Re: MSVC++ 2005 bug

From:

"Larry Smith" <no_spam@_nospam.com>

Newsgroups:

microsoft.public.vc.language

Date:

Thu, 7 Jun 2007 10:41:55 -0400

Message-ID:

<e9wI#HRqHHA.3356@TK2MSFTNGP03.phx.gbl>

There is a bug in msvc++ 2005 when running the code below, compiled in
release mode:

void foo(int) {
  throw "error";
}

void bar()
{
  int a = 0;
  try
  {
      foo(a++);
  } catch (...) {
      cout << "a=" << a;
  }
}

This will, in release mode, output "a=0", when it should be "a=1".
Guess that the optimization tried to save a temporary by incrementing
'a' after the function call.

It might actually qualify as an error after all though the rule might be
tricky to pin down (the standard isn't always clear on all matters but I
haven't looked). I'm speculating that "a" need not be incremented until the
next sequence point is encountered which I suspect is the end of the
function call itself. All arguments need to be evaluated before calling the
function (in undefined order) but in this case they are. "a" is evaluated to
zero and passed. Whether it needs to be incremented before the function
actually completes however is another matter. Even if there is no error
however (I'm not sure) I'd argue that it's taking the path of "most"
surprise instead of "least" surprise so it should be "corrected" barring any
compelling reasons against it (notiwithstanding the latitude compilers have
in these areas). To be safe however you really shouldn't rely on potential
side-effects like this in the first place.