[Bug libstdc++/20758] operator-(const T&, const complex<T>&) vs operator-(const complex<T>&, const complex<T>&)

["gdr at integrable-solutions dot net" <gcc-bugzilla at gcc dot gnu dot org>]

------- Additional Comments From gdr at integrable-solutions dot net  2005-04-08 14:15 -------
Subject: Re:  operator-(const T&, const complex<T>&) vs operator-(const complex<T>&, const complex<T>&)

"kreckel at ginac dot de" <gcc-bugzilla@gcc.gnu.org> writes:

| (In reply to comment #11)
| > I think we need more careful analysis and tracking of both C99, C++ and
| > LIA-3. 
| 
| Apart from looking at standards, we could also try to use our brains, right?  It
| must be possible to answer the question whether the current behavior is right or
| not by analogy with real numbers, ie. simply by looking at the imaginary part alone.
| 
| On systems without signed zero, there is no problem.
| 
| On systems with -0.0, the code is trying to compute 0.0 - 0.0.  Can that
| possibly be -0.0?  If the answer is _no_, then this is a bug and it ought to be
| fixed.  Period.  If the asnwer is _yes_, then, well, then I'm bemazed and confused.

Thatis the mathematical question/answer.  The real issue is this:

  * in operator-(const T&, const complex<T>&), should the imaginary
    part eve be touched?

there are vairous ansewrs.  And, yes we've been using our brains.

| 
| BTW: I've always tought that systems that distinguish between 0.0 and -0.0, but
| not between 0.0 and +0.0 are slightly broken from a mathematical point of view.

If you ask me, any system with signed zeros is broken, to start with.

-- Gaby


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20758