• Sean Eskapp (8/8) Feb 13 2011 I remember in C++, I had to do double-dispatch using the visitor pattern...
• Simen kjaeraas (10/19) Feb 13 2011 Andrei Alexandrescu (yes, the same one) wrote a generic implementation o...
• Lutger Blijdestijn (19/29) Feb 13 2011 There isn't really, but you can take out some of the boilerplate with a
• bearophile (4/5) Feb 13 2011 Andrei has recently said no one needs double dispatch (in D) :-) So Andr...
• Sean Eskapp (3/8) Feb 13 2011 The age-old collision handling problem is how I'm using it.
• Peter Alexander (3/11) Feb 16 2011 Use a 2D array of function pointers, with the indices coming from unique...

Sean Eskapp <eatingstaples gmail.com> writes:

I remember in C++, I had to do double-dispatch using the visitor pattern. This
is cumbersome, just because each subclass has to have the exact same
singly-dispatched code that looks like:

void dispatch(Base& other)
{
   other.dispatch(*this);
}

Is there a nicer way to do this in D, or am I stuck with the same thing?

"Simen kjaeraas" <simen.kjaras gmail.com> writes:

Sean Eskapp <eatingstaples gmail.com> wrote:

 I remember in C++, I had to do double-dispatch using the visitor  
 pattern. This
 is cumbersome, just because each subclass has to have the exact same
 singly-dispatched code that looks like:

 void dispatch(Base& other)
 {
    other.dispatch(*this);
 }

 Is there a nicer way to do this in D, or am I stuck with the same thing?

Andrei Alexandrescu (yes, the same one) wrote a generic implementation of
multimethods[1] for his book Modern C++ Design[2]. You might want to take
a look at that. If you decide to re-implement it in D, it might also be
worth incorporating in Phobos.

[1]:  
http://loki-lib.cvs.sourceforge.net/loki-lib/loki/include/loki/MultiMethods.h?view=markup
[2]: http://www.amazon.com/a/dp/0201704315/

-- 
Simen

Lutger Blijdestijn <lutger.blijdestijn gmail.com> writes:

Sean Eskapp wrote:

 I remember in C++, I had to do double-dispatch using the visitor pattern.
 This is cumbersome, just because each subclass has to have the exact same
 singly-dispatched code that looks like:
 
 void dispatch(Base& other)
 {
    other.dispatch(*this);
 }
 
 Is there a nicer way to do this in D, or am I stuck with the same thing?

There isn't really, but you can take out some of the boilerplate with a 
mixin, which goes a long way:

// The template parameter Visitor is not actually needed, depending on 
// what you want to do
mixin template Accept(Visitor)
{
    void accept(Visitor v)
    {
        v.visit(this);
    }
}

class Stuff
{
    mixin Accept!IStuffVisitor;
}



With some ctfe it's also not too hard to take out the boilerplate of 
creating the visitor interface with a bit of code generation, though I'm not 
sure if it's worth it.

bearophile <bearophileHUGS lycos.com> writes:

Sean Eskapp:

 Is there a nicer way to do this in D, or am I stuck with the same thing?

Andrei has recently said no one needs double dispatch (in D) :-) So Andrei will
be interested in your use case.

Bye,
bearophile

Sean Eskapp <eatingstaples gmail.com> writes:

== Quote from bearophile (bearophileHUGS lycos.com)'s article
 Sean Eskapp:
 Is there a nicer way to do this in D, or am I stuck with the same thing?

 Andrei has recently said no one needs double dispatch (in D) :-) So Andrei will

be interested in your use case.
 Bye,
 bearophile

The age-old collision handling problem is how I'm using it.

Peter Alexander <peter.alexander.au gmail.com> writes:

On 13/02/11 9:32 PM, Sean Eskapp wrote:
 == Quote from bearophile (bearophileHUGS lycos.com)'s article
 Sean Eskapp:
 Is there a nicer way to do this in D, or am I stuck with the same thing?

 Andrei has recently said no one needs double dispatch (in D) :-) So Andrei will

 be interested in your use case.
 Bye,
 bearophile

 The age-old collision handling problem is how I'm using it.

Use a 2D array of function pointers, with the indices coming from unique 
IDs on the shapes.