Raising an exception: use instance or class?

Question

I have seen Ruby code that raises exceptions using a class:

raise GoatException,    "Maximum of 3 goats per bumper car."

Other code uses an instance:

raise GoatException.new "No leotard found suitable for goat."

Both of these are rescued the same way. Is there any reason to use an instance vs a class?

Answer 1

It makes no difference; the exception class will be instiantiated in either case.

If you provide a string, either as the argument to new or as the second argument to raise, it be passed to initialize and will become the exception instance's .message.

For example:

class GoatException < StandardError
  def initialize(message)
    puts "initializing with message: #{message}"
    super
  end
end

begin
  raise GoatException.new "Goats do not enjoy origami." #--|
                                                        #  | Equivilents
  raise GoatException,    "Goats do not enjoy origami." #--|
rescue Exception => e
  puts "Goat exception! The class is '#{e.class}'. Message is '#{e.message}'"
end

If you comment the first raise above, you'll see that:

• In both cases, initialize is called.
• In both cases, the exception class is GoatException, not class as it would be if we were rescuing the exception class itself.