[ruby-talk:7280] RE: Multiple inheritance and mixins

> -----Original Message-----
> From: [e-mail] [mailto:[e-mail]]On Behalf Of
> Adam Spitz
> Sent: Thursday, December 14, 2000 12:40 PM
> To: ruby-talk ML
> Subject: [ruby-talk:7252] Multiple inheritance and mixins
> 
> 
> I'm having trouble understanding the difference between Ruby's mixins
> and "ordinary" multiple inheritance. Can someone explain to me how
> mixins are different from, say, C++'s version of multiple inheritance?
> Which of C++'s problems has Ruby avoided? Is there anything you can do
> with multiple inheritance that you can't do with mixins?
> 

Basically Ruby's include  corresponds to C++'s  virtual inheritance.  
For example,  you can have an inheritance graph of Ruby Classes 
BC, XBC,XYBC and Ruby  Modules X,Y,XYB.

    	         X         B      C
             //   \\   //   \\ /       
            Y       XB       BC
             \\  //   \\   /  
              XYB      XBC
		   \\     /      
                  XYBC       	

"--" is Ruby inheritance and "==" is Ruby include.
In C++ land this translates to
"--" is public inheritance and "==" is virtual public inheritance.  
Note that you are not allowed to create instances of Ruby Modules
(sort of like C++ abstract classes ).
Ruby has a pretty straightforward way to resolve the ambiguity
problem resulting form it's restricted form  of  MI ... the code
snippet 
module A
	def f()
		 p 'A'
	end
end	
module  B
	def f()
		 p 'B'
	end
end	
class C
	def f()
		 p 'C'
	end
end
class D < C
end
x = D.new 
x.f
class D
include A
end
x.f
class D
include B
end
x.f 
has the output - C A B.
Don't forget that Ruby's object model is in some respect more complicated 
then C++'s object model  - there is no  C++ equivalent for  Metaclasses.

Christoph