Question

In many ruby projects and even ruby itself I’ve encountered the using of dup method. For example, in some project I’ve met this construction:

class Array
  def flush
    self.dup.tap { self.clear }
  end
end

The question is why we should use dup method i.e. create copy of object instead of just use the same object? Thanks

Answer 1

Use dup or clone if part of the codebase is written by an idiot that doesn’t know you shouldn’t modify objects if you can help it (instead, you should create a new object):

class Greeter

  def initialize
    @greeting_cache = {}
  end

  def expensive_greeting_calculation(formality)
    case formality
      when :casual then "Hi"
      when :formal then "Hello"
    end
  end

  def greeting(formality)
    unless @greeting_cache.has_key?(formality)
      @greeting_cache[formality] = expensive_greeting_calculation(formality)
    end
    @greeting_cache[formality]
  end

end

def memoization_mutator
  greeter = Greeter.new
  first_person = "Bob"
  # Mildly contrived in this case,
  # but you could encounter this in more complex scenarios
  puts(greeter.greeting(:casual) << " " << first_person) # => Hi Bob
  second_person = "Sue"
  puts(greeter.greeting(:casual) << " " << second_person) # => Hi Bob Sue
end

memoization_mutator

In my case, the idiot is past-me.