What does class_eval <<-"end_eval", __FILE__, __LINE__ mean in Ruby?

__FILE__ and __LINE__ are sort of dynamic constants that hold the file and line that are currently executing. Passing them in here allow errors to properly report their location.

instance_eval <<-end_eval, __FILE__, __LINE__
  def foo
    a = 123
    b = :abc
    a.send b
  end
end_eval

foo

When you run this

$ ruby foo.rb 
foo.rb:5:in `send': undefined method `abc' for 123:Fixnum (NoMethodError)
    from foo.rb:5:in `foo'
    from foo.rb:11

Note it says the file and line #5 even though that was just text in an eval. Without those the file/line trick the output would look like this:

$ ruby foo.rb 
(eval):5:in `send': undefined method `abc' for 123:Fixnum (NoMethodError)
    from (eval):5:in `foo'
    from foo.rb:11

The stack trace simply shows (eval) which isn't as helpful.

The << is the start of a heredoc. That line is the start of a multiline string. The string is evaled to create the function. The class_eval function uses the __FILE__ and __LINE__ to add debug information.