Can some one explain the code snippet given below in ruby ?

Question

class ApplicationController < ActionController::Base
  class InvalidParam < StandardError;end
end

What is the reason for including another class inside application controller?? What will be the behavior for other controllers?

Answer 1

Can it be declaration of nested class that will not be used anywhere else?

Answer 2

The reason for including another class inside application controller is that the class will be visible only inside the controller (scoping). Other controllers will not be influenced.

Answer 3

Adding Semantics - Private exceptions v System generated exceptions

There's no structural difference between InvalidParam and StandardError - I'm betting that the developer is layering his/her own semantics on InvalidParam. That's naughty because it will just confuse the reader.

The code declares a nested class called InvalidParam for the purposes of Exception handling. The developer wants to be able to raise and rescue exceptions with InvalidParam rather than StandardError - most likely because they want to distinguish between system exceptions and their own.