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Subject: Re: [boost] Boost.Multimethod proposal
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2009-08-24 15:31:53

Edward Diener wrote:
> Jeffrey Bosboom wrote:
> > Edward Diener wrote:
> >> What is a practical use for multi-methods ?
> >
> > Multimethods (assuming this library uses the term as it's commonly
> > understood) are like virtual functions that are dispatched on the
> > dynamic types of their arguments as well as the type of *this. You can
> > get this effect through double dispatch, but it requires a bit of
> > plumbing and requires two virtual calls (which for some people is a
> > large amount of overhead). (Of course, multimethods have overhead too.)
> This does not explain the practical purpose of multimethods.
> I know what virtual functions are, of course. What in multimethods
> improves on the polymorphic capabilities of virtual functions that make them
> a practical choice for use over normal polymorphism.

If it isn't obvious yet, you missed the bit about dispatching on the *dynamic type of the arguments*. Virtual functions can dispatch on the dynamic type of this, but not on the arguments. Double dispatch uses two virtual calls to dispatch on this and then, by switching argument order, on the dynamic type of the argument-cum-this-type. Extending that beyond two types, however, becomes nearly untenable.

Rob Stewart robert.stewart_at_[hidden]
Software Engineer, Core Software using std::disclaimer;
Susquehanna International Group, LLP

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