Minitest中的mc变量名称是什么意思？

Question

In the minitest.rb file is inside the module Minitest the variable mc defined. What is this variable doing? I can't derive it from the context.

Here is a part of the code:

##
# :include: README.rdoc

module Minitest
  VERSION = "5.8.2" # :nodoc:
  ENCS = "".respond_to? :encoding # :nodoc:

  @@installed_at_exit ||= false
  @@after_run = []
  @extensions = []

  mc = (class << self; self; end)

Answer 1

Way back in the old days, before we had the Object#singleton_class method, the only way to get access to the singleton class was to open up the singleton class and return self from the class definition expression.

Remember, a class definition expression, like all other definition expressions, evaluates to the value of the last expression evaluated inside the definition, eg:

class Foo
  42
end
# => 42

Remember also that inside a class definition expression, the value of the special variable self is the class object itself:

class Bar
  self
end
# => Bar

And last but not least, remember that class << some_expression opens up the singleton class of whatever object some_expression evaluates to.

Putting it all together:

class << self # Here, self is Minitest
  self        # Here, self is the singleton class of Minitest
end

Opens up the singleton class of self , which is the Minitest module itself, then it returns self from the singleton class definition, which is the class itself, ergo the singleton class of Minitest .

As to why the variable is named mc : that's short for "metaclass", which is one of the few dozen names that were proposed for what we now call the singleton class . ( Some of the other suggestions were eigenclass (my personal favorite), shadow class, virtual class (yuck!), ghost class, own class, … ) It's a bit of an unfortunate name, because the term metaclass has a well-defined meaning that does not fully match what a singleton class is in Ruby.

Nowadays, we would probably just call singleton_class directly, or, if we want to cache it for performance reasons, do something like

sc = singleton_class

instead.