Example seeming to demonstrate that unary ampersand operator holds on to obj references in Ruby 2.1.x

unary_ampersand_keeping_refs.rb

# Create some special classes to facilitate tracking allocated objects.
class TrackedArray < Array; end
class TrackedString < String; end
STRANG = "a" * 5000

class ClingyObjects
  def generate(should_cling = false)
    strs = TrackedArray.new
    30000.times { strs << TrackedString.new(STRANG) }

    char_count = 0
    # I'm not sure why, but using the unary & operator on the Array, whether
    # through #each or #map, prevents the allocated objects from being GC'd.
    # Maybe I'm missing something, but after this method returns nothing
    # should refer to the strs Array or any of the objects contained in the
    # Array, so GC should proceed without issue. What gives?

    strs.each(&:length) if should_cling
    strs.each {|x| char_count += x.length }
    char_count
  end

  # Helper to print object allocation stats.
  def object_stats(tag)
    puts "#{tag}:"
    puts "TrackedArray: #{ObjectSpace.each_object(TrackedArray).count}"
    puts "TrackedString: #{ObjectSpace.each_object(TrackedString).count}"
  end

  def print_with_stats(char_count)
    object_stats("Before GC")
    # Run the garbage collector.
    GC.start
    object_stats("After GC")
    puts char_count
  end
end

def wrapper
  clinger = ClingyObjects.new
  puts "Non-clingy:"
  count = clinger.generate
  clinger.print_with_stats(count)
  puts "\nClingy:"
  count = clinger.generate(:should_cling)
  clinger.print_with_stats(count)
  # Try to GC again for fun
  puts "\nTry GC again"
  GC.start
  clinger.print_with_stats(count)
  puts "\nDitch clinger and try GC again"
  clinger = nil
  5.times do
    GC.start
    puts "TrackedArray: #{ObjectSpace.each_object(TrackedArray).count}"
    puts "TrackedString: #{ObjectSpace.each_object(TrackedString).count}"
    puts "\nSleep a bit and try again"
    sleep 3
  end
  puts "TrackedArray: #{ObjectSpace.each_object(TrackedArray).count}"
  puts "TrackedString: #{ObjectSpace.each_object(TrackedString).count}"
end
wrapper
puts "TrackedArray: #{ObjectSpace.each_object(TrackedArray).count}"
puts "TrackedString: #{ObjectSpace.each_object(TrackedString).count}"

# Non-clingy:
# Before GC:
# TrackedArray: 1
# TrackedString: 30000
# After GC:
# TrackedArray: 0
# TrackedString: 0
# 150000000

# Clingy:
# Before GC:
# TrackedArray: 1
# TrackedString: 30000
# After GC:
# TrackedArray: 1
# TrackedString: 30000
# 150000000

# Try GC again
# Before GC:
# TrackedArray: 1
# TrackedString: 30000
# After GC:
# TrackedArray: 1
# TrackedString: 30000
# 150000000

# Ditch clinger and try GC again
# TrackedArray: 1
# TrackedString: 30000

# Sleep a bit and try again
# TrackedArray: 1
# TrackedString: 30000

# Sleep a bit and try again
# TrackedArray: 1
# TrackedString: 30000

# Sleep a bit and try again
# TrackedArray: 1
# TrackedString: 30000

# Sleep a bit and try again
# TrackedArray: 1
# TrackedString: 30000