Visual C++ Unmanaged Code: Use /EHa or /EHsc for C++ exceptions?

Question

If I\'m creating a new project in unmanaged C++, Visual Studio 2008 or greater, which exception handling model do I want to go with?

I understand that /EHa option re

Answer 1

I use /EHa because it's safe with .NET interop , whereas /EHsc may not be; see Destructors not called when native (C++) exception propagates to CLR component for example.

However, if for a specific bit of code the extra performance really matters and you don't need .NET (or whatever else) compatibility, then sure, /EHsc sounds fine.

Neither /EHsc nor /EHa catch most memory errors, so using these to catch access violations is a hopeless case.

Answer 2

/EHa does two things. First and foremost, it suppresses an optimization that omits exception filters that automatically call the destructors of local class variables if the code analyzer cannot see any code that might throw a C++ exception. This makes stack unwinding safe for any kind of exception, not just a C++ exception. Overhead for these exception filters is time on x86 and space on both x86 and x64.

And yes, it alters the behavior of catch(...), it now also filters any SEH exception, not just C++. This is indeed a gamble because you catch all the really nasty stuff, the asynchronous hardware exceptions. Although I personally don't think that catching all C++ exceptions is very defensible either, you still have but a vague idea to what degree the program state got mutated and why it failed.

Realistically, you'll need to switch to using __try/__except so that you can write your own exception filter and avoid catching the bad ones. The exception code for C++ exceptions is 0xe04d5343 ("MSC"). Using _set_se_translator() would be another approach.