[boost] Re: Including abs() in rational.hpp

From: Paul Moore (gustav_at_[hidden])
Date: 1999-12-18 18:16:43

From: Alan Griffiths [mailto:alan_at_[hidden]]
> Let me relate a problem from "real life"...
>
> Cast: two "public" programmers Janet and John, and a library implementor
> (me).
>
> Janet writes a class using std::vector<>, and provides a swap() method
> and a free swap() function (as someone on this thread has already
> described). She'd been paying attention during my seminar on exception
> safety. The swap function looked something like:
>
>    C& C::swap(C& that)
>    {
>       using std::swap;
>       swap(a, that.a);
>       swap(b, that.b);
>       swap(c, that.c);
>       swap(d, that.d);
>       . . .
>    }
>
> Some months later John comes along and decides that a container (call it
> arg::vector) that I wrote would be better than std::vector. He changes
> the class definition, re-runs the unit test harness and commits the
> change.
>
> The problem is, of course, that the resulting code /1/ no longer meets
> the strong exception safety guarantee and /2/ is a lot slower.  (I had
> provided an efficient, non-throwing swap<> in namespace arg.)

At the risk of exposing my ignorance, isn't that what Koenig lookup does for
you? I assume that you are saying that a etc were std::vector and were changed
to arg::vector. Doesn't Koenig lookup say that swap() is searched for in the
namespace containing a's type - ie std:: or arg:: as appropriate?

If that ISN'T what the lookup rules imply, then I agree with you. But I'm
working on the basis that Janet's code above, even without the using std::swap,
works whatever namespace a is in.

Regardless of the rights and wrongs, I'd have to say that this is all extremely
subtle, and *not* the sort of stuff people could be expected to get right
without better guidelines.

Thanks for the comments,
Paul.