class_eval_problem.rb

def create_class
  Class.new
end

MyClass = create_class

MyClass.class_eval do
  p self #=> #<Class:0xb775f6a4>
  def greet
    puts 'hi'
  end
end

obj = MyClass.new
obj.greet

class_eval_problem2.rb

def create_class
  cl = Class.new

  cl.class_eval do
    p self #=> #<Class:0xb775f6a4>
    yield
  end

  return cl
end

p self #=> main

MyClass = create_class do
  p self #=> main
  def greet
    puts 'hello'
  end
end

obj = MyClass.new
obj.send(:greet)
# obj.greet #=> class_eval_problem2.rb:23: private method `greet' called for #<MyClass:0xb78b9644> (NoMethodError)