Omitting Variables on Saving

As with YAML serialization, it is possible to limit the variables that are saved when serializing using Marshal. In YAML, you did this by writing a method called to_yaml_properties. With Marshal, you need to write a method named marshal_dump. In the code of this method you should create an array containing the actual variables to be saved (in YAML, you created an array of strings containing the variable names).

This is an example:

def marshal_dump
    [@variable_a, @variable_b]
end

Another difference is that, with YAML, you were able simply to load the data in order to re-create an object. With Marshal, you need to add a special method called marshal_load to which any loaded data is passed as an argument. This will be invoked automatically when you call Marshal.load, and it will be passed the data in the form of an array. The previously saved objects can be parsed from this array. You can also assign values to any variables that were omitted (such as @some_other_variable here) when the data was saved:

def marshal_load(data)
    @variable_a = data[0]
    @variable_b = data[1]
    @some_other_variable = "a default value"
end

Here is a complete program that saves and restores the variables @num and @arr but omits @str:

limit_m.rb

class Mclass
    def initialize(aNum, aStr, anArray)
        @num = aNum
        @str = aStr
        @arr = anArray
    end

    def marshal_dump
        [@num, @arr]
    end

    def marshal_load(data)
        @num = data[0]
        @arr = data[1]
        @str = "default"
   end
end

ob = Mclass.new( 100, "fred", [1,2,3] )
p( ob )
#=> #<Mclass:0x2be7278 @num=100, @str="fred", @arr=[1, 2, 3]>

marshal_data = Marshal.dump( ob )
ob2 = Marshal.load( marshal_data )
p( ob2 )
#=> #<Mclass:0x2be70e0 @num=100, @str="default", @arr=[1, 2, 3]>

Note that although the serialization is done here in memory, the same techniques can be used when using Marshal to save and load objects to and from disk.