C / C++ / MFC

23-Jun-10 2:04

Can anyone point me to any open source application using exception handling? I have never used EH myself nor ever felt the need to use it. Just wondering what and when is the right use for it?
I want to see how other ppl use exception handling in their code and how does it make their program robust?
one good thing about EH is to quickly exit from deeply nested hierarchy. but how do experts use it?
How often do you use EH in your code?

Richard MacCutchan23-Jun-10 3:02

Richard MacCutchan

23-Jun-10 3:02

For questions like this it's always worth checking Google and/or CodeProject articles[^].

It's time for a new signature.

Emilio Garavaglia23-Jun-10 4:47

Emilio Garavaglia

23-Jun-10 4:47

One example is the standard library itself:
A stream ask a buffer to read.
If the buffer cannot complain, throws a runtime error that the stream catches, and set its own badbit, so that further reading are discarded in advance.
Believable or not, every software using C++ streams uses exception handling, at least just because of that.
----

Going more personal, wherever you're implementing something at low level that may -in certain RARE condition or in certain WRONG USAGE- not behave correctly, you can throw an exception.
The idea is to manage all those "rare events" not in the "return chain", but at "some point enough hight", typically not necessarily to have a plai n recover, but attempt a clean exit, at least at a level that allows a clean restart.
This works great especially when tighted to technics like RAII, and self-cleaning objects.

2 bugs found.
> recompile ...
65534 bugs found.
D'Oh! | :doh:

Aescleal23-Jun-10 7:10

23-Jun-10 7:10

Sorry, most of the open source code I use comes from the Linux community and they're not great fans of C++. As a few guidelines:

You generally don't handle exceptions that often. Usually an exception means that something has gone horribly wrong and it's game over folks, let's just clean up and hope a restart gives a better result.

So generally you'd use an exception handler around main() for a console application and that's about it:

int main()
try
{
    // Much good stuff in here...
}
catch( std::exception &e )
{
    std::cout << "Something went wrong: " << e.what() << std::endl;
}
catch( ... )
{
    std::cout << "Something went wrong, no idea what!" << std::endl;
}

If you're writing a shared library or DLL with a C interface you'll want to catch exceptions before exiting a cdecl function. C (and other programming languages) don't know about C++ exceptions so it's a bit rude to through one at them.

The same goes for callbacks which go through any third party libraries (e.g. windows procedures in Windows). They may or may not have the same exception strategy and may not be able to handle them.

Cheers,

Ash

VeganFanatic25-Jun-10 16:11

VeganFanatic

25-Jun-10 16:11

I tend to use assert a lot to check on things as its one way to keep code from crapping out.

http://www.contract-developer.tk

Aescleal25-Jun-10 20:17

25-Jun-10 20:17

If you mean C style assert then I'd have to disagree with you and say it's just about useless, except for one edge case. The reason being is that C style assert is meant to be turned off when you build your final code, which can change your program's observable behaviour and I don't want the safety checks I think are important pulled out when I need them - i.e. when my code meets a user!

The edge case is using C style assert to as your tool to do Test Driven Development in a tools challenged environment. Having something that does a boolean test and informs the user if it's false is the minimum you need to do TDD and C style assert fits the bit adequately.

If you mean "assert" as in the general concept of checking preconditions, postconditions and class invariants then you've bought into Design By Contract and haven't got a lot of choice. In that case though you use assertions to generate an exception if the pre/postcondition is not true or the invariant is broken. I don't tend to use DBC that formally (it's a pain in the arse with C++ as the conditions end up embedded in every method and you have to manually check invariance) but the concepts are good. I've found that having a good set of unit tests from TDD is a good way of checking post conditions for example.

Cheers,

Ash

Stephen Hewitt27-Jun-10 18:13

27-Jun-10 18:13

In general I don't approve of this use of catch (...). As with any rule of thumb there are exceptions but here's my policy on catch (...):

In general you should only catch what you expect could be thrown because catching an unknown exception could result in continued execution when the application is in an inconsistent state. Since catch (...) catches everything it should be avoid. One notable exception is the pattern shown below:

try
{
}
catch (...)
{
   // Cleanup resources.
   throw; // Re-throw.
}

Also note that by not using catch (...) or using it only when re-throwing means that the call stack to the unexpected exception is available in the crash dump. Another pitfall is centred around the /EH[^] switch, or older MS compilers, but I won't go into that now.

Steve

Aescleal28-Jun-10 6:09

28-Jun-10 6:09

Which of the three uses do you disagree with? The first use (catch(...) at the end of main) is only when every other remedial action has failed and the program's going to crash anyway. If you have other base exceptions or exceptions that aren't derived from std::exception feel free to add catches for them at the top level.

Another type is where something throws in a destructor. You can't let exceptions come out of a destructor, std::terminate will end up being called in most cases. So the program's going to crash - you might as well handle it and tell the user that something's gone wrong then bail out cleanly.

A final example is where you don't know where you're throwing. If you have a C interface there's no guarentee that something on the other side knows what to do with it. So catch the exception and translate it to an error code - even if the error code is "Buggered if I know what went wrong in here mate..." which should be translated by the caller into stick up an error message and bail out of the program.

One final point - you shouldn't need to manually clean up resources in the face of an exception. We're talking C++ here and not Java (and even then you've got finally, not catch( ... ) ). The general idiom with C++ is to handle cleanup of resources using RAII, don't do it manually 'cause you're bound to miss something.

Cheers,

Ash

Stephen Hewitt28-Jun-10 15:00

28-Jun-10 15:00

On the issue of a catch (...) in main as follows (I also have objections on other grounds, but we'll address one issue at a time):

#include "stdafx.h"
#include <tchar.h>
#include <iostream>
#include <stdexcept>

using namespace std;

class MyException : public runtime_error
{
public:
	MyException(const char *pMsg) : runtime_error(pMsg) {}
};

void SomeFunction()
{
	throw MyException("SomeFunction()");
}

int _tmain(int argc, _TCHAR* argv[])
{
	try
	{
		SomeFunction(); // Unexpected exception thrown in here (assume it's deeply nested).
	}
	catch (...)
	{
		cerr << "Caught exception..." << endl;
	}

	return 0;
}

All the error information you get from the unexpected event is the following message:

Caught exception...

If the catch (...) is removed a crash dump is generated and the stack extracted from it (using WinDBG) looks as follows:

0:000> .ecxr
eax=00000000 ebx=0024f2d4 ecx=00000000 edx=0008dcd8 esi=770a030c edi=011915e3
eip=742dbb47 esp=0024f1e8 ebp=0024f214 iopl=0         nv up ei pl nz na po nc
cs=0023  ss=002b  ds=002b  es=002b  fs=0053  gs=002b             efl=00000202
msvcr90!terminate+0x33:
742dbb47 e879100100      call    msvcr90!__SEH_epilog4 (742ecbc5)
 
0:000> kvn 1000
 # ChildEBP RetAddr  Args to Child              
00 0024f214 0119161f 0024f2a4 77009d57 0024f2d4 msvcr90!terminate+0x33 (FPO: [Non-Fpo]) (CONV: cdecl) [f:\dd\vctools\crt_bld\self_x86\crt\prebuild\eh\hooks.cpp @ 130]
01 0024f21c 77009d57 0024f2d4 b21db5fb 00000000 Console!__CxxUnhandledExceptionFilter+0x3c (FPO: [Non-Fpo]) (CONV: stdcall) [f:\dd\vctools\crt_bld\self_x86\crt\prebuild\eh\unhandld.cpp @ 72]
02 0024f2a4 77b10727 0024f2d4 77b10604 00000000 kernel32!UnhandledExceptionFilter+0x127 (FPO: [SEH])
03 0024f2ac 77b10604 00000000 0024f894 77acc3d0 ntdll!__RtlUserThreadStart+0x62 (FPO: [SEH])
04 0024f2c0 77b104a9 00000000 00000000 00000000 ntdll!_EH4_CallFilterFunc+0x12 (FPO: [Uses EBP] [0,0,4])
05 0024f2e8 77af87b9 fffffffe 0024f884 0024f424 ntdll!_except_handler4+0x8e (FPO: [4,5,4])
06 0024f30c 77af878b 0024f3d4 0024f884 0024f424 ntdll!ExecuteHandler2+0x26
07 0024f3bc 77ab010f 0024f3d4 0024f424 0024f3d4 ntdll!ExecuteHandler+0x24
08 0024f3bc 772bb727 0024f3d4 0024f424 0024f3d4 ntdll!KiUserExceptionDispatcher+0xf (FPO: [2,0,0]) (CONTEXT @ 0024f424)
09 0024f75c 742ddbf9 e06d7363 00000001 00000003 KERNELBASE!RaiseException+0x58 (FPO: [4,20,0])
0a 0024f794 0119106a 0024f7b4 01192460 b21db00a msvcr90!_CxxThrowException+0x48 (FPO: [Non-Fpo]) (CONV: stdcall) [f:\dd\vctools\crt_bld\self_x86\crt\prebuild\eh\throw.cpp @ 161]
0b 0024f804 011912ca 00000001 005920e8 00591e00 Console!SomeFunction+0x65 (CONV: cdecl) [c:\users\stephen.hewitt\documents\visual studio 2008\projects\scratch\console\console.cpp @ 19]
0c 0024f848 76fe3677 7efde000 0024f894 77ad9d72 Console!__tmainCRTStartup+0x10f (FPO: [Non-Fpo]) (CONV: cdecl) [f:\dd\vctools\crt_bld\self_x86\crt\src\crtexe.c @ 579]
0d 0024f854 77ad9d72 7efde000 72bf8284 00000000 kernel32!BaseThreadInitThunk+0xe (FPO: [1,0,0])
0e 0024f894 77ad9d45 01191412 7efde000 00000000 ntdll!__RtlUserThreadStart+0x70 (FPO: [SEH])
0f 0024f8ac 00000000 01191412 7efde000 00000000 ntdll!_RtlUserThreadStart+0x1b (FPO: [2,2,0]))

This shows what went wrong and where. What's more, this information is sent to Microsoft and can be analysed by the software publisher using Microsoft's Windows Error Reporting[^] website. You get real crash data from real users as well as statistical information on the most common problems.

In short, a pretty message saying "oops" isn't as helpful — crashing is a feature.

Steve

theCPkid29-Jun-10 3:20

29-Jun-10 3:20

thanks for the detailed reply but for a lay user, crash is just an act of cheating by the application. if a software crashes 2-3 times, I normally uninstall it (except the windows OS and ibm rational clearcase coz' i do not hv options in these 2 cases)
If one use catch all in main and then give some kind of error message to the user, that this module has failed. we're sorry. our chipmunks are running to solve your problem, then it is better.
and regarding your stack trace, yes it can be very helpful if you can get the log but I dont know much about it. I think there are ways using SEH to get stack trace etc.
anyway, you made a good point Smile | :)

)

Stephen Hewitt1-Jul-10 14:31

1-Jul-10 14:31

Sure, if it crashes frequently you ditch it. Nothing unusual about you so far. What do you do if it doesn't crash per se, but instead fails with a generic error message just as frequently? I suspect the same. I say let it crash, suck down the crash dumps and fix the problem.

Steve

theCPkid1-Jul-10 17:45

1-Jul-10 17:45

yes, you are right but correct me if I am wrong. Windows 7 seems to be using lot of exception handling and displaying message about why it crashed with what error code. eg. in windows xp, if your explorer crashes, you get a message like "An unhandled win32 exception occured in explorer.exe".. but in windows 7, i hope you have observed it's different. they give you error message etc. You can argue that even information in latter case is not very helpful but in my experience, it is less annoying.
same is the case with world's best browser Wink | ;)

opera. whenever it crashes (once a year or two maybe.. to be honest, opera never crashed before v9. after that i think lot of advancements have made it complex, so it has started crashing once every 3 months or so), an option is provided to restart the browser, along with sending crash dump to opera where it is analyzed and the reason for crash is displayed to the user.
In nutshell, exception handling when done properly can make your application more polite and responsible. If you are not handling exceptions and your application crashes without a hint, how the user will know which dump files to send to which address and in fact, will they ever bother to do it themselves? Poke tongue | ;-P

Stephen Hewitt1-Jul-10 17:59

1-Jul-10 17:59

Windows Error Reporting[^] allows you to publish a solution for a particular stack (it classifies crashes). When you do this and a user gets that particular crash, they get the message you associated with it. This allows you to tell the user what's wrong and what to do about fixing it. The dumps are send automatically (the user has to agree). Sure you could use some custom error reporting framework as some products do, or you could just let it crash and use the one that comes with the OS.

Steve

theCPkid1-Jul-10 19:03

1-Jul-10 19:03

yes, it is a good alternative. One of the requirements of using it is that "You must have administrative privileges on the machine you are using". That should not be a problem in majority of cases though.

second thing is it is "windows" specific. Exception handling is platform neutral.

third thing is I never send data to microsoft website. however, if any of my fav. app crashes, i do send the crash dump to them.

thanks for the info. btw.

Stephen Hewitt1-Jul-10 19:38

1-Jul-10 19:38

theCPkid wrote:
One of the requirements of using it is that "You must have administrative privileges on the machine you are using". That should not be a problem in majority of cases though.

I'm not sure what you're referring to here. The user doesn't need to have administrative privileges to submit a dump file.

theCPkid wrote:
second thing is it is "windows" specific. Exception handling is platform neutral.

Yeah, Windows Error Reporting is certainly Windows specific. That said this message board is primarily about Windows. The general principle of crashing as close to the initiating fault still applies.

Steve

Aescleal29-Jun-10 7:01

29-Jun-10 7:01

This is a complete strawman - the exception you're throwing is derived from std::exception so the first catch clause I wrote would have caught it.

Stephen Hewitt1-Jul-10 14:17

1-Jul-10 14:17

Aescleal wrote:
This is a complete strawman - the exception you're throwing is derived from std::exception so the first catch clause I wrote would have caught it.

You seem to be missing the point. The choice of exception was arbitrary, the point is that it's not expected. Sure the first catch in your code would have caught it, but there is no such beast in the code I posted. My code is different to yours; you seem to be misrepresenting my argument. Who's augment is the straw man? You asked me to elaborate on my position and I did: instead of the user getting a generic error message (of no use to anyone) I get a dump file. I prefer the latter.

Steve

theCPkid1-Jul-10 19:06

Re: Exception Handling in C++ [modified]

1-Jul-10 19:06

Stephen Hewitt wrote:
crashing is a feature.

and I provide that feature absolutely free with no hidden costs in all of my products. Laugh | :laugh:

theCPkid29-Jun-10 3:11