Design Patterns discussion

[oleg shteynbuk <oshteynbuk AT nyc.rr.com>]

Example that you describe is from the “Sample Code” of the Visitor and 
in the first paragraph of this section you can find: “The Equipment 
classes are so simple that using Visitor isn't really necessary, but 
they make it easy to see what's involved in implementing the pattern”. 
They probably anticipated your email :) Motivation section has other 
example that shows that advantage of the Visitor.

Oleg



Chris Finlayson wrote:

> Hi:
>
> I don't understand the advantage a Visitor pattern provides in the context
> of the Go4 example - I was wondering if someone could help clarify this for
> me.
>
> If one has a composite - such as the Equipment composite that the Go4
> discuss and one wants to compute the cost of equipment, I don't get why the
> PricingVisitor is advantageous.  Why bother with the double-dispatch
> mechanism over this well-defined Composite interface?
>
> For example, instead of the PricingVisitor, we could have something like:
>
> class EquipmentPricingPolicy
> {
>
> public:
>         EquipmentPricingPolicy(Equipment* oEquipment)
>         {
>                 m_oEquipment = oEquipment;
>         }
>         Currency GetNetPrice()
>         {
>                 return m_oEquipment->NetPrice();
>         }
>
>         Currency GetDiscountPrice()
>         {
>                 return m_oEquipment->DiscountPrice();
>         }
>
>         Currency GetSavings()
>         {
>                 return GetNetPrice() - GetDiscountPrice();
>         }
>
>         //etc. etc.
>
> private:
>         Equipment* m_oEquipment
> }
>
> So, when m_oEquipment->NetPrice() is called, it gets the net price of the
> equipment - since Equipment is implemented following the Composite design
> pattern.  This seems more straightforward.
>
>
> What I CAN understand is how a visitor is applicable in working with many
> disparate classes of distinct interfaces.  I.e., if one is piecing together
> disparate modules of some legacy code, each having their own definition of
> an Equipment and how to return its net price.  I do not, however, understand
> how it is advantageous for a set of classes adhering to a well-defined
> interface, as the Equipment class' adhere to.
>
>
> --Chris.
>
>
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