How to exclude an instance variable from marshal dump

Question

My object contains an instance variable that point to a File object amongst several other attributes. Because of this Marshal cannot serialize it. How can I write a bespoke dump method so to exclude only that instance variable?

class Area
  attr_accessor :area
  attr_reader :several_other_attributes
  ........

  def initialize(name)
    @area = name
    @several_other_attributes = ....

    @log = File.open( 'test.log', 'w')
  end
end

Answer 1

You can very easily write a marshal_dump and marshal_load method that marshals all but a specified number of instance variables. Here's a fairly generic example of something you could do:

class MarshalTest
  attr_reader :foo, :bar, :baz, :t
  UNMARSHALED_VARIABLES = [:@foo, :@bar]

  def initialize(foo = nil, bar = nil, baz = nil, t = nil)
    @foo = foo
    @bar = bar
    @baz = baz
    @t = t
  end

  def marshal_dump
    instance_variables.reject{|m| UNMARSHALED_VARIABLES.include? m}.inject({}) do |vars, attr|
      vars[attr] = instance_variable_get(attr)
      vars
    end
  end

  def marshal_load(vars)
    vars.each do |attr, value|
      instance_variable_set(attr, value) unless UNMARSHALED_VARIABLES.include?(attr)
    end
  end
end

Example usage:

1.9.3-p550 :026 > m = MarshalTest.new(1, 2, 3, 4)
 => #<MarshalTest:0x007ff73194cae8 @foo=1, @bar=2, @baz=3, @t=4>
1.9.3-p550 :027 > m.foo
 => 1
1.9.3-p550 :028 > m.bar
 => 2
1.9.3-p550 :029 > m.baz
 => 3
1.9.3-p550 :030 > m.t
 => 4
1.9.3-p550 :031 > s = Marshal.dump(m)
 => "\x04\bU:\x10MarshalTest{\a:\t@bazi\b:\a@ti\t"
1.9.3-p550 :032 > n = Marshal.load(s)
 => #<MarshalTest:0x007ff73112b828 @baz=3, @t=4>
1.9.3-p550 :033 > n.foo
 => nil
1.9.3-p550 :034 > n.bar
 => nil
1.9.3-p550 :035 > n.baz
 => 3
1.9.3-p550 :036 > n.t
 => 4

As you can see, the instance variables foo and bar were ignored in the marshaling/unmarshaling.