Re: [Boost-users] Re: swapping keys in multiindex

From: Joaquín Mª López Muñoz (joaquin_at_[hidden])
Date: 2005-04-15 07:48:56

David Gruener ha escrito:

> Joaquin M Lopez Munoz wrote:
> > There's no particular provision for swapping keys. You
> > could do something like:
> >
> > blah.modify_key(blah.find(foo1),         _1 = unused_value);
> > blah.modify_key(blah.find(foo2),         _1 = foo1);
> > blah.modify_key(blah.find(unused_value), _1 = foo2);
>
> Well, i did something like this (in my case unused_value is INT_MIN).
> But i really dont find that a clean solution. I think
> the container must not assume that i always have something
> like an "unused" value in the particular domain, it seems
> easy for int, but that might not be true for other types.

I totally agree with you; it's an ugly workaround (but at least is
more than you can do with an std::container, since you retain
iterator and reference stability.)

>
>
> > Providing a general key_swap facility is not as simple as it
> > might seem
>
> Thats what I expected. Not simple, but don't you think thats possible,
> do you?

It's possible, at the expense of fattening the internal implementation
of all indices :(

> I think we need a swap() for values too, because a value might
> be used as a uniq key in another index. IMHO such a swap method is valuable.
>

>From the point of view of the internal implementation, a value swap is
entirely equivalent to a key swap (same relationship as between modify
and modify_key, internally the implementation is the same.) The practical
usefulness of a value swap AFAICS is limited, though, since it only would
have any important effect on sequenced indices, and for those you can
use relocate().

In any case, I'm writing this down into my list of things to consider.

Joaquín M López Muñoz
Telefónica, Investigación y Desarrollo