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From: Corrado Zoccolo (czoccolo_at_[hidden])
Date: 2008-02-02 07:51:55

Hi Joaquin

On Feb 2, 2008 12:57 PM, "JOAQUIN LOPEZ MU?Z" <joaquin_at_[hidden]> wrote:

> 2. Indeed GoF introduces a key type K into the pattern that
> is used to retrieve the actual values of T. So, we have
> a one-to-one relation K-->T, i.e. there exists a stateless
> function f of the form
> T f(const K&);
> that can be used to construct a T from a given K. And,
> additionally, K is cheaper to construct than T. This is
> the map approach, right? My question now is: is this a realistic
> scenario? If K is actually cheaper to construct than T and
> we can univocally get the associated T from any K, why work
> with Ts and not just use Ks in the first place? The only
> plaussible justifications I can think of is that f() is
> computationally expensive or that T is more convenient to
> work with than K, but these seem (to me) not so likely
> concerns --more on the second concern on point 3 below, though.
> Note that I explicitly observed that f must be *stateless*, i.e.
> a K object contains exactly the same information as its associated
> T value. This is not the case in most usages of std::map, which
> is a reason why std::maps are useful :)
> I am not plainly denying the existence of sensible K-->T
> scenarios, but I thought long and hard and couldn't find
> any. If you can come up with one I'll be happy to know.
> So, my analysis led me to conclude that the right approach
> is to assume that K==T, that is, the set approach, or at
> most than K and T are just different representations of the
> same information.

One example for all:
K = const char *
T = std::string

I have seen many of those examples in which an object is actually built from
something simpler (in all the cases f will be a one-argument, non-explicit
constructor T(K) ).

What about defining a trait, construction_arg<T>::type that can give K in
such cases, and T if not specified?
It could be generally useful also in other cases, expecially in the
transition phase, in which we still lack r-value references.


dott. Corrado Zoccolo                          mailto:zoccolo_at_[hidden]
PhD - Department of Computer Science - University of Pisa, Italy
The self-confidence of a warrior is not the self-confidence of the average
man. The average man seeks certainty in the eyes of the onlooker and calls
that self-confidence. The warrior seeks impeccability in his own eyes and
calls that humbleness.
                              Tales of Power - C. Castaneda

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