ruby-core

Issue #14541 has been updated by jwmittag (J旦rg W Mittag).


Hanmac (Hans Mackowiak) wrote:
> @Eregon: the problem is the other way around ...
> 
> a class variable in a superclass DOES NOT removes class variables in subclasses.
> 
> BUT if you define the class variable in the superclass BEFORE the one is defined in the subclasses,
> 
> THEN the subclass will use the parent one instead

In addition to what Benoit said, there is another problem, namely that class definitions in Ruby are never "finished", so when talking about making changes to classes, "before" and "after" doesn't even make sense. You could always add a variable to either a subclass or a subclass at any time!

----------------------------------------
Bug #14541: Class variables have broken semantics, let's fix them
https://bugs.ruby-lang.org/issues/14541#change-74347

* Author: Eregon (Benoit Daloze)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: ruby 2.6.0dev (2018-01-29 trunk 62091) [x86_64-linux]
* Backport: 2.3: UNKNOWN, 2.4: UNKNOWN, 2.5: UNKNOWN
----------------------------------------
Class variables have the weird semantics of being tied to the class hierarchy and being inherited between classes.
I think this is counter-intuitive, dangerous and basically nobody expects this behavior.

To illustrate that, we can break the tmpdir stdlib by defining a top-level class variable:

    $ ruby -rtmpdir -e '$SAFE=1; @@systmpdir=42; p Dir.mktmpdir {}'
    -e:1: warning: class variable access from toplevel
    Traceback (most recent call last):
    	3: from -e:1:in `<main>'
    	2: from /home/eregon/prefix/ruby-trunk/lib/ruby/2.6.0/tmpdir.rb:86:in `mktmpdir'
    	1: from /home/eregon/prefix/ruby-trunk/lib/ruby/2.6.0/tmpdir.rb:125:in `create'
    /home/eregon/prefix/ruby-trunk/lib/ruby/2.6.0/tmpdir.rb:125:in `join': no implicit conversion of Integer into String (TypeError)

Or even simpler in RubyGems:

    $ ruby -e '@@all=42; p Gem.ruby_version'
    -e:1: warning: class variable access from toplevel
    Traceback (most recent call last):
    	3: from -e:1:in `<main>'
    	2: from /home/eregon/prefix/ruby-trunk/lib/ruby/2.6.0/rubygems.rb:984:in `ruby_version'
    	1: from /home/eregon/prefix/ruby-trunk/lib/ruby/2.6.0/rubygems/version.rb:199:in `new'
    /home/eregon/prefix/ruby-trunk/lib/ruby/2.6.0/rubygems/version.rb:199:in `[]': no implicit conversion of String into Integer (TypeError)

So defining a class variable on Object removes class variables in all classes inheriting from Object.
Maybe @@systmpdir is not so prone to conflict, but how about @@identifier, @@context, @@locales, @@sequence, @@all, etc which are class variables of the standard library?

Moreover, class variables are extremely complex to implement correctly and very difficult to optimize due to the complex semantics.
In fact, none of JRuby, TruffleRuby, Rubinius and MRuby implement the "setting a class var on Object removes class vars in subclasses".
It seems all implementations but MRI print :foo twice here (instead of :foo :toplevel for MRI):

~~~ ruby
class Foo
  @@cvar = :foo
  def self.read
    @@cvar
  end
end

p Foo.read
@@cvar = :toplevel
p Foo.read
~~~


Is there any library actually taking advantage that class variables are inherited between classes? I would guess not or very few.
Therefore, I propose to give class variable intuitive semantics: no inheritance, they behave just like variables of that specific class, much like class-level instance variables (but separate for compatibility).

Another option is to remove them completely, but that's likely too hard for compatibility.

Thoughts?



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