Re: [boost] [serialization] a proposal for an alternative to the new c

From: JOAQUIN LOPEZ MU?Z (joaquin_at_[hidden])
Date: 2005-06-25 00:26:37

Hello Dave,

----- Mensaje original -----
De: brangdon_at_[hidden] (Dave Harris)
Fecha: Sábado, Junio 25, 2005 2:37 am
Asunto: Re: [boost] [serialization] a proposal for an alternative to
        the	new	c

> In-Reply-To: <42BBC406.1A3FB240_at_[hidden]>
> joaquin_at_[hidden] (=?iso-8859-
> 1?Q?Joaqu=EDn=20M=AA=20L=F3pez=20Mu=F1oz?=) 
> wrote (abridged):
> > The hardest part is checking for equality. My proposal is
> > to follow a hash-based approach, which is effective both in
> > terms of complexity and space (one word per tracked object.),
> > and does not impose any special requirement on the serialized
> > objects (for instance, an approach based on operator== would
> > require that objects be equalitycomparable).
> > [...]
> >   if(hash_addr!=0){
> >     boost::hash_combine(*hash_addr,x);
> >   }
> 
> Why do we need the test against 0? Is it supposed to improve 
> performance? 
> Hashing ought to be cheap - it is just bit-twiddling on primitives 
> - and 
> conditional branches are relatively expensive. It may be quicker 
> to always 
> hash.

Well, the proposed pseudocode is only meant to illustrate
the idea, certainly it could prove faster to hash
unconditionally.

> 
> It might be worth using #ifndef _NDEBUG or similar, though.
> 

Yes. As this is merely a protection against incorrect user
code, one should be able to turn it on/off or at least
have it only in debug mode, much in the same way as
the safety checks done by some STL implementations.

Joaquín M López Muñoz
Telefónica, Investigación y Desarrollo