Re: Is run-time switching on type traits bad?

On 15 Apr., 08:53, DeMarcus <use_my_alias_h...@hotmail.com> wrote:


No, I don't think so. Traditionally, compile-time
traits are very popular in C++, but there is no
reason at all to restrict to use runtime traits.
In fact, even the library provides them: The
regex_traits belong to the rare library traits
that are intrinsically stateful, which can be
recognized by noticing the non-const function

locale_type imbue(locale_type loc);

and the fact that the majority of it's functions
are non-static functions. Usually this kind of
customization point is not strictly considered
as a *type* trait, because stateful traits are
usually object specific because of the potentially
individual state. In the way to view at it,
allocators could be considered as *traits* as well,
but normally they are not named this way. In fact,
stateful/polymorphic traits are often called
"policies".

In regard to your question related to type-switching:
There is always the danger that type-switching can be
abused. It requires some discipline to stay away
from that and the real danger is that these switches
tend to occur in many different code locations which
makes them extremely hard to maintain.

Therefore I always try first one of the following
alternatives:

a) Direct virtual function calls
b) Delegation to some registry that hides the switch
or replaces it by a more general mapping

Following (b), in your example it would be much more
better to define a registry/provider, that is queried
for the correct visitor, so the code would look like
this:

int main()
{
     std::vector<VisitableInferface*> vec;

     for( auto i = vec.begin(); i != vec.end(); ++i )
     {
        IVisitor& visitor = visitors.get_visitor(*i);
        (*i)->accept(visitor);
     }
}

One might also consider to replace the two lines
within the loop by a single one:

        visitors.set_visitor(*i);

This completely hides the actual switch and ensures
that there exists a single point of responsibility.

HTH & Greetings from Bremen,

Daniel Kr?gler

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