an example about TOPLEVEL_BINDING

gistfile1.rb

class A
  eval 'def f; "hello world"; end'
end  

A.new.f # => "hello world"
A.f # NoMethodError: undefined method `f' for A:Class
f # NameError: undefined local variable or method `f' for main:Object

class A
  eval 'self', TOPLEVEL_BINDING # => main
end

class A
  eval 'def g; "hello world"; end', TOPLEVEL_BINDING
end  

g # => "hello world"
A.new.g # NoMethodError: private method `g' called for #<A:0x007f81af845e58>

I don't know why the last error msg is " private method `g'"