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From: SeeWebsiteForEmail@moderncppdesign.com ("Andrei Alexandrescu (See Website For Email)")
Newsgroups: comp.std.c++
Subject: Re: Is this really unspecified behavior?
Date: Wed, 21 Dec 2005 04:02:43 GMT
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Hyman Rosen wrote:
> Andrew Koenig wrote:
>  > The arguments (and corresponding parameters) could,
>  > it seems to me, be evaluated in any order.
> 
> Only if the goal was fixing the f(auto, auto) problem and
> nothing else. But that's not the main goal, that's a side
> effect of achieving the main goal. The main goal is to make
> C++ expression evaluation a fully deterministic and defined
> process so that code is maximally portable and unambiguous,
> because there is no value in leaving some programming language
> semantics unspecified.

I agree. That brings me to another gratuitous source of problems. The 
language should finally fix the postfix ++ and -- to give a guarantee. 
As of now,

a = b++;

has undefined behavior if a and b alias the same lvalue. There's no 
particular reason for which things have to be that way anymore. The 
PDP-7 had some postincrement assembler instruction that did something 
like that, and it was very advisable to use it because it was fast. I 
don't think we still need to cling to such anachronisms. The semantics 
of postfix operator++ should be those of a function:

int & increment(int & i) {
   int old(i);
   ++i;
   return old;
}

that does the postincrement. Then, writing a = b++ would be equivalent 
to a = increment(b); which has defined semantics, and I doubt there will 
be any loss in efficiency with today's compilers.


Andrei

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