From: Jean-Hugues ROBERT <jean_hugues_robert@...>
Date: 2002-05-05T01:15:45+09:00
Subject: Re: Class methods defined in a module, how-to ?

Hello,

At 00:31 05/05/2002 +0900, you wrote:
> >>    class MyClass
> >>       extend MyModule
> >>    end
>
> > Thanks. However I would like to *include* the module, not extending it.
>See the recepie at
>         http://www.rubycookbook.org/showrecipe.rb?recipeID=166
>// Niklas

Thanks. I am using it in my CanValue module. When that module is included 
by YourClass, YourClass.create( an_obj ) will create a new object using 
an_obj's instance variables (or .clone() when appropriate). This is the 
equivalent of C++'s Copy Constructor (a Constructor that create an object 
whose "value" is the same as the original).

Yours,

Jean-Hugues

# Typical synopsis:
# class MyClassWithCopyConstructor
#   include CanValue
#   def initialize( some_stuff )
#     return self if initValue( some_stuff)
#     ... init somehow differently here ...
#   end
#   ...
# end
# The module basically makes it easy to write copy constructors. Nota: the new
# object is not a deep copy of the original, its instance variables have the
# values of the original object's ones.
# The module also defines a class.create() method that behaves like the
# class's .new() method but will act as a copy constructor if called with
# a parameter that is of the same class (or of a class derived from self).
#
# Bugs: The copy gets the instance variables of the source, including the ones
# the ones that were added by derived class. The .create() does not handle
# blocks the way .new() does.

module CanValue

   class Value < Hash
     def toValue() self end
   end

   def initValue( a_value )
     return nil unless a_value.respond_to? :toValue
     a_value.toValue().each_pair do | var_name, value |
       instance_eval "#{var_name}= value"
       # p "Setting #{var_name} instance variable"
     end
     self
   end

   def toValue()
     a_value = Value.new
     instance_variables().each do | var_name |
       a_value[var_name.intern()] = instance_eval( var_name)
     end
     a_value
   end

   # Defines a create() class method that will act as a copy constructor if
   # called with a parameter that is of the same kind.
   module ClassMethods
     def create( *param )
       # p "self #{self} is a #{self.class()}"
       if param.length != 1 then
         self.new( *param)
       else
         if (src = param[1]).kind_of? self then
           # Use faster .clone() if source object's class is the good one
           return src.clone() if src.class() == self
           obj = self.new()
           src.toValue().each_pair do | var_name, value |
             obj.instance_eval "#{var_name}= value"
           end
           obj
         else
           self.new( param[1])
         end
       end
     end
   end
   def self.append_features( includer )
     super( includer)
     includer.extend ClassMethods
   end

end # CanValue


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