Hi Joaquin On Feb 2, 2008 12:57 PM, "JOAQUIN LOPEZ MU?Z" <joaquin@tid.es> 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@di.unipi.it 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