Class Variables shared by all instances

class_variables.rb

# 1. Create a file class_variables.rb with the contents below
# 2. Run the file with `ruby class_variables.rb` in Terminal
# 3. Review the output and the code below and explore why:
#
#    - the class variable @@total_results_count correctly counts the number
#      of all Result instances.
#
#    - the class variable @@errors does not track the errors for
#      a single instance, instead it wrongly tracks *all* errors
#      for all Result instances.
#
# 4. Note that class variables can be used to track state that is shared
#    by all instances of the class (in this case the total count of
#    all Result instances).
#
# 5. Note that class variables cannot be used to track state is
#    individual to an instance of the class (in this case errors
#    should not be shared between different Result instances).

class Result
  # There is only one object with this name @@errors,
  # and it is shared by all Result instances:
  @@errors = []

  @@total_results_count = 0

  def initialize
    @@total_results_count = @@total_results_count + 1
  end

  def add_error(reason)
    @@errors << reason
  end

  def errors_message
    @@errors.join(', ')
  end

  def total_results_count
    @@total_results_count
  end
end

r1 = Result.new
puts "Just constructed the 1st instance of Result"
puts "Total results count: #{r1.total_results_count}"

r2 = Result.new
puts "Just constructed a 2nd instance of Result"
puts "Total results count: #{r1.total_results_count}"

r3 = Result.new
puts "Just constructed a 3rd instance of Result"
puts "Total results count: #{r1.total_results_count}"


puts "r1 errors message: '#{r1.errors_message}'"
puts "r2 errors message: '#{r2.errors_message}'"

puts "Add an error to r1 only"
r1.add_error('result 1 has a bad foo')

puts "r1 errors message: '#{r1.errors_message}'"

puts "Why is this printing r1's error message?"
puts "r2 errors message: '#{r2.errors_message}'"

puts "Add an error to r2 only"
r2.add_error('result 2 is missing crucial data')

puts "Why are these lines printing both r1 and r2s error messages combined?"
puts "r1 errors message: '#{r1.errors_message}'"
puts "r2 errors message: '#{r2.errors_message}'"