Is it possible for a library consumer to override C++ exceptions handling?

Question

I have a C++ DLL with code like this:

LogMessage( "Hello world" );
try {
    throw new int;
} catch( int* e ) {
    LogMessage( "Caught exception" );
    delete e;
}
LogMessage( "Done" );

This DLL is loaded by some third-party application and the code above is invoked. The problem is only the first LogMessage is invoked - even though there's an exception handler the control flow is transferred to the unknown.

I see this and can't decide whether it's some obscure bug to be investigated or just the evil force of the consumer application.

Is it really possible for a consumer application to override the C++ exception handling in a DLL?

EDIT: The problem resolved after thinking on all things to check outlined in the answers. In real code it's not just throw, there'a a special function for throwing exceptions that has a call to MessageBoxW() Win32 call in debug version. And the consumer application had troubles showing the message box (it's an NT service) and effectively hung up. So it's not a problem of C++ exceptions handling in any way.

Answer 1

The code is OK. I executed it with this function:

void LogMessage( char* s )
{
  cout << s << endl;
}

And got the correct output:

Hello world

Caught exception

Done

Is it possible that there is some problem with your LogMessage function?

I would suggest to investigate (by debugger or adding debug prints), whether your flows reaches the try block, and whether it reaches the catch block.

Not being a problem, it still isn't clear, what do you achieve by throwing a pointer and not a value. I would recommend not to use catch-by-pointer, see the reasoning here, at the C++-FAQ-lite.

Answer 2

First, please check whether the LogMessage call flushes the output (use << endl).

Although far-fetched, is it possible that the new call is throwing an exception (out of memory)? I'd consider that as one reason not to new exceptions (just to add to what Igor said).

Answer 3

The code looks okay. But can you check if there any unhandled exception before the structured exception throw new int; is thrown.

LogMessage( "Hello world" );
try {

     //some unhandled exception here 

    throw new int;
} catch( int* e ) {
    LogMessage( "Caught exception" );
    delete e;
}
LogMessage( "Done" );

If that is the case then you can check the same using SetUnhandledExceptionFilter function.

If the DLL already provides an unhandled exception filter and if that returns EXCEPTION_EXECUTE_HANDLER then your exception handler will not be called.

You need to use _set_se_translator to convert the win32 exceptions to c++ structured exceptions.

Answer 4

The giveaway is actually in the EDIT. "MessageBoxW() ... NT service ... effectively hung up.". That's a fair description. The message box is on a desktop not associated with a logged-in user. Hence, it's still there, waiting for a mouseclick that will never happen.