Catching all exceptions and logging information

Question

I am a Java programmer working with C++ code, and need some help with exception handling.

I have code in the following structure:

try{
...
}
catch( ... ) 
{
    log("Exception occurred");
}

An exception is occurring, but the try block is really massive and debugging is not an option, so I need to minimally modify the code to give me relevant info on the exception.

So I added the following catch block before the existing catch block (working with my Java knowledge and referring to C++ guides):

catch(exception e)
{
    log(e.what());
}

However, I am still getting the old message - "Exception occurred". The code in try block contains a lot of low level functions like strncpy, memcpy etc.

Why is this catch block not working as expected? What can I do to get information on the exception that is occurring and on which line, similar to the stack trace that Java so easily gives.

There is no such thing as an "exception" in `C` (at least not in the sense of a control-flow mechanism). — Mankarse, May 10 '12 at 07:26
@Mankarse, `std::exception` does exist in C++ standard library. To Shailesh, you can use gdb to debug such non-standard exceptions. — iammilind, May 10 '12 at 07:29
possible duplicate of [Getting the backtrace from the catch block](http://stackoverflow.com/questions/4283943/getting-the-backtrace-from-the-catch-block) — BЈовић, May 10 '12 at 07:33
@iammilind: This question was previously tagged C, and talked about handling exception handling in "C/C++" code. It has since been edited, as Shailesh Tainwala is clearly writing C++, not C. — Mankarse, May 10 '12 at 07:34
worth to mention that you should put your catch block before the catch(...), also you should catch by reference and not by value — Alessandro Teruzzi, May 10 '12 at 07:36
@AlessandroTeruzzi: In the question the guy specified that he put his catch block _before_ `catch( ... )`. — Jagger, May 10 '12 at 07:45

score 6 · Answer 1 · answered May 10 '12 at 08:04

First, you should catch by reference (generally const), so your new catch block should read:

try {

} catch(std::exception const& e) {
    log(e.what());
} catch(...) {
    log("Exception occurred");
}

Second, in C++ you may throw any value. Even of type int. If your codebase include such unsavvy throw statements, I pity you.

Since you come from Java, I would check if you mistakenly used a throw new XXXX which would throw a pointer (to a dynamically allocated value) instead of a value. The new is unnecessary in C++.

score 3 · Answer 2 · answered May 10 '12 at 07:27

3

Probably because those exceptions are not derived from exception class. In C++ any type can be an exception, for example string, int, etc. Anyway if you want to catch exception you should probably catch a reference to the exception &.

answered May 10 '12 at 07:27

Jagger

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If the existing catch-all handler didn't catch the exception, the exception thrown is not an c++ exception at all. – Alok Save May 10 '12 at 07:34

score 2 · Accepted Answer · answered May 10 '12 at 07:28

2

You will have to debug and determine if the exception is an C++ exception.
Note that divide by 0 etc are runtime exceptions not supported by C++, so it can be anything literally.

A catch-all handler will only catch valid C++ exceptions not all exceptions(which can be plenty).

answered May 10 '12 at 07:28

Alok Save

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How can I get information about the exception that occurred from the catch-all handler? – Shailesh Tainwala May 10 '12 at 07:28
3

@ShaileshTainwala: You cannot, and hence catch-all is to be avoided to begin with.It just hides your problems, not solves it.Your best bet is to determine what goes wrong by debugging, there is no easy way. – Alok Save May 10 '12 at 07:30
@Als: I got from the question that the catch-all did catch the exception. – Matthieu M. May 10 '12 at 08:00
@MatthieuM.: I didn't get the same actually, perhaps the OP should clarify. – Alok Save May 10 '12 at 08:35

Catching all exceptions and logging information

3 Answers3