Semantics of toString
Justin Johansson
no at spam.com
Thu Nov 12 10:47:57 PST 2009
Andrei Alexandrescu Wrote:
> Denis Koroskin wrote:
> > On Thu, 12 Nov 2009 16:23:22 +0300, Steven Schveighoffer
> > <schveiguy at yahoo.com> wrote:
> >
> >> On Thu, 12 Nov 2009 08:22:26 -0500, Steven Schveighoffer
> >> <schveiguy at yahoo.com> wrote:
> >>
> >>> On Tue, 10 Nov 2009 18:49:54 -0500, Andrei Alexandrescu
> >>> <SeeWebsiteForEmail at erdani.org> wrote:
> >>>
> >>>> I think the best option for toString is to take an output range and
> >>>> write to it. (The sink is a simplified range.)
> >>>
> >>> Bad idea...
> >>>
> >>> A range only makes sense as a struct, not an interface/object. I'll
> >>> tell you why: performance.
> >>>
> >>> Ranges are special in two respects:
> >>>
> >>> 1. They are foreachable. I think everyone agrees that calling 2
> >>> interface functions per loop iteration is much lower performing than
> >>> using opApply, which calls one delegate function per loop. My
> >>> recommendation -- use opApply when dealing with polymorphism. I
> >>> don't think there's a way around this.
> >>
> >> Oops, I meant 3 virtual functions -- front, popNext, and empty.
> >>
> >> -Steve
> >
> > Output range has only one method: put.
> >
> > I'm not sure, but I don't think there is a performance difference
> > between calling a virtual function through an interface and invoking a
> > delegate.
> >
> > But I agree passing a delegate is more generic. You can substitute an
> > output range with a delegate (obj.toString(&range.put, fmt)) without any
> > performance hit, but not vice versa (obj.toString(new
> > DelegateWrapRange(&myput), fmt) implies an additional allocation and
> > additional indirection per range.put call).
>
> I think that, on the contrary, working with a delegate is less generic.
> A delegate is cost-wise much like a class with only one (non-final)
> method. Since we're taking that hit already, we may as well define
> actual interfaces and classes that have multiple methods. That makes
> things more flexible and more efficient.
>
> Andrei
"Since we're taking that hit already, we may as well define
> actual interfaces and classes that have multiple methods."
Which you mean -- interfaces, classes or both?
Don't interfaces have a higher cost than classes?
Justin
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