From: Robert Klemme <shortcutter@...>
Date: 2007-08-20T17:42:35+09:00
Subject: Re: Named Constructor Idiom in Ruby?

2007/8/18, Stefano Crocco <stefano.crocco@alice.it>:
> Alle sabato 18 agosto 2007, Arlen Christian Mart Cuss ha scritto:
> > Hi all,
> >
> > I'm almost certain there's a good way to do this, but I seem to have
> > lost hold of the thought of how -- is there a straight-forward/neat way
> > to do the named constructor idiom in Ruby?
> >
> > In C++ terms, a class like this implements it:
> >
> > class A {
> > public:
> >     static A something() {
> >         A a;
> >         // do something special to a
> >         return a;
> >     }
> >
> >     static A otherthing() {
> >         A a;
> >         // some other special thing to a
> >         return a;
> >     }
> >
> > protected:
> >     A();
> > };
> >
> > The result is that we can't construct A directly, only by one of the two
> > 'named' constructors. Is there any way to achieve a similar thing using
> > protection in Ruby?
> >
> > Thanks,
> >
> > Arlen.
>
> You need to make the new class method of class A private. Then, you implement
> the class methods you want to make public to create the instances of A:
>
> class A
>
>   private_class_method :new
>
>   def self.special_method_1
>     a = A.new
>     #do something special
>     a
>   end
>
>   def self.special_method_2
>     a = A.new
>     #do something else special
>     a
>   end
>
> end
>
> a1 = A.special_method1
> a2 = A.special_method2
>
> I hope this helps

A small glitch:

irb(main):020:0> a1 = A.special_method_1
NoMethodError: private method `new' called for A:Class
        from (irb):6:in `special_method_1'
        from (irb):20
        from :0

You need to remove the "A." when calling "new" after making it
private. It's not needed anyway.

irb(main):021:0> def A.special_method_1
irb(main):022:1> a=new
irb(main):023:1> a
irb(main):024:1> end
=> nil
irb(main):025:0> a1 = A.special_method_1
=> #<A:0x7ff63428>

Kind regards

robert