Re: When and where to use Visitor Pattern?

On 28 Apr 2005 12:46:35 -0700, "Daniel Parker"
<danielaparker@xxxxxxxxxxx> wrote:

>
>Robert C. Martin wrote:
>> On Wed, 27 Apr 2005 00:51:29 -0400, "Daniel Parker"
>> <danielaparker@spam?nothanks.windupbird.com> wrote:
>>
>> >wouldn't "instanceof" do just as well? And be a little
>> >clearer? With less clutter in the core class (no visit method)?
>>
>>
>> if (x instanceof D1)...
>> else if (x instanceof D2)...
>> else if (x instanceof D3)...
>> ...
>
>> No, instanceof does not do just as well. To use instanceof you have
>> to create a if/else statement that depends on every possible
>> derivative:
>
>Well, no, you need to query for the interfaces of interest. In most
>cases that wouldn't be every possible derivative, it would be a small
>number of interfaces that every possible derivative implements.

I'm not following you here, unless you are talking about Acyclic
Visitor.

I thought that you were proposing:
if (x instanceof D1)...
else if (x instanceof D2)...
else if (x instanceof D3)...

Clearly in this case, if there are N derivative, then we will do N/2
instanceof calls on average. Visitor, on the other hand, make just
two polymorphic dispatches (one to accept, and the other to visit).
Acyclic visitor makes *one* instanceof call in addition to these two
polymorphic dispatches.


> But
>lets say you do, take the modem example in your article...
>>
>> Not only is this time consuming, it's also heavy with dependencies.
>> visitor (especially acylic visitor) mitigates both of these problems.
>>
>I don't see it. Your UnixModemConfigurator has to implement visit
>methods for all the derivatives of interest, much as instanceof has to
>query for the derivatives of interest.

Implementing a visit method does not take execution time. Instanceof
does.

>There doesn't seem to be a fundumental difference here.

There is a huge difference in the execution timeline. There is also a
big difference in the dependency structure.



-----
Robert C. Martin (Uncle Bob) | email: unclebob@xxxxxxxxxxxxxxxx
Object Mentor Inc. | blog: www.butunclebob.com
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