Re: [boost] [flyweight] Review period extended to February 3

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.

Corrado

-- 
__________________________________________________________________________
dott. Corrado Zoccolo                          mailto:zoccolo_at_[hidden]
PhD - Department of Computer Science - University of Pisa, Italy
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