explicit @c try/catch blocks around
+        OnInit() body to be able to handle the exception earlier.
+
+        The default implementation dumps information about the exception using
+        wxMessageOutputBest.
     */
     virtual void OnUnhandledException();