Question

In Ruby 1.8.7, the documentation does not list ARGF under classes and modules, and ARGF isn’t a class or a module:

ARGF.class # => Object

In Ruby 1.9.3, the documentation has ARGF under classes and modules, but I see this:

ARGF.class # => ARGF.class
ARGF.superclass # => NoMethodError: undefined method `superclass' for ARGF:ARGF.class
ARGF.class.superclass # => Object

• Why does Ruby 1.9 documentation place ARGF as a class when the actual class is something else? Or are they the same thing?
• Is ARGF.class a metaclass, a virtual class, singleton class, or something else?

Answer 1

ARGF is implemented in C and you can do weird things in it. The ARGF class is defined there first. It is not set to any constant in Ruby, but its name is set to “ARGF.class”.
Then ARGF constant is set to an instance of that class.

rb_cARGF = rb_class_new(rb_cObject);
rb_set_class_path(rb_cARGF, rb_cObject, "ARGF.class");
/* ... */
argf = rb_class_new_instance(0, 0, rb_cARGF);
rb_define_global_const("ARGF", argf);

Here is a Ruby code that is doing roughly the same thing.

argf_class = Class.new
def argf_class.name
  "ARGF.class"
end
argf = argf_class.new
ARGF = argf

It does not look reasonable in Ruby, but in C it is fine. Although, I think the class could be set to ARGFClass like NilClass, TrueClass, FalseClass, so that it is not confusing.

I don’t know the history of the change. I think Ruby core folks wanted to get ARGF into the docs and this was the simplest way. (RDoc can’t show documentation for singleton objects.)