Question

What’s the difference between these classes? What’s the difference between their purposes?

Answer 1

BasicObject was introduced in Ruby 1.9 and it is a parent of Object (thus BasicObject is the parent class of all classes in Ruby).

BasicObject has almost no methods on itself:

::new
#!
#!=
#==
#__id__
#__send__
#equal?
#instance_eval
#instance_exec

---

BasicObject can be used for creating object hierarchies independent of
Ruby’s object hierarchy, proxy objects like the Delegator class, or
other uses where namespace pollution from Ruby’s methods and classes
must be avoided.

BasicObject does not include Kernel (for methods like puts) and
BasicObject is outside of the namespace of the standard library so
common classes will not be found without a using a full class path.

---

Object mixes in the Kernel module, making the built-in kernel
functions globally accessible. Although the instance methods of Object
are defined by the Kernel module…

You can use BasicObject as a parent of your object in case if you don’t need methods of Object and you would undefine them otherwise:

# when you inherit Object
class Tracer
  instance_methods.each do |m|
    next if [:__id__, :__send__].include? m
    undef_method m
  end

  # some logic
end

# when you inherit BasicObject
class Tracer < BasicObject
  # some logic
end