Question

This snippet of code creates method as a private method, why?

a = %q{def hello() "Hello there!" end}
class A; end
A.class.send(:eval, a)
A.new.hello #=> NoMethodError: private method `hello' called for A

Platform: ruby 1.9.3p125, tested in pry

Answer 1

Methods which are defined outside of any module (so called global methods) are actually defined as private instance methods of Object. That way, the can be called everywhere (since everything inherits from Object), and they can only be called without an explicit receiver.

This includes methods like require, load, puts, print, p, gets, and eval. (Note: most of those are actually defined in Kernel and mixed into Object but the effect and the objective is the same.)

In your case, you are defining a method outside of any module: there is no mention of a module in your a string. The fact that you are calling eval on A.class is completely irrelevant. Like I said above: eval is a global method defined on Object for convenience reasons (so that it may be called everywhere). Your A.class.send(:eval) is just a very convoluted way of calling the global private eval method. It does not somehow magically set the context of the evaluated string to A.class.

You could do 42.send(:eval) instead and the result would still be the same, just like puts('Hello') and 42.send(:puts, 'Hello') are exactly the same, because they end up calling the exact same method.

And by the way: even if it did, it still wouldn’t do what you want. A.class is just Class (the class of any class is always Class), so if it did work as you expect, the method would be defined in Class, not in A.