From: George Ogata <george.ogata@...>
Date: 2007-02-14T11:17:29+09:00
Subject: Re: Question about rb_cvar_set

On 2/14/07, Daniel Berger <djberg96@gmail.com> wrote:
> Hi,
>
> Ruby 1.8.5 p12
>
> It looks like there was a prototype change in rb_cvar_set() at some
> point in the 1.8.x branch. It now takes a 4th parameter (an int). What
> is the 4th parameter supposed to be?

Hi Dan,

If you poke your nose into the rb_cvar_set() function, you'll see it's
just a warning enabler flag.  Specifically, if this argument is
nonzero, then in verbose mode (-v, -w) you'll get a warning if you
modify a class variable from a subclass.  As in:

$ ruby -w -e 'class B; @@x = 1; end; class D < B; @@x = 5; end'
-e:1: warning: already initialized class variable @@x

The flag appears to be used to distinguish between cases such as:

  class B
    @@x = 1
  end

  class D < B
    @@x = 2    # warning

    def f
      @@x = 3  # no warning
    end

    def self.f
      @@x = 4  # no warning
    end
  end

Here, although the last 3 assignments to @@x all modify B's @@x, the
ones inside a def are considered "assignment-style", whereas the "@@x
= 2" is "declaration-style".  Only the latter generates a warning.

Regards,
George.