From: Stefano Crocco <stefano.crocco@...>
Date: 2007-08-18T22:41:57+09:00
Subject: Re: Named Constructor Idiom in Ruby?

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

Stefano