Ruby performance: define method with define_method or eval

Question

While looking through ActiveSupport source code I've noticed that sometimes eval is used in places where define_method is enough.

Example: ActiveSupport: Module.delegate

I consider define_method more clean and safe way of doing things. What is the benefits of eval over define_method? Perfomance, memory usage, something else?

Answer 1

When you use define_method, the method you're defining can't accept a block.

It’s pretty well known that because of a deficiency in blocks arguments in Ruby 1.8 Class#define_method cannot define methods that take blocks.

def x *args, █ end  # => works!
define_method(:x) {|*args,&block| } # => SyntaxError: compile error

The method being defined requires a block:

"def #{prefix}#{method}(*args, &block)" # def customer_name(*args, &block)

So define_method can't be used.

Answer 2

I found this to be a very nice article on the subject: http://blog.grayproductions.net/articles/eval_isnt_quite_pure_evil.

Answer 3

I don't know what the reason in that particular case, but define_method takes a block, which is a closure (carries local variables of the place it was defined), and that can lead to considerably higher memory consumption comparing to plain eval.